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El siguiente estudio cualitativo investigó las percepciones con respecto a los desafíos y adecuaciones realizadas por un grupo de 16 docentes de inglés como lengua extranjera durante la crisis sanitaria del COVID-19 durante el año 2020. Esta investigación se basó en la teoría de distancia transaccional (TDT) propuesta por Moore (2016) y aplicada a la enseñanza en línea, la cual considera que el éxito en la enseñanza de lenguas en ambientes virtuales debe considerar tres aspectos: la presencia de enseñanza, la presencia social y cognitiva. Basado en estas dimensiones, y luego de determinar la moda a través del software estadístico JASP (2021), los resultados permiten concluir que las mayores adecuaciones de los docentes se abocaron a aquellos relacionados a la enseñanza, por ejemplo la adecuación de contenido para la enseñanza del inglés. Por otro lado, se puede señalar que, los principales desafíos corresponden a aspectos sociales y cognitivos por cuanto se advierte una falta específica de estrategias para promover la vinculación de los educandos en entornos virtuales tanto con los docentes como con sus pares, así como pocas opciones de los educandos para interactuar con el contenido en formatos digitales. Es significativo indicar que en relación al contenido lingüístico, los participantes sostienen que la oralidad puede ser desarrollada más efectivamente en un formato presencial. Los resultados de este estudio permitirán tomar mejores decisiones al momento de diseñar cursos de enseñanza de idiomas asistidos por tecnología y, además ratifica la necesidad de fomentar el desarrollo de las dimensiones sociales y cognitivas.

14 páginas.- 7 figuras.- 2 tablas.-71 referencias.- Additional supporting information can be found online in the Supporting Information section at the end of this article., Maritime Antarctic King George Island (South Shetland Islands) has experienced rapid warming in recent decades, but the impacts on soil organic matter (SOM) decomposition remain ambiguous. Most vegetation cover is dominated by bryophytes (mosses), whereas a few vascular plants, such as Deschampsia antarctica and Colobanthus quitensis grow interspersed. Therefore, SOM is mainly enriched with carbohydrates and C-alkyl, provided by mosses, which lack lignin as a precursor for aromatic compounds and humus formation. However, there is no clear answer to how substrate and temperature increase changes in Antarctic microbial respiration. We determined in what way SOM mineralization changes with temperature and substrate addition by characterizing the temperature sensitivity (Q10) of soil respiration in an open-top chamber warming experiment. We hypothesized that: (a) cold-tolerant microorganisms are well adapted to growing in maritime Antarctic soils (~ 0°C), so would not respond to low and moderate temperature increases because they undergo various metabolic mechanism adjustments until they experience increasing temperatures toward optimum growth (e.g., by enzyme production); and (b) cellulose, as a complex carbonaceous substrate of vegetated areas in Maritime Antarctic soils, activates microorganisms, increasing the Q10 of soil organic carbon (SOC) mineralization. Soils (5–10 cm) were sampled after four consecutive years of experimental warming for SOC composition, microbial community structure, and C mineralization at 4, 12, and 20°C with and without cellulose addition. Functional group chemoheterotrophs, represented mainly by Proteobacteria, decomposed more refractory SOC (aromatic compounds), as indicated by nuclear magnetic resonance (NMR) spectroscopy, in ambient plots than in warming plots where plants were growing. The C-CO2 efflux from the incubation experiment remained stable below 12°C but sharply increased at 20°C. Q10 varied between 0.4 and 4 and was reduced at 20°C, whereas cellulose addition increased Q10. In conclusion, as confirmed during field studies in a climate scenario, cold-tolerant microorganisms in maritime Antarctic soils were slightly affected by increasing temperature (e.g., 4–12°C), with reduced temperature sensitivity, as summarized in a conceptual model., We are indebted to the project Antarctic Plant Ecophysiology:Unraveling the biological consequences of climate change on plantpopulations of the Maritime Antarctic, Anillos de Investigación en Ciencia Antártica, Programa de Investigación Asociativa CONICYT-INACH ART1102.

Despite the global expansion of forest plantations in Chile, their effect on biology properties of soil has still been only scarcely studied. Land use change in the Chilean Coastal Cordillera (36° to 40° S) is mainly attributed to the conversion of native forest to agriculture and forest plantations (Eucalyptus globulus and Pinus radiata de Don). The aim of this paper was to evaluate the changes in microbial composition (PCR-DGGE) and enzyme activity after the substitution of a native forest (e.g., Nothofagus spp.) by fast-growing exotic species and cropping. The most important factors that influence the abundance and diversity of bacteria and the fungi community were the soil organic matter (SOM) content, phosphorous (P-Olsen), calcium (Ca), boron (B), and water-holding capacity. These variables can better predict the microbial community composition and its enzymatic activity in the surface Ah horizon. Land use change also affected chemical soil properties of biogeochemical cycles. However, to deeply understand the connection between chemical and physical soil factors and microbial community composition, more research is needed. On the other hand, the expansion of forest plantations in Chile should be subject to legislation aimed to protect the biological legacy as a strategy for forest productivity as well as the soil microbial biodiversity.

When studying carbon (C) sequestration in soil, it is necessary to recognize the maximal storage potential and the main influencing factors, including the climate, land use, and soil properties. Here, we hypothesized that the silt and clay contents in soils as well as the clay mineralogy are the main factors affecting the maximal C and N storage levels of soils. This hypothesis was evaluated using a database containing the organic C contents of topsoils separated by ultrasonic dispersion to determine the particle size fractions. The slopes of the linear regressions between the C contents in silt and clay to the soil organic C (SOC) and between the N contents in silt and clay to the total N content were independent of the clay mineralogy (2:1, 1:1, calcareous soil, amorphous clays), climate type (tropical, temperate, and Mediterranean), and land use type (cropland, grassland, and forest). This clearly shows that the silt and clay content is the main factor defining an upper SOC level, which allowed us to propose a generalized linear regression (R2 > 0.95) model with a common slope, independent of the land use and climate type, to estimate the soil C sequestration potential. The implications of these findings are as follows: (1) a common slope regression was accurately calculated (0.83 ± 0.02 for C-silt + clay

The effect of mineral properties on soil organic carbon (SOC) destabilization processes is poorly known. Andisols developed under temperate old-growth rain forest store important amounts of SOC due to strong mineral adsorption capacity. We evaluated the effect of mineral soil properties on microbial activity and native SOC mineralization in an incubation experiment with and without the addition of labile organic carbon (C) and nutrients. Top- and subsoil horizons of two Andisols with contrasting age from temperate old-growth rain forest were sampled. We used the selective dissolution methods to determine the metals complexed with SOC and allophane content. Radiocarbon analysis activity was performed to determine the 14C activity and 14C ages of all soils as a proxy for SOC persistence. Additionally, we carried out a laboratory incubation under controlled conditions with and without 13C labeled cellulose and nutrient solution (NH4NO3, KH2PO4) addition. A lower microbial activity in the subsoils of both Andisols compared to topsoils in accordance with the lower 14C activity at depth was recorded. After cellulose and nutrient additions, both subsoils horizons showed native SOC mineralization after 80 days of incubation. In contrast, these amendments did not induce native SOC mineralization in topsoils showing differences on C turnover rates between top- and subsoil horizons. In both soils, the native SOC mineralization after labile C and nutrient additions was negatively related to Na-pyrophosphate extractable C and may therefore be controlled by the formation of organo-mineral complexes with increasing soil C age. We conclude that the physicochemical soil characteristics evolving during pedogenesis control the SOC stabilization and also destabilization processes in Andisols developed under temperate old-growth rain forests.

Para analizar la dinámica del carbono orgánico de los suelos (COS) asociada a su cambio de uso, de vegetación y prácticas de manejo, es necesario desarrollar modelos para usarse en forma predictiva. Un enfoque de modelación es la distribución del COS presente en los complejos organominerales primarios (arcillas, limos y arenas) ligado a las fracciones físicas. Para la separación de las fracciones físicas se emplea la dispersión del suelo por ultrasonido, paso que requiere optimizar las energías de sonicación para lograr la completa dispersión del suelo. En este trabajo se discute el modelo COLPOS y su hipótesis, así como sus posibles extensiones que consideran las masas y los enriquecimientos de las fracciones físicas del suelo, adicionalmente al análisis de las relaciones entre esas fracciones. Para analizar los patrones asociados al modelo COLPOS y sus extensiones, se analizan resultados disponibles de fraccionamientos de suelos mexicanos realizados con ultrasonido, además de tres bases de datos de fraccionamientos similares publicados en la literatura. Éstos muestran que el modelo COLPOS puede ser parametrizado en función del tamaño y masa de las partículas del suelo; aunque para el caso de las masas algunos resultados son inconsistentes. Del análisis de los cocientes del carbono orgánico entre fracciones para dos cinéticas lineales diferentes (organomineral y organomineral más partícula) las relaciones muestran mayor dispersión que para el caso de solo considerar fracciones de una cinética.

Drying and rewetting (D/W) of soil have significant impacts on soil organic matter (SOM) turnover. We hypothesised that frequent D/W cycles would release the labile organic matter locked away in soil aggregates, increasing the priming effect (PE) (acceleration or retardation of SOM turnover after fresh substrate addition) due to preferential utilisation by microbes. 13C-labelled lignocellulose was added to the soil, and the effects of 0, 1, or 4 cycles of D/W were evaluated at 5 °C and 25 °C after a 27-day incubation of undisturbed soil cores from a temperate forest (Araucaria araucana). Following the incubation, macroaggregates (> 250 μm), microaggregates (250–53 μm), and silt + clay materials ( 2.0 g cm−3) were determined. D/W cycles caused macroaggregates to increase and a decrease in microaggregates (> 15%) at warm temperatures, and preferential use of the novel particulate organic matter (13C labelled), formerly protected fPOM. CO2 efflux was three times higher at 25 °C than at 5 °C. The D/W cycles at 25 °C had a strong negative impact on cumulative CO2 efflux, which decreased by approximately − 30%, induced by a negative PE of −50 mg C kg−1 soil with 1 D/W cycle and − 100 mg C kg−1 soil with 4 D/W cycles, relative to soil under constant soil moisture receiving 13C-labelled lignocellulose, but no cycles. Increasing the temperature and the number of D/W cycles caused a decrease in substrate use efficiency of particulate lignocellulose. In conclusion, D/W cycles at warm temperatures accelerated OM turnover due to preferential use from fPOM, increasing macroaggregates at the expense of microaggregates. A novel pathway of OM release and PE due to the D/W cycles is discussed.

Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our objective was to investigate N losses and N acquisition strategies along a unique ecosystem-sequence (ecosequence) ranging from arid shrubland through Mediterranean woodland to temperate rainforest. These ecosystems differ in mean annual precipitation, mean annual temperate, and vegetation cover, but developed on similar granitoid soil parent material, were addressed using a combination of molecular biology and soil biogeochemical tools. Soil N and carbon (C) contents, δ15N signatures, activities of N acquiring extracellular enzymes as well as the abundance of soil bacteria and fungi, and diazotrophs in bulk topsoil and rhizosphere were determined. Relative fungal abundance in the rhizosphere was higher under woodland and forest than under shrubland. This indicates toward plants' higher C investment into fungi in the Mediterranean and temperate rainforest sites than in the arid site. Fungi are likely to decompose lignified forest litter for efficient recycling of litter-derived N and further nutrients. Rhizosphere—a hotspot for the N fixation—was enriched in diazotrophs (factor 8 to 16 in comparison to bulk topsoil) emphasizing the general importance of root/microbe association in N cycle. These results show that the temperate rainforest is an N acquiring ecosystem, whereas N in the arid shrubland is strongly recycled. Simultaneously, the strongest 15N enrichment with decreasing N content with depth was detected in the Mediterranean woodland, indicating that N mineralization and loss is highest (and likely the fastest) in the woodland across the continental transect. Higher relative aminopeptidase activities in the woodland than in the forest enabled a fast N mineralization. Relative aminopeptidase activities were highest in the arid shrubland. The highest absolute chitinase activities were observed in the forest. This likely demonstrates that (a) plants and microorganisms in the arid shrubland invest largely into mobilization and reutilization of organically bound N by exoenzymes, and (b) that the ecosystem N nutrition shifts from a peptide-based N in the arid shrubland to a peptide- and chitin-based N nutrition in the temperate rainforest, where the high N demand is complemented by intensive N fixation in the rhizosphere.

Simple Summary Scientific knowledge should transcend the barriers between the laboratory and the field to act in the service of humanity. Considering the enormous potential that soil offers for organic carbon (SOC) sequestration for the mitigation of greenhouse gas (GHG) emissions, and considering the recognized ecological importance of biological soil crusts (biocrusts) to be applied in the soil–plant continuum, we propose three perspectives to apply biocrusts to sustainable agriculture. Abstract The major priority of research in the present day is to conserve the environment by reducing GHG emissions. A proposed solution by an expert panel from 195 countries meeting at COP 21 was to increase global SOC stocks by 0.4% year−1 to compensate for GHG emissions, the ‘4 per 1000′ agreement. In this context, the application of biocrusts is a promising framework with which to increase SOC and other soil functions in the soil–plant continuum. Despite the importance of biocrusts, their application to agriculture is limited due to: (1) competition with native microbiota, (2) difficulties in applying them on a large scale, (3) a lack of studies based on carbon (C) balance and suitable for model parameterization, and (4) a lack of studies evaluating the contribution of biocrust weathering to increase C sequestration. Considering these four challenges, we propose three perspectives for biocrust application: (1) natural microbiome engineering by a host plant, using biocrusts; (2) quantifying the contribution of biocrusts to C sequestration in soils; and (3) enhanced biocrust weathering to improve C sequestration. Thus, we focus this opinion article on new challenges by using the specialized microbiome of biocrusts to be applied in a new environment to counteract the negative effects of climate change.

Simple Summary Manganese (Mn)-oxidizing bacteria (MnOxb) are an essential group of microorganisms that oxidize soluble Mn(II) to form precipitate Mn(III) minerals, playing a crucial role in soil formation. The Fildes Peninsula is one of the fastest-warming areas globally and, therefore, the maritime Antarctic soils from this pivotal location allow for the examination of the effect of temperature on bacterial communities. The temperature causes an increase in the microbial respiratory rate, producing reactive oxygen species (ROS), which are harmful to bacteria. We evaluate an evasive secondary non-enzymatic mechanism for ROS production under increasing temperature in MnOxb isolated from Antarctic soils. Bacteria produce ROS capable of oxidizing Mn(II) as temperature increases, contributing to the enzymatic pathway protecting microbial cells from Mn(II) toxicity. In addition, we determine that certain strains, such as Arthobacter oxydans, can use these ROS as mechanisms to protect themselves from Mn toxicity at high concentrations. In conclusion, we describe a secondary mechanism of Mn(II) oxidation in bacterial strains of Antarctic soils. Abstract Manganese (Mn) oxidation is performed through oxidative Mn-oxidizing bacteria (MnOxb) as the main bio-weathering mechanism for Mn(III/IV) deposits during soil formation. However, with an increase in temperature, the respiration rate also increases, producing Reactive Oxygen Species (ROS) as by-products, which are harmful to microbial cells. We hypothesize that bacterial ROS oxidize Mn(II) to Mn(III/IV) as a secondary non-enzymatic temperature-dependent mechanism for cell protection. Fourteen MnOxb were isolated from Antarctic soils under the global warming effect, and peroxidase (PO) activity, ROS, and Mn(III/IV) production were evaluated for 120 h of incubation at 4 °C, 15 °C, and 30 °C. ROS contributions to Mn oxidation were evaluated in Arthrobacter oxydans under antioxidant (Trolox) and ROS-stimulated (menadione) conditions. The Mn(III/IV) concentration increased with temperature and positively correlated with ROS production. ROS scavenging with Trolox depleted the Mn oxidation, and ROS-stimulant increased the Mn precipitation in A. oxydans. Increasing the Mn(II) concentration caused a reduction in the membrane potential and bacterial viability, which resulted in Mn precipitation on the bacteria surface. In conclusion, bacterial ROS production serves as a complementary non-enzymatic temperature-dependent mechanism for Mn(II) oxidation as a response in warming environments.

Microaerophilic white-rot fungi (WRF) are impacted by oxygen depletion because of fluctuating redox occurrence in southern temperate forest soils of Chile (1500–5000 mm year−1). How these conditions influence WRF survival has been scarcely examined. We explored the contributions of WRF to greenhouse gas (GHG) emissions of N2O and CH4 and soil organic C oxidation (CO2) in five sterilized and inoculated forest soils derived from various parent materials and climates. The soil was incubated for 20 days following (i) oxic, (ii) anoxic, and (iii) fluctuating redox conditions. Fungi contributed to 45% of the total GHG under redox fluctuating conditions, including the contribution of bacteria, while the opposite (26%) was valid for oxic treatment. On average, the highest gas emission (62%) was N2O for WRF under redox treatment, followed by anoxic (22%) and oxic (16%) treatments, while CO2 and CH4 emissions followed oxic &gt, redox &gt, anoxic. These data suggest that indigenous microbial WRF communities are well adapted to fluctuating redox milieu with a significant release of GHG emissions in humid temperate forests of the southern cone.

Microbial activity and functioning in soils are strongly limited by carbon (C) availability, of which a great proportion is released by living roots. Rhizodeposition and especially root exudates stimulate microbial activity and growth, and may shift the stoichiometric balance between C, N, and P. Thereby, exudates heighten microbial nutrient demand and acquisition of N and P from organic matter, leading to an increase in enzyme production. Aim of this study was to determine environmental controls of extracellular enzyme production, and hence on potential enzyme activities (Vmax) and substrate affinities (Km). To determine the controlling factors, we worked on four spatial scales from the microscale (i.e. rhizosphere) through the mesoscale (i.e. soil depth) and landscape scale (relief positions), and finally to the continental scale (1200 km transect within the Coastal Cordillera of Chile). Kinetics of seven hydrolyzing enzymes of the C, N, and P cycles (cellobiohydrolase, β‑glucosidase, β‑xylosidase, β‑N‑acetylglucosaminidase, leucine‑aminopeptidase, tyrosine‑aminopeptidase, and acid phosphatase) were related to soil texture, C and N contents, pH, and soil moisture via redundancy analysis (RDA). Potential activities of C, N, and P acquiring enzymes increased up to 7-times on the continental scale with rising humidity of sites and C and N contents, while substrate affinities simultaneously declined. On the landscape scale, neither Vmax nor Km of any enzyme differed between north and south slopes. From top- to subsoil (down to 120 cm depth) potential activities decreased (strongest of aminopeptidases under humid temperate conditions with up to 90%). Substrate affinities, however, increased with soil depth only for N and P acquiring enzymes. Affinities of cellobiohydrolase and β‑xylosidase, on the contrary, were 1.5- to 3-times higher in top- than in subsoil. Potential activities of N and P acquiring enzymes and β‑glucosidase increased form bulk to roots. Simultaneously, substrate affinities of N and P acquiring enzymes declined, whereas affinities of β‑glucosidase increased. These trends of activities and affinities in the rhizosphere were significant only for acid phosphatase. The RDA displayed a strong relation of potential activities of C and P acquiring enzymes and β‑N‑acetylglucosaminidase to C and N contents in soil as well as to the silt and clay contents. Aminopeptidase activity was mainly dependent on soil moisture and pH. We conclude that substrate availability for microorganisms mainly determined enzyme activity patterns on the continental scale by the humidity gradient. Patterns on the meso- and microscale are primarily controlled by nutrient limitation, which is induced by a shift of the stoichiometric balance due to input of easily available C by roots in the rhizosphere.

We evaluated the abiotic formation of dissolved organic nitrogen (DON) by the fast reaction of iron (Fe) with nitrate (NO3−) in the dissolved organic matter (DOM) of volcanic soils in a temperate rainforest (>5000 mm precipitation per year). During five days, the educts and products of abiotic reactions under anoxic conditions were measured in a microcosm experiment depending on the Fe and NO3− concentrations. A control zero-Fe was not used because there was no chemical reaction with nitrate addition. Using a novel technique of automated sample preparation for inorganic N (SPIN) attached to a membrane inlet quadrupole mass spectrometry (MIMS), the 15N abundances and inorganic N concentrations were determined directly in aqueous solutions. The results were explained in the context of the Ferrous Wheel Hypothesis which states that Fe(II) is utilized to reduce NO3− to nitrite (NO2−) that is incorporated into DOM. Fe(II) is regenerated from Fe(III) in anaerobic soil microsites. Here we tested one part of this hypothesis, the processes occurring in DOM (instead of soil organic matter). Using the SPIN-MIMS technique, we could overcome Ferrous Wheel Hypothesis criticism regarding possible Fe interference during NO3− analysis. The total recovery of 15N added as NO3− fluctuated between 63 and 101%, and the remaining 15N was measured as gaseous N2O. The 15N-labelled NO3− added decreased immediately after 15 min of incubation. After five days of incubation, approximately 25% of the labelled NO3− (e− acceptors) added was transformed to DON in the presence of a high amount of Fe(II) (e− donors). Small amounts of N2O and CO2 provided further evidence of NO3− reduction and DOM oxidation, respectively. From these results, we propose a new theoretical model that includes the Ferrous Wheel Hypothesis, where only the transformation of NO3− to DON was proven. The present results explain the high retention of NO3− in DOM from volcanic soils in ecosystems with high precipitation.

The diversity of orchid mycorrhizal fungi (OMF) and other beneficial root-associated fungi in temperate forests has scarcely been examined. This study aimed to analyze the diversity of mycorrhizal and rhizosphere-associated fungal communities in the terrestrial orchids Gavilea lutea and Chloraea collicensis growing in high-orchid-population-density areas in the piedmont of the Andes Cordillera with native forest (Nothofagus-Araucaria) and Coastal Cordillera with an exotic plantation (Pinus-Eucalyptus) in south-central Chile. We focused on rhizosphere-inhabiting and peloton-associated OMF in a native forest (Andes Cordillera) and a mixed forest (Coastal Cordillera). The native terrestrial orchids G. lutea and C. collicensis were localized, mycorrhizal root segments were taken to isolate peloton-associated OMF, and rhizosphere soil was taken to perform the metabarcoding approach. The results revealed that Basidiomycota and Ascomycota were the main rhizosphere-inhabiting fungal phyla, showing significant differences in the composition of fungal communities in both sites. Sebacina was the most-abundant OMF genera in the rhizosphere of G. lutea growing in the native forest soil. In contrast, Thanatephorus was the most abundant mycorrhizal taxa growing in the rhizosphere of orchids from the Coastal Cordillera. Besides, other OMF genera such as Inocybe, Tomentella, and Mycena were detected. The diversity of OMF in pelotons differed, being mainly related to Ceratobasidium sp. and Tulasnella sp. These results provide evidence of differences in OMF from pelotons and the rhizosphere soil in G. lutea growing in the Andes Cordillera and a selection of microbial communities in the rhizosphere of C. collicensis in the Coastal Cordillera. This raises questions about the efficiency of propagation strategies based only on mycorrhizal fungi obtained by culture-dependent methods, especially in orchids that depend on non-culturable taxa for seed germination and plantlet development.

Microaerophilic white-rot fungi (WRF) are impacted by oxygen depletion because of fluctuating redox occurrence in southern temperate forest soils of Chile (1500–5000 mm year−1). How these conditions influence WRF survival has been scarcely examined. We explored the contributions of WRF to greenhouse gas (GHG) emissions of N2O and CH4 and soil organic C oxidation (CO2) in five sterilized and inoculated forest soils derived from various parent materials and climates. The soil was incubated for 20 days following (i) oxic, (ii) anoxic, and (iii) fluctuating redox conditions. Fungi contributed to 45% of the total GHG under redox fluctuating conditions, including the contribution of bacteria, while the opposite (26%) was valid for oxic treatment. On average, the highest gas emission (62%) was N2O for WRF under redox treatment, followed by anoxic (22%) and oxic (16%) treatments, while CO2 and CH4 emissions followed oxic > redox > anoxic. These data suggest that indigenous microbial WRF communities are well adapted to fluctuating redox milieu with a significant release of GHG emissions in humid temperate forests of the southern cone.

14 páginas.- 4 figuras.- 5 tablas.- 61 referencias., Wildfire induces soil alterations that have a long-term impact on soil organic matter (SOM) quality. We postulated that after different fire severities, the neutral sugars in soils can be used as an indicator of soil organic matter quality after fire. The aim of this study was to determine the contribution of neutral sugar, bulk and occluded particulate organic matter (oPOM) affected by wildfire, at different soil depths in an Araucaria–Nothofagus Forest, four years post-after fire. The concentration and composition of the neutral sugars in the soils clearly comprised the major fraction in the unburned soil. Medium-and high-severity fires caused a drastic reduction in soil sugars in the bulk soil as well as in the oPOM fractions. The13C-CPMAS NMR spectroscopy analysis revealed a high contribution of recalcitrant carbon to the decomposition such as aryl–C and aryl–O derived from charred material, whereas the abundance of O–alkyl C and alkyl C functional groups were decreased. The neutral sugars (Galactose+Mannose/Xilose+Arabinose) revealed a major microbial origin in fire affected areas as the ratio was >2. Therefore. Therefore, we suggest that the neutral sugar content of soil should be used for monitoring both short-and long-term changes in SOM altered by fires. © 2021 by the authors. Licensee MDPI, Basel, Switzerland., The first author also thanks CONICYT (National Commission for Scientific and Technological Research) and FONDECYT Postdoctoral Project # 3140161 (National Fund for Scientific and Technological Development). D.R. was funded by the Water Research Center for Agriculture and Mining, CRHIAM (ANID/FONDAP/15130015).

In temperate rainforest soils of southern Chile (38 °S), there are high rates of soil organic carbon (SOC) mineralization under oxygen (O2) limitation. We study the combined effects of Fenton reactions and the activity of two enzymes manganese peroxidase (MnP) and lignin peroxidase (LiP), which was hypothesised potentiate SOC mineralization under anoxic conditions leading to carbon dioxide (CO2) release. Both mechanisms produce free radicals when competing for SOC oxidation in the absence of microorganisms. We quantify the CO2 release by induced Fenton reactions in combination with MnP and LiP under aerobic and anaerobic conditions (20 °C, 36 h) in temperate rainforest soils. CO2 levels released by Fenton reactions and enzyme activity were eight times higher than those released by Fenton reaction and peroxidase enzymes in individual treatment. Approximately 31% of the CO2 released under aerobic soil incubation was found to be abiotic (sterilized), while 69% was biotic (non-sterilized soils), and respective values of 17% and 83% were recorded under anaerobic conditions. The relative fluorescence intensity clearly shows ·OH radicals production from Fenton reactions. In conclusion, levels of MnP and LiP coupled with Fenton reactions strongly increase SOC mineralization under long periods of O2 limitation in temperate rainforest soils.

Arbuscular mycorrhizal fungi (AMF) are important partners in plant nutrition, as they increase the range to scavenge for nutrients and can access resources otherwise occlude for plants. Under water shortage, when mobility of nutrients in soil is limited, AMF are especially important to acquire resources and can modulate plant drought resistance. Strategies of plants to cope with water and nutrient restrictions are shaped by the intensity of aridity. To investigate the effect of aridity on plant-AMF associations regarding drought resistance and plant nutrient acquisition, a 13CO2 pulse labeling was conducted across an aridity gradient. In a semiarid shrubland (66 mm a-1), a Mediterranean woodland (367 mm a-1), and a humid temperate forest (1500 mm a-1), root and soil samples were taken from 0-10 cm and 20-30 cm soil depth before labeling and at 1 day, 3 days, and 14 days after labeling. Carbon (C), nitrogen (N), and phosphorus (P) stocks as well as AMF root colonization, extraradical AMF biomass (phospho- and neutral lipid fatty acids (PLFA and NLFA) 16:1w5c), specific root length (SRL), and root tissue density (RTD) were measured. Plant C investment into AMF and roots was determined by the 13C incorporation in 16:1w5c (PLFA and NLFA) and root tissue, respectively. Soil C:N:P stoichiometry indicated a N and P limitation under humid conditions and a P limitation in the topsoil under Mediterranean conditions. N stocks were highest in the Mediterranean woodland. A strong correlation of the AMF storage compound NLFA 16:1w5c to C:P ratio under semiarid conditions pointed to a P limitation of AMF, likely resulting from low P mobility in dry and alkaline soils. With increasing aridity, the AMF abundance in root (and soil) decreased from 45% to 20% root area. 13C incorporation in PLFA 16:1w5c was similar across sites, while relative AMF abundance in topsoil (PLFA 16:1w5c:SOC) was slightly higher under semiarid and humid than under Mediterranean conditions, pointing to the importance of AMF for plant nutrition under nutrient limitation. Additionally, PLFA 16:1w5c contents in soil were higher with lower P availability in each site, underlining the role of AMF to supply P for plants under P deficiency. Under humid conditions (with strong N and P limitation) and semiarid conditions (with strong water limitation), root AMF colonization increased with lower N availability, displaying the role of AMF for plant N nutrition under nutrient and/or water shortage. Under humid and Mediterranean conditions, SRL decreased (0.5 and 0.3 times, respectively) and RTD increased (1.9 and 1.7 times, respectively) with depth, indicating a drought tolerance strategy of plants to sustain water shortage. Under semiarid conditions, SRL increased with depth (2.3 times), while RTD was consistently high, suggesting an increasing proportion of long-living fine roots with depth as scavenging agents for water. These relations point to a drought avoidance strategy of plants as adaptation to long-term water limitation. Under strong nutrient limitation, as under humid and semiarid conditions, AMF are crucial to sustain plant nutrition and to enhance plant resistance to water shortage.

Temperate rain forest soils (>8000 mm yr -1 ) of south of Chile in the East Andes range areintensively affected by increasing freezing and thawing cycles (FTC) due to increasingclimate variability in the last 20 years. Most of these volcanic forests soils are unpolluted(pristine) and receive seasonal snow-cover. In spite of pollutant free precipitations, thesnow cover in these ecosystems contains aerosols, nutrients and microorganisms fromcircumpolar south west winds. These inputs and FTC generate specific conditions at theshallow layer at the soil surface for soil microbiology and biochemistry. The objectives ofthe study were to compare (micro)biological and chemical properties of topsoil and snowcover in an pristine forest and after clear-cut. The organic matter mineralization wasmonitored in a microcosm experiment to explore the effects of FTC and snow melting onredox potential and other topsoil parameters. FTC for soil+snow released more CO 2 inclosed forest (81.9 mg CO 2 kg -1 ) than that after clear-cut (20.5 mg CO 2 kg -1 ). Soil textureand soil organic matter accumulation played a crucial role for organic matter mineralizationand CO 2 fluxes. Gradually increase of temperature after freezing reveled that loamy soilswith certain amount of available C maintain active microbial population that response veryfast to temperature change. Sandy soils with very low C content showed the oppositeresults – very slow response of microbial community and CO 2 fluxes. In conclusion,microbial community structure and functions have distinct transition from snow to the soilin temperate snow-covered forest ecosystem. FTC showed that different microbial groupsare responsible for organic matter mineralization in soil under forest and clear-cut, becausethe pH and redox potential are influenced by snow melting.

Electron acceptors (NO3–, SO42–, Fe3+, Mn4+) play a crucial function in the oxidation of soil recalcitrant organic compounds. Soils that present large amount of total Fe (8-57 g kg-1soil) and organic (C) (10-110 g kg-1soil), iron-reducing bacteria (IRB) may play a importan role. In the present study we hypothesized that IRB which reduce Fe(III)(oxyhydr)oxide of low solubility to soluble Fe(II), can contribute substantially to the degradation of lignin from soil organic matter (SOM). The aim of this study was to isolate IRB and evaluate their importance in lignin degradation. IRB were obtained from topsoils of different climates (humid temperate, cold temperate, subpolar), vegetation type (steppe, rainforest) and parent materials (granitic, volcanic, fluvio-glacial, basaltic-Antartic and metamorphic). The potential of IRB to reduce Fe(III) was assessed with lactate substrate as source of carbon (C) and anthraquinone-2,6-disulfonate (AQDS) as electron acceptor. The contribution of IRB to lignin degradation was assessed in an anaerobic microcosms experiment for 36 h. The CO2 efflux from sterilized and reinoculated soil with IRB was compared with sterilized (abiotic), non-sterilized (biotic) and induced Fenton reaction. Lignin degradation by IRB was examined by: 1) bacterial growth containing alkali lignin and alkali lignin disappearance during incubation, 2) Lignin peroxidase and manganese peroxidase activities originated from IRB, 3) cells abundance estimated from ATP synthase from bacteria growing in alkali lignin and 4) lignin degradation monitored by fluorescence disappearance intensity. The major microbial group for Fe(III) reduction, as essayed by PLFA and nested-PCR and sequencing different species were Geobactericeae-strains (G. metallireducens and G. lovleyi) in all studied. The CO2 respiration in reinoculated soils was 140% higher than the CO2 release by abiotic and Fenton reaction and, 40% lower than biotic treated soil. The Fe(II) extractable in HCl in soil derived from basaltic-Antarctic parent material showed 362 % more Fe(II) solubilisation than that of biotic treatment. Fluorescence intensity decreased during lignin degradation and it was closely correlated with CO2 release in the same sample. We conclude that IRB community such as Geobacter spp. Uses intensively Fe(III) as an electron acceptor to oxidize lignin compounds, and this process is especially active in Fe rich soils.

Besides nitrogen, phosphorus (P) is the major limiting nutrient of terrestrial primary productivity, with major P stocks being bound in soils. Stocks, speciation, and bioavailability of soil P differ among ecosystem types and with rock weathering status, which are both driven by climatic conditions. Microorganisms and plants have developed a range of strategies to mobilize P from organic and inorganic sources, e.g. expression of extracellular phosphatases and excretion of low-molecular-weight organic acids (LMWOA). However, the impact of precipitation, vegetation type, and soil P speciation on plant P acquisition strategies is not well understood, yet.A semi-desert-to-humid-temperate-rainforest ecosystem sequence was investigated. Soil samples were taken from three sampling sites, all developed on granodiorite, comprising a precipitation gradient (66 mm a-1 to 1469 mm a-1) along the Chilean Coastal Cordillera. Small-scale gradients (mm) from single roots to bulk soil in three depths were sampled to examine changes in P speciation, enabling the identification of local P depletion by plant roots and differences in P-speciation between rhizosphere and non-rhizosphere soil. Phosphorus speciation was examined by X-ray absorption near edge structure analysis. LMWOA as biotic weathering agents, and acid phosphatase kinetics as proxy for organic P recycling, were quantified. The aim was to disentangle the impact and functions of roots and associated microorganisms on driving agents of P-cycling.Rhizosphere P speciation in soil changed considerably along the precipitation gradient from mainly primary P minerals in the semi-desert ecosystem to a dominance of organic P species in the humid-temperate rainforest. Contents of organically bound P were higher in root proximity compared to bulk soil in the humid-temperate rainforest soils (320 mg kg-1 and 70 mg kg-1, respectively) and in the topsoil of the Mediterranean woodland ecosystem (134 mg kg-1 and 62 mg kg-1, respectively). In contrast, the rhizosphere soil was depleted in sesquioxide-adsorbed P in comparison to root-free bulk soil.The content of LMWOA was correlated with inorganic P in soils of the semi-desert ecosystem, indicating intensive LMWOA exudation for biogenic P weathering of primary and secondary minerals. Under temperate rainforest LMWOA content, phosphatase activity, and microbial biomass carbon exhibited a negative correlation with secondary inorganic P forms but were positively linked to organic P species. We therefore conclude that P nutrition in this ecosystem relies less on weathering of P bearing minerals by LMWOA but is mainly based on organic P sources.In terms of process understanding, these findings clearly show that LMWOA fundamentally change their role in the rhizosphere depending on the nutrient acquisition strategy of the respective ecosystem, which is affected by mean annual precipitation. While LMWOA facilitate biogenic weathering of P bearing minerals in the semi-desert, they mainly contribute to P recycling in the humid-temperate rainforest by preventing its precipitation and sorption. We conclude that P acquisition and cycling depends on the nutritional constrains of the given ecosystem: from biological weathering of inorganic P forms in the semi-desert driven by LMWOA and plant uptake to intensive P recycling from organic forms in the humid-temperate rainforest.

Temperate forests in Chile have experienced increasing temperatures and extreme climatic events, such as severe drought and short winters in unique Araucaria araucana forest in Nahuelbuta National Park. Therefore, it is relevant to understand the impact of drying and rewetting (D/R) or freezing and thawing (F/T) on SOM turnover in these ecosystems. Particularly important is the destabilization of soil organic matter (SOM) by microbial activity, which is highly heterogeneous and influenced by soil properties and water cycles. Drying and rewetting or F/T cycles accelerate particulate organic matter (POM) decomposition by aggregate disruption, thereby, decreasing carbon (C) availability for soil microorganism. We hypothesized that frequent D/R and F/T cycles release labile organic C locked away in the aggregates for microbial consumption. We assumed that a repeated number of D/R and F/T cycles enhance the preferential C utilization of fresh organic substrate. In the present study an incubation experiment was conducted for 27 days to assess the effect of F/T (-18 ºC to room temperature) and D/R (-500 kPa to 33 kPa, field capacity) cycles on labelled 14C glucose and 13C lignocellulose decomposition, soil aggregates size and POM fractions distributions. CO2 efluxes and priming effect (PE), i.e. the turnover acceleration or retardation of native C mineralization, C use efficiency (CUE) and C allocation in soil aggregate classes as POM-light, POM-occluded and heavy fractions were also determined. Labelled glucose was mainly allocated in macro (> 250 mm) and microaggregates (< 250 mm) as part of the POM-light fraction. In contrast, labelled lignocellulose was allocated in microaggregate in the POM-occluded and heavy fraction. CUE was similar amongst all treatments. The PE was negative in soil with and without cycles and it was much more pronounced (-125 mg C kg-1 soil) for F/T cycles than D/R (-50 mg C kg-1 soil) at the end of incubation. The C:N ratio of soil following mining theory is further discussed. We conclude that D/R cycles clearly retarded the native C mineralization by preferential use of labelled 13C-lignocellulose, while F/T cycles led to preferential use of 14C-glucose.

Mathematical–statistical problem on estimation of soil-forming processes. This paper raises a mathematical–statistical problem that finally can be found in any chronosequence of a specific Quaternary-based process (e.g. soil mass, clay content accumulation). The problem arises when interval-specific rates of a component f(t) are intended by dividing the stock by the given age t. The procedure implies that only an average specific rate is calculated in a linear form, for example when soil organic carbon often tends to an asymptotic end-value. For any other parameters (e.g. sedimentation rates, soil horizons thickness), the same holds true. The mathematical–statistical problem arises if the rates are derived using a linear calculation (individual stocks, f(t) divided by time, t) and then plotted as a function of time, in circumstances where there is no correlation between t and the components f(t). To illustrate this problem, we used a random generator function where the plot t versus f(t) are not correlated. We also give examples of the chronosequence approaches of soil organic carbon and soil mass, where, at a given point, the real specific rate corresponds to the slope (first derivative) of the obtained non-linear regression function. The random generator function for the plot t versus f(t) showed no significant relationship (R2 = 0.0), but f(t)t−1 and t showed highly significant correlation (R2 = 0.62). The error in calculating the component rate by dividing by time instead of using the derivative function when the processes are not linear ranged between 22% and > 500%. We conclude that non-independent variables plotted in a chronosequence can infer correlations even when they might not exist. Carefully observation is needed on the dataset when the time is involved, particularly in Quaternary-based studies, since the mass component not necessarily is related with time. Avoid in calculating the change of the mass component simply dividing by time, because an over- or under-estimation from real rate (obtained by derivative function) occurs when the process under study is not linear.

RESUMEN: El auge de la producción intensiva del avestruz, comenzó en la década de los noventa impulsada por la calidad de su carne y potencialidad de sus subproductos. La raza empleada para producción por la calidad nutricional y sabor de su carne es el híbrido llamado African black (Struthio camelus var. domesticus). En cuanto a la reproducción, el avestruz hembra alcanza su madurez sexual a partir de los 2,5 años. Es importante considerar el aparato genital en aves de producción, ya que una alteración en él, puede generar deficiencias en la fertilidad que se traducen en un menor número de crías. El estudio histológico del aparato reproductor de la hembra será una herramienta más que permitirá resolver problemas reproductivos. Para este análisis se obtuvo muestras de los diferentes segmentos del aparato reproductor de 6 avestruces hembras en edad reproductiva y se procesaron de acuerdo a las técnicas histológicas de rutina. Los cortes fueron observados, fotografiados y analizados bajo microscopio de luz. Obtenidas las fotografías, se analizó comparativamente su morfología con la descrita en la gallina (Gallus gallus). El aparato reproductor de la hembra tiene la particularidad de tener desarrollado solo el ovario y oviducto izquierdo. El ovario es de gran tamaño y en forma de racimo, el cual varía según la estacionalidad. Presenta folículos primordiales, previtelogénicos, vitelogénicos y atrésicos. Los folículos vitelogénicos presentan células de la granulosa y de la teca interna y externa. El oviducto presenta de cefálico a caudal los siguientes segmentos: infundíbulo, magnum, istmo, útero y vagina, que desemboca en la cloaca a nivel del urodeo. En ellos hay pliegues de variada longitud, grosor y número que comprometen la mucosa y submucosa, con glándulas de secreción mucosa y serosa a excepción de la vagina. El análisis histológico comparativo, permitió establecer que la morfología del aparato reproductor de la hembra es semejante a la observada en la gallina con ciertas diferencias microscópicas (Gallus gallus).

Biodiversity of phototrophic microorganisms in South American biological soil crusts (BSCs) and their role in the biogeochemical phosphorus (P)-cycle are unknown. Richness of BSC green algae and cyanobacteria was investigated at four climatically different Chilean sites (arid, semi-arid, Mediterranean, humid). Carbon (C), nitrogen (N), sulfur (S), and P contents, P pools and P speciation as well as spatial P species distribution within the BSCs were investigated. Morphological identification of enrichment cultures revealed 24 green algal and 18 cyanobacterial taxa in total. Irrespective of climatic conditions, each BSC comprised 12 to 15 different phototrophic species. Thereby, green algal richness increased, while cyanobacterial richness decreased with increasing humidity/decreasing mean annual temperature (North to South). Total C, N, and S contents ranged between 6.7 and 41.1 g C kg−1, 0.6–2.8 g N kg−1 and 0.2–0.7 g S kg−1, respectively, and increased in the order crust-free soil

Little is known about the community structure of bacteria in the rhizosphere of canola during growth. This study was conducted to characterize the community structure of soil bacteria in relation to soluble-C dynamics in a Podzol during the growth of canola. Next generation sequencing of 16S rDNA characterized the main bacterial communities in a Podzol that was unplanted (soil alone), and planted to canola (Brassica napus) with and without N-fertilizer addition. PERMANOVA and ANOVA analysis showed that the number of operational taxonomic units and the relative abundance of bacteria were higher in canola rhizosphere than in soil alone for phyla, classes, orders and families. In soil alone and at the family level, communities of Acidobacteriaceae, Micrococcaceae, Nocardioidaceae, Intrasporangiaceae and Nitrosomonadaceae had the highest relative abundances (sequence reads ranged from 1,000 to 3,000). In the rhizosphere, communities of Sphingomonadaceae, Bacillaceae, Gaiellaceae, Micrococcaceae and Intrasporangiaceae presented the highest relative abundance (sequence reads ranged from ~8,000 to 23,000). The addition of N-fertilizer increased the relative abundance of bacteria in 5 families, decreased those in 39 and did not affect the communities in 56 other families. A two-way ANOVA showed that management, inclusive of canola and N-fertilizer, had the largest (87%) effect on the total relative abundance of soil bacteria and soluble-C content. Polynomial regression models of second order showed a close relation between the relative abundance of bacteria and soluble-C content during crop growth. We suggest that diffusional constraints depleted soluble-C in the vicinity of microbial communities to a growth limiting concentration more frequently in the soil alone than in canola rhizosphere.

Comparing the belowground allocation of assimilated carbon (C) to roots and mycorrhizal fungi across biomes can reveal specific plant nutrient acquisition strategies in ecosystems and allows to predict consequences of environmental changes. Three natural ecosystems (arid shrubland, coastal matorral, humid-temperate forest) distinct in annual precipitation and vegetation cover and compositions were selected to conduct a 13CO2 pulse labeling of natural woody vegetation to chase the allocation of assimilated C to arbuscular mycorrhizal (AM) fungi and fine roots. Further, nitrogen (N) and phosphorus (P) availability, root traits, root colonization, and the extraradical AM fungal mycelium (PLFA and NLFA 16:1ω5c) were analyzed to evaluate the efficiency of nutrient acquisition strategies. AM fungal colonization decreased with increasing aridity by up to 55% intraradical and by up to 90% extraradical. High root tissue densities – indicating longevity of roots – and low specific root lengths – indicating a low nutrient uptake capacity – pointed to a slow and resource conservative acquisition strategy of plants in the arid shrubland. Plants in the matorral, on the contrary, had lower root tissue densities but higher specific root lengths and higher root N contents, pointing to a fast nutrient acquisition strategy. The expression of abundant acquisitive fine roots of plants in the matorral, however, comes at the cost of larger C investment, shown by high 13C incorporation into root tissue. High root tissue densities and greater root diameter indicated that plants in the humid-temperate forest followed a resource-conservative strategy and outsource their nutrient acquisition to AM fungi. This outsourcing provides an efficient pathway to compensate a low uptake capacity of thick and dense roots. These ecosystem-specific acquisition strategies and distinct mutualism with AM fungi across the biomes will likely affect the sensitivity of plants to abiotic and biotic stressors and, thus, ecosystem responses to future climatic and environmental changes.

A disturbance due to selective logging causes the release of carbon from forest soils. Here, we examine this effect under Nothofagus stands in Andisols of southern Chile (39°38′S, 72°02′W, > 4000 mm yr − 1 ). We postulate that soil carbon destabilization caused by forest management is counteracted by two contrasting carbon stabilization mechanisms linked to pedogenic processes: (1) the formation of allophane-imogolite-like materials (i.e., short-range ordered aluminosilicate particles with a specific Al/Si molar ratio, for example in the present study 3.8) and (2) formation of Al- and Fe-carbon complexes. The aim of this study was to compare the effects of selective logging on the organic carbon storage of forest Andisols. We estimated that carbon storage occurred in the uppermost 1 m of soils under evergreen (EGF) and secondary growth mixed deciduous forests (DMF). The selective dissolution method was used to estimate the mechanisms of soil carbon storage. The soil carbon storage under the unmanaged EGF did not differ significantly from that of the stands measured 10 or 50 years after selective logging. Likewise, the soil carbon storage under DMF was similar to that under stands measured 2, 6 or 50 years after selective logging. Overall, soil carbon storage was 24% lower under DMF than under EGF, which was attributed to the faster decomposition of organic matter produced by deciduous forests. The soil pH of the Ah-horizon was significantly lower (4.5–5.1) than that of the subsoil (5.5–6.0) due to the leaching of cations, leading to the development of less allophanic properties (i.e., an aluandic horizon). This effect increased the formation of organo-mineral complexes and reduced the content of allophane-imogolite-like precursors (e.g., non-crystalline Si and Al). As a result, an inverse relationship was found between metals (Al, Fe) complexed with carbon and the content of non-crystalline Si and Al, both of which were within single horizons and the entire soil profile in general. We conclude that disturbances caused by forest management did not alter carbon storage in these Andisols, which can be explained by (1) the potential mechanisms of carbon stabilization studied here and (2) humid climate, which protected the bare horizon from irreversible drying after the management of these ecosystems, thus preventing potential carbon loss.

International audience; To date the effect of aluminium (Al) on carbon (C) mineralization has not been extensively studied in Chilean temperate rainforests. Here, we examine to what extent the C mineralization of water extractable organic matter (WEOM) and mineral Ah horizon are affected by differing molar ratios of Al and C (Al:C). It has been postulated that an increase in the concentration of Al in the soil solution does not affect the mineralization of WEOM and Ah horizon until a Al:C threshold is reached, at which point mineralization decreases. We hypothesized that dissolved organic C binds Al that is otherwise toxic to soil microorganism in the soil solution. Subsequently, the organic C remaining is mineralized or lost in water sheds. A microcosm experiment was designed where Al was added to WEOM or Ah horizon (0-8 mg L-1 or kg(-1)). Potential biodegradation was evaluated by trapping CO2 emitted during 15 days of incubation. Additionally, the amount of living fungal cells in the WEOM and microbial biomass of Ah horizon were quantified. The Al complexation was determined by selective dissolution, and characterized by infrared spectroscopy. In general, the carbon mineralization rates and the amount of fungal cells of WEOM were similar to that of the control (0 Al addition) when the Al:C ratio was

One of the critical aspects in modelling soil organic carbon (SOC) predictions is the lack of access to soil information which is usually concentrated in regions of high agricultural interest. In Chile, most soil and SOC data to date is highly concentrated in 25 % of the territory that has intensive agricultural or forestry use. Vast areas beyond those forms of land use have few or no soil data available. Here, we present a new database of SOC for the country, which is the result of an unprecedented national effort under the frame of the Global Soil Partnership that help to build the largest database on SOC to date in Chile named “CHLSOC" comprising 13,612 data points. This dataset is the product of the compilation from numerous sources including unpublished and difficult to access data, allowing to fill numerous spatial gaps where no SOC estimates were publicly available before. The values of SOC compiled in CHLSOC range from 6×10−5 to 83.3 percent, reflecting the variety of ecosystems that exists in Chile. Profiting from the richness of geochemical, topographic and climatic variability in Chile, the dataset has the potential to inform and test models trying to predict SOC stocks and dynamics at larger spatial scales., Earth System Science Data Discussions, ISSN:1866-3591

Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisition of mineral phosphorus by plants. However, the role of LMWOAs in mobilizing organic phosphorus, which is the predominant phosphorus form in at least half of the world's ecosystems, especially in humid climates, is unclear. The mechanisms of phosphorus mobilization by LMWOAs depend on climate, mainly precipitation, and shape the phosphorus nutrition strategies of plants. We disentangled the impact of roots and associated microorganisms on mechanisms of phosphorus cycling mediated by LMWOAs by studying soils along an ecosystem-sequence (ecosequence) from arid shrubland (~70 mm yr−1), and Mediterranean woodland (~370 mm yr−1) to humid-temperate forest (~1470 mm yr−1). Phosphorus speciation in soil was examined by X-ray absorption near edge structure analysis (XANES). LMWOAs were quantified as biological rock-weathering and organic phosphorus mobilization agents and compared to kinetics of acid phosphatase as a proxy for organic phosphorus mineralization. Calcium-bound phosphorus in topsoils decreased from 126 mg kg−1 in the arid shrubland, to 19 mg kg−1 in the Mediterranean woodland and was undetectable in the humid-temperate forest. In contrast, organic phosphorus in topsoils in close root proximity (0–2 mm distance to roots) was absent in the arid shrubland but raised to 220 mg kg−1 in the Mediterranean woodland and to 291 mg kg- 1 in the humid-temperate forest. The organic phosphorus content in topsoils was 1.6 to 2.4 times higher in close root proximity (0–2 mm distance to roots) compared to bulk soil (4–6 mm distance to roots) in the Mediterranean woodland and humid-temperate forest, showing intensive phosphorus bioaccumulation in the rhizosphere. Redundancy analysis (RDA) revealed that LMWOAs were explained by the content of hydroxyapatite and variscite phosphorus-species in the arid shrubland, indicating that LMWOAs contribute to mineral weathering in this soil. LMWOA contents, phosphatase activity, and microbial biomass carbon correlated strongly with organic phosphorus in the humid-temperate forest soil, which implies a high relevance of LMWOAs for organic phosphorus recycling. In the Mediterranean woodland soil, however, oxalic acid correlated with organic phosphorus in the topsoil (suggesting phosphorus recycling), whereas in the subsoil malic and citric acid were correlated with primary and secondary phosphorus minerals (implying mineral weathering). We conclude that phosphorus acquisition and cycling depend strongly on climate and that the functions of LMWOAs in the rhizosphere change fundamentally along the precipitation gradient. In the arid shrubland LMWOAs facilitate biochemical weathering (rock eating), while in the humid-temperate forest their functions change towards supporting organic phosphorus recycling (vegetarian).

Mechanisms of carbon dioxide (CO2) release from soil in the absence of oxygen were studied considering the Fenton process, which encompasses the reaction of H2O2 with Fe(II) yielding a hydroxyl radical ( OH), in combination with manganese peroxidase (MnP) and lignin peroxidase (LiP). This study aimed to explain the high rate of soil organic matter (SOM) mineralisation and CO2 release from humid temperate rainforest soils under oxygen-limited conditions. The investigated mechanisms challenge the traditional view that SOM mineralisation in rainforest is slow due to anaerobic (micro)environments under high precipitation and explain intensive CO2 release even under oxygen limitation. We hypothesised that the Fenton reaction (FR) greatly contributes to the CO2 released from SOM mineralised under anaerobic conditions especially in the presence of ligninolytic enzymes. We used a novel technique that combines labelled H218O2 and Fe(II) to induce the FR and measured CO18O, Fe(II) solubilisation, and peroxide consumption in a closed gas circulation system for 6 h. Maximal CO2 amount was released when the FR was induced in combination with LiP addition. The CO2 efflux with LiP was 10-fold that of abiotic FR reactions without enzymes, or in soils amended with MnP. This was consistent with i) the contribution of 18O from peroxide to CO2 release, ii) peroxide consumption, and iii) Fe(II) solubilisation by FR. The amount of consumed peroxide was closely correlated with the CO18O derived from soil without enzyme addition or with LiP addition. Concluding, abiotic Fenton Reaction coupled with oxidative enzymes, such as LiP, are crucial for SOM oxidation under anaerobic conditions, e.g. in temperate rainforest soils.

Iron-reducing bacteria (IRB) are crucial for electron transfer in anaerobic soil microsites. The utilization of the energy gathered by this mechanism by decomposers of organic matter is a challenging and fascinating issue. We hypothesized that bacteria reducing Fe(III) (oxyhydr)oxides to soluble Fe(II) obtain electrons from reduced soil organic matter (SOMr) involving lignin oxidation. Iron-reducing bacteria were isolated from topsoils of various climates (humid temperate, cold temperate, subpolar), vegetation types (mostly grasslands and forests), and derived from various parent materials treatments assigned as Granitic, Volcanic-allophanic, Fluvio-glacial, Basaltic-Antarctic and Metamorphic. After the screening of IRB by phospholipid fatty acid (PLFA) analysis and PCR identification (full-length 16S rDNA), the IRB were inoculated to 20 samples (five soils and 4 replicates) and a broad range of parallel processes were traced. Geobacter metallireducens and Geobacter lovleyi were the main Geobacteraceae-strains present in all soils and strongly increased the activity of ligninolytic enzymes: lignin peroxidase and manganese peroxidase. Carbon dioxide (CO2) released from IRB-inoculated soils was 140% higher than that produced by Fenton reactions (induced by H2O2 and Fe(II) addition) but 40% lower than in non-sterile soils. CO2 release was closely correlated with the produced Fe (II) and H2O2 consumption. The highest CO2 was released from Basaltic-Antarctic soils with the highest Fe content and was closely correlated with lignin depolymerization (detection by fluorescence images). All IRB oxidized the lignin contained in the SOM within a wide pH range and in soils from all parent materials. We present a conceptual model showing electron shuttling from SOM containing lignin (as a C and energy source) to IRB to produce energy and promote Fe(III) (oxyhydr)oxides reduction was proposed and discussed.

The silt and clay particles play a key role as stabilizing agents of soil organic carbon (SOC). Several lines of evidence indicate a theoretical maximum or C saturation in individual particles. In the present study, we hypothesized that a C fraction displaying linear accumulation relative to the SOC is not influenced by C saturation, while a fraction displaying an asymptotic relationship is regarded as saturated (Stewart et al., 2008). The aim of the present study was to compare the amount of C in the silt and clay sized fractions in temperate and subtropical cropping soils across a range of textures with different mineralogy. Twenty-one and 18 soil samples containing 1:1 and 2:1 clay of temperate soil from Chile under monoculture of maize (Zea maiz L.) for at least 30 years and 9 subtropical soils from Mexico under maize and bean (Phaseolus vulgaris L.) cropping for 9 years having mixed clay were collected at 0-0.1 m. The SOC of 2:1 soils was significantly higher (14±0.5 g kg-1 dry soil) than 1:1 soils (10±0.7 g kg-1). However, subtropical soils showed the highest values (59±0.5 g kg-1). A positive (P < 0.01) relationship was observed between the SOC and the C in the silt fraction (R2 0.80-0.97, P < 0.01). In contrast, the clay fraction remained constant or showed asymptotic behavior. We conclude that the silt fraction, unlike clay, showed no evidence of C saturation, while clay accumulates C to a maximum. On average, the 2:1 clay was saturated at 1-2 g C kg-1 and 1:1 at 1 g C kg-1, and subtropical soils at 14 g C kg-1.

Las emisiones de gases efecto invernadero asociadas al carbono orgánico del suelo (COS) son importantes para estudiar cambios en los ecosistemas terrestres. Hay modelos simples que, ofrecen la oportunidad de analizar la dinámica de las alteraciones provocadas por los cambios en el uso de suelo, la vegetación y las prácticas de manejo. En México se dispone de información del COS total y la textura del suelo. En el presente trabajo se analizan diferentes esquemas para parametrizar el modelo COLPOS, el cual se desarrolló para caracterizar la distribución del COS en fracciones físicas del suelo. El modelo considera el COS de diferentes almacenes: materia orgánica particulada (MOP, materia orgánica lábil) y complejos organominerales (MO + arena-limo-arcilla). El carbono orgánico inerte (COI), con tiempos medios de residencia de cientos a miles de años, y el tamaño de partícula asociado, permiten precisar los parámetros del modelo COLPOS y determinar las pérdidas potenciales de carbono orgánico del suelo. La validación de los métodos de parametrización del modelo COLPOS permite su implementación operativa empleando solamente la información disponible en México (COS y textura del suelo). La parametrización se desarrolló con base en los datos de experimentos de distribución del carbono asociadas al fraccionamiento físico de los suelos, después de la aplicación de dispersión ultrasónica para lograr fraccionamientos completos.

10 páginas.-- 3 figuras.-- 2 cuadros.-- 24 referencias, [EN]: In high-altitude montane forests, the vascular plants undergo mycorrhizal symbiosis with mycorrhizal fungi as a strategy to face extreme soil and climate conditions. In this study, glomalin content related to soil proteins (GRSP) was studied, an insoluble glycoprotein produced by hyphae of arbuscular mycorrhizal fungi and deposited on the soil, four years after a wildfire of variable severity, in Araucaria araucana forest in south-central Chile (38° S). For the previously stated, the aim proposed was to determine the content of this glycoprotein as well as to evaluate and relate the composition of soil organic matter to the content of this glycoprotein. Samples were collected at different depths (0-5, 5-10, 10-20 and 20-30 cm, respectively) from areas presenting various fire severities (low, medium and high) and an unburned soil area (control soil). Furthermore, composition and structure of soil organic matter was studied by Nuclear Magnetic Resonance Spectroscopy of 13C (NMR 13C). As main results, total GRSP concentration showed significant differences between the burned soil and the control soil. A significant correlation between the composition of the organic matter and GRSP was found. The high concentration of GRSP obtained in burned forests could answer to an ecological-evolutionary strategy from the Araucaria araucana forest in their adaptation to both a soil with low nutrient availability and to periodical fire catastrophic events; suggesting their key role in the recovery of these ecosystems and should be considered in restauration programs., [ES]: En bosques de alta montaña, las plantas vasculares establecen simbiosis con hongos micorrícicos como una estrategia a condiciones edafo-climáticas extremas. En este estudio, se analizó el contenido de glomalina relacionada a las proteínas del suelo (GRSP), una glicoproteína producida por hongos micorrícicos arbusculares y depositada en el suelo, después de cuatro años de ocurrido un incendio de severidad variable en un bosque de Araucaria araucana en el Centro-Sur de Chile (38°S). Para lo anterior, se propuso como objetivo determinar el contenido de esta glicoproteína en el suelo así como evaluar y relacionar la composición de la materia orgánica con el contenido de esta. Se analizó muestras de suelo a distintas profundidades (0-5, 5-10, 10-20 y 20-30 cm, respectivamente) en sectores con diversos niveles de severidad de incendio (baja, media, alta) y un área no afectada por el incendio (control). Adicionalmente, se estudió la estructura y composición de la materia orgánica mediante espectrometría de resonancia magnética nuclear del 13C (13C RMN). Como principales resultados, la concentración de GRSP mostró diferencias significativas entre el suelo quemado y suelo control. Se encontró una significativa correlación entre la composición de la materia orgánica y GRSP. La alta concentración de GRSP en los bosques quemados, podría responder a una estrategia ecológico-evolutiva de los bosques de A. araucana en su adaptación a suelos con limitantes nutricionales y sometidos a eventos catastróficos periódicos de incendios, que sugieren su rol clave en la recuperación de estos ecosistemas y deberían ser considerados en programas de restauración, Proyecto FONDECYT Postdoctorado # 3140161, al Dr. F. Borie por su apoyo en los análisis químicos en el Laboratorio de Ciencias Químicas, de la Universidad de la Frontera, Temuco; Proyecto DID-UACH (D-2006-17), Corporación Nacional Forestal (CONAF)

Enzyme activities (EAs) respond to contamination in several ways depending on the chemical form and content of heavy metals and metalloids (HMs) and their interactions with various soil properties. A systematic and mechanistic understanding of EA responses to HM contamination in soil is necessary for predicting the consequences for nutrient availability and the cycling of carbon (C), nitrogen (N), phosphorus (P) and sulphur (S). In this study, a meta-analysis based on 671 observations found the activities of seven enzymes to decrease in response to soil contamination with Pb, Zn, Cd, Cu and As. HM contamination linearly reduced the activities of all enzymes in the following order: arylsulfatase > dehydrogenase > β-glucosidase > urease > acid phosphatase > alkaline phosphatase > catalase. The activities of two endoenzymes: arylsulfatase (partly as exoenzyme) and dehydrogenase were reduced by 72% and 64%, respectively. These reductions were two times greater than of exoenzymes: β-glucosidase, urease, acid phosphatase, alkaline phosphatase and catalase (partly endoenzyme). This reflects the much stronger impact of HMs on living microorganisms and their endoenzymes than on extracellular enzymes stabilized on clay minerals and organic matter. Increasing clay content weakened the negative effects of HM contamination on EAs. All negative effects of HMs on EAs decreased with soil depth because HMs remain mainly in the topsoil. EAs involved in the cycling of C and S were more affected by HMs than the enzymes associated with the cycling of N and P. Consequently, HM contamination may alter the stoichiometry of C, N, P and S released by enzymatic decomposition of organic compounds that consequently affect microbial community structure and activity.

The effects of climate and topography on soil physico-chemical and microbial parameters were studied along an extensive latitudinal climate gradient in the Coastal Cordillera of Chile (26°–38°S). The study sites encompass arid (Pan de Azucar), semiarid (Santa Gracia), mediterranean (La Campana) and humid (Nahuelbuta) climates and vegetation, ranging from arid desert, dominated by biological soil crusts (biocrusts), semiarid shrubland and mediterranean sclerophyllous forest, where biocrusts are present but do have a seasonal pattern to temperate-mixed forest, where biocrusts only occur as an early pioneering development stage after disturbance. All soils originate from granitic parent materials and show very strong differences in pedogenesis intensity and soil depth. Most of the investigated physical, chemical and microbiological soil properties showed distinct trends along the climate gradient. Further, abrupt changes between the arid northernmost study site and the other semi-arid to humid sites can be shown, which indicate non-linearity and thresholds along the climate gradient. Clay and total organic carbon contents (TOC) as well as Ah horizons and solum depths increased from arid to humid climates, whereas bulk density (BD), pH values and base saturation (BS) decreased. These properties demonstrate the accumulation of organic matter, clay formation and element leaching as key-pedogenic processes with increasing humidity. However, the soils in the northern arid climate do not follow this overall latitudinal trend, because texture and BD are largely controlled by aeolian input of dust and sea salts spray followed by the formation of secondary evaporate minerals. Total soil DNA concentrations and TOC increased from arid to humid sites, while areal coverage by biocrusts exhibited an opposite trend. Relative bacterial and archaeal abundances were lower in the arid site, but for the other sites the local variability exceeds the variability along the climate gradient. Differences in soil properties between topographic positions were most pronounced at the study sites with the mediterranean and humid climate, whereas microbial abundances were independent on topography across all study sites. In general, the regional climate is the strongest controlling factor for pedogenesis and microbial parameters in soils developed from the same parent material. Topographic position along individual slopes of limited length augmented this effect only under humid conditions, where water erosion likely relocated particles and elements downward. The change from alkaline to neutral soil pH between the arid and the semi-arid site coincided with qualitative differences in soil formation as well as microbial habitats. This also reflects non-linear relationships of pedogenic and microbial processes in soils depending on climate with a sharp threshold between arid and semi-arid conditions. Therefore, the soils on the transition between arid and semi-arid conditions are especially sensitive and may be well used as indicators of long and medium-term climate changes. Concluding, the unique latitudinal precipitation gradient in the Coastal Cordillera of Chile is predestined to investigate the effects of the main soil forming factor – climate – on pedogenic processes.

The Chilean Coastal Cordillera features a spectacular climate and vegetation gradient, ranging from arid and unvegetated areas in the north to humid and forested areas in the south. The EarthShape project (“Earth Surface Shaping by Biota”) uses this natural gradient to investigate how climate and biological processes shape the Earth's surface. We explored the Critical Zone, the Earth's uppermost layer, in four key sites located in desert, semidesert, Mediterranean, and temperate climate zones of the Coastal Cordillera, with the focus on weathering of granitic rock. Here, we present first results from 16 approximately 2 m-deep regolith profiles to document: (1) architecture of weathering zone; (2) degree and rate of rock weathering, thus the release of mineral-derived nutrients to the terrestrial ecosystems; (3) denudation rates; and (4) microbial abundances of bacteria and archaea in the saprolite. From north to south, denudation rates from cosmogenic nuclides are ~10 t km−2 yr−1 at the arid Pan de Azucar site, ~20 t km−2 yr−1 at the semi-arid site of Santa Gracia, ~60 t km−2 yr−1 at the Mediterranean climate site of La Campana, and ~30 t km−2 yr−1 at the humid site of Nahuelbuta. A and B horizons increase in thickness and elemental depletion or enrichment increases from north (~26°S) to south (~38°S) in these horizons. Differences in the degree of chemical weathering, quantified by the chemical depletion fraction (CDF), are significant only between the arid and sparsely vegetated site and the other three sites. Differences in the CDF between the sites, and elemental depletion within the sites are sometimes smaller than the variations induced by the bedrock heterogeneity. Microbial abundances (bacteria and archaea) in saprolite substantially increase from the arid to the semi-arid sites. With this study, we provide a comprehensive dataset characterizing the Critical Zone geochemistry in the Chilean Coastal Cordillera. This dataset confirms climatic controls on weathering and denudation rates and provides prerequisites to quantify the role of biota in future studies.

There are scientific benefits for using an automated closed steady-state flow 13C (carbon; C) plant–soil labeling system (PSLS) for online recording and adjustment of 13CO2 (carbon dioxide) concentration equivalents to those used by photosynthesis, as compared to widely used uncontrolled open pulse labeling systems. Wheat plants were labeled at four growth stages (four leaves, tillering, late boot, and early dough) with 13CO2 for 2 and 6 h continuously. Analyses of the 13C/12C isotope using isotope ratio mass spectrometry of plant, soil solution, and rhizosphere soil samples showed that the injected 13CO2 was closely correlated with the 13C pools. The labeled 13C root exudates at tillering stage to rhizosphere soil increased from 0.4 ± 0.1 mg 13C pot−1 (four plants) after 2 h to 2.5 ± 1.0 mg 13C pot−1 after 6 h of labeling. The steady-state PSLS permits rapid control and maintenance of 13C by 13CO2 injection, enabling detailed examination of the C budget over short labeling periods.

Recent studies with Andisols show that the carbon (C) stabilization capacity evolves with soil age relative to the evolution of the mineral phase. However, it is not clear how soil mineralogical changes during pedogenesis are related to the composition of soil organic matter (SOM) and 14C activity as an indicator for the mean residence time of soil organic matter (SOM). In the present study, we analyzed the contribution of allophane and metal–SOM complexes to soil C stabilization. Soil organic matter was analyzed with solid-state 13C nuclear magnetic resonance spectroscopy. Additionally, the soil was extracted with Na-pyrophosphate (Alp, Fep) and oxalate (Alo, Sio, and Feo). Results supported the hypothesis that allophane plays a key role for SOM stabilization in deep and oldest soil, while SOM stabilization by metal (Al and Fe) complexation is more important in the surface horizons and in younger soils. The metal/Cp ratio (Cp extracted in Na-pyrophosphate), soil pH, and radiocarbon age seemed to be important indicators for formation of SOM–metal complexes or allophane in top- and subsoils of Andisols. Changes in main mineral stabilization agents with soil age do not influence SOM composition. We suggest that the combination of several chemical parameters (Alp, Fep and Cp, metal/Cp ratio, and pH) which change through soil age controls SOM stabilization.

The effects of cultivation in hillside (> 30% slope) on soil C and N stocks after forest clearance and subsequent cultivation in new re-growth forest sites not always induce rapid soil organic matter (SOM) loss in subtropical areas. In the present study we evaluated the sensitivity to SOM changes of material floatable in water (Fw), coarse sand (> 250 mm), fine sand (50-250 mm), silt (2-50 mm) and clay (0-2 mm) fractions by the impact of cultivation of Mexican Ferrasols. This research also determined the relative degree of C and N saturation in the silt and clay size particles. The soil physical fractions were obtained after ultrasonic vibration and sedimentation. Soil organic matter stock (0-20 cm depth) in cultivated sites ranged from 54 to 146 Mg C ha-1 and from 3.9 to 7.1 Mg N ha-1 and in the forest soil 46-94 Mg C ha-1 and 3.7-5.1 Mg N ha-1. The relative distribution of C and N was lower in the coarse sand and highest in the silt fraction; the latter paralleled the changes of whole soil C and N. Coarse sand was the most sensitive fraction responding to cultivation, thus representing a diagnostic pool to assess the SOM shifts under cultivation. Half of our studied soil showed a low degree of C and N saturation in the silt and clay fraction indicating potential for accumulation of C and N, while the other half were well above the saturation limit.Los efectos del cultivo de ladera (> 30% de pendiente) sobre el C y N del suelo y su almacenamiento después de la tala del bosque y sucesivo cultivo en nuevos sitios de crecimiento del bosque no siempre induce una pérdida de materia orgánica del suelo (SOM) en áreas subtropicales. En el presente estudio se evaluó la sensibilidad del cambio de la SOM del material que flota en agua (Fw), arena gruesa (> 250 µm), arena fina (50-250 µm), limo (2-50 µm) y arcilla (0-2 µm) por el impacto del cultivo en los Ferrasols mexicanos. Este estudio también determinó el grado de saturación relativo de C y N en las partículas del tamaño de las arcillas y limo. Las fracciones físicas del suelo se obtuvieron con vibración ultrasónica y posterior sedimentación. El stock de materia orgánica en los suelos cultivados varió desde 54 hasta 146 Mg C ha-1 y desde 3,9 hasta 7,1 Mg N ha-1 y en suelos del bosque 46-94 Mg C ha-1 y 3,7-5,1 Mg N ha-1. La distribución relativa de C y N fue menor en la arena gruesa y mayor en la fracción de limo; esta última siguió los cambios del C y N del suelo. La arena gruesa fue la fracción más sensitiva que responde a los efectos de la cultivación, lo que representa un reservorio de diagnóstico para evaluar los cambios de la SOM bajo cultivación. La mitad de las muestras de suelo estudiadas presentaron un bajo grado de saturación de C y N en las fracciones de limo y arcilla indicando que existe un potencial de estas fracciones para la acumulación de C y N, mientras que la otra mitad fueron muy por encima del límite de saturación.

Wildfires have shaped the biogeography of south Chilean Araucaria–Nothofagus rainforest vegetation patterns, but their impact on soil properties and associated nutrient cycling remains unclear. Nitrogen (N) availability shows a site-specific response to wildfire events indicating the need for an increased understanding of underlying mechanisms that drive changes in soil N cycling. In this study, we selected unburned and burned sites in a large area of the National Park Tolhuaca that was affected by a stand-replacing wildfire in February 2002. We conducted net N cycling flux measurements (net ammonification, net nitrification and net N mineralization assays) on soils sampled 3 years after fire. In addition, samples were physically fractionated and natural abundance of C and N, and 13C-NMR analyses were performed. Results indicated that standing inorganic N pools were greater in the burned soil, but that no main differences in net N cycling fluxes were observed between unburned and burned sites. In both sites, net ammonification and net nitrification fluxes were low or negative, indicating N immobilization. Multiple linear regression analyses indicated that soil N cycling could largely be explained by two parameters: light fraction (LF) soil organic matter N content and aromatic Chemical Oxidation Resistant Carbon (CORECarom), a relative measure for char. The LF fraction, a strong NH4+ sink, decreased as a result of fire, while CORECarom increased in the burned soil profile and stimulated NO3- production. The absence of increased total net nitrification might relate to a decrease in heterotrophic nitrification after wildfire. We conclude that (i) wildfire induced a shift in N transformation pathways, but not in total net N mineralization, and (ii) stable isotope measurements are a useful tool to assess post-fire soil organic matter dynamics.

In Mexico, 70% ofthe land surface shows some degree of degradation. A substantial portion of these degraded soils are located in the central part of the country, where a high population density exerts unusual pressure on the land. The study of these degraded soils is important because of the ecological, social and economic consequence of this ecosystem component Two types of degraded volcanic soils were studied in the present research: one coming from tepetates (a volcanic tuff, partially altered and ameliorated for production purposes) and the other, a highly eroded Acrisol developed from old volcanic materials. These soils have not been much studied and are here explored due to their potential to sequester carbon. In studies to focus the relationship between mineralogy and the carbon sequestration it will be necessary to clarify the characteristics of the fine fraction of the soil (