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Colloidal organo-mineral associations contribute to soil organic matter (OM) preservation and mainly occur in two forms: (i) as water-dispersible colloids that are potentially mobile (free colloids) and (ii) as building units of soil microaggregates that are occluded inside them (occluded colloids). However, the way in which these two colloidal forms differ in terms of textural characteristics and chemical composition, together with the nature of their associated OM, remains unknown. To fill these knowledge gaps, free and occluded fine colloids220 nm were isolated from arable soils with comparable organic carbon (C

Engineered nanoparticles (ENPs) released into the environment can affect phosphate (Pi) availability in soils. In this study, we evaluated the effect of silver (Ag) or copper (Cu) ENPs (3 and 5%, w/w) on Pi sorption processes in soil particle size fractions. The 2000–32 μm, 32–2 μm, and

The Atacama Desert is a temperate desert restricted by the Pacific Ocean and the Andeans, which is an ideal place to study the biogeochemical phosphate-water dynamics in the conditions with extreme limited water and biomass. We hypothesized that phosphate pools and oxygen signature change along with the increasing distance to the coast and thus aridity. The surface soils (0-10 cm) were sampled along the transect with distance to coast in Paposo region (~25°S) which is located in the Coastal Cordillera nearby the Pacific Ocean from 2.3 to 22.9 km, including 9 altitude sites (600 m, 900 m, 880 m, 920 m, 1000 m, 1200 m, 1450 m, 1700 m, 2110 m). Each site involved 3 samples surrounding the plant with a distance of 0-10 cm and other 3 samples far from the plant with 1 m. The Ca-bound P (HCl-extracted P followed the Hedley sequential P fractionation) accumulated along the increasing distance to coast within 37.9 km and could be described by a mono-exponential regression mode. However, an initial declining trend was detected for phosphate 18O of HCl-Pi and it reached a steady-state condition beyond 10 km from the coastline, which was the maximum distance that advective fog could penetrate inland. Only the nearest site at 2.3 km (600 m.a.s.l) to coast showed an isotope value within the range of full isotopic equilibrium with biologically cycled phosphate. Furthermore, the effects of the present plant distribution on surface soil Hedley P stocks and phosphate 18O signatures were very limited. We concluded that both P stocks and phosphate 18O signatures followed primarily the aridity gradient but phosphate 18O signatures could work as a tracer for long-term climate conditions.

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory is typically used to quantify surface interactions between engineered nanoparticles (ENPs), soil nanoparticles (SNPs), and/or porous media, which are used to assess environmental risk and fate of ENPs. This study investigates the co-transport behavior of functionalized multiwalled carbon nanotubes (MWCNTs) with positively (goethite nanoparticles, GNPs) and negatively (bentonite nanoparticles, BNPs) charged SNPs in quartz sand (QS). The presence of BNPs increased the transport of MWCNTs, but GNPs inhibited the transport of MWCNTs. In addition, we, for the first time, observed that the transport of negatively (BNPs) and positively (GNPs) charged SNPs was facilitated by the presence of MWCNTs. Traditional mechanisms associated with competitive blocking, heteroaggregation, and classic DLVO calculations cannot explain such phenomena. Direct examination using batch experiments and Fourier transform infrared (FTIR) spectroscopy, asymmetric flow field flow fractionation (AF4) coupled to UV and inductively coupled plasma mass spectrometry (AF4-UV-ICP-MS), and molecular dynamics (MD) simulations demonstrated that MWCNTs-BNPs or MWCNT-GNPs complexes or aggregates can be formed during co-transport. Non-DLVO interactions (e.g., H-bonding and Lewis acid-base interaction) helped to explain observed MWCNT deposition, associations between MWCNTs and both SNPs (positively or negatively), and co-transport. This research sheds novel insight into the transport of MWCNTs and SNPs in porous media and suggests that (i) mutual effects between colloids (e.g., heteroaggregation, co-transport, and competitive blocking) need to be considered in natural soil; and (ii) non-DLVO interactions should be comprehensively considered when evaluating the environmental risk and fate of ENPs.

The Atacama Desert (Chile) is one of driest places on Earth, with a hyper-arid climate and less than 2 mm yr−1 precipitation; nevertheless, it has experienced rare periods of sporadic rainfall. These periods shortly enhanced vegetation growth and microbial activity, which must have utilized major nutrients such as phosphorus (P). However, any biological cycling of P involves an oxygen exchange with water, which should now reside in the hyperarid soils as tracer of life. In order to identify such evidences, we performed sequential P fractionation and analyzed the oxygen isotope composition of HCl-extractable phosphate (δ18OHCl–P) in the surface soil (0–15 cm) of a climatic gradient along the rising alluvial fans of the Central Depression to the Precordillera, Chile. At the driest sites, the δ18OHCl-P values were constant with depth and deviated from biologically-driven isotopic equilibrium. In contrast, we observed a considerable increase of δ18OHCl-P values below the soil surface at less arid sites, where some isotope values were even within the range of full isotopic equilibrium with biologically cycled phosphate. For the latter sites, this points to most efficient biological P cycling right below the uppermost surface of the desert. Critically, the absolute concentrations of this biologically cycled P exceeded those of P potentially stored in living microbial cells by at least two orders of magnitude. Therefore, our data provides evidence that δ18OHCl-P values trace not recent but past biological activity, making it a powerful tool for assessing the existence, pathways and evolution of life in such arid ecosystems on Earth and, thus, potentially on other planets such as Mars.

Organic carbon (OC) in the hyper-arid Atacama Desert soils is known to be extremely low (0.1-0.01%). OC can accumulate on soil colloids (1-1000 nm) and nanoparticles (1-100 nm) due to its high specific surface area. Small-sized colloids may be transferred to deeper depth through the macropores in the soil. However, little is known about the colloidal-OC soil transfer under hyper-arid conditions. In this study, the Water Dispersible Colloids (WDCs, 4) coupled online to an Organic Carbon Detector (OCD). The experimental site is located at 1450 m altitude near Paposo (Antofagasta region, Chile) and receives -1 compared with the content found in crust-related older fan (0.24 mg OC kg-1) or at the edge between the fan sections (0.19 mg OC kg-1). Notably high WDC-OC in the fan near to the few isolated plant remains were also observed. The increase of biological activities and debris near to the plant contributes to more colloidal-OC (26.8 mg kg-1). The relatively flat hard impermeable surface of the crust-related old fan section may induce colloids loss during high-intensity rainfalls, e.g. occurring during past El Niño periods. Furthermore, the relative percentage of WDC-OC as a part of the total was highest in the upper layer (0-1 cm) of the active fan (27-48%) and at the edge (69%), while in the older crust-related sections the highest values were observed in the subsurface (5-10 cm) (19%-29%). Near the plant remains, nano-colloids were dominated in the upper soil accounting for 48% of the WDC-OC, whereas medium colloids were predominant in the older crust-related sections (64%). Dust (colloidal-sized) particles may be deposited at the surface and then are easily trapped near plants. We conclude that WDC-OC depth profiles within the hyper-arid Atacama Desert reflects the differential surface characteristics and the age of the fan surface, i.e., the period of geomorphological inactivity. During the extremely rare rainfall events in the Atacama, both factors will lead to differential infiltration rates, which thus in turn affect the size distribution of colloidal-OC with profile depth.

Microaggregates (250 µm) that resisted 60 J mL−1 ultrasonic dispersion. Afterwards, we assessed the C, N, Fe, and Si stable isotope composition in each size fraction. After four weeks we found a rapid build-up of stable macroaggregates comprising almost 50 % of soil mass in the treatment with plants and respective soil rooting, but only 5 % when plants were absent. The formation of these stable macroaggregates proceeded with time. Soil organic carbon (SOC) contents were elevated by 15 % in the large macroaggregates induced by plant growth. However, the recovery of EPS-derived 13C was below 20 % after 4 weeks, indicating rapid turnover in treatments both with and without plants. The remaining EPS-derived C was mainly found in macroaggregates when plants were present and in the occluded small microaggregates (

There is an intimate relationship between microorganisms and the formation and stability of soil microaggregates, realized by the immobilization and occlusion of microorganisms. Little is known about the initial aggregate formation phase and the role of microorganisms in this process under the impact of environmental changes such as wetting and drying. We investigated this initial aggregate formation process of montmorillonite and goethite in combination with two bacterial strains, Pseudomonas protegens strain CHA0 and Gordonia alkanivorans strain MoAcy 2, in the presence or absence of stress conditions in form of wetting and drying cycles for up to eight days. Montmorillonite and goethite formed microaggregates instantaneously, the size of these aggregates being enhanced in the presence of microorganisms, resulting in up to twofold larger aggregates. This increase in aggregate size was strain-dependent. However, the aggregates that developed during the first 48 h broke into smaller structures later on. A microscopic analysis of the microaggregates revealed that notably the larger microaggregates harbored bacteria and that microaggregates had a sheltering effect on living cells, especially when exposed to desiccation stress. Additionally, aggregate formation was analyzed in the presence of a Pseudomonas protegens mutant strain (CHA211) with increased production capability of extracellular polymeric substances (EPS). About fivefold higher survival rates of culturable cells were observed after desiccation for this EPS overproducing mutant strain in comparison to the wild-type. Our results hint at an aggregate formation process characterized by a rapid occlusion of mineral compounds, and, after the addition of microorganisms, the bacterial colonization of small microaggregates, leading to an increase in aggregate size. The further development of the aggregate size distribution varied depending on the presence of microbial taxa and was modulated by environmental conditions like desiccation events.

Engineered nanoparticles (ENPs) present in consumer products are being released into the agricultural systems. There is little information about the direct effect of ENPs on phosphorus (P) availability, which is an essential nutrient for crop growthnaturally occurring in agricultural soils. The present study examined the effect of 1, 3, and 5% doses of Cu0 or Ag0 ENPs stabilized with L-ascorbic acid (suspension pH 2–3) on P ad- and desorption in an agricultural Andisol with total organic matter (T-OM) and with partial removal of organic matter (R-OM) by performing batch experiments. Our results showed that the adsorption kinetics data of H2PO4− on T-OM and R-OM soil samples with and without ENPs were adequately described by the pseudo-second-order (PSO) and Elovich models. The adsorption isotherm data of H2PO4− from T-OM and R-OM soil samples following ENPs addition were better fitted by the Langmuir model than the Freundlich model. When the Cu0 or Ag0 ENPs doses were increased, the pH value decreased and H2PO4− adsorption increased on T-OM and R-OM. The H2PO4− desorption (%) was lower with Cu0 ENPs than Ag0 ENPs. Overall, the incorporation of ENPs into Andisols generated an increase in P retention, which may affect agricultural crop production.

Soil colloids < 220 nm including nanoparticles (1-100 nm), mainly composed of mineral particles and organic matter (OM) as well as their associations, have been gradually recognized as primary building units of the hierarchically organized soil aggregate system. As these colloidal building units are normally occluded inside soil aggregates, we refer to them as occluded colloids. Meanwhile, a large proportion of soil colloids is free from aggregate occlusion and mobile in the soil matrix. These free colloids can potentially serve as carriers for adsorbed nutrients and contaminants, mediating their translocation in the subsurface. However, the differences between free and occluded colloids remain unclear.Here, both occluded and free colloids were isolated from soil samples of an arable field with different clay contents. The occluded colloids were released from soil macroaggregates (>250 µm) with ultrasonic treatments. The free and occluded colloids were sequentially characterized for their size-resolved elemental composition using flow field-flow fractionation inductively coupled plasma mass spectrometry and organic carbon detector (FFF-ICP-MS/OCD). Besides, selected samples were also subjected to transmission electron microscopy (TEM) and pyrolysis field ionization mass spectrometry (Py-FIMS).Both free and occluded colloids mainly consisted of three size fractions: the first size fraction (0.6–60 nm), the second sized fraction (60–170 nm), and the third size fraction (>170 nm). The first size fraction was dominated by organic carbon and Ca, which were likely to be present as Ca-bridged OM associations. The elemental composition of the second and third size fractions was similar, which mainly consisted of organic carbon, Al, Si, and Fe, indicating the presence of mineral-mineral or mineral-organic associations. However, the ratios of organic to inorganic components in each size fractions varied among colloidal samples. TEM-EDX revealed that particles from free colloids were mainly present as mineral-mineral associations, while particles from occluded colloids tended to be mineral-organic associations. The C and N analysis showed higher N contents and narrower C/N ratios in free colloids when compared with occluded ones, suggesting different OM compositions in free and occluded colloids. The Py-FIMS results suggested that alkyl aromatics, phenols, lignin monomers, and lipids were the major OM compound classes in both free and occluded colloids. The relative abundance of carbohydrates, amides, heterocyclic nitrogen, and nitriles were higher in occluded colloids, whereas suberin and free fatty acids were relatively abundant in free colloids. Moreover, thermograms of OM compounds showed that occluded colloids possessed a higher proportion of thermal stable fractions of OM compounds, while the proportion of thermal liable fractions of OM compounds was relatively higher in free colloids. Overall, shedding light on the differences between free and occluded colloids may help us to gain insight into soil aggregate formation.

Nano and colloidal particles (1-1000 nm) play important roles in phosphorus (P) migration and loss from agricultural soils; however, little is known about their relative distribution in arable crop soils under varying agricultural geolandscapes at the regional scale. Surface soils (0-20 cm depth) were collected from 15 agricultural fields, including two sites with different carbon input strategies, in Zhejiang Province, China, and water-dispersible nanocolloids (0.6-25 nm), fine colloids (25-160 nm), and medium colloids (160-500 nm) were separated and analyzed using the asymmetrical flow field flow fractionation technique. Three levels of fine-colloidal P content (3583-6142, 859-2612, and 514-653 μg kg-1) were identified at the regional scale. The nanocolloidal fraction correlated with organic carbon (Corg) and calcium (Ca), and the fine colloidal fraction with Corg, silicon (Si), aluminum (Al), and iron (Fe). Significant linear relationships existed between colloidal P and Corg, Si, Al, Fe, and Ca and for nanocolloidal P with Ca. The organic carbon controlled colloidal P saturation, which in turn affected the P carrier ability of colloids. Field-scale organic carbon inputs did not change the overall morphological trends in size fractions of water-dispersible colloids. However, they significantly affected the peak concentration in each of the nano-, fine-, and medium-colloidal P fractions. Application of chemical fertilizer with carbon-based solid manure and/or modified biochar reduced the soil nano-, fine-, and medium-colloidal P content by 30-40%; however,the application of chemical fertilizer with biogas slurry boosted colloidal P formation. This study provides a deep and novel understanding of the forms and composition of colloidal P in agricultural soils and highlights their spatial regulation by soil characteristics and carbon inputs.

Redox fluctuations in hydromorphic soils can influence ecosystem functions by altering the cycling of organic carbon (OC) and other elements in both the aqueous and solid phases throughout the soil profile. Most studies focusing on the mobility of dissolved OC in rice paddy soils have often disregarded the contribution of colloidal particles. We provide a detailed chemical and size-related characterization of water-dispersible soil colloids and their depth distribution in two soil profiles under long-term temperate paddy and non-paddy management, by asymmetric flow field-flow fractionation. Anoxic conditions enhanced colloid dispersion with a preferential release of the finer colloid fractions (5 and 4-fold increase in the

Nowadays engineered silver nanoparticles (AgNP) are being widely used for a multitude of purposes. At certain times during their life-cycle they might enter soils and freshwater resources and thus recent research has focused on their transport and fate in soils and the vadose zone as well as the saturated zone. AgNP retention in the subsurface depends on a multitude of parameters including the type and shape of the sediments through the nanoparticles are exposed to, the chemical composition of pore and groundwater acting as background solution or the type and quantity of soil organic matter present. One aspect that has received little attention so far is their transport behaviour in the presence of nutrients.Here we study PVP-AgNP transport and retention in saturated columns containing silicate-dominated aquifer material that is also exposed to orthophosphate (NaH2PO4) or myo-inositol hexakisphosphate (IP6) via the background solution. In particular, we compare PVP-AgNP transport behaviour for different pH (6 and 4.5) in the background solution, for different mass concentrations of sediments Results of our experiments show that PVP-AgNP exhibit a higher mobility through the columns in the presence of phosphate as the latter can block attachment sites otherwise available to the nanoparticles. In the presence of SOM this mobility is even higher than in the absence of SOM as SOM and phosphate anions are both negatively charged and potentially bound to the same attachment sites. PVP-AgNP mobility also increased for both P-species when an increase in pH occurred but this increase was more pronounced in columns with orthophosphate. Results further show that PVP-AgNP are more mobile in columns with IP6 than orthophosphate in the absence of sediments

Rice paddy soils are known to represent a large proportion of global terrestrial carbon (C) stocks (ca.10 Pg), accumulating organic C in the topsoil due to cultivation under submerged conditions. Apart from the limited mineralization under anoxic soil conditions resulting from frequent field flooding, other mechanisms involving the dynamic interactions between organic C and redox-active minerals particularly Fe (oxy)hydroxides, together with the transport of organic C to deep mineral horizons, can lead to long-term C stabilization. Our previous studies have shown that up to 30-50 g m-2 of dissolved organic C (DOC, defined as -2 of Fe2+ may be mobilized and translocated into the subsoil over a rice cropping season in temperate rice paddies, contributing to an increase in belowground C stocks. However, little is yet know on influence of frequent redox fluctuations on the contribution of colloidal organo-mineral associations to C mobilization and accrual in paddy subsoils.We hypothesized that (i) redox fluctuations may lead to an overall increase in colloid dispersion (via reductive dissolution of Fe oxides, changes in soil pH, as well as neoformation of colloidal organo-mineral associations), and that (ii) colloidal mobility may represent an important C input to paddy subsoils. In order to evaluate the effects of redox fluctuations on colloid dynamics in situ, water-dispersible fine colloids (WDFC) were isolated from soils collected from different horizons along two profiles opened in adjacent plots under long-term paddy (P) and non-paddy (NP) management in NW Italy. Moreover, WDFC were also isolated from anaerobically-incubated topsoil samples to evaluate the changes in colloid dispersion under reducing conditions as a function of management. Colloidal size-fractionation and their elemental compositions were evaluated by asymmetric flow field-flow fractionation (AF4) coupled with OCD or ICP-MS. Our results evidenced that redox cycling favours colloidal stability in the topsoils, with a preferential dispersion of the smallest-sized colloidal C (240 nm) contributes predominantly to the WDFC. Consequently, under long-term paddy management colloidal dispersion and transport along the soil profile were probably responsible for the lower amounts of colloidal C (and Fe) observed in the Ap topsoil horizons of P with respect to NP, as well as for the significant accumulation of colloidal C in correspondence with the Brd subsoil horizons just beneath the plough pan. These illuvial horizons were also particularly rich in small-sized (30-240 nm) colloidal Fe, Al and Si possibly due to mineral phase changes induced by redox fluctuations.Our findings therefore indicate that downward mobilization of colloidal C associated with Fe (hydr)oxides (e.g. coprecipitates) or small aluminosilicate minerals, rather than dissolved organic C, may represent an important process driving organic C accrual in paddy subsoils. However, further insights are still required to entangle the contribution of the different mechanisms involved.

Several beneficial soil functions are linked to aggregates, but how the formation and stability depend on the presence of colloidal building blocks is still understood poorly. Here, we sampled subsites from an arable toposequence with 190 and 340 g kg-1 clay, and isolated small soil microaggregates (small SMA; < 20 µm) from larger macroaggregate units (> 250 µm) using an ultrasonic dispersion energy of 60, 250, and 440 J mL-1 , respectively. We then allowed these small SMA to reaggregate after chemical removal of organic carbon (OC) as well as of Fe- and Al (hydr)oxides, respectively. The size distribution of the reaggregated small SMA and fine colloids (< 0.45 µm) was analyzed via laser diffraction and asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry and OC detection, respectively. We found elevated amount of both fine colloids and stable SMA at subsites with larger clay contents. The size distribution of small SMA was composed of two distinct fractions including colloids (< 1 µm) and SMA with an average size of 5 µm. The removal of Fe with Dithionite-Citrate-Bicarbonate (DCB) shifted the size of the small SMA to a larger equivalent diameter, while destruction of OC with NaOCl reduced it. After three wetting and drying cycles, the concentration of colloids declined, whereas the small SMA without chemical pre-treatments reaggregated to particles with larger average diameters up to 10 µm, with the size depending on the clay content. Intriguingly, the gain in size was more pronounced after Fe removal, but it was not affected by OC removal. We suggest that Fe (hydr)oxides impact the stability of small SMA primarily via cementing the aggregates to smaller size. In contrast, the effect of OC was restricted to the size of colloids, gluing them together to small SMAs within defined size ranges when OC was present but releasing these colloids when OC was absent.

Although nanoscale surface roughness has been theoretically demonstrated to be a crucial factor in the interaction of colloids and surfaces, little experimental research has investigated the influence of roughness on colloid or silver nanoparticle (AgNP) retention and release in porous media. This study experimentally examined AgNP retention and release using two sands with very different surface roughness properties over a range of solution pH and/or ionic strength (IS). AgNP transport was greatly enhanced on the relatively smooth sand in comparison to the rougher sand, at higher pH, and lower IS and fitted model parameters showed systematic changes with these physicochemical factors. Complete release of the retained AgNPs was observed from the relatively smooth sand when the solution IS was decreased from 40mM NaCl to deionized (DI) water and then the solution pH was increased from 6.5 to 10. Conversely, less than 40% of the retained AgNPs was released in similar processes from the rougher sand. These observations were explained by differences in the surface roughness of the two sands which altered the energy barrier height and the depth of the primary minimum with solution chemistry. Limited numbers of AgNPs apparently interacted in reversible, shallow primary minima on the smoother sand, which is consistent with the predicted influence of a small roughness fraction (e.g., pillar) on interaction energies. Conversely, larger numbers of AgNPs interacted in deeper primary minima on the rougher sand, which is consistent with the predicted influence at concave locations. These findings highlight the importance of surface roughness and indicate that variations in sand surface roughness can greatly change the sensitivity of nanoparticle transport to physicochemical factors such as IS and pH due to the alteration of interaction energy and thus can strongly influence nanoparticle mobility in the environment.

Colloids and their subset nanoparticles are key soil constituents for nutrient and Organic Carbon (OC) storage and transport, yet little is known about their specific role in overall transfer of elements under hyper-arid conditions. We analyzed the Water Dispersible Colloids (WDCs) of two adjacent soil profiles, located either on the active (named: Fan) or passive (named: Crust) sections of an alluvial fan. Colloidal particles (

Soil organic matter (SOM) and pH are key ecosystem drivers, influencing resilience to environmental change. We tested the separate effects of pH and SOM on nutrient availability, plant strategies, and soil community composition in calcareous and acidic Grey dunes (H2130) with low, intermediate, and/or high SOM, which differ in sensitivity to high atmospheric N deposition. Soil organic matter was mainly important for biomass parameters of plants, microbes, and soil animals, and for microarthropod diversity and network complexity. However, differences in pH led to fundamental differences in P availability and plant strategies, which overruled the normal soil community patterns, and influenced resilience to N deposition. In calcareous dunes with low grass-encroachment, P availability was low despite high amounts of inorganic P, due to low solubility of calcium phosphates and strong P sorption to Fe oxides at high pH. Calcareous dunes were dominated by low-competitive arbuscular mycorrhizal (AM) plants, which profit from mycorrhiza especially at low P. In acidic dunes with high grass-encroachment, P availability increased as calcium phosphates dissolved and P sorption weakened with the shift from Fe oxides to Fe-OM complexes. Weakly sorbed and colloidal P increased, and at least part of the sorbed P was organic. Acidic dunes were dominated by nonmycorrhizal (NM) plants, which increase P uptake through exudation of carboxylates and phosphatase enzymes, which release weakly sorbed P, and disintegrate labile organic P. The shifts in P availability and plant strategies also changed the soil community. Contrary to expectations, the bacterial pathway was more important in acidic than in calcareous dunes, possibly due to exudation of carboxylates and phosphatases by NM plants, which serve as bacterial food resource. Also, the fungal AM pathway was enhanced in calcareous dunes, and fungal feeders more abundant, due to the presence of AM fungi. The changes in soil communities in turn reduced expected differences in N cycling between calcareous and acidic dunes. Our results show that SOM and pH are important, but separate ecosystem drivers in Grey dunes. Differences in resilience to N deposition are mainly due to pH effects on P availability and plant strategies, which in turn overruled soil community patterns.

Dissolved and colloidal phosphorus (P) represent the mobile P fractions in soils, but their role in P cycling in forests is still largely unclear. In this study of four calcareous forest soil profiles, the elemental compositions of different size fractions of water dispersible colloids (WDC) were investigated by asymmetric field flow fractionation. Nuclear magnetic resonance spectroscopy (NMR) was applied to identify the organic P compounds in soils, WDC, and soil solutions. Carbon was the dominant element in WDC of all soil horizons, including mineral soils that were rich in Ca or Si. Although chemical composition of P varied dramatically with increasing depth, the colloidal P composition remained unchanged. This contrasting difference between mineral soil and its WDC fraction indicated that the colloids were not locally generated but originated from the overlying organic soil horizons. Carbonate minerals were unlikely involved in colloid formation under acidic condition. Instead, Ca2+ probably drove colloid formation by bridging organic matter, including P-containing compounds released from litter degradation. Colloid formation was influenced by climate, vegetation, and soil characteristics. No dissolved P was detected in deeper mineral soil horizons due to efficient retention by Ca minerals. Colloidal P was still present in deeper soil layers and thus of significance for potential P vertical transfer.

Background Colloid-facilitated phosphorus (P) transport is recognized as an important pathway for the loss of soil P in agricultural systems; however, information regarding soil aggregate-associated colloidal P (Pcoll) is lacking. To elucidate the effects of aggregate size on the potential loss of Pcoll in agricultural systems, soils (0–20 cm depth) from six land-use types were sampled in the Zhejiang Province in the Yangtze River Delta region, China. The aggregate size fractions (2–8 mm, 0.26–2 mm, 0.053–0.26 mm and Results Our study demonstrated that 0.26–2 mm small macroaggregates had the highest total P (TP) content. In acidic soils, the highest Pcoll content was observed in the 0.26- to 2-mm-sized aggregates, while the lowest was reported in the coll than other land-use types. The proportion of Pcoll in total dissolved P (TDP) was dominated by coll in paddy soils, where Pcoll contributed up to 83% TDP in the silt + clay-sized particles. The Pcoll content was positively correlated with TP, Al, Fe, and the mean weight diameter. Aggregate-associated total carbon (TC), total nitrogen (TN), C/P, and C/N had significant negative effects on the contribution of Pcoll to potential soil P loss. The Pcoll content of the aggregates was controlled by the aggregate-associated TP and Al content, as well as the soil pH value. The potential loss of Pcoll from aggregates was controlled by its organic matter content. Conclusion We concluded that management practices that increase soil aggregate stability or its organic carbon content will limit Pcoll loss in agricultural systems.

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at the laboratory scale. Initial column experiments were conducted to investigate the influence of chemical heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence of soil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two retention sites. Results demonstrate that an increase in CCS content in the artificially mixed porous medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesive interaction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominated aquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation bridging. Surprisingly, SOM had no significant impact on PVP-AgNP retention, which suggests that a reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance of other binding mechanisms. Our findings are important for future studies related to AgNP transport in shallow unconsolidated calcareous and siliceous sands.

Due to their extreme aridity, high rate of UV irradiation and low soil carbon (C) content, the soils of the Atacama Desert represent one of the world's most hostile environments for microbial life and its survival. Although infrequent, climatic conditions may, however, prevail which temporarily remove these stresses and allow life to briefly flourish. In this study we investigated the response of soil microbial communities to water and C availability across an aridity gradient (semi-arid, arid, hyper-arid) within the Atacama Desert. We simulated the impact of hyper-dry spells, humid fogs and precipitation events on the activation of the microbial community and the subsequent mineralization of low (glucose) and high (plant residues) molecular weight C substrates. Our results showed that mineralization rate followed the trend: semi-arid > arid > hyper-arid. Some glucose mineralization was apparent under hyper-arid conditions (water activity, aw = 0.05), although this was 10-fold slower than under humid conditions and ca. 200-fold slower than under wet conditions. A lag phase in CO2 production after glucose-C addition in the hyper-arid soils suggested that mineralization was limited by the low microbial biomass in these soils. No lag phase was apparent in the corresponding semi-arid or arid soils. In contrast, the breakdown of the plant residues was initially much slower than for glucose and involved a much longer lag phase in all soils, suggesting that mineralization was limited by low exoenzyme activity, particularly in the humid and hyper-dry soils. Our results also showed that microbial C use efficiency followed the trend: hyper-arid > arid > semi-arid. In conclusion, we have shown that even under hyper-arid conditions, very low levels of microbial activity and C turnover do occur. Further, the microbial communities are capable of rapidly responding to available C once water becomes more abundant, however, this response is both biomass and metabolically limited in hyper-arid soils.

Nanoparticles and colloids affect the mobilisation and availability of phosphorus for plants and microorganisms in soils. We aimed to give a description of colloid sizes and composition from forest soil profiles and to evaluate the size-related quality of colloids for P fixation. We investigated the size-dependent elemental composition and the P content of water-dispersible colloids (WDC) isolated from five German (beech-dominated) forest soil profiles of varying bulk soil P content by field-flow fractionation (FFF) coupled to various detectors. Three size fractions of WDC were separated: (i) nanoparticles

Biogeochemical cycling of elements largely occurs in dissolved state, but many elements may also be bound to natural nanoparticles (NNP, 1-100 nm) and fine colloids (100-450 nm). We examined the hypothesis that the size and composition of stream water NNP and colloids vary systematically across Europe. To test this hypothesis, 96 stream water samples were simultaneously collected in 26 forested headwater catchments along two transects across Europe. Three size fractions (~1-20 nm, >20-60 nm, and >60 nm) of NNP and fine colloids were identified with Field Flow Fractionation coupled to inductively coupled plasma mass spectrometry and an organic carbon detector. The results showed that NNP and fine colloids constituted between 2 ± 5% (Si) and 53 ± 21% (Fe; mean ± SD) of total element concentrations, indicating a substantial contribution of particles to element transport in these European streams, especially for P and Fe. The particulate contents of Fe, Al, and organic C were correlated to their total element concentrations, but those of particulate Si, Mn, P, and Ca were not. The fine colloidal fractions >60 nm were dominated by clay minerals across all sites. The resulting element patterns of NNP

The hyperarid soils of the Atacama Desert, Chile, contain the largest known nitrate deposits in the world. They also represent one of the most hostile environments for microbial life anywhere in the terrestrial biosphere. Despite known for its extreme dryness, several heavy rainfall events causing localised flash flooding have struck Atacama Desert core regions during the last five years. It remains unclear, however, whether these soils can support microbial denitrification. To answer this, we sampled soils along a hyperaridity gradient in the Atacama Desert and conducted incubation experiments using a robotized continuous flow system under a He/O2 atmosphere. The impacts of four successive extreme weather events on soil-borne N2O and N2 emissions were investigated, i) water addition, ii) NO3− addition, iii) labile carbon (C) addition, and iv) oxygen depletion. The 15N–N2O site-preference (SP) approach was further used to examine the source of N2O produced. Extremely low N2O fluxes were detected shortly after water and NO3− addition, whereas pronounced N2O and N2 emissions were recorded after labile-C (glucose) amendment in all soils. Under anoxia, N2 emissions increased drastically while N2O emissions decreased concomitantly, indicating the potential for complete denitrification at all sites. Although increasing aridity significantly reduced soil bacterial richness, microbial potential for denitrification and associated gene abundance (i.e., napA, narG, nirS, nirK, cnorB, qnorB and nosZ) was not affected. The N2O15N site preference values based on two end-member model suggested that fungal and bacterial denitrification co-contributed to N2O production in less arid sites, whereas bacterial denitrification dominated with increasing aridity. We conclude that soil denitrification functionality is preserved even with lowered microbial richness in the extreme hyperarid Atacama Desert. Future changes in land-use or extreme climate events therefore have a potential to destabilize the immense reserves of nitrate and induce significant N2O losses in the region.

Natural soils have frequently been considered to decrease the mobility of engineered nanoparticles (NPs) in comparison to quartz sand due to the presence of colloids that provide additional retention sites. In contrast, this study demonstrates that the transport and release of silver nanoparticles (AgNPs) in sandy clay loam and loamy sand soils were enhanced in the presence of soil colloids that altered soil grain surface roughness. In particular, we found that the retention of AgNPs in purified soils (colloid-free and acid-treated) was more pronounced than in raw (untreated) soils or soils treated to remove organic matter (H2O2 or 600 °C treated). Chemical analysis and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy demonstrated that the grain surfaces of raw and organic matter-removed soils were abundant with metal oxides and colloids compared to purified soil. Column transport and release experimental results, SEM images, and interaction energy calculations revealed that a significant amount of concave locations on purified soils hindered AgNP release by diffusion or ionic strength (IS) reduction due to deep primary energy minima. Conversely, AgNPs that were retained in soils in the presence of soil colloids were more susceptible to release under IS reduction because the primary minimum was shallow on the tops of convex locations created by attached soil colloids. Additionally, a considerable fraction of retained AgNPs in raw soil was released after cation exchange followed by IS reduction, while no release occurred for purified soil under the same conditions. The AgNP release was highly associated with soil colloids and co-transport of AgNPs and soil colloids was observed. Our work is the first to show that the presence of soil colloids can inhibit deposition and facilitate the release and co-transport of NPs in soil by alteration of the soil grain surface morphology and shallow primary minimum interactions.

The concentration of nonionic surfactants like Triton X-100 (TX100) can influence the transport and fate of emerging contaminants (e.g., carbon nanotubes) in porous media, but limited research has previously addressed this issue. This study investigates the co-transport of functionalized multi-walled carbon nanotubes (MWCNTs) and various concentrations of TX100 in saturated quartz sand (QS). Batch experiments and molecular dynamics simulations were conducted to investigate the interactions between TX100 and MWCNTs. Results indicated that the concentration ratio of MWCNTs and TX100 strongly influences the dispersion of MWCNTs and interaction forces between MWCNTs and QS during the transport. Breakthrough curves of MWCNTs and TX100 and retention profiles of MWCNTs were determined and simulated in column studies. MWCNTs strongly enhanced the retention of TX100 in QS due to the high affinity of TX100 for MWCNTs. Conversely, the concentration of TX100 had a non-monotonic impact on MWCNT retention. The maximum transport of MWCNTs in the QS occurred at an input concentration of TX100 that was lower than the critical micelle concentration. This suggests that the relative importance of factors influencing MWCNTs changed with TX100 sorption. Results from interaction energy calculations and modeling of competitive blocking indicate that the predictive ability of interaction energy calculations and colloid filtration theory may be lost because TX100 mainly altered intermolecular forces between the MWCNT and porous media. This study provides new insights into the co-transport of surfactants and MWCNTs in porous media, which can be useful for environmental applications and risk management.

The release and stability of soil water-dispersible colloids (WDC) in the soil structure are critical for colloid-facilitated soil organic carbon sequestration and contaminants transport. In this study, the potential effects of temperature and associated organic carbon (OC) on the release of WDCs in three silt loam topsoils with the same clay content (~ 20%) under different land uses were investigated. A soil fractionation method was used for simulating the release of colloids from the soil under environmental conditions where mobilization and sedimentation processes occur sequentially. The surface loading of OC has been characterized by the analysis of organic carbon content of WDC with the measurements of the specific surface area (SSA). The effects of fractionation temperature on colloidal properties (e.g., particle size and zeta potential) were systematically investigated and the aggregation kinetics of WDC in salt electrolyte influenced by temperature was assessed by dynamic light scattering (DLS). Experimental results demonstrated that the amount of extracted WDC from three soils decreased when the fractionation temperature increased. A more rapid sedimentation of WDC at higher temperatures outweighed the effect of temperature on WDC mobilization from bulk soil in the shaking step. The sedimentation of WDC at various temperatures indicated that the temperature dependence of the water viscosity ( η ) was a dominate parameter and caused lower efficiency of WDC mass gained at higher temperature according to the Stoke's law. After introducing the factor of η 7 °C / η T , the temperature effect only on WDC mobilization during shaking step could be described and the whole fractionation process could be successfully timely determined along the two shaking and sedimentation steps. Activation energies ( E a ) of about 10 kJ mol − 1 could be now calculated for the WDC mobilization processes from the three topsoils. The associated organic carbon contents of WDC (WDC(OC)) and the mineral surface of WDC blocked by organic carbon ( SSA OC - block ) after various shaking temperatures and shaking time were further determined in order to examine the WDC(OC) effect on the release of WDC from soil matrix. The results demonstrated that the escape of the mobile clay fraction ( F ) from soil at short shaking times is favored by the presence of effective surface loading by an OC layer ( SSA OC - block ), which is known to stabilize its colloidal state through electrosteric effects. The WDC(OC) surface concentration has been also used to estimate the clay-associated OC distribution in the three topsoils. In Ca 2 + solution, an increase of temperature favors the colloidal stability of WDC as measured from the shift of critical coagulation concentration ( CCC ) to higher concentrations of Ca 2 + . In total, the results from this study revealed that temperature and WDC(OC) distribution are critical parameters when considering soil WDC release and stability in natural bulk soils.

Saturated soil column experiments were conducted to investigate the transport, retention, and release behavior of a low concentration (1mgL-1) of functionalized 14C-labeled multi-walled carbon nanotubes (MWCNTs) in a natural soil under various solution chemistries. Breakthrough curves (BTCs) for MWCNTS exhibited greater amounts of retardation and retention with increasing solution ionic strength (IS) or in the presence of Ca2+ in comparison to K+, and retention profiles (RPs) for MWCNTs were hyper-exponential in shape. These BTCs and RPs were well described using the advection-dispersion equation with a term for time- and depth-dependent retention. Fitted values of the retention rate coefficient and the maximum retained concentration of MWCNTs were higher with increasing IS and in the presence of Ca2+ in comparison to K+. Significant amounts of MWCNT and soil colloid release was observed with a reduction of IS due to expansion of the electrical double layer, especially following cation exchange (when K+ displaced Ca2+) that reduced the zeta potential of MWCNTs and the soil. Analysis of MWCNT concentrations in different soil size fractions revealed that >23.6% of the retained MWCNT mass was associated with water-dispersible colloids (WDCs), even though this fraction was only a minor portion of the total soil mass (2.38%). More MWCNTs were retained on the WDC fraction in the presence of Ca2+ than K+. These findings indicated that some of the released MWCNTs by IS reduction and cation exchange were associated with the released clay fraction, and suggests the potential for facilitated transport of MWCNT by WDCs.

Silver nanoparticles (AgNPs) are used in a growing number of applications and products. Previous studies showed AgNPs can leach from these products to the environment. As a result of AgNPs leaching, sediment, soil and sludge-treated soils may be contaminated with AgNPs. Methods to detect, quantify and characterize AgNPs in soil are urgently needed. This study describes the development and validation of a method for the extraction, quantification and particle size determination of AgNPs in soils. The final method consists of pre-wetting the sample followed by an aqueous extraction, using sonication to re-suspend adsorbed AgNPs, and analysis of the aqueous extract with single particle ICP-MS. Validation of the method showed that the recovery of AgNPs spiked to soil was 44% for sandy soil and 42% for clayey soil. Although this recovery is relatively low, the repeatability and reproducibility values of the particle concentration were within the limits of Horwitz ratio, which makes the method suitable for its purpose. Further, the method concentration detection limit, LODc., is 5 μg kg −1 soil. The developed method can be applied in eco-toxicological and risk-assessments studies for AgNP in the soil environment.

Transport and retention behavior of multi-walled carbon nanotubes (MWCNTs) was studied in mixtures of negatively charged quartz sand (QS) and positively charged goethite-coated sand (GQS) to assess the role of chemical heterogeneity. The linear equilibrium sorption model provided a good description of batch results, and the distribution coefficients (KD) drastically increased with the GQS fraction that was electrostatically favorable for retention. Similarly, retention of MWCNTs increased with the GQS fraction in packed column experiments. However, calculated values of KD on GQS were around 2 orders of magnitude smaller in batch than packed column experiments due to differences in lever arms associated with hydrodynamic and adhesive torques at microscopic roughness locations. Furthermore, the fraction of the sand surface area that was favorable for retention (Sf) was much smaller than the GQS fraction because nanoscale roughness produced shallow interactions that were susceptible to removal. These observations indicate that only a minor fraction of the GQS was favorable for MWCNT retention. These same observations held for several different sand sizes. Column breakthrough curves were always well described using an advective-dispersive transport model that included retention and blocking. However, depth-dependent retention also needed to be included to accurately describe the retention profile when the GQS fraction was small. Results from this research indicate that roughness primarily controlled the retention of MWCNTs, although goethite surfaces played an important secondary role.

The article “Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities”, written by Timothy S George et al., was originally published with incorrect affiliation information for one of the co-authors, E. Klumpp.

The roles of graphene oxide (GO) particle geometry, GO surface orientation, surface roughness, and nanoscale chemical heterogeneity on interaction energies, aggregation, retention, and release of GO in porous media were not fully considered in previous studies. Consequently, mechanisms controlling the environmental fate of GO were incompletely or inaccurately quantified. To overcome this limitation, plate-plate interaction energies were modified to account for these factors and used in conjunction with a mathematical model to interpret the results of GO aggregation, retention, and release studies. Calculations revealed that these factors had a large influence on the predicted interaction energy parameters. Similar to previous literature, the secondary minimum was predicted to dominate on smooth, chemically homogeneous surfaces that were oriented parallel to each other, especially at higher ionic strength (IS). Conversely, shallow primary minimum interactions were sometimes predicted to occur on surfaces with nanoscale roughness and chemical heterogeneity due to adsorbed Ca2+ ions, especially when the GO particles were oriented perpendicular to the interacting surface. Experimental results were generally consistent with these predictions and indicated that the primary minimum played a major role in GO retention and the secondary minimum contributed to GO release with IS reduction. Cation exchange (Na+ replacing Ca2+) enhanced GO release with IS reduction when particles were initially deposited in the presence of Ca2+ ions. However, retained GO were always completely recovered into the excess deionized water when the sand pore structure was destroyed during excavation, and this indicates that primary minima were shallow and that the pore structure also played an important role in GO retention. Further evidence for the role of pore structure on GO retention was obtained by conducting experiments in finer textured sand and at higher input concentrations that induced greater aggregation. In both cases, greater GO retention occurred, and retention profiles became more hyperexponential in shape.