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Summary In tropical montane forests nutrients released from the organic layers of the soil can supply a large part of the vegetation's requirements. We have examined concentrations, storage, and turnover times of nutrients in the organic layer and the fluxes of nutrients by the fall of small litter (leaves, seeds, flowers, small twigs, and plant debris that passed an opening of 0.3 m × 0.3 m) in such a forest in Ecuador. The times taken for litter to turn over were estimated by relating nutrient storage in the organic layer to rate of litterfall and by incubating samples in the laboratory. The organic layer had a thickness of 2–43 cm, a mass of 30–713 t ha-1 , and a nutrient storage of 0.87–21 t N, 0.03–0.70 t P, 0.12–2.5 t K, 0.09–3.2 t Ca, and 0.07–1.0 t Mg ha-1 . The pH (in H2 O) ranged between 3.1 and 7.4 and was correlated with the concentrations of Ca and Mg (r = 0.83 and 0.84, respectively). The quantity of small litter (8.5–9.7 t year-1 ) and mean concentrations of nutrients in litter (19–22 g N, 0.9–1.6 g P, 6.1–9.1 g K, 12–18 g Ca, and 3.5–5.8 g Mg kg-1 ) were larger than in many other tropical montane forests. The mean turnover times of elements in the organic layer increased in the order, Mg (7.0 years) < Ca (7.9) < K (8.5) < P (11) < N (14) < S (15) when calculated as the quotient of storage in the organic layer to flux by litterfall; they were < 12 years for N, P, and S in the incubation experiment. Under optimum conditions in the laboratory, the mineralization of S was just as large as the S deposition by litterfall. In weakly acid soils Mn and Zn and in strongly acid soils Ca added in a nutrient solution were immobilized during incubation. Thus, lack of S, Mn, Zn, and Ca might limit plant growth on some soils. [ABSTRACT FROM AUTHOR]

Describes the soil organic matter (SOM) composition as influenced by deforestation and agricultural management in the subhumid highlands in Ethiopia. Impact of land use changes on the amount and structural composition of SOM; Occurrence of depletion from the labile SOM; Details on the oxidation states of lignin.

It is generally accepted that particulate organic matter derives from plants. In contrast, the enriched labile fraction is thought by many to derive from microbes, especially fungi. However, no detailed chemical characterization of these fractions has been done. In this study, we wanted to assess the sources (plants or microbes; fungi or bacteria) and degree of microbial alteration of (i) three particulate organic matter fractions – namely the free light fraction (1.85 g cm-3), the coarse (250–2000 μm) and the fine (53–250 μm) intra-aggregate particulate organic matter fractions – and of (ii) three density fractions of fine-silt associated carbon – namely < 2.0, 2.0–2.2 (i.e. enriched labile fraction) and > 2.2 g cm-3 – by analysing the amino sugars, by CuO oxidation analyses, and by 13C-, 1H- and 31P-NMR analyses. Macroaggregates (250–2000 μm) were separated by wet-sieving from a former grassland soil now under a no-tillage arable regime. The three particulate organic matter fractions and the three density fractions were isolated from the macroaggregates by a combination of density flotation, sonication and sieving techniques. Proton NMR spectroscopy on alkaline extracts showed that the enriched labile fraction is not of microbial origin but is strongly degraded plant material that is enriched in aliphatic moieties partly bound to aromatics. In addition, the enriched labile fraction had a glucosamine content less than the whole soil, indicating that it is not enriched in carbon derived from fungi. Decreasing yields of phenolic CuO oxidation products and increasing side-chain oxidation in the order coarse intra-aggregate particulate organic matter < fine inter-aggregate particulate organic matter < fine-silt fractions indicate progressive alteration of lignin as particle size decreases. The light fraction was more decomposed than the coarse inter-aggregate particulate organic matter, as indicated by (i) its larger ratio of acid-to-aldehyde of the vanillyl units released by CuO oxidation, (ii) the smaller contribution of H in carbohydrates to total extractable H as estimated by 1H-NMR spectroscopy, and (iii) a larger contribution of monoester P to total extractable P in the 31P-NMR spectra. In conclusion, the four fractions are derived predominantly from plants, but microbial alteration increased as follows: coarse inter-aggregate particulate organic matter < light fraction ≈ fine inter-aggregate particulate organic matter < enriched labile fraction. [ABSTRACT FROM AUTHOR]

Dissolved organic matter is important in translocation and export of nutrients from forest ecosystems. Its mobility in soil is restricted by sorption to mineral surfaces which depends on its chemical properties. Carboxyl and hydroxyl groups form strong bondings to mineral surfaces, whereas the role of N-containing functional groups in the sorption process is less well understood. We examined in laboratory experiments the binding of dissolved organic matter from the forest floor to amorphous Al(OH)3, goethite, kaolinite, and illite and to subsoils in order to compare the sorption and desorption of dissolved organic C with that of dissolved organic N. The mineral samples were equilibrated with acidic solutions of organic matter at pH 4. In the equilibrium solutions organic C and N and their contribution to two operationally defined fractions, namely the so-called hydrophilic and hydrophobic fractions, were determined. We measured neutral and acidic amino sugars to discover the nature of the binding of organic N. Within the hydrophilic and hydrophobic fractions, the sorption and desorption of organic C and N did not differ, indicating that there was no preferential binding of N-containing compounds. The hydrophilic fraction contained more N and sorbed less than the hydrophobic fraction, and so the overall retention of organic N by the mineral phases and subsoils was smaller than that of organic C. Among the amino sugar compounds, muramic acid was preferentially removed from the solution, whereas the neutral amino sugars were sorbed similar to organic C. The results suggest that the sorption of N-containing compounds is favoured by acidic groups and not by amino groups. [ABSTRACT FROM AUTHOR]

Secondary forests are increasingly wide-spread on neotropical soils.In this study, we investigated if, and how, the establishment of a secondary forest on abandoned pasture affect the quality of soil organic matter (SOM). We approached this by a combination of physical fractionation of soil, where particulate SOM (light fraction and sand-associated SOM) is separated from mineral-bound SOM (silt- and clay-associated SOM), and structural chemical analyses, including measurementsof well-decomposable carbohydrates and the more refractory lignin. Particle-size separation revealed that agricultural use of a soil being formerly under primary forest resulted in a depletion of the particulate SOM pool, whereas clay- and silt-bound SOM was less affected. Abandonment of the pasture and growth of a secondary forest raised theC content in all separates to a pre-cultivation level within 18 years, and sand-associated C was even higher as compared to the primary forest. The lignin and carbohydrate signature showed that the land userarely affected the chemical composition of SOM within the differentfractions. This was corroborated by solution 13C NMR spectroscopy of the NaOH-soluble SOM. The results suggested that land useprimarily influences the C balance across the light fraction and thesize separates, with the particulate SOM pool being the most significant SOM component in the context of management impacts on these soils. While the gross chemical composition of SOM within the fractions remained unaffected, some molecular differences indicated a shift in the microbial community and/or activity at transformation of primary forest into pasture and after abandonment of the pasture with growth of secondary forest. [ABSTRACT FROM AUTHOR]

Abstract: Soils with deep and dark brown to black humic surface horizons, derived from sandy-silty aeolian and alluvial sediments, are relatively fertile and store huge amounts of carbon. Globally, they mainly correlate with steppe ecosystems. Because their deep and dark epipedons are frequently rich in Black Carbon, it was recently suggested that they developed due to prevalent burning of the semiarid vegetation. This paper describes six soils with deep and dark humic surface horizons, located in catchments of the Selenga and Uda rivers in Buryatia, southeastern semiarid Siberia. The organic matter of most of these soils not only originates from grass steppe, but in addition from trees and shrubs. Five soils can be classified as Chernozems and Kastanozems (); one soil is a deep and black Anthrosol, occurring in patches in the region. All soils have calcic horizons below the mollic epipedon, some with cambic Bw horizons in between. As the parent material does not contain carbonates, aeolian input and hydrolysis of Ca-silicates contribute to the development of the secondary carbonate accumulation, which is frequently cryoturbated. All profiles are more or less stratified due to solifluction. Texture ratios (<6.3/63-2000 μm) show maxima in the dark A horizons, indicating advanced paedogenesis. This is supported by weathering proxies, including SiO2/Al2O3 and Al2O3 × 100/Al2O3 + Na2O. Increased concentrations of TiO2, MgO and Fe2O3 presumably indicate advanced formation of chlorite. Radiocarbon analyses indicate that these deep and dark mollic horizons started to develop during the more humid-Holocene Climatic Optimum, ca. 7000–5000 BP. Palynological studies confirm expansion of forests in the Lake Baikal region in this period. This is in agreement with higher alkane ratios nC27/nC31 in most of the deep and dark A horizons, indicating that these epipedons, diagnostic for Chernozems and Kastanozems, did not develop exclusively under grass steppe, thus supporting the hypothesis that a significant part of the soil organic matter is inherited from trees and shrubs. This seems to be in contrast to Middle Europe and the Russian plain where Chernozems presumably developed during the early and mid-Holocene under grass steppe and subsequent degraded with increasing humidity and invasion of forests. High contents of Black Carbon in the Calcic Chernozem P4 and lower ones in the long-lasting occupied Hortic Anthrosol indicate that the deep and dark A horizons of Mollisols most likely do not result from frequent burning, but from biochemical processes. [Copyright &y& Elsevier]