Organic matter stabilization in a Xanthic Ferralsol of the central Amazon as affected by single trees: chemical characterization of density, aggregate, and particle size fractions

Not only the amount of organic carbon in soil is important for soil organic matter (SOM) stability, but also its physical and chemical properties. The appropriate technique for the assessment of SOM dynamics can vary between soil types, and information about this is lacking for Ferralsols of the cen tral Amazon basin. First, this work identified SOM pools which are sensitiv e to land-use changes on the terra firme in the central Amazon. In a second step, the effects of single trees on SOM properties were evaluated in a mi xed tree crop plantation in comparison to secondary and primary forest site s. Thus, the processes of organic matter stabilization could be studied in the highly aggregated soils. A combination of aggregate and density fractio nation was found to be most suitable for physical SOM characterization. The particulate organic matter (POM, density less than 1.6 Mg m(-3)) varied by one order of magnitude between sites and could be used as a sensitive indi cator of land-use changes. Aggregate stability was not related to SOM conte nts or bulk SOM properties. The incorporation of plant material into stable SOM, however, was enhanced by aggregation. Among aggregate separates, the fraction, 0.25-0.5 mm, showed single-tree effects the most. SOM replenishme nt was higher under tree species with law quality litter, i.e, high C-to-N and polyphenol-to-N ratios. High quality litter from a leguminous ground co ver, however, showed low soil nitrogen and carbon replenishment but increas ed nitrogen concentrations in light fractions. Litter with a high quality m ay improve soil nitrogen availability but not amounts of total SOM, which c ould only be shown for low quality litter. The results indicate the importa nce of aggregation and POM dynamics for SOM stabilization in the studied Xa nthic Ferralsols of the central Amazon basin. (C) 2001 Elsevier Science B.V . All rights reserved.