Barbara Wick, Edzo Veldkamp, William Zamboni de Mello:
Soil Organic Matter Turnover in Pasture Sites Following Deforestation in the Humid Central Amazon of Brazil

BARBARA WICK¹, EDZO VELDKAMP¹, WILLIAM ZAMBONI DE MELLO^2
1Humboldt University Berlin, Division of Soil Science and Site Science, Germany
²Universidade Federal Fluminense, Brasil

We studied the organic matter cycling characteristics in the humid central Amazon of Brazil. Total C and natural ¹³C abundance ($\delta^{13}$C _V-PDB) in the whole soil and in the microbial biomass (labile pool) was followed along a chronosequence of pasture sites of 6 months up to 60 years after deforestation.

The $\delta^{13}$C natural abundance in the whole soil decreased from [-27] under undisturbed forest sites to [-23] in the 60-year-old pasture site, and from [-30] to [-19.4] in the microbial biomass.

An increase of total soil organic C in the whole soil occurred during the first 2 years of pasture establishment and was due to an increase in grass-derived C (C₄). After this 2 years, C₃-C decreased by [25]% and remained at this level up to 8 years following deforestation. Thereafter, C₃-C declined steadily. Grass derived carbon was similar at all sites ($\sim$ 8 g kg^-1) once the pastures were established after 2 years. In the microbial biomass, C₃-C declined by [60]% during the first 2 years of pasture establishment. From 2 to 4 years following forest cutting C₃-C increased for a short period and declined then steadily with pasture age. Contrary to the whole soil, C₄-C in the microbial biomass increased during the first 6 years following pasture establishment. Thereafter, it decreased continuously. The overall decrease of total C in the pastures with time following deforestation was due primarily to the loss of C₃-derived carbon. The loss of C₃-C from the system started much earlier in the microbial biomass compared to the whole soil. Sixty years following deforestation and pasture establishment the loss of C₃-C was [60]% in the whole soil and [80]% in the microbial biomass.

Keywords: Amazon, microbial biomass, organic matter transformation, pasture sites, whole soil

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Contact Address: Barbara Wick, Humboldt University Berlin, Division of Soil Science and Site Science, Invalidenstraße 42, 10115 Berlin, Germany, e-mail: barbara.wickagrar.hu-berlin.de