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DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY

Soil Carbon-13 Natural Abundance under Native and Managed Vegetation in Brazil

^a Dep. of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 11-12, D-10587 Berlin, Germany
^b Synergy Resource Solutions, 1755 Hymer Ave., Sparks, NV 89431

^* Corresponding author (wolfgang.wilcke{at}tu-berlin.de).

The conversion of native Cerrado vegetation (a mixed C₃ and C₄ vegetation) to Pinus caribaea Morelet plantations (a pure C₃ vegetation) and to Brachiaria decumbens stapf pastures (a pure C₄ vegetation) likely affects the C cycle. We used the natural abundance of ¹³C (¹³C) in vegetation and soil to: (i) quantify the contributions of C₃ and C₄ plants to the organic matter input into the soil under Cerrado vegetation and (ii) determine the degree of the replacement of original Cerrado-derived C by Pinus plantations and pastures. The mean ¹³C value of the soils (Anionic Acrustoxes) under Cerrado vegetation ranged from –20.5 to –19.7, which was dissimilar to the mass-weighted mean ¹³C signal of the aboveground biomass (–25.8). This was because grasses being C₄ plants contributed 11% to the aboveground biomass but about 50% of the organic matter input to the soil, which was estimated with a simple mixing model of the C₃ and C₄ ¹³C signals. After 12 and 20 yr, only 30% of the original organic matter in the topsoil was replaced by new organic matter under pasture or Pinus plantation, respectively. This turnover took place without significantly changing the C storage of the top 2 m of the soil (17–19 kg m^–2). The C replacement under Pinus affected only the top 0.15 m. Our results demonstrate that the C replacement in soils following land-use change in the Oxisols of this study takes several decades and is considerably slower under Pinus than under pasture.