Soil organic matter dynamics in density and particle size fractions as revealed by the C-13/C-12 isotopic ratio in a Cerrado's oxisol

In order to better understand the dynamics of soil organic matter (SOM) in Oxisols and the impact of converting native cerrado (savannah) into pasture , we studied the dynamics of different physically separated SOM pools at di fferent depths in a cerrado oxisol (Typic Haplustox), under natural conditi ons and after 23 years of cultivated pasture (Brachiaria spp.) via the repl acement of the native C (C-3-derived) by pasture C (C-4-derived). Organic C stocks of the original cerrado (15 +/- 3 kg m(-2)) and pasture (17 +/- 3 k g m(-2)) were not significantly different, which was attributed to the high biomass production of the tropical grasses and the protective effect of th e high clay content (> 800 g kg(-2)). We observed that 89-91% of the total organic C accumulated in the clay + silt fraction. The replacement of cerra do-derived C by pasture-derived C was in average 36%, 34%, and 19% for A(p) , AB(1), and B-w2 horizons, respectively, suggesting a fast turnover rate o f organic C regardless of the high clay content. The replacement decreased in the order: free low-density organic matter (LDOM) > heavy fractions (san d, silt, clay) > occluded-LDOM. The lower replacement of the occluded-LDOM compared to the heavy fractions was attributed to protection inside aggrega tes and to a possible accumulation of C-3-derived charcoal (black carbon). After 23 years of pasture, about 50% of the total organic C in the free-LDO M in the topsoil was still from cerrado, indicating that a significant part of this fraction was relatively recalcitrant. Charcoal fragments observed in the fraction suggested that the recalcitrance was probably due to charre d material. (C) 2001 Elsevier Science B.V. All rights reserved.