Soil organic matter dynamics in density and particle size fractions as revealed by the 13C/12C isotopic ratio in a Cerradoʹs 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 different depths in a cerrado oxisol (Typic Haplustox), under natural conditions and after 23 years of cultivated pasture (Brachiaria spp.) via the replacement of the native C (C3-derived) by pasture C (C4-derived). Organic C stocks of the original cerrado (15±3 kg m−2) and pasture (17±3 kg m−2) were not significantly different, which was attributed to the high biomass production of the tropical grasses and the protective effect of the 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 cerrado-derived C by pasture-derived C was in average 36%, 34%, and 19% for Ap, AB1, and Bw2 horizons, respectively, suggesting a fast turnover rate of organic C regardless of the high clay content. The replacement decreased in the order: free low-density organic matter (LDOM)>heavy fractions (sand, silt, clay)>occluded-LDOM. The lower replacement of the occluded-LDOM compared to the heavy fractions was attributed to protection inside aggregates and to a possible accumulation of C3-derived charcoal (black carbon). After 23 years of pasture, about 50% of the total organic C in the free-LDOM 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 charred material.