Soil carbon stock and humification in pastures under different levels of intensification in Brazil

Intensive management of tropical pastures has shown potential for greenhouse gas (GHG) mitigation due to high forage production and C accumulation in the soil. This study aimed to evaluate different pasture management options in relation to their effect on soil C stocks and soil organic matter (SOM) humification. Pastures in four beef cattle production systems were assessed: intensive and irrigated pasture with high stocking rate (IHS); dryland pasture with high stocking rate (DHS); dryland pasture with moderate stocking rate (DMS); degraded pasture (DP). The soil under the native forest was also evaluated and soil carbon stocks from the 0-100 and 0-30 cm layers were assessed. Carbon stocks (0-100 cm) ranged from 99.88 to 142.33 Mg ha(-1) in DP and DMS, respectively and were, respectively, 14 % and 24 % higher compared to the soil under the forest and indicate the capacity of adequately managed tropical pastures to mitigate GHG emissions from livestock production. Humification indexes indicated the presence of more labile C in pastures with greater C accumulation (DHS and DMS), mainly in the upper soil layers, indicating recent C accumulation resulting from correct management. However, more labile C can be easily lost to the atmosphere as CO2, depending on pasture management. Low C stocks associated with high humification indexes are characteristics of DP in which significant amounts of SOM are lost. It is necessary to develop technologies to improve C sequestration in IHS and results indicate the importance of quantifying C stocks in association with C stability. (AU)