Labile soil organic carbon loss in response to land conversion in the Brazilian woodland savanna (cerradão)

Conversion of native vegetation to agriculture may change the carbon cycle by reducing carbon soil storage capacity and increasing CO2 emissions. We aimed to comparatively assess the impact of land use change on labile pools and dynamics of soil organic carbon (SOC) in two land uses (Urochloa pastures and Eucalyptus forestry) relative to the native reference ecosystem (Brazilian woodland savanna, the cerradão), as a function of soil depth and season. For three replicated study sites, each of them including a control area of the native vegetation (Cerrado) and two land uses (Pasture, Eucalyptus), we sampled soil from 0 to 2 m depth in both dry and wet seasons. We quantified dissolved organic carbon (DOC) and microbial biomass carbon (MBC), estimated the microbial quotient (MBC/SOC) and DOC/SOC ratio, and evaluated C dynamics by assessing soil basal respiration and the metabolic quotient (qCO2). Compared with Cerrado, DOC, MBC and MBC/SOC decreased in both Pasture and Eucalyptus. Differences between land uses vanished below 30 cm soil depth. Seasonality affected most analyzed variables, with lower values for DOC, DOC/SOC and qCO2, and slightly higher values for MBC and MBC/SOC in the wet season. In the dry season qCO2 increased in the Eucalyptus topsoil as compared to Cerrado, suggesting higher stress in the microbial community and/or lower decomposition efficiency in Eucalyptus. Overall, our results show that cerradão conversion to pastures and Eucalyptus plantations negatively affects labile pools and dynamics of SOC, with the effects surpassing a strong spatial and seasonal variability in the soil response to land conversion.