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 CO 2 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 (qCO 2 ). 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 qCO 2 , and slightly higher values for MBC and MBC/SOC in the wet season. In the dry season qCO 2 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.