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The expansion of short rotation eucalypt plantations in low soil organic matter (SOM) sandy soils may offer an alternative to improve soil C sequestration. The goal of this study was to estimate the changes in C stocks and emissions in different SOM fractions following conversion of the native Cerrado to pasture and then to eucalypt plantation. Therefore, we studied soils under native Cerrado, planted pasture (cultivated for 34years following the clearing of the Cerrado) and eucalypt plantation (4years). The C and N stocks in particulate organic matter and mineral‐associated organic matter (MAOM) were determined 4years after eucalypt planting. Soil CO₂‐C, CH₄‐C fluxes and CO₂‐C concentrations in soil profile were measured in different seasons over 4years. Variation in the natural abundance of ¹³C was used to partition the SOM‐C. The soil CO₂‐C and CH₄‐C fluxes were influenced by soil surface moisture (r= 0.185ᵒ and r= 0.430**, respectively), whereas only the soil CH₄‐C fluxes correlated with soil surface temperature (r= 0.355**). The highest soil CO₂‐C flux in soil under eucalypt occurred after 4years of eucalypt planting (2.5 kg ha⁻¹h⁻¹, approximately 70%). The pasture soil acted as a CH₄‐C source to the atmosphere. The pasture MAOM‐C losses in the 0.0–1.0‐m soil layers were not compensated by the new eucalypt C inputs (MAOM‐C lost ~9.6 Mg ha⁻¹). In summary, the recent worldwide expansion of short rotation eucalypt plantations should be carefully considered, particularly under pasture degraded soil sandy soils, because land uses able to increase SOM are priorities.

The objective of this study was to investigate the effect of maize rhizosphere in the dynamics of soil organic matter (SOM) fractions (particulate organic matter—POM and mineral associated organic matter—MAOM) under different soil nitrogen (N) availability. The experiment was laid out as a factorial scheme 2 × 2: (i) without (-Plant) and with maize plant (+Plant); (ii) without (-N) and with N fertilization (+N) throughout 56 d under greenhouse conditions. Soil surface CO2-C efflux was monitored by static PVC chambers, and soil- and root-derived CO2-C efflux was determined using the mass-balance approach. In the absence of plants, N addition reduced total soil CO2-C efflux over the 56-d experiment and increased MAOM-C. This was followed by an increase of 366% in microbial biomass (MB-C), indicating a high C assimilation by soil microbes. In the presence of plants, there was evidence of positive rhizosphere priming effect (RPE) at 42-d after planting when N was applied. The new C input by rhizodeposition promoted an increase of 495% and 50% in MB-C under -N and +N addition, respectively. The short-term effect of maize rhizosphere appeared to preserve native SOM and allowed higher incorporation of microbial residues-derived C into the MAOM.