Isotopically constrained soil carbon and nitrogen budgets in a soybean field chronosequence in the Brazilian Amazon region

The impacts of large‐scale conversion of cattle pastures to cropland on soil carbon (C) and nitrogen (N) stocks are poorly understood in the Amazon region. The objective of this research was to determine whether soybean cultivation on a previously deforested and pastured soil has changed C and N stocks and dynamics. We sampled a chronosequence of soybean fields in 2009 and again in 2013. We hypothesized that detecting statistically significant changes in total soil C and N stocks would be difficult but that fluxes of C and N through the soil would be sufficiently large to significantly decrease the stable isotope ratios of soil organic matter. We observed statistically significant decreases in the 13C and 15N enrichments and C:N ratio. When combined with estimates of crop biomass production, harvest yield, and biological nitrogen fixation, these measurements provided sufficient constraints for C and N budgets to infer modest rates of net change in soil N (+15 to +27 kg N ha−1 yr−1) and soil C (−0.15 to −0.30 Mg C ha−1 yr−1) in the top 10 cm of soil. These results indicate that this intensive soybean cropping system is having minimal impacts on N loss to the environment but likely is a small net source of C to the atmosphere. Key Points Repeated isotopic measurements across a soybean field chronosequence constrained C and N budgets Small increases in soil N and decreases in C were inferred from a budget with multiple constraints This intensively managed soybean field in the Brazilian Amazon may be a small net source of CO2