Depth profiles of soil carbon isotopes along a semi-arid grassland transect in northern China

Soil is an important organic carbon (C) pool in terrestrial ecosystems, but knowledge on soil organic carbon (SOC) decomposition rate and influencing factors remains limited, particularly in arid and semi-arid grasslands. Models show that global semi-arid regions are experiencing more extreme climate events, which may trigger more C loss from soils. We used the stable carbon isotope (delta C-13) of plant and soil depth profile to examine decomposition rates of SOC along a 2200 km semi-arid grassland transect in northern China. Beta (beta) was calculated from the relationship between delta C-13 and C concentrations of plant and soil profile (0-100 cm). Partial correlation analysis and structure equation models were used to analyze correlations between beta and climatic and edaphic variables. We found that the delta C-13 values increased from plant tissues to surface soil and then deep soil at all sites, while the SOC concentration decreased. The beta values ranged from -0.33 to -2.27, with an average value at -1.37. There was a positive correlation between beta value and SOC decomposition rate constant (k), supporting the hypothesis that enrichment of delta C-13 along soil depth profile was mainly due to isotopic fractionation during microbial SOC decomposition. The correlation between beta value and aridity index was mainly due to the variations of edaphic properties such as soil C/N ratio with aridity index, pointing to the more important role of edaphic properties in beta value than climatic factors. Despite uncertainties associated with the interpretation of soil delta C-13 along its depth profile, our results demonstrated that soil delta C-13 could provide an independent benchmark for examining model-based predictions of SOC decomposition in semi-arid grasslands. Incorporation of both climatic and edaphic variables into models may enhance the predictions for SOC dynamics under global climate changes.