Carbon input from 13C-labelled soybean residues in particulate organic carbon fractions in a Mollisol

Understanding the decomposition processes of crop residues and the quantity of residue carbon (C) incorporated into soil organic C (SOC) pools in the soil is crucial for optimizing C management in agricultural systems. This study is highly valuable in Mollisols in northeastern China, where SOC is markedly decreasing. Soybean is a major crop in this region; however, the decomposition processes of soybean residues and their contributions to physically separated SOC pools remain unknown. Thus, a 150-day incubation experiment was conducted with different C-13-labelled residues of soybean, i.e., leaf, stalk, and root, incorporated into a Mollisol. The leaves had the highest decomposition rate. At the end of the incubation, cumulative respiration reached 7.76 mg CO2-C g(-1) in the leaf-incorporated soil, but only 5.98 and 5.51 mg CO2-C g(-1) was recorded for the stalk- and root-amended soils. Furthermore, similar trends were found for the microbial biomass C and dissolved organic C. Different residue sources greatly affected the residue-derived C incorporation in the SOC fractions, resulting in a ranking of root > stalk > leaf. The root-derived C incorporation values were 49.5, 17.2, and 5.0 g residue C kg(-1) in the coarse particulate organic C (POC), fine POC, and mineral-associated C (MOC) fractions, respectively, which were significantly higher than those for the stalk- and leaf-derived C. These results indicate that C input from roots can play an important role in C stability in this Mollisol by incorporating more C in the POC and MOC.