Interactive effects of nitrogen and root exudate addition on soil organic carbon fractions in wheat agroecosystems

Nitrogen (N) deposition alters the composition and release of plant root exudates, thereby influencing the dynamics of soil organic carbon (SOC). However, the effects of specific root exudate compounds on SOC fractions under different N levels remain unclear. In this study, we conducted an incubation experiment to investigate how oxalic acid, citric acid, D-tryptophan, D(+) maltose, and p-hydroxybenzoic acid interact with different N addition levels (0, 90, 180, and 270 kg N ha−1) in affecting SOC fractions. The results revealed that N addition altered SOC through changes in soil physicochemical properties and microbial activity, reduced soil microbial biomass C, and increased CO2 emissions. The addition of root exudates resulted in an average 2.5% reduction in SOC. The interaction with N addition significantly (p < 0.05) increased soil microbial biomass C and dissolved organic C by 60% and 9.1%, respectively. Oxalic acid, citric acid, and p-hydroxybenzoic acid significantly (p < 0.05) stimulated greater CO2 release than D-tryptophan and D(+) maltose. Under N enrichment, root exudates influenced SOC dynamics primarily by regulating soil microbial biomass N. These findings highlight the interactive effects of N deposition and root exudate composition on soil C stability in agricultural ecosystems.