Effect of the combined addition of mineral nitrogen and crop residue on soil respiration, organic carbon sequestration, and exogenous nitrogen in stable organic matter

Nitrogen fertilization can change soil respiration via microbial biomass, which affects the decomposition of soil organic matter (SOM) and plant residues. The quantitative influence and underlying mechanisms of mineral N and the combination of straw and N on stable organic N (humus) are still unclear. To explore the mechanisms of the conversion of mineral N to stable organic N (humus N), we performed incubation experiments using C-13 natural abundance and N-15 isotope labeling techniques with four treatments: control, added straw (St), added N (N), and added straw and N (St + N). The effect of N addition on the CO2 emission rate and cumulative CO2 emissions showed a prominent two-phase characteristic: both the CO2 emission rate and cumulative production were stimulated by exogenous mineral N during the initial stage, while the CO2 emission rate gradually decreased during the middle and later stages; however, residual C may remain in the soil for a longer time, leading to great differences in soil C/N ratios at different stages. This indicates that added N was rapidly utilized by microorganisms. At the end of the incubation period, the proportion of N-15 derived from labeled N to total N in humus was 0.063% (N) and 0.085% (St + N), while the proportion of N-15 in humus to total mineral N added was 0.41% and 0.64%, respectively. This indicated that even if only mineral N was added, the stable organic N in humus was converted by microbes. Compared with the addition of only mineral N, the combination of N and straw promoted the conversion of mineral N to stable organic N. The results of this study clarify the effect of mineral N on stable organic N and associated SOC mineralization, and can thus be used to develop strategies to improve soil fertility or mitigate climate change by increasing the SOC.