Dynamics of microbial-derived nitrogen to increasing straw addition in two typical arable soils in Northeast China

Background and Aims Microbial-derived nitrogen (N) is now recognized as an important contributor to the organic N pool in croplands, yet its dynamics under straw addition remain poorly quantified. Methods We inoculated microorganisms from two typical arable soils (Alfisol and Mollisol) in Northeast China separately in ¹⁵ N-containing M9 liquid media to generate labeled microorganisms. The harvested ¹⁵ N-labeled microorganisms were subsequently introduced into their native soils for 288 days of incubation under four levels of straw addition: 0, 4, 8 and 12 t ha ^-1 (S0, S4, S8 and S12), respectively. Results The mineralization rate of microbial-derived N decreased with increasing straw addition (except for the Mollisol under S4), with a range of 0.008–0.113 d ^-1 in the Alfisol and 0.026–0.051 d ^-1 in the Mollisol. By the end, 38.45–57.15% of the labeled N was retained in the organic N pool in the Alfisol, whereas 72.03–78.33% was recovered from the NO ₃ ^- -N pool in the Mollisol. Straw addition significantly increased the labeled N stored in the organic N pool in the Alfisol and had little effect in the Mollisol. When the function of biotic processes on the microbial-derived N retention or supply is relatively weak in the soil, abiotic processes will fill the deficit and play a compensatory role. Conclusion The utilization strategies of microbial-derived N differ between the two soils—the Alfisol tends to maintain its original organic form, while Mollisol tends to convert it into mineral N pool.