Edaphic properties and nitrogen use efficiency are associated with microbial residue nitrogen accumulation during vegetation restoration

Vegetation restoration is a key driver of soil nitrogen accumulation in drylands, yet the processes governing the formation and stabilization of microbial-derived nitrogen are still poorly quantified. Here, we studied a 70-year restoration chronosequence of Caragana korshinskii in China’s Mu Us Desert to quantify the dynamics of microbial residue nitrogen (MRN), its contribution to total soil nitrogen (TN), and the associations of MRN with biotic and abiotic factors. MRN increased more than 35-fold (from 5.41 to 191.89 mg kg⁻¹) across the restoration years, showing pronounced stage-dependent patterns of accumulation. The apparent rate of MRN accumulation varied across restoration years, rising from 0.07 to its peak of 0.42 kg ha⁻¹ yr⁻¹ during the 28–54 years restoration interval, before falling to 0.08 kg ha⁻¹ yr⁻¹ in the 54–70 years interval. Fungal residue nitrogen (FRN) dominated the microbial residue N pool throughout vegetation restoration, with its contribution to TN increasing from 5.17% to 32.38%. The FRN contribution to TN consistently surpassed that of bacterial residue N (BRN), which peaked at 11.29%, thus indicating a progressively greater accumulation of fungal-derived residues in the soil N pool. Partial least squares structural equation modeling indicated that microbial nitrogen use efficiency, soil organic carbon, ammonium nitrogen and silt–clay content were most strongly associated with MRN accumulation, likely reflecting linkages with nitrogen assimilation and retention, substrate availability, and residue stabilization, respectively. Overall, these empirical findings suggest that MRN accumulation during dryland vegetation restoration is tied to microbial characteristics as well as edaphic properties, highlighting MRN as a potential process-based indicator for assessing nitrogen stabilization in arid ecosystems.