Grazing-N addition interactions drive soil carbon priming and balance via bacterial assimilation in a meadow steppe

Grassland carbon storage depends on microbial-mediated interactions between grazing and nitrogen (N) addition, which regulates the balance between soil organic carbon (SOC) retention and priming effects. However, uncertainties regarding these interactive mechanisms constrain projections of SOC vulnerability under global change. We conducted a factorial field experiment involving grazing and N addition in a Leymus chinensis meadow in north-eastern China. In the fifth year of the experiment, we collected soil to conduct a 70-day soil incubation combined with labile carbon (glucose) addition to examine the effects of the grazing and N addition treatments soil carbon priming and carbon retention. Grazing consistently increased priming effects regardless of N addition. In contrast, N addition strongly reduced priming by 41.0% in ungrazed plots but had minimal increase effects (3.2%) under grazing. Mechanistically, bacterial glucose assimilation capacity primarily mediated grazing-dependent N effects on priming, explaining 65.0% of the variation and correlating positively with priming intensity. Grazing notably decreased the net SOC balance (35.7 mg kg⁻¹ soil) and diminished the beneficial effect of N addition on SOC (+79.6% in ungrazed vs. +12.3% in grazed plots). Priming effects and bacterial glucose assimilation were dominant drivers of SOC responses under grazing, exhibiting negative correlations with net SOC balance. Synthesis and applications. Our results show that grazing-induced bacterial dominance in carbon assimilation alters priming effects and net soil carbon balance under N addition, offsetting potential carbon sequestration benefits by accelerating native organic matter decomposition. Thus, microbial carbon assimilation capacity, particularly bacterial substrate assimilation, may serve as an indicator of SOC vulnerability under global change.