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 northeastern 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.