Grazing impacts on desert steppe prokaryotic communities: nitrogen speciation, depth stratification, and rhizosphere dynamics

Grazing-induced changes in soil prokaryotic communities are well documented at the surface, yet how grazing reorganizes microbial assembly across soil depth gradients and between bulk soil and rhizosphere compartments remains poorly resolved. This study examines whether prokaryotic community assembly in desert steppe is more closely governed by bulk edaphic variables or by specific nitrogen speciation, and whether microbial sensitivity to grazing is compartment- and depth-dependent. Integrating a 3-year demographic survey of Convolvulus ammannii (2020–2022) with 16S rRNA gene sequencing, we characterized prokaryotic communities in the rhizosphere and bulk soil (0–30 cm) under four stocking rates spanning rest to heavy grazing (0, 0.93, 1.82, 2.71 sheep ha $$^{-1}$$ ; June–October). Community dynamics were quantified via $$\alpha$$ -diversity indices, PERMANOVA for compositional turnover, and redundancy analysis (RDA) to partition the explanatory power of SOC, TN, NH $$_4^+$$ -N, NO $$_3^-$$ -N, and pH. Heavy grazing reduced plant biomass by 68–86% over the 3-year monitoring period, accompanied by significant contraction of rhizosphere Shannon diversity (from 9.40 ± 0.07 to 8.98 ± 0.08). Bulk soil microbial sensitivity was strongly depth-stratified: the 20–30 cm layer exhibited the most pronounced Chao1 richness decline (18%), disproportionately exceeding losses in surface soils. PERMANOVA confirmed significant effects of grazing intensity and soil depth on community composition, with a notable interaction in bulk soil ( $$R^2=0.16$$ , $$P=0.028$$ ). RDA identified NH $$_4^+$$ -N as the strongest measured predictor of community variance (14.9%), substantially exceeding TN (4.0%) and SOC (1.9%), although its explanatory power is partially entangled with the depth gradient ( $$r = -0.88$$ ). Grazing-induced microbial restructuring is both compartment-specific and depth-stratified. The stronger association of inorganic nitrogen speciation—relative to bulk carbon pools—with community variance suggests that NH $$_4^+$$ -N merits consideration as a sensitive indicator of microbial stability in arid grasslands under grazing pressure, though its independence from soil depth remains only partially resolved because of strong collinearity.