Nitrogen-Driven Acidification and Microbial Shifts Contribute to a Breakdown in Carbon-Phosphorus Trade in the...

Anthropogenic nitrogen (N) deposition is reshaping terrestrial phosphorus (P) cycling. This effect is pronounced in the rhizosphere, where plants respond to P status by releasing root exudates. Yet it remains unclear whether these interactions drive P mobilization or immobilization. To capture both the chronic effects of N deposition and the transient effects of root exudates, we integrated a decade-long N addition experiment (a single annual application of (NH₄)₂SO₄ at 0, 2, and 5 g N m^-2 year^-1) with a 14-day laboratory incubation. Specifically, rhizosphere and bulk soils from a Eurasian meadow steppe were amended with ¹³C-labeled low-molecular-weight organic acids (LMWOA; a single addition of acetic and succinic acids). Phosphorus transformation was quantified using sequential extraction and ³¹P NMR, while underlying mechanisms were assessed via priming effects, phosphatase activity, and microbial functional genes. Nitrogen enrichment promoted the dissolution of mineral-bound inorganic P (P_i) through acidification (pH drop > 1 unit), leading to the accumulation of secondary mineral P_i and organic P (P_o). This was accompanied by reduced microbial phosphatase activity and lower abundances of the P-cycling genes phoC and phoD. The above effects were stronger in the rhizosphere than in bulk soil. In the rhizosphere, LMWOA enhanced microbial P immobilization and P_o accumulation, even as their positive priming effects accelerated soil organic carbon mineralization. These results were partly driven by the recruitment of key P-cycling taxa such as Burkholderia and Rhodoplanes, suggesting a shift in carbon-for-P exchange between plants and microbes under N deposition. Collectively, our findings indicated that while N-induced P solubilization may temporarily alleviate P limitation, synergy with increased LMWOA exudation promotes P_o accumulation. Though P_o can be labile compared to mineral-bound forms, suppressed phosphatase activity likely constrains its mineralization. This inhibition potentially decelerates P cycling, thereby compromising the medium- to long-term P supply within the Eurasian steppe.