Nitrogen‐Driven Acidification and Microbial Shifts Contribute to a Breakdown in Carbon–Phosphorus Trade in the Rhizosphere

Abstract

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 (NH 4 ) 2 SO 4 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 13 C‐labeled low‐molecular‐weight organic acids (LMWOA; a single addition of acetic and succinic acids). Phosphorus transformation was quantified using sequential extraction and 31 P 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.

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