Nitrogen enrichment buffers phosphorus limitation by mobilizing mineral‐bound soil phosphorus in grasslands

Abstract Phosphorus (P) limitation is expected to increase due to nitrogen (N)‐induced terrestrial eutrophication, although most soils contain large P pools immobilized in minerals (P i ) and organic matter (P o ). Here we assessed whether transformations of these P pools could increase plant available pools alleviating P limitation under enhanced N availability. The mechanisms underlying these possible transformations were explored by combining results from a 10‐year field N addition experiment and a 3700‐km transect covering wide ranges in soil pH, soil N, aridity, leaching, and weathering that could affect soil P status in grasslands. Nitrogen addition promoted the dissolution of immobile P i (mainly Ca‐bound recalcitrant P) to more available forms of P i (including Al‐ and Fe‐bound P fractions and Olsen P) by decreasing soil pH from 7.6 to 4.7, but did not affect P o . Soil total P declined by 10% from 385 ± 6.8 to 346 ± 9.5 mg kg −1 , whereas available P increased by 546% from 3.5 ± 0.3 to 22.6 ± 2.4 mg kg −1 after the 10‐year N addition, associated with an increase in P i mobilization, plant uptake, and leaching. Similar to the N addition experiment, the drop in soil pH from 7.5 to 5.6 and increase in soil N concentration along the grassland transect were associated with an increased ratio between relatively mobile P i and immobile P i . Our results provide a new mechanistic understanding of the important role of soil P i mobilization in maintaining plant P supply and accelerating biogeochemical P cycles under anthropogenic N enrichment. This mobilization process temporarily buffers ecosystem P limitation or even causes P eutrophication, but will extensively deplete soil P pools in the long run.