Stepwise shift in root phosphorus‐acquisition strategies with nitrogen input in temperate forests

Intensified human‐derived nitrogen (N) loading may induce extensive phosphorus (P) uptake limitations in temperate forests. It remains unclear how plants will acclimate to such progressively deprived P environments under N input, especially in terms of adjustments in root P‐acquisition strategies. Here, we show, conducting N input experiments in two temperate forests (natural and secondary forest), that low, medium and high N inputs reduced plant‐available soil P concentrations by 9.3%, 15.7% and 16.3% in natural forests, and by 29.0%, 31.0% and 28.2% in secondary forests, respectively. This suggested that the natural forest had a stronger buffering capacity for N inputs, consequently resulting in a relatively lower impact on soil P availability. Importantly, continuous N input stepwise altered the P‐acquisition strategy of temperate forest plant roots. This transition moved from an initial dependence on mycorrhizal symbiosis for soil P acquisition to the mobilization of soil inorganic P by root‐released carboxylates, and ultimately to the inorganic P acquisition through the facilitation of the mineralization of organic P by rhizosheath phosphatases and by the enhancement of the ability of roots to scavenge the soil matrix. Simultaneously, plant rhizosheath phosphomonoesterase, phosphodiesterase and phytase activities responded divergently to declined soil P availability, suggesting that increased N inputs altered plant mineralization preference and strategy for soil organic P with different chemical forms. Synthesis . These shifts in root P‐acquisition strategy reveal the adaptive strategies adopted by plants when soil P becomes increasingly limiting, also reflecting the profound effects of N inputs on plant allocation of below‐ground carbon (C) resources. Together, this study elucidated that N inputs remodelled C‐P coupling in temperate forests by altering root plasticity and C‐investment strategies.