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.