Contrasting effects of long-term nitrogen and phosphorus fertilization on phosphorus availability in soil

Distinct mechanisms regulate soil P availability under long-term N and P inputs. Microbial biomass and NaOH-Pi sustain soil available P under N fertilization. P fertilization increases soil available P mainly via direct input and labile Pi. Soil phosphorus (P) availability is essential for forest productivity and stability, yet the long-term effects of nitrogen (N) and P fertilization on its mobilization remain unclear. To address this, we conducted a 10-year field experiment in Metasequoia glyptostroboides plantations under a subtropical monsoon climate, assessing the responses of soil chemical property, enzyme activity, microbial biomass, and P cycling indices and fractions to five independent levels each of N (0, 56, 168, 280, 336 kg ha−1) and P (0, 7.8, 31, 93, 155 kg ha−1) fertilization. P fertilization strongly increased available P and inorganic P fractions (Resin-Pi, NaHCO3-Pi, NaOH-Pi), while N fertilization had minimal direct effects. Under N fertilization, available P was primarily influenced by microbial biomass P and carbon, with NaOH-Pi as the dominant supply source. In contrast, under P fertilization, available P was regulated by total P, nitrate nitrogen, and organic carbon, with NaOH-Pi, NaOH-Po, NaHCO3-Pi, and NaHCO3-Po as key fractions. The direct effects of P fertilization, with a pathway coefficient of 0.37, slightly exceeded indirect effects. Overall, our decade-long fertilization trial shows that N fertilization sustains soil P availability chiefly through adaptive microbial processes, whereas P fertilization maintains it largely via direct nutrient inputs. © Higher Education Press 2026.