Objectives Phosphorus (P)-enriching plants can accumulate high levels of P, and nitrogen (N) fertilization has been found to enhance the accumulation. In this study, we investigated the rhizosphere biochemical properties of a P-accumulating plant under N fertilization, aiming to provide a scientific basis for understanding how N nutrition promotes the remediation of P-excessive soils by P-accumulating plants.

Methods A pot experiment was conducted using a P-enriching plant species, Pilea sinofasciata. The mining ecotype (ME) and a non-mining ecotype (NME) of Pilea sinofasciata were grown in soils under N doses of 0 and 140 mg/kg and P doses of 0, 400, 800, and 1200 mg/kg, respectively. The P accumulation in roots and shoots of the two ecotypes, as well as P composition, enzyme activity, and microbial activity in the rhizosphere and bulk soils, were analyzed.

Results 1) When exposed to soil applied with 800 and 1200 mg/kg of P, the growth and P uptake of both ecotypes were inhibited. N application increased their tolerance to high P treatments, showing increased biomass and P accumulation. The biomass and P accumulation improvement in ME were 72.5% and 78.7%, and in NME were 80.1% and 125.6%, compared to no N application. Under N application, the maximum shoot biomass (27.2 g/plant) and P accumulation (193.8 mg/plant) of ME were recorded at P 800 mg/kg, which were 1.18 and 1.24 times higher than those of NME, respectively. 2) N application increased the concentrations of H₂O-P and NaHCO₃-Pi, and decreased the concentrations of NaHCO₃-Po, did not significantly affect the concentrations of NaOH-P, HCl-P, and residual-P in the rhizosphere soil of both ecotypes. And the concentrations of H₂O-P and NaHCO₃-Pi in rhizosphere soil of ME were lower than those of NME. 3) N application increased the activities of acid phosphatase, alkaline phosphatase, and phytase in the rhizosphere soil of both ecotypes, as well as the concentrations of microbial biomass carbon and nitrogen in the rhizosphere soil and the intensity of soil respiration, indicating that N application could improve the phosphatase and microbial activities in the rhizosphere soil of both ecotypes. Under N application, the activities of acid phosphatase, alkaline phosphatase, and phytase in the rhizosphere soil of ME were higher than those of NME, while microbial biomass carbon, nitrogen, and soil respiration intensity were not significantly different between the two ecotypes.

Conclusions The growth and P accumulation ability of Pilea sinofasciata were inhibited at soil P rate of 800 mg/kg and 1200 mg/kg. N application could alleviate the growth inhibition caused by high P circumstance, and increase rhizosphere P availability by promoting the activity of phosphatase, phytase, and soil microbial communities, further promoting the P accumulation of Pilea sinofasciata, especially that of the ME.