Effects of nitrogen fertilizer topdressing time on the phosphorus-accumulation capacity and soil phosphorus-removal potential of Polygonum hydropiper under phosphorus-enriched soils

Objectives Nitrogen (N) fertilization enhances the phytoextraction efficiency of phosphorus (P) by plants. We investigated the effects of N topdressing stages on P accumulation and P removal efficiency in the P-accumulating plant, Polygonum hydropiper, to provide a scientific basis for utilizing N fertilization to promote phytoremediation of soil P excess.

Methods A soil-based pot experiment was conducted using mining ecotype (ME) and non-mining ecotype (NME) as study materials. Under the condition of N application at 100 mg per kg soil and a base-topdressing N ratio of 1∶1, five topdressing N stages were established, specifically at the 5th, 6th, 7th, 8th, and 9th weeks after transplantation. The treatment without N was used as the control (CK). P was applied at 800 mg per kg soil for all treatments. At 10 weeks after transplanting, the plants were harvested for investigation of biomass, P accumulation, and soil available P concentration. The plant effective number, P extraction ratio, maximum potential yield and maximum potential P removal were calculated.

Results Compared with CK, all the N topdressing treatments increased the biomass and P accumulation of ME and NME plants. ME achieved maximum shoot biomass and P accumulation at the 8th week topdressing treatment, reaching 52.11 g/plant and 219.71 mg/plant, respectively, which were 1.78 and 1.80 times those of CK and significantly higher than those of NME. Rhizosphere soil available P concentration in ME increased significantly at the 6th and 8th weeks topdressing treatment, exceeding both the control and the non-rhizosphere soil value, facilitating P uptake. The 8th week topdressing treatment in ME yielded the lowest plant effective number (significantly lower than that of NME) but the highest aboveground P extraction rate (significantly higher than that of NME), along with peak values of both maximum potential yield and maximum potential P removal.

Conclusions N topdressing enhanced P accumulation and P removal efficiency in Polygonum hydropiper by promoting rhizosphere P activation and absorption. The 8th week post-transplantation topdressing stage optimized ME’s P accumulation and soil P removal efficiency, with maximum shoot P accumulation (219.71 mg/plant) and maximum potential P removal (97.45 kg/hm²), and the P extraction capacity of ME was higher than that of NME.