Phosphorus transport under different irrigation modes and urea application regimes in rice-duckweed systems

A clear understanding of phosphorus transport in rice-duckweed systems is critical to enhancing phosphorus utilization efficiency (PUE) and mitigating phosphorus non-point source pollution (PNPSP). Here, we conducted a two-year field experiment with two irrigation modes (alternate wetting and drying, AWD; continuous flooding, CF), two nitrogen application rates (193 kg ha −1 , N2; 135 kg ha −1 , N1), and two nitrogen split application ratios (tiller fertilizer: panicle fertilizer = 70%: 30%, F2; basal fertilizer: tiller fertilizer: panicle fertilizer = 50%: 30%: 20%, F3). Results indicated that phosphorus fertilizer accounted for approximately 97% of total phosphorus input in rice-duckweed systems, with other input (irrigation, wet deposition, and seedling transplanting) being negligible. Rice accumulated 30.4–41.5 kg ha −1 of phosphorus, while the average PUE was only 12.0%–13.9%. Soil retention accounted for 46.0% of phosphorus output, and soil total phosphorus (TP) content generally decreased with increasing soil depth. TP concentration in surface water peaked immediately after phosphorus fertilizer application, whereas that in groundwater peaked 20–25 days later, with concentrations dropping significantly with increasing depth. Phosphorus drainage and leaching accounted for less than 1% of phosphorus output, with leaching representing only approximately 10% of drainage; yet they still induced severe PNPSP by high phosphorus-to-nitrogen ratios. AWD regulated duckweed’s phosphorus uptake and release to alleviate the phosphorus supply-demand mismatch for rice, thereby boosting PUE, and increasing the nitrogen application rate most notably enhanced phosphorus utilization. The AWDN2F3 treatment performed optimally in improving PUE and mitigating PNPSP in rice-duckweed systems. • Groundwater phosphorus concentrations peaked 20–25 days after phosphorus fertilizer application. • Phosphorus drainage and leaching accounted for less than 1% of phosphorus output. • Alternate wetting and drying irrigation effectively regulated duckweed's phosphorus uptake and release. • Increasing the nitrogen application rate most effectively improved phosphorus utilization efficiency. • Increasing rice panicle biomass directly boosted phosphorus utilization efficiency.