Effect of a reduced fertilizer rate on the water quality of paddy fields and rice yields under fishpond effluent irrigation

Aquaculture effluent irrigation has been widely adopted to replace freshwater irrigation to save water and providing additional fertilizer to the crop. There is limited information on the performance of fertilizer supply levels under fishpond effluent irrigation. The objectives of this study were to investigate the effect of reducing a rate of fertilizer on the purification of wastewater from fishponds by paddy fields and the yield of rice under fishpond effluent irrigation in central China in 2015. The treatments included 100 %, 80 %, and 60 % of the normal fertilizer rate (NFR, 150 kg N ha−1, 120 kg P2O5 ha−1 and 75 kg K2O ha−1) with fishpond (freshwater-pond aquaculture) effluent as an irrigation source, designated NFR-E, 0.8NFR-E, and 0.6NFR-E, respectively; with an additional NFR with freshwater as an irrigation source (NFR-F). The results showed that 5700 m3 ha-1 freshwater was saved by the use of the NFR-E, 0.8NFR-E and 0.6NFR-E treatments. The concentrations of total nitrogen (TN), total phosphorus (TP), dissolved phosphorus (DP), ammonia-nitrogen (NH4+-N), nitrate-nitrogen (NO3--N) and particulate phosphorus (PP) in the surface water and seepage water of the paddy field and the residual soil N and P in the 0−60 cm soil depth after the rice harvest decreased with the decreasing NFR. The removal rates of the TN in the surface water across the tillering, booting, heading and filling stages were 25.1 %, 38.9 % and 50.5 % on average for the NFR-E, 0.8NFR-E and 0.6NFR-E treatments, respectively. The corresponding removal rates of the TP were 56.4 %, 71.2 % and 76.2 %, respectively. These increased removal efficiencies were related to the lower N and P concentrations in the surface water of the paddy field and the efficient use of nutrients by rice under the reduced fertilization treatments. Compared to the NFR-F treatment, the 0.8NFR-E treatment resulted in a comparable accumulation of N and P and grain yield of rice, while decreasing the contents of N and P in the water of paddy fields and the residual soil N and P in the 0−60 cm soil depth after the rice harvest. Thus, reducing the normal fertilizer rate by 20 % could improve the water quality of the paddy field without deleterious effects on the rice yield and save 5700 m3 ha-1 of fresh water under fishpond effluent irrigation. These results can also provide a basis for in-depth understanding of the mechanism of aquaculture effluent purification through paddy field ecosystem in response to fertilizer supply levels.