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Surface Runoff Phosphorus Losses in Rice-Wheat Crop Systems as Influenced by Rainfall Patterns and Fertiliser Applications

RK Dey

Phosphorus loss from agricultural soils contributes to surface water eutrophication. This field study investigated the effects of precipitation patterns and P application on P loss through surface runoff from rice and wheat growing systems in the Taihu Lake region, China. The study was conducted on two types of rice growing land with different P status, the duration of 3 years in low, high and normal rainfall regimes. Four ratios of inorganic phosphate fertilizers, i.e. P-free, 30 kg P ha–1 for rice and 20 kg P ha–1 for wheat, 75 plus 40 and 150 plus 80 were applied as treatments. Water runoff from individual plots and spill events were recorded and analyzed for total and dissolved reactive P concentrations. Total P and soluble P losses reacted significantly with precipitation depth and P level. Total annual P losses ranged from 0.36–0.92 kg ha–1 in the control group to 1, 13–4.67 kg ha–1 at P150+80 at Anzhen, and accordingly 0.36– 0.48 kg ha–1 to 1.26–1.88 kg ha–1 at Xinzhuang, with 16–49 % of total P as dissolved reactive P. In particular, a large amount of P was lost during heavy rains that occurred immediately after P application in Anzhen. The average of all P treatments, the rice crop accounted for 37–86% of the total annual P loss in Anzhen and 28–44% of that in Xinzhuang. In both crops, P concentrations peaked during the first runoff events and decreased over time. During the rice growing season, the P concentration in the runoff is positively correlated with the P concentration in the field water which is intentionally closed by the construction of a dike around the field. The relatively high P loss during the wheat growing season in Xinzhuang is due to the high P status of the soil. In summary, P should be applied at a rate that balances between crop removal and at a time when heavy rainfall is eliminated. In addition, irrigation and drainage must be managed appropriately to reduce P loss through runoff from the rice-wheat farming system.