Abstract

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.