Phosphorus losses via surface runoff in rice-wheat cropping systems as impacted by rainfall regimes and fertilizer applications

Phosphorus (P) losses from agricultural soils contribute to eutrophication of surface waters. This field plot study investigated effects of rainfall regimes and P applications on P loss by surface runoff from rice (Oryza sativa L.) and wheat (Triticum aestivum L.) cropping systems in Lake Taihu region, China. The study was conducted on two types of paddy soils (Hydromorphic at Anzhen site, Wuxi City, and Degleyed at Xinzhuang site, Changshu City, Jiangsu Province) with different P status, and it covered 3 years with low, high and normal rainfall regimes. Four rates of mineral P fertilizer, i.e., no P (control), 30 kg P ha-1 for rice and 20 kg P ha-1 for wheat (P30+20), 75 plus 40 (P75+40), and 150 plus 80 (P150+80), were applied as treatments. Runoff water from individual plots and runoff events was recorded and analyzed for total P and dissolved reactive P concentrations. Losses of total P and dissolved reactive P significantly increased with rainfall depth and P rates (P<0.0001). Annual total P losses ranged from 0.36-0.92 kg ha-1 in control to 1.13-4.67 kg ha-1 in P150+80 at Anzhen, and correspondingly from 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, large amounts of P were lost during heavy rainfall events that occurred shortly after P applications at Anzhen. On average of all P treatments, rice growing season constituted 37-86% of annual total P loss at Anzhen and 28-44% of that at Xinzhuang. In both crop seasons, P concentrations peaked in the first runoff events and decreased with time. During rice growing season, runoff P concentrations positively correlated (P<0.0001) with P concentrations in field ponding water that was intentionally enclosed by construction of field bund. The relative high P loss during wheat growing season at Xinzhuang was due to high soil P status. In conclusion, P should be applied at rates balancing crop removal (20-30 kg P ha-1 in this study) and at time excluding heavy rains. Moreover, irrigation and drainage water should be appropriately managed to reduce runoff P losses from rice-wheat cropping systems.