Effects of agricultural production on phosphorus losses from paddy soils: a case study in the Taihu Lake Region of China

A field plot experiment was conducted on two types of paddy soils in the Taihu Lake Region of China from June 2000 through 2002 to assess phosphorus (P) losses by runoff and drainage flow and the effectiveness of rice–wheat double cropping on reducing P losses from paddy soils. Commercial NPK compound fertilizer and single superphosphate fertilizer were applied to furnish 0, 30, 150, and 300 kg P ha−1 for rice season trials, and 0, 20, 80, and 160 kg P ha−1 for wheat season trials. The experiments consisted of four replicates (plots of 5 × 6 m in a randomized block design) of each treatment in Argic stagnic anthrosols (Anzhen site) and six replicates in Cumulic stagnic anthrosols (Changshu site). P30 and P20 treatments (30 and 20 kg P ha−1 in rice and wheat seasons, respectively) were considered as conventional P application rates in this area. Higher P treatments, such as P150 and P300 for rice and P80 and P160 for wheat, were intended to simulate the status of soil P in ~10–20 years with an application of P30 or P20 kg P ha−1 each season. Results revealed that the average concentration of total P (TP) in runoff samples was 0.870 mg P l−1 from P30 plots during the rice season, and 0.763 mg P l−1 from P20 plots during the wheat season in both years at the Anzhen site, while it was 0.703 and 1.292 mg P l−1, respectively, at the Changshu site. Average TP load (mass loss) at the Anzhen site with conventional P application rates was 220.9 and 439.5 g P ha−1 during rice season in 2000/2001 and 2001/2002, respectively, but was 382.3 and 709.4g P ha−1 during wheat season, respectively. Mass loss at the Changshu site was 140.4 and 165.7 g P ha−1 during the rice season and 539.1 and 1184.6 g P ha−1 during the wheat season, respectively. P losses from paddy soils were significantly greater during the wheat season, especially at the Changshu site, indicating that planting rice reduced P. Phosphate fertilizer levels significantly affected P concentrations and P loads in runoff both seasons. Both mean concentrations and average seasonal P loads from the P150/P80 plots were lower than that from the P300/P160 plots, but significantly higher than that from the P30/P20 and P0 plots. This implied that runoff P loads would be greatly increased in 10–20 years as a result of the accumulation of soil P if 50 kg P ha−1 (rice season plus wheat season) is applied each year.