Experiments on nitrogen and phosphorus losses from paddy fields under different scales

In order to analyze loss laws of nitrogen and phosphorus from the rice farming system under different scales, six nesting scales composing of subbasins were selected for water balance analysis as well as nitrogen and phosphorus losses monitoring during rice growing season in Zhanghe Irrigation Area, Hubei Province. Water quantity of inlet and outlet of different scales were measured and water samples were collected for nitrogen and phosphorus analysis at the same time during the whole rice growing season from May to September in 2009. The results showed that there was scale effect in the process of nitrogen and phosphorus losses from the rice farming system. The loss loads of total nitrogen, ammonia nitrogen, nitrate nitrogen, total phosphorus, particulate phosphorus and dissoluble phosphorus decreased by 80.5%, 73.4%, 39.7%, 73.8%, 75.0%, and 50.0% from field to watershed scale, respectively. The reason of scale effect was caused by removal the effect of nitrogen and phosphorus pollutants in both ponds and drainage ditches and the reuse of drainage water. The scale effect should be taken into consideration when evaluating the contribution of nitrogen and phosphorus pollution loads from farmland to river body.