Experimental study on nitrogen leaching in a direct-seeding rice paddy of Taihu Lake Basin

Field experiment of nitrogen leaching loss in a dry direct-seeding rice paddy was conducted in Danyang region of Taihu Lake Basin. Results show that the nitrogen leaching loss is mainly occurred during the early stage of rice growing season under local water and fertilizer management, especially for the seedling fertilization. The base fertilization has not rapidly hydrolyzed without standing water in about two weeks after rice sown, that resulted in part of the fertilizers still exists in surface soil. Even though part of the base fertilizers is transformed into nitrate-nitrogen in wet soil, the nitrate nitrogen still resides in surface soil without downward water-leaching. Both the residual base fertilizers and the added seedling fertilizers increase the chance of nitrogen leaching loss when irrigation or precipitation occurred. The mean concentrations of ammonium-nitrogen and nitrate-nitrogen, at 40 cm soil depth, are 5.79 and 0.49 mg/L respectively, and nitrate-nitrogen is the main part of nitrogen loss through leaching, whose maximal concentration is 21.8 mg/L on day 7 after the seedling fertilization. The mean quantities of leaching loss of ammonium-nitrogen and nitrate-nitrogen at soil depth of 40 cm, in the whole rice growing season, are about N 3.8 and 28.4 kg/ha respectively, which indicate that the nitrate-nitrogen accounts for a majority proportion. As a result, in direct-seeding rice paddies, the superposition effect of both base and seedling fertilizers should be paid more attention. Due to the low nitrogen-uptake of rice at its seedling stage, the prophase fertilizers may be advisably reduced to decrease the risk of nitrogen leaching.