Study On Crop Productivity And Nitrogen Use Efficiency In The Rice-Wheat Cropping System In Danyang

National conditions in China determine that agriculture in our country has to constantly improve resource use efficiency under high crop yield. Nitrogen (N) is the most limiting nutrient for crop production. High quantity of N has contributed to a large increase of world food production. However, overuse of N in agriculture has led to serious environmental pollution, and jeopardized human health. Rice(Oryza sativa L.)-wheat {Triticum aestivum L.) cropping system plays an important role in grain production in China, particularly in Tai Lake basin which has been a main grain production area from long time ago. At the same time, the highest rate of N application in China also takes place in this area. Long-term overuse of N in agriculture has threatened water environment and people's life in this region. Thus, researches on productivity and N use efficiency (NUE) in rice-wheat cropping system are necessary and urgent. The present study taking Danyang municipality, in Jiangsu Province, a typical region of Tai Lake basin, for example analyzed dynamics of grain production, fertilizer application and soil fertility. Effects of precision fertilization (PF) on rice yield and NUE under different soil fertility conditions and effects of PF on wheat yield and NUE under different soil types and fertility conditions were also studied through field experiments. These results are critical to providing scientific support for improve productivity as well as NUE in rice-wheat cropping system. The following are the main conclusions.(1) Grain yield in Danyang has been declining since 1997, which came to 392 thousand tons with a 28.8% decrease compared to that of 1997. Dynamics of rice yield and wheat yield showed the same trends as total grain yield. Rice yield and wheat yield were respectively 102 and 276 thousand tons in 2005, which decreased by 12.4% and 27.2% relative to those of 1997. The total amount of applied N, phosphorus (P) potassium (K) fertilizers in Danyang has increased quickly in the last two decades, which were respectively 1.2,2.7 and 7.6 times larger in 2005 than those in 1997. Contents of soil total N, organic carbon and available P were respectively 16.6%,9.3% and 63.3% higher in 2005 than those in 1982. soil pH did not show obvious change, while soil available K experienced a 25.4% decrease in 2005 compared to that in 1982.(2)Compared with no N treatment (NN), precision fertilization (PF) and conventional fertilization (CF) significantly increased rice yield. Rice yield in PF was also significantly higher than in CF. Increasing extent of rice yield in PF compared to CF under medium and low fertility soil was respectively 9.9% and 9.3% higher than that under high fertility soil with only 3.2%. Recovery efficiency of applied N (REN) averaged 25.3%, which was higher in PF than that in CF. The increasing rate of REN in PF compared to CF showed differences under different fertility conditions, which reached 27.9% under low fertility soil higher than those in medium and high fertility soils with 14.4% and 16.5%, respectively. Agronomic efficiency of applied N (AEN) was 8.4 kg kg-1 in PF with a significant increase of 56.7% relative to CF. Furthermore, increasing extent varied under different fertility condition, which was highest under medium fertility soil and lowest under high fertility condition. NN owned the highest physiological efficiency of N (PEN). Results showed that applying sound management and utilization to farmland with relative lower fertility might be an effective way to increase rice yield as well as NUE.(3) Compared with NN, PF and CF significantly increased wheat yield, while difference between PF and CN was not significant. Increasing extent of wheat yield in PF and CN compared to NN was higher under medium and low fertility soil than that under high fertility soil. REN in the present research reached to 25.6% on average. REN in PF treatment was 28.4%, significantly higher than in CF. AEN in PF was 13.1 kg kg-1, and increased by 35.4% compared to that in CF. Soil types and fertility conditions did not significantly affect REN or AEN. PEN in NN treatment was higher than that in PF and CF. Soil types also significantly altered PEN with 51.1 kg kg-1 in yellow mud soil higher than that in powder sandy soil.