Effects Of Returning Organic Materials To Field And Site-specific Nitrogen Management On Grain Yield Of Direct-sowing Rice And Emission Of Greenhouse Gases From Paddy Field

Rice is one of the most important food crops. Returning crop straw to field plays an important role in crop production in China. However, there had been few studies on the effects of returning organic materials to field and interaction between returning organic materials to field and nitrogen (N) fertilizer management on grain yield and morphological and physiological indices of direct-sowing rice, nutrient absorption, soil properties and emission of greenhouse gases from paddy field. A long term fixed field experiment was conducted to investigate the effects of returning maize straw and organic materials made of maize straw (compost, biogas residue and black carbon) to field and site-specific nitrogen management (SSNM) on grain yield, morphological and physiological indices of direct-sowing rice, nutrient absorption, soil properties and emission of greenhouse gases from paddy field. The main results are as follows.1. Effects of returning various organic materials to field and SSNM on the grain yield of direct-sowing rice were different. Grain yield ranged from 4.91 t ha-1 to 9.74 t ha-1 under different treatments. When N fertilizer was not applied (ON) and when compared with the control (without organic materials returning), grain yield was increased by 32.5%,20.7%,29.7% and 5.1%, respectively, for maize straw, compost, biogas residue and black carbon in 2013, and by 21.6%,21.0%,31.2% and 2.2%, respectively, in 2014, and the difference in grain yield among maize straw, compost and biogas residue treatments were significant, whereas application of black carbon showed no significant effect on grain yield. Under the SSNM and when compared with the control, grain yield was increased by 19.7%,16.3%,9.3% and 2.4%, respectively, for maize straw, compost,biogas residue and black carbon in 2013, and by 2.5%,3.1%,9.1% and 1.4%, respectively, in 2014. Application of maize straw, compost and biogas residue significantly decreased the amount of N application when compared with the control. Increases in number of stems and tillers per unit area, SPAD value, dry matter accumulation, leaf area index (LAI) after heading, grain-leaf ratio, photosynthetic rate of the flag leaf after heading, root dry weight, root oxidation activity and amount of root bleeding sap contributed to the increase in grain yield when maize straw, compost and biogas residue were incorporated into the field.2. Nutrient absorption varied with organic materials. When compared with the control, maize straw, compost and biogas residue treatments significantly increased the contents of nitrogen, phosphorus and potassium uptake, nitrogen use efficiency (agronomic use efficiency, nitrogen recovery use efficiency, and partial factor productivity), and partial factor productivity of phosphorus and potassium, and significantly decreased nitrogen physiological use efficiency. Application of black carbon showed no significant effect on the use efficiency of nitrogen, phosphorus and potassium.3. Returning organic material to field had obvious effects on soil organic matter content, and the effects varied with types of organic materials. In comparison with the control, maize straw, compost and biogas residue treatments significantly increased contents of organic matter and soil respiration, and reduced soil bulk density, whereas using inorganic nitrogen reduced soil respiration and increased soil bulk density.4. The emission of methane (CH4), carbon dioxide (CO2), or nitrous oxide (N2O) from the field exhibited a similar trend among all the organic materials treatments. Application of organic material significantly increased the emission of each greenhouse gas and global warming potential (GWP) except the N2O emission which was not increased under the maize straw returning treatment, relative to that under the control. Under the same organic material treatment, SSNM increased emissions of all the greenhouse gases and GWP, but maize straw treatment decreased greenhouse gas intensity (GWP per unit grain yield, amount of GWP/grain yield) when compared with ON.The results indicate that application of organic materials, expect black carbon, could increase grain yield and improve physical and chemical properties in the soil, but also enhance emissions of greenhouse gases from the paddy field. Both higher grain yield and higher environmental efficiency would be achieved by the adoption of SSNM and the application of maize straw treatment.