Pattern selection of water and nitrogen practices to reduce greenhouse gas emission and increase profit in a double rice system

Rice paddy field has been an important emission source of greenhouse gas. A combination of water-saving irrigation, controlled release urea, and reduced nitrogen can offer the promising potential to decrease the emissions of greenhouse gas, while, to increase grain yield simultaneously in rice cultivation. Aiming to verify which the water and nitrogen practice can achieve the goal of "low input-low emission-high benefit", a field experiment was conducted in a double rice cropping system in the Jianghan Plain, Hubei province, China. Four nitrogen practices were designed: 1) urea (U), 2) polymer-coated controlled release urea (CRU), 3) 20% reduced urea application (US), and 4) 20% reduced polymer-coated controlled release urea application. Two water practices were integrated, 1) the conventional irrigation with mid-season drainage (CI), and 2) water-saving irrigation with shallow water depth and alternation of wetting and drying (SWD). The automatic static chamber method equipped with gas chromatography was applied to the sample, further to measure the emissions of greenhouse gas (CH4 and N2O) during the rice growing season under various treatments. A life cycle analysis (LCA) was used to calculate the carbon footprint in the rice production system. The intensity of carbon emission per unit grain yield was estimated using cost-benefit analysis, together with the unit net income. The results showed that the controlled release urea and reduced rate of nitrogen application can alleviate both emissions of CH4 and N2O, while, the water-saving irrigation decreased CH4 emissions but increased N2O emissions. The yield of grain increased with the application of controlled release urea, while, decreased with the water-saving irrigation, and 20% reduction in nitrogen application, but these negative effects can be ignored in the practical case. The LCA indicated that CH4 and N2O emissions in the double rice cultivation contributed the highest portion to total carbon footprint (50.7%-69.9%), followed by nitrogen input (21.6%-33.4%). The carbon footprint and net income decreased at varied levels under the treatments of water-saving irrigation, controlled release urea, and reduced rate of nitrogen application. Compared with the treatment of U + CI, the CRUS + SWD treatment achieved the highest reduction in carbon emissions (P<0.05), followed by CRU + SWD, US + SWD, CRU + CI, and U + SWD. It infers that the application of controlled-release urea led to much higher input cost, while, the reduction of nitrogen application amount can decrease this cost. The water-saving irrigation directly saved the water, electricity and manpower consumption, showing a lower cost. Considering both input cost and yield production, the controlled-release urea contributed to the increase of net profit for the double rice, where the CRUS + SWD achieved the highest net profit, followed by CRU + CI. The net income of carbon footprint decreased noticeably under all other treatments, compared with the urea and conventional irrigation. Particularly, the treatment of CRUS + SWD achieved the lowest intensity of carbon emission (0.51 kg/yuan), 62.7% lower than that of U + CI. The data confirmed that the combination of water-saving irrigation, controlled release urea with 20% reduction in nitrogen application rate can be used to save the input cost, while to improve net profit, and thereby to effectively reduce carbon emission intensity in a double rice cropping system. These findings can also provide a promising theoretical support for the production of low carbon rice in China.