Nitrous oxide and methane emissions from optimized and alternative cereal cropping systems on the North China Plain: a two-year field study.

The impacts of different crop rotation systems with their corresponding management practices on grain yield, greenhouse gas emissions, and fertilizer nitrogen (N) and irrigation water use efficiencies are not well documented. This holds especially for the North China Plain which provides the staple food for hundreds of millions of people and where groundwater resources are polluted with nitrate and depleted through irrigation. Here, we report on fertilizer N and irrigation water use, grain yields, and nitrous oxide (N2O) and methane (CH4) emissions of conventional and optimized winter wheat-summer maize double-cropping systems, and of three alternative cropping systems, namely a winter wheat-summer maize (or soybean)-spring maize system, with three harvests in two years; and a single spring maize system with one crop per year. The results of this two-year study show that the optimized double-cropping system led to a significant increase in grain yields and a significant decrease in fertilizer N use and net greenhouse gas intensity, but the net greenhouse gas N2O emissions plus CH4 uptake and the use of irrigation water did not decrease relative to the conventional system. Compared to the conventional system the net greenhouse gas emissions, net greenhouse gas intensity and use of fertilizer N and irrigation water decreased in the three alternative cropping systems, but at the cost of grain yields except in the winter wheat-summer maize-spring maize system. Net uptake of CH4 by the soil was little affected by cropping system. Average N2O emission factors were only 0.17% for winter wheat and 0.53% for maize. In conclusion, the winter wheat-summer maize-spring maize system has considerable potential to decrease water and N use and decrease N2O emissions while maintaining high grain yields and sustainable use of groundwater. Crown