Effects of the partial replacement of chemical fertilizer with manure on the yield and nitrogen emissions in leafy vegetable production

Vegetables are the second largest crop in China, with leafy vegetables accounting for a large proportion, for which the yield is increasingly dependent on large inputs of chemical fertilizer, especially nitrogen. However, there are problems concerning the high application rate of chemical fertilizers and low nitrogen use efficiency of leafy vegetable production. Continuously high inputs of chemical nitrogen fertilizer can easily lead to soil quality degradation, environmental pollution, and other problems, which seriously restrict the sustainable development of vegetable production. The partial replacement of chemical fertilizer with manure could potentially increase efficiency and reduce emissions. To illuminate the best ratio for the nitrogen replacement of chemical fertilizer by manure, and to evaluate the economic benefits and environmental effects of leafy vegetable production, field experiments were conducted. To study the effects of the partial replacement of chemical fertilizer with manure on the yield, economic benefits, ammonia volatilization, and nitrous oxide emissions of Brassica oleracea and Brassica chinensis production; 5 treatments were used:pure fertilizer (0M), and chemical fertilizers replaced with 25%, 50%, 75%, and 100% organic fertilizers (25%M, 50%M, 75%M, and 100%M, respectively). The results showed that among the different treatments, the yield of B. oleracea and B. chinensis were both highest under the 25%M treatment. Compared with the 0M treatment, the yield of B. oleracea and B. chinensis significantly increased by 15.0% and 16.3%, respectively (P < 0.05). During the B. oleracea and B. chinensis seasons, compared with the 0M treatment, the 25%M treatment increased the economic benefit by 11.7% and 5.4%, respectively. However, the economic benefit was negative under the 50%M, 75%M, and 100%M treatments. Under the 25%M treatment, the cumulative ammonia volatilization emissions were 42.1 and 12.9 kg·hm^-2 during the B. oleracea and B. chinensis seasons, which were 23.4% and 41.6% lower than that of the 0M treatment, respectively (P < 0.05). There were no significant differences between the 0M and 25%M treatments concerning the cumulative nitrous oxide emissions. Under the 25%M treatment, the cumulative nitrous oxide emissions were 0.74 and 3.06 kg·hm^-2 during the B. oleracea and B. chinensis seasons, respectively. Compared with the 25%M treatment, the nitrous oxide emissions increased by 33.7%-60.8%, 50.0%-134.3%, and 56.8%-185.6% in the 50%M, 75%M, and 100%M treatments, respectively. Based on this, it is proposed that for leafy vegetable production the appropriate replacement ratio of chemical nitrogen fertilizer by manure is approximately 25%, which can achieve the greatest efficiency increase and emission reduction.