Enhanced Nitrogen Use and Economic Benefits in Organic Vegetable Fields via Appropriate Manure Application in Northwestern China

Clarify soil nitrogen (N) nutrient dynamics for organic production to guide manure application in organic monocultures with three crop rotations per annum in northwestern China. Field experiments were conducted to clarify the fate of ¹⁵N-labeled manure N in organic vegetable fields, and evaluate the effects of manure N application levels from 0 to 1200 kg N ha⁻¹ on the yield of organic Chinese flowering cabbage and the economic benefits to producers. Average NO₃^–-N concentrations in the topsoil (0–20 cm depth) increased significantly with manure N level and was positively correlated with abundance of ammonia-oxidizing bacterial (AOB) amoA genes and potential nitrification rates. The NO₃^–-N concentration in the 20–100 cm profile was not significantly changed at manure N levels below 600 kg N ha⁻¹; however, at 1200 kg N ha⁻¹, the NO₃^–-N concentration in the profile increased significantly, enhancing the deep leaching of NO₃^–-N. Compared with 600 kg N ha⁻¹ application, percentage manure ¹⁵N loss increased by 77.3% and 104.5% (P < 0.01) following manure applications of 900 and 1200 kg N ha⁻¹, respectively. The ¹⁵N residual percentage decreased by 28.6% and 31.7% (P < 0.01), respectively, and percentage uptake by crops decreased by 26.3% and 71.4% (P < 0.01), respectively. The biomass, yield, and economic benefits of Chinese flowering cabbage initially increased, and thereafter decreased with increase in manure N level, peaking at 900 kg N ha⁻¹. The optimal manure N level of 600–900 kg N ha⁻¹ is recommended for application in organic cultivation in northwestern China.