Effects of different nitrogen supply levels on the yield of Orychophragmus violaceus, soil residual inorganic nitrogen, and nitrogen balance.

Abstract

Understanding the responses of winter green manure February orchid (Orychophragmus violaceus) to different levels of nitrogen (N) supply in Northern China and determining the optimal soil N supply level to meet N demands of green manure production with high yield and efficiency, could provide a theoretical foundation and practice reference for maximizing ecological effects of green manure and optimizing N management for spring maize-green manure rotation system in intensive farmland in Northern China. We carried out a field experiment in a site which had received no fertilizer for many years. The aboveground biomass accumulation, N uptake of February orchid and soil residual inorganic N before green manure incorporation, as well as the N balance during the green manure growing season were determined under different levels of N supply. The results showed that N fertilizer application significantly increased the biomass and N uptake of February orchid under low soil inorganic N content (15 kg·hm-2 in 0-90 cm soil layer). At the application rate of 90 kg·hm-2, the biomass (dry mass) and N uptake reached the maximum, being 2031 and 42 kg·hm-2, respectively. The soil residual inorganic N amount rose with the increases of N fertilizer application before sowing, growing very rapidly once the application rate was over 60 kg·hm-2. With the increases of N application rate, the calculated apparent N balance changed from deficit to surplus in the growing season of February orchid. The inputs and outputs of N reached a balance at the application rates of 60 to 90 kg·hm-2. The relationships between February orchid biomass, N uptake, soil inorganic N before green manure incorporation, and soil N supply amount (0-90 cm preplant soil inorganic N content plus N application rate) could be fitted by the quadratic, linear plus plateau and exponential models respectively. Based on the simulation, we calculated the preplant soil N supply and soil residual inorganic N content before green manure incorporation would be 136 and 78 kg·hm-2 individually, as the biomass of February orchid reached the maximum (2010 kg·hm-2). While N uptake was at the highest level of 40 kg·hm-2, the biomass of February orchid was 95% of the maximum biomass mentioned above (1919 kg·hm-2) and the soil residual inorganic N before green manure incorporation decreased to 57 kg·hm-2 whose corresponding minimum soil N supply amount was 105 kg·hm-2. This value was quite near to the recommended soil residual inorganic N (100 kg·hm-2) after maize harvest under optimized N management in Nor-thern China. Taken together, our results showed that the level of soil N supply should be at approximately 100 to 105 kg·hm-2 in spring maize-winter green manure system for improving tradeoffs between agronomic and environmental impacts.