Overwintering grass-clover as intercrop and moderately reduced nitrogen fertilization maintain yield and reduce the risk of nitrate leaching in an organic cauliflower (Brassica oleracea L. var. botrytis) agroecosystem

Summary

Cauliflower (Brassica oleracea L. var. botrytis) has high nitrogen (N) demand during growing, but due to its low N use efficiency and N-rich residues, it may cause N losses by leaching after harvest. A field experiment was conducted to investigate the effect of overwintering grass-clover as an intercrop and moderately reduced N fertilization on organic cauliflower yield and risk of nitrate leaching. The experiment tested sole-cropped (S) and intercropped (I) systems using a substitution design and two N fertilizer rates (180 and 230 kg N ha−1). To investigate the competition between cauliflower and grass-clover, cauliflower grown alone with empty grass-clover rows (Semp) and grass-clover grown alone with empty cauliflower rows (GCemp) were included. The results showed that marketable yield was maintained in the I system compared to in the S system when calculated per meter crop row, and when N fertilization was moderately reduced. The Relative Competition Index (RCI) values in the I system were lower than 0, indicating that cauliflower growth was not interfered in the I system. The ability for aboveground N accumulation in the S and I systems was similar, but the N content of cauliflower residues in the S system was 33 and 23 kg ha−1 higher than in the I system in 2012 and 2013, respectively. The S system had 168 kg Ninorg ha−1 in the soil layer of 0–1.5 m at the start of leaching in 2013, which was 40 kg N ha−1 higher than the I system. Reduced N fertilizer level led to 22 kg ha−1 lower soil Ninorg at the end of leaching. However, the effect of system and N level on soil residual Ninorg per unit marketable yield was not significantly different. Intercropping cauliflower with overwintering grass-clover and moderately reduced N fertilization was feasible in organic cauliflower production, with regard to maintaining marketable yield and reducing the risk of nitrate leaching in the agroecosystem.