Optimization of nitrogen nutrition of cauliflower intercropped with clover and in rotation with lettuce

Cauliflower (Brassica oleracea var. botrytis L.) requires large amounts of nitrogen (N) fertilizer to produce high yields. The intercropping of cauliflower with a leguminous species may help farmers to reduce N fertilizer rates, production costs and environmental pollution. Moreover, the recycling of crop residues can contribute to the nutritional support of subsequent crops in a rotation. In this study, cauliflower was cultivated in year one either alone or intercropped with annual clover (Trifolium resupinatum L.), using four N fertilization rates: 0, 75, 150, and 300 kg N ha−1 (referred to as N0, N1, N2, and N3 respectively). Following crop residue incorporation in year two, iceberg lettuce (Lactuca sativa var. capitata (L.) Janchen) was cropped on the same field without the supply of N fertilizer to assess the effect of cauliflower and clover residues as well as of residual N fertilizer rates on the growth and N uptake of lettuce. The presence of clover did not affect the marketable yields of the intercropped cauliflower, which were similar to the cauliflower sole-cropped. The N1 and N2 fertilizer rates maximized both growth and N uptake of the intercropped cauliflower, while with the N3, crop development was limited. In the N0 plot, the legume competed for N and depressed growth and N absorption of cauliflower. The sole-cropped cauliflower was, on the contrary, unaffected by the N fertilizer rates. The iceberg lettuce cultivated in succession benefited from the presence of clover in plots N1 and N2. When succeeded to the sole-cropped cauliflower, however, the iceberg lettuce produced less biomass and absorbed less N. Results from this study suggest that the intercropping system cauliflower-clover can be a sustainable tool to optimize N input and reduce N fertilizer requirements for the successive crop.