Nitrogen (N) transfer is well documented in legume-cereal intercropping but this is less often reported for legume-Brassica intercrops even though Brassica crops require higher levels of N fertilizers. The present study was carried out to quantify N transfer from legumes (Lupinus albus L., Trifolium incarnatum L. or Vicia sativa L.) to rapeseed (Brassica napus L.) using the split-root 15N-labelling method. After three months we observed that legumes did not alter the growth of rapeseed. Vetch showed the lowest growth and demonstrated low 15N shoot to root translocation and no significant N transfer to rapeseed. In contrast, significant 15N enrichment was found in lupine and clover and 15N was transferred to the associated rapeseed plants (around 6 and 4 mg N plant−1, respectively), which contributed 2 to 3% of the rapeseed total N. Additionally, the data revealed that N2 fixation dominated the N nutrition in lupine despite the high N level provided in the donor compartment, suggesting a greater niche segregation between companion plants. Based on the results of this study we suggest that intercropping can be a relevant contributor to rapeseed N nutrition. Among the three legumes tested, clover and lupine seemed to be the best intercropping candidates.

In this study we tested whether legumes can improve the growth and N and S nutrition of rapeseed in an intercropping system and compared the effect of mixtures on legume N-fixation and soil N-resources. Rapeseed was cultivated in low N conditions in monocrops using one (R) or two plants (RR) per pot and in mixtures with lupine, clover or vetch.The R monocrop was the most relevant control, intraspecific competition inducing a significant growth delay resulting in a significantly lower leaf number, in RR monocrop compared to R and the three mixtures considered. Plant biomass, and the N and S contents of rapeseed grown in mixtures were the same than those measured in R monocrop. Compared to the monocrop, the proportion of N derived from the atmosphere was increased by 34, 140 and 290% in lupine, clover and vetch, respectively when intercropped with rapeseed. In mixture with clover and lupine, the soil N pool at harvest was higher than in other treatments, while N export by crop was constant. Legumes suffered from competition for soil S resulting in a decrease of 40% in their S content compared to the monocrop. Compared to rapeseeds grown in R monocrop and in mixture with lupine and vetch, rapeseed mixed with clover showed significantly higher SPAD values in old leaves.In our conditions, mixing legumes with rapeseed is relevant to reduce N fertilization and improve nutrition and growth of rapeseed.

Rapeseed (Brassica napus L.) is a crop requiring high levels of nitrogen (N) fertilizer for growth and to optimize yield and seed quality. To limit the environmental pollution associated with intensive N fertilizer use, rapeseed–clover (Trititcum incarnatum L.) mixtures were grown in lysimeters under low N conditions (100 kg N ha−1). Considering the high sulfur (S) requirements of both rapeseed and clover, two inputs of S fertilizer (30 and 60 kg S ha−1) were applied. The effects S input on the agronomic performance of rapeseed in mixture and monocrops considered as reference, the N2‐fixing capacity of clover, and the leaching of nitrate and sulfate were monitored. This study showed that the N2‐fixing capacity (%Ndfa) of clover was improved (1.3‐fold) when it was grown in mixture with rapeseed at S60. However, irrespective of the type of cropping (monocrops or mixtures) and S application level (30 or 60 kg S ha−1), the biomasses and total N and S contents of both plants were not significantly different, nor was the rapeseed seed quality. Moreover, the yield of rapeseed grown in mixture at S60 was significantly lower than the yield of rapeseed grown as a monocrop (331.5 ± 9.8 versus 380.8 ± 3.5 g DW m−2, respectively). The results demonstrate that, in our field conditions, rapeseed mixed with clover required only 30 kg S ha−1 to maintain yield and seed quality, despite the high S needs of both plants. More surprisingly, compared to the rapeseed monocrop, the rapeseed–clover mixture led to an increase in N ( NO3 - ‐N) and S ( SO4 2- ‐S) leaching during the early winter period of cultivation.