Impact of sulfur applications on the agronomic performance of rapeseed–clover mixtures

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 N 2 - fixing capacity of clover, and the leaching of nitrate and sulfate were monitored. This study showed that the N 2 - 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 DWm- 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 (NO 3 - N) and S (SO24 - S) leaching during the early winter period of cultivation.