Effects of Winter Rapeseed - Faba-bean intercrop and litter mulch on soil Nitrogen
Authors: Al Naemi Moza , Garnier Patricia , Jullien Alexandra , Richard-Molard Celine 
Published: February 2023

Intercropping management strategy involves the use of legumes alongside a commercial crop to achieve various benefits, such as improved soil nutrient circulation, water retention, and pest control. However, research has shown that there is a lack of understanding of the long-term benefits of legumes within cropping systems and their specific interactions with the soil.&#160;In our experimental setup, we conducted four treatments using soil columns that were 24 cm in diameter and 1 m in length. We grew two winter rapeseeds as a monocrop, an intercrop of one rapeseed and one faba-bean, and two faba-beans as a monocrop in each soil column. We killed the faba-bean during the winter frost and left it as green mulch in the rapeseed intercrop. The final treatment was bare soil control columns. We measured soil nitrogen, the biomass, and nitrogen content of living plants and plant litter in September, October, November, and June. Additionally, we studied the decomposition of rapeseed and faba-bean residues in the soil in a laboratory incubation experiment to measure carbon and nitrogen mineralization.&#160;The majority of mineral nitrogen leaching occurred during late autumn at the beginning of the growing season (September to October) in all treatments. The soil mineral nitrogen contents over the growing season for the rapeseed-faba-bean intercrop system were similar to the rapeseed monocrop system, but lower than in the faba-bean monocrop system. The nitrogen balance in the columns for each treatment revealed that bare soil lost the most nitrogen over time due to leaching and lack of plants to uptake mineral nitrogen and immobilize it as biomass. The second one which lost the most nitrogen by leaching is The faba-bean monocrop. In this treatment, the plant did not use soil nitrogen in the lower portions of the soil column and this portion of the nitrogen in the soil may have been lost by leaching. The nitrogen was better conserved in the soil column and in the plants in the treatments with rapeseed intercrop and monocrop. Soil nitrogen was removed by the plant more efficiently leading to less leaching.&#160;The nitrogen content and biomass of one rapeseed plant in the intercrop was nearly double the one rapeseed plant in monocrop. Indeed, the total biomass and nitrogen content of the two rapeseeds in monocrop was equivalent to the single rapeseed in the intercrop. Conversely, rapeseed mulch had less nitrogen in intercrops than in the monocrop system.&#160;Lastly, the incubation of crop residues initially immobilized soil mineral nitrogen. The faba-bean mulch started releasing more mineral nitrogen than the bare soil after day 70. The release of mineral nitrogen of rapeseed and rapeseed-faba-bean mulch mixture exceeded the nitrogen of bare soil after day 90.&#160;Overall, it is clear that intercropping with legumes can have positive effects on soil nitrogen and plant growth, but more research is needed to fully understand the long-term benefits and interactions between legumes, the soil, and commercial crops.