Cover crops, soil fertility and straw management influence rice nutrition, weed competition and greenhouse gas emissions in a mesocosm study

In organic rice systems, cover crop (CC) cultivation is commonly adopted to increase rice productivity. However, factors such as CC species and their management, co-incorporation with rice straw, and intrinsic soil fertility status may influence the benefits and trade-offs related to this practice. A mesocosm study was therefore setup to simulate different scenarios and evaluate the best CC management practice to improve nutrient inputs and efficiency, limit weeds and mitigate greenhouse gases (GHG) emissions during paddy rice cropping. The treatments included two CC species (hairy vetch and Italian ryegrass), two CC termination practices (green manuring and mulching), two soil types (high and low fertility), with and without rice straw incorporation. Cover crop biomass and nutrient contents were determined prior to termination, while rice and weed growth, grain yields, N availability and nutrient uptake, and methane (CH4) and nitrous oxide (N2O) emissions were measured and compared. Green mulching with hairy vetch on the high fertility soil showed the best results in terms of nutrient inputs with CC and rice yield. On the other hand, ryegrass resulted in a substantially higher biomass that allowed for better weed control, although no significant differences in CC nutrient inputs, rice grain and straw yields were observed. Furthermore, the high CC biomass of ryegrass resulted in highest CH4 emissions over the rice cropping season. The green mulching was more effective than green manuring in controlling weeds, which also influenced rice growth and yield. Avoiding the incorporation of rice straw prior to cultivating hairy vetch and green mulching did not have an impact on weed infestation during rice cropping, but decreased soil available N although this did not affect rice growth and grain yields. Soil fertility was an important factor positively affecting CC development and nutrient inputs, rice growth and grain yields, but was also responsible for higher CH4 emissions. Together these findings indicate that CCs could have a positive effect on plant nutrition, rice grain yields and weed control. However, high CH4 emissions associated with the additional organic matter inputs require the implementation of appropriate management strategies to mitigate the environmental impact.