Cover crop mixtures including legumes can self-regulate to optimize N2 fixation while reducing nitrate leaching

Summary

Cover crop (CC) mixtures including both legume and non-legume species have the potential to reduce nitrate leaching and increase N availability in the system through biological N2 fixation (BNF). However, the provision of multiple services depends on the biomass expression of functionally diverse species. Cover crop growth can be manipulated through management, but more knowledge is needed on species expression, and the effects on nitrate leaching and BNF. We investigated this in a long-term organic crop rotation experiment, where an undersown legume-based CC mixture, composed of perennial ryegrass (Lolium perenne L.), chicory (Chicorium intybus L.), white clover (Trifolium repens L.) and red clover (Trifolium pratense L.), was grown under different cropping system and fertility management in three consecutive years. We found a linear complementary relation between legume and non-legume biomass when CC growth was the greatest. Non-legume biomass was high under non-N-limiting conditions, while legume biomass and N input via BNF were high where the risk of nitrate leaching was low, as shown by the low and stable nitrate leaching above a threshold of 0.4 Mg clover DM ha−1. Above this threshold, the percentage of N derived from the atmosphere (%Ndfa) was high and stable (91 %), while lower and more variable with lower legume biomass. Overall, our results show that legume-based CC mixtures are self-regulating in terms of nitrate leaching reduction and N input via BNF.