Plant identity and evenness affect yield and trace gas exchanges in forage mixtures

Aims We explore the potential effect of plant diversity on yield and greenhouse gas exchanges in forage mixtures, identifying potential co-benefits between functions. Methods Using the biodiversity-ecosystem function (BEF) modelling framework (Connolly et al. 2013), we analyse results from a field experiment where the relative sown proportion of three forage species (a grass, a legume, and a non-legume forb) was varied to obtain a gradient in species proportions and evenness. The exchange rates of N2O, CH4, NH3 and CO2 were measured, together with forage yield and soil inorganic N content. We conducted gas measurements 4 years after sward establishment during the expected period of highest emissions. Results Results support the hypothesis that emission rates are affected by plant diversity. Sown evenness was found to decrease N2O exchange rates, and some evidence was found for a similar negative effect on CH4. Plant composition affected gas exchanges, with legume monocultures showing the highest emission rates for all gases. Diversity also increased yield and modulated NO3− and NH4+ soil concentrations. Conclusions The integrated analysis of yield and emission response to species diversity allows the identification of a range of species proportions for which both functions are optimized. Diversifying forage legume-based systems could contribute to mitigation while improving ecosystem productivity.