Balancing productivity and greenhouse gas emissions : evaluating legume-grass mixtures in managed boreal grasslands

Although the agricultural sector is a major source of greenhouse gas (GHG) emissions, mitigation options exist through altering management practices. One of these options is the selection of forage species, as their varying functional traits may alter the dynamics of carbon and nitrogen in grassland ecosystem. Perennial legumes have a strong potential to reduce nitrogen fertilizer needs by fixing atmospheric nitrogen (N2) through rhizobia, while deep-rooted species can enhance soil carbon inputs by allocating carbon into deeper soil layers. In this study, we investigated the effects of supplementing a timothy (Phleum pratense) grass mixture with either deep-rooted grass species (tall fescue, Festuca arundinacea Schreb.), or a legume species (red clover, Trifolium pratense L.) on yield, net ecosystem carbon balance (NECB), and the contribution of nitrous oxide (N2O) in the greenhouse gas balance (GHGB). A two-year field experiment was conducted on a managed boreal grassland with mineral soil. GHG fluxes were measured using manual chambers during snow-free seasons and the snow gradient method during snow-covered periods. Inclusion of legume resulted in lower cumulative N2O emissions (with legume 1.7 vs. without legume 3.1 kg N2O-N ha−1) and similar yields compared to the deep-rooted grass mixture, despite requiring 4.2 times less nitrogen fertilization. However, carbon dioxide (CO2) was the dominant GHG, and legume inclusion led to higher respiration rates, resulting in a higher GHGB. These results highlight the potential of legume mixtures to reduce fertilizer use and N2O emissions in boreal grasslands.