Managing Legume Nitrogen Fixation to Reduce Nitrogen Losses from Farms

Abstract:

Nitrogen (N) losses from agriculture are a persistent driver of global change, highlighting that fundamental changes to farm management are necessary to mitigate N pollution. For instance, long-term experiments show that agroecosystems with greater crop functional diversity, and legume cover crops (i.e., non-harvested crops) as the primary N source, have smaller N losses and larger stocks of soil organic matter (SOM). However, predicting the response of legume N₂ fixation and other N cycling processes to variable soil conditions is needed to increase cover crop adoption on farms. In an experiment on 10 farms spanning a gradient of SOM levels, we used ¹⁵N natural abundance to quantify N₂ fixation in a hairy vetch cover crop. The legume downregulated N₂ fixation rates with increasing N availability in intra-aggregate particulate organic matter (POM), and with increasing N mineralization rates, suggesting a mechanism for internal N cycling efficiency. In a separate field experiment, we found that increasing plant functional diversity with a legume-grass mixture reduced N mineralization rates and pulse emissions of N₂O after tilling cover crop biomass into soil, while maintaining inputs from N₂ fixation similar to a sole legume. In an incubation study, we decomposed cover crop litter in two soils with contrasting management histories. The soil with a longer history of cover cropping had greater total N release and microbial biomass C, indicating that diversification enhances soil N cycling. However, there was a greater microbial response to litter addition (i.e., CO₂ production and extracellular enzyme activity) in the lower fertility soil, suggesting the importance of targeting conservation practices to restore ecosystem functions in degraded soils. Finally, we discuss the need for ecosystem-based approaches to assessing and modeling links between management practices and N pollution, and the need for policy changes to reduce the social and economic constraints to farm diversification.