Feedbacks between nitrogen fixation and soil organic matter increase ecosystem functions in diversified agroecosystems

Nitrogen (N) losses from intensified agriculture are a major cause of global change, due to nitrate (NO3-) export and the eutrophication of aquatic systems as well as emissions of nitrous oxide (N2O) into the atmosphere. Diversified agroecosystems with legume cover crops couple N and carbon (C) inputs to soil and reduce N pollution, but there is a need to identify controls on legume N2 fixation across ecosystems with variable soil conditions. Here, I tested the hypothesis that N mineralization from turnover of soil organic matter (SOM) regulates legume N2 fixation across 10 farms that spanned a gradient of SOM levels. I separated soil samples into two SOM fractions, based on size and density, which are indicators of soil nutrient cycling and N availability (free particulate organic matter and intra-aggregate particulate organic matter (POM)). This study indicates downregulation of legume N2 fixation in diversified agroecosystems with increasing N availability in intra-aggregate POM and ...