Data from: 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
increasing N mineralization. Intercropping the legume with a grass
weakened the relationship between N in POM and N2 fixation due to N
assimilation by the grass. Further, mean rates of N and C mineralization
across sites increased with two seasons of a legume-grass cover crop
mixture, which could enhance this stabilizing feedback between soil N
availability and N2 fixation over time. These results suggest a potential
mechanism for the diversity-ecosystem function relationships measured in
long-term studies of agroecosystems, in which regular use of legume cover
crops increases total soil organic C and N and reduces the environmental
costs of crop production.