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

Area of Science:

• Agricultural Science
• Soil Science
• Ecology

Background:

• Intensified agriculture causes nitrogen losses, leading to environmental issues like eutrophication and nitrous oxide emissions.
• Legume cover crops in diversified agroecosystems can improve nitrogen and carbon cycling but require understanding of nitrogen fixation controls.
• Variability in soil conditions necessitates identifying key regulators of legume nitrogen fixation.

Purpose of the Study:

• To test the hypothesis that nitrogen mineralization from soil organic matter turnover regulates legume nitrogen fixation.
• To investigate the role of soil organic matter fractions in controlling nitrogen fixation across diverse farm soil conditions.

Main Methods:

• Collected soil samples from 10 farms along a soil organic matter (SOM) gradient.
• Separated SOM into free particulate organic matter and intra-aggregate particulate organic matter (POM) fractions.
• Assessed nitrogen (N) mineralization and legume nitrogen fixation rates in relation to SOM fractions.

Main Results:

• Legume nitrogen fixation was downregulated with increased N availability in intra-aggregate POM and higher N mineralization rates.
• Intercropping legumes with grasses weakened the link between POM-N and N fixation due to grass N uptake.
• A legume-grass cover crop mixture increased N and C mineralization rates over two seasons.

Conclusions:

• Soil organic matter dynamics, particularly N mineralization, play a crucial role in regulating legume nitrogen fixation in diversified agroecosystems.
• Legume cover crops can enhance soil organic carbon and nitrogen, potentially creating a stabilizing feedback loop for N availability and fixation.
• Findings provide a mechanism for observed diversity-ecosystem function relationships in long-term agroecosystem studies, highlighting reduced environmental impact.