Microbial Metropolis: Understanding how legume pasture systems interact with soil microbial communities, and subsequent greenhouse gas emissions

Non-Peer ReviewedCattle producers may graze animals on mixed pastures of non-bloat legumes and grasses. This approach can increase dietary protein uptake, improve animal value, and reduce cattle methane emissions by decreasing pasture bloat. The introduction of legumes to a grass pasture can also affect greenhouse gas (GHG) fluxes from the soil by shifting the structure of the microbial communities responsible for nitrous oxide (N2O) emissions and methane consumption, and by altering mineralization rates and soil nutrient content. Two novel forage legume-grass mixes and a grass-alfalfa control were sampled throughout the 2017 and 2018 grazing seasons and analyzed for microbial community structure, nutrient cycling rates, as well as for N2O and methane GHG fluxes. Results suggest microbial community structure, rather than microbial abundance, as one factor regulating GHG emissions. Reduced phosphorous and nitrogen supply rates were key factors limiting microbial abundance, and communities experiencing these environmental stressors were correlated with reduced N2O fluxes. Increasing microbial abundance in response to substrate availability results in depletion of soil phosphorous and nitrogen. This in turn upregulates the carbon and nitrogen cycling activities of communities. Nitrogen and soil moisture content were correlated with increasing nitrous oxide emissions, suggesting that denitrification processes are the major contributor to pasture N2O emissions. In addition, decreasing moisture increased methane consumption, providing a partial sink for cattle-derived methane emissions. Sainfoin treatments had lower cumulative methane consumption when compared to cicer milkvetch and control treatments. Further analysis suggests that different interactions between environmental factors may be involved in shaping microbial communities within each legume treatment, and that local environmental conditions at each sampling point were more important than plant cover treatments in determining daily GHG fluxes. Understanding the microbial processes at play when considering net GHG emissions within a pasture system will contribute to the future sustainability of beef production systems