Non-bloat legumes alter pasture soil greenhouse gas fluxes, nutrient cycling rates, & microbial community structure

Non-Peer ReviewedPasture grazing systems can act as both sources and sinks for greenhouse gases (GHGs) — cattle emit enteric methane (CH4), while soil microorganisms emit nitrous oxide (N2O) and consume ambient CH4. Producers seed non-bloat legumes into pasture forage to increase cattle protein uptake and decrease enteric CH4 emissions, but with undetermined impacts on soil N cycling and N2O emissions. While the introduction of legumes to a grass system can shift soil microbial community structure, it remains unclear how non-bloat legumes affect soil microbial community structure, nutrient cycling rates, and GHG fluxes. To answer this, a two-year survey of microbial community structure, enzyme activity, and GHG emissions was conducted within pastures containing a grass control treatment and two non-bloat legume treatments consisting of sod-seeded Astragalus cicer (cicer milkvetch) and Onobrychis viciifolia (common sainfoin). Seasonal changes and precipitation had the largest effect on microbial community structure, while including non-bloat legumes was responsible for 5% of the community variation, partially through increased soil nitrate levels. Structural equation models (SEMs) revealed that cicer milkvetch lowered soil nitrogen cycling rates and decreased arbuscular mycorrhizal fungi (AMF) abundance, which was associated with higher N2O emissions. Soil moisture was the strongest predictor of N2O emissions but did not guarantee a large N2O flux. While cicer milkvetch increases soil nitrate and potential N2O emissions, this must be weighed against the expected benefits of reduced cattle CH4 emissions