Microbial Regulation of Soil Greenhouse Gas Emissions in a Non-Bloat Legume Grazing System

Abstract

Cattle pastures are a source of greenhouse gas (GHG) emissions, including enteric methane from ruminating cattle, carbon dioxide (CO2), and nitrous oxide (N2O) from microbial respiration of soil carbon (C) and nitrogen (N). Producers may introduce non-bloat legumes to cattle pastures to improve soil N content, increase cattle protein uptake, and decrease enteric methane emissions. However, such land management changes can alter soil microbial communities, potentially increasing net system GHGs. The research goal was to determine whether non-bloat legumes alter soil microbial community structure, activity, and N2O emissions. Grazed pastures with introduced Veldt cicer milkvetch and common sainfoin were surveyed for differences in GHGs, microbial community structure, and extracellular enzyme activity for two growing seasons. Seasonal shifts explained most microbial community changes; however, communities structured according to legume treatment and legume microbial community structure correlated with increasing soil nitrate (NO3-) content, particularly in cicer milkvetch plots. Soil N2O fluxes did not differ on sampling dates, however cicer milkvetch tended towards larger N2O emissions. Pasture soil microbial community changes did not translate to increased N2O emissions on sampling dates. Additional protein found in non-bloat legumes may increase urine urea-N content in cattle affecting soil processes. Urine containing low or high concentrations of 15N and 13C labeled urea was added to soil microcosms under controlled conditions to better understand the impact of urine on soil N2O emissions, microbial N cycling communities, and N2O sources. Ammonia oxidizing bacteria (AOB) were the most active nitrifiers following urine addition. Denitrifiers contributed the most N2O in urine amended soils and their dynamics varied. After urine deposition, nirS gene abundance and transcript increases were greater and more sustained than nirK, while nirK increased activity more rapidly but did not increase gene abundance. Urine toxicity and increased clade II nosZ transcription likely reduced initial N2O emissions at high urea concentrations, resulting in no difference between cumulative soil N2O fluxes between urea rates and a lower urine-N emission factor for high urea soils. These findings support the use of non-bloat legumes in pastures, particularly when considering the potential reduction in enteric methane emissions, reducing net pasture system GHG emissions.