Long-Term Grazing and Nitrogen Management Impacted Methane Emission Potential and Soil Microbial Community in Grazing Pastures

Achieving sustainable development in livestock agriculture by balancing livestock production, reduction of greenhouse gas (GHG) emissions, and effective utilization of nitrogen nutrient has indeed been challenging. This study investigated the long-term effects of continuous cattle grazing, stocking rates, and fertilization regimens on methane (CH₄) emissions, soil microbial communities, and soil organic carbon (SOC) stocks in Bermudagrass pastures in East Texas, USA. Pastures were subjected to high or low stocking rates for over 50 years, with further subdivision based on fertilization: nitrogen-based fertilizer application or no fertilizer but with the growth of annual clover. Seasonal soil cores (0–60 cm) were collected, and laboratory microcosm incubation studies revealed unexpectedly high in vitro CH₄ emissions in surface soils, particularly in the top 0–5 cm soil layer, reaching up to 300 nmol of CH₄ mL^–1. Higher CH₄ emissions and methanogen abundance, along with lower SOC stocks, were observed in pastures subjected to high stocking rates compared to those with low stocking rates and in clover pastures compared to those with N-fertilized ryegrass. On the contrary, in low-stocked, N-fertilized annual ryegrass pastures, methanogen abundance was lowest, CH₄ emissions were negligible, and SOC stocks were highest. Furthermore, animal excreta deposition significantly contributed to increased CH₄ emissions. Prokaryotic and potential methanotrophic taxa, as compared to fungi, exhibited greater responsiveness to N-fertilization than to cattle stocking treatments with higher levels of methanotrophs observed in pastures subjected to high stocking rates and clover growth. This study suggests that strategic management practices such as optimal grazing and nitrogen management could effectively mitigate CH₄ emissions in grazing lands.