Differentiating Methane Emissions from California Dairy Farms and Their δ13CCH4 Source Signatures from the Farm-Scale to Regional-Scale

Abstract:

Since 2007, the global mole fraction of atmospheric methane (CH₄) has steadily increased meanwhile the ¹³C/¹²C isotopic ratio of methane (expressed as δ¹³C_CH4) has shifted to more negative values. This suggests that methane emissions are primarily driven by biogenic sources, however more in situ isotopic measurements of CH₄ are needed at the local level to identify which biogenic sources dominate CH₄ emissions. In California, dairies contribute to a substantial amount of CH₄ emissions from enteric fermentation in dairy cows and manure management. In this study, we present seasonal atmospheric measurements of δ¹³C_CH4 from dairy farms in the San Joaquin Valley in California using a mobile platform, with cavity ring-down spectrometers, GPS unit for geolocation, and 2D sonic anemometer for wind direction and wind speed. We used δ¹³C_CH4 to characterize emission signatures from enteric fermentation by measuring downwind of cattle housing (e.g., corrals, freestall lanes) and from manure management areas (e.g., anaerobic manure lagoons). Across seasons, enteric fermentation from dairy cows in corrals and freestall lanes were consistently more depleted of δ¹³C_CH4 than manure lagoons. The mean δ¹³C_CH4 from manure lagoons ranged from -40.29‰ to -49.12‰, whereas the mean δ¹³C_CH4 from cattle housing ranged from -49.78‰ to -67.85‰. Preliminary results at the regional-scale suggest that δ¹³C_CH4 signatures can be used to identify CH₄ emissions from both enteric fermentation and anaerobic manure lagoons from individual dairy farms. Our findings could improve spatial allocation of CH₄ emissions from dairy farms and constrain the relative contributions from these different sources of emissions in the methane budget.