UC Riverside

Dairies are an important source of greenhouse gas, such as methane (CH4) and nitrous oxide (N2O), and ammonia (NH3) emissions but remain highly uncertain across spatial and temporal scales. Stable isotope measurements and enhancement ratios between different trace gases can be used to characterize and identify relative magnitudes, trends, and sources of emissions. In the first study, CH4 emissions from enteric fermentation and manure management source areas from dairy farms in California's San Joaquin Valley (SJV) are characterized using isotopic signatures of CH4 (δ13CCH4). Methane from manure lagoons was more enriched in δ13C than CH4 from enteric fermentation across seasons on average by 14 ± 2‰. The second study quantified and characterized seasonal CH4, N2O, and NH3 emissions from SJV dairy farms using enhancement ratios of trace gases (ΔN2O:ΔCH4; ΔNH3:ΔCH4). The average ΔNH3:ΔCH4 from freestall barns and corrals observed across all seasons is 0.58 ± 0.19 ppbv ppbv-1 and 0.48 ± 0.05 ppbv ppbv-1, respectively. Manure lagoons had an average ΔNH3: ΔCH4 of 0.09 ± 0.01 ppbv ppbv-1 , whereas dry bedding had ΔNH3:ΔCH4 of 2.71 ± 0.85 ppbv ppbv-1. ΔN2O:ΔCH4 also show distinct signatures between livestock housing, manure management, cropland, and silage, with the highest enhancement ratios observed in cropland (1.65 ± 0.17 ppbv ppbv-1). The third study estimated seasonal and diurnal CH4 fluxes from dairy manure lagoons in Southern California. Diurnal CH4 fluxes were closely correlated with latent heat fluxes and show a positive relationship with lagoon temperatures and wind speed; however, the temperature relationship differs at seasonal timescales. Methane fluxes decreased over the study period, with the highest CH¬4 fluxes measured during the spring season (6.89 µmol m-2s-1; 95% CI: 6.41 - 7.45 µmol m-2s-1), following precipitation events. These findings highlight how isotopic measurements and enhancement ratios of co-located emissions can be used for source apportionment of dairy emissions. Lastly, it underscores the importance of long-term measurements of CH4 fluxes from dairy manure lagoons to determine the principal environmental factors influencing the magnitude of seasonal and diurnal trends.