These source signatures were comparable across season, particularly from manure lagoons, and were always different from one another by at least ~8‰. Additionally, isotopic signatures from CH4 hotspots observed from remote mobile surveys were consistent with on-farm isotopic signatures and captured CH4 source areas. Our downwind observations revealed that enteric fermentation derived CH4 contributed from 0 to 93% of CH4 in plumes that varied with the amount of animal housing and lagoon in the emission footprint . Measurements of 13C of CH4 downwind of dairy farms may be a useful tool to monitor and quantify enteric:manure ratios with changes in mitigation . As shown in this study, isotopic signatures of CH4 downwind of dairy farms can be used to estimate the fraction of contributing sources, such as from manure lagoons and enteric fermentation source areas. We measured that the fraction of enteric CH4 to total CH4 from a mixed cluster of dairy farms ranged from 0.33 to 0.53 similar to model predictions of 0.5 for this region . Most CH4 mitigation strategies separately address CH4 emitted from enteric fermentation, such as through feed additives , or manure emissions by changing management techniques . As governing bodies undertake mitigation strategies to reduce CH4 emissions from enteric fermentation or dairy manure management, it is essential to verify mitigation effectiveness. In California, for example, numerous dairy farms have recently adopted or plan to install digesters in the near future to capture and convert CH4 from manure lagoons into fuel. Although digesters are designed to capture most CH4 emissions, studies have detected notable CH4 leaks from biogas plants . An important area of future research is to quantify the effect of mitigation strategies by comparing δ13CCH4 downwind of dairy farms before and after installation of digesters.
Isotopic signatures in this study agree with previous research showing that manure CH4 is more enriched in 13C than enteric CH4. Our on-farm measurements, however, weed dryers show that manure lagoon CH4 is relatively more enriched in 13C than previously reported in Southern California . Townsend-Small et al. reported a 13CCH4 range of -52.4‰ to -50.3‰ from manure bio-fuel from a manure digester facility and Viatte et al. reported 13C of CH4 of about -57‰ near manure lagoons. This may be explained by differences in CH4 generation processes and manure management differences between Southern California and San Joaquin Valley. Dairies in the San Joaquin Valley predominately use flush systems and store manure in lagoons, while Southern California dairies typically operate dry lots that forgo flushing manure from the feed lanes such that less manure is stored in anaerobic lagoons . Nevertheless, all California farms produce liquid manure from flushing solids in the milking parlor . Although Viatte et al. reported a more depleted 13C of CH4 of about -57‰ near manure lagoons compared to this study, they also observed an ~8‰ fractionation between enteric CH4 and manure CH4, consistent with our findings of isotopic fractionation between manure lagoons and enteric CH4 from free stall barns. There may also be differences in the stable carbon isotope composition of feed and differences in biogeochemical factors that play a key role in determining which microbial communities and pathways promote or inhibit CH4 generation from dairy manure management, and in turn affect the isotopic signature of CH4 emissions. These include pH, dissolved oxygen level, temperature, volatile fatty acids, chemical composition of the substrate, total nitrogen, and nutrient composition .Substrate depletion may also explain this variation, but additional measurements of δ 13C of volatile solids or CO2 concentrations would be needed to confirm isotopically fractionated substrates.
During acetate fermentation, CH4 and CO2 are commonly formed simultaneously. Reduction of CO2 may further transform the generated CO2 into CH4. In the influential study conducted by Whiticar et al. , CH4 generated from pure acetate fermentation resulted in δ 13C-CH4 ranging from -60 to -33‰, whereas CH4 from pure CO2 reduction had δ13C-CH4 values ranging from -110 to -60‰. However, bacterial oxidation in the substrate may affect these pathways before being emitted to the atmosphere, and consequently enrich 13C values of CH4. Measurements of δ2H-CH4 can provide information about partial oxidation since this process enriches δ13C-CH4 and δ2H-CH4 values . Possible explanations for the subtle differences of the manure isotopic signatures between seasons at the reference site may be influenced by changes in diet composition of the milking cows, substrate depletion, perterbations in the lagoon , or a combination of these factors. A future study examining δ 13C and δ2H of methane and δ 13C-CO2 from dairy manure lagoon waste is necessary to confirm the dominant processes contributing to the enriched δ 13CCH4 signatures from California dairy manure lagoons. Isotopic signatures of CH4 from enteric fermentation depend on the C isotopic ratio of foods, specifically with the proportion of plants with C3 and C4 photosynthetic pathways in cattle diets . A diet consisting mostly of C3 plants has been shown to generate more depleted δ13CCH4 than a diet of C4 plants . A database of studies found that ruminants fed a diet of more than 60% C4 plants emit CH4 with δ13CCH4 signatures of -54.6 ± 3.1‰, whereas ruminants fed a C3 diet emit CH4 with δ13CCH4 signatures of -69.4 ± 3.1‰ . This ~15‰ difference is about the same difference between 13C of C3 and C4 feeds. Furthermore, there is a ~41‰ difference between feed and CH4 regardless of ruminant species and diet . Future studies could explore the relationship between diet and CH4 isotope composition across seasons from different cattle production groups. To improve source apportionment of regional CH4 emissions in top-down studies, it is important to consider direct measurements of δ13CCH4 of enteric methane given that it varies depending on diet composition. We have shown that δ13C measurements of atmospheric CH4 using a mobile platform can be used for source attribution of enteric and manure methane. Our findings show that CH4 from manure lagoons is more enriched in δ13C than CH4 from enteric fermentation across seasons on average by 14 ± 2‰. This has implications to track the effectiveness of mitigation strategies by measuring δ13CCH4 to quantify enteric:manure ratios over time. In addition, drying cannabis this study contributes to a body of knowledge dedicated to investigating the sources and processes responsible for the increasing global mole fraction of atmospheric methane.
Future work could explore whether δ13CCH4 signatures change with mitigation efforts. Additional measurements using δ13C and δ2H of CH4 and δ13C-CO2 could elucidate which methane generation processes drive manure lagoon emissions.Major differences in δ13CCH4 from dairy farms among regions underscore the importance of δ13CCH4 measurements at local scales for global analyses.Livestock agriculture is a major source of ammonia and greenhouse gas emissions, such as methane and nitrous oxide . In the United States, livestock contributes an estimated 66% of total agricultural GHG emissions . Methane is more efficient at trapping infrared radiation than carbon dioxide , with a lifetime of about 10 years in the troposphere and a global warming potential about 28 times that of CO2 on a 100-year scale . Ammonia is a gas-phase precursor to fine particulate matter, impacting human health and posing a threat to terrestrial and aquatic systems . As such, there is a need for accurate observations of GHG and NH3 emissions from the agricultural sector are imperative to address poor air quality and climate change. The San Joaquin Valley of California is a region with significant CH4, N2O, and NH3 emissions . Currently, there is disagreement whether state inventories accurately represent these gases across spatial and temporal scales. For example, atmospheric studies often report dairy CH4 emissions in California up to two times higher than bottom-up inventories . Meanwhile, other studies have reported that CH4 observations were comparable to inventories during the summer but not winter seasons, or using ground observations but not airborne measurements . A similar case is observed for NH3 in the SJV, where chemical transport models substantially underestimate gas-phase NH3 observations compared to airborne and satellite measurements . These results suggest that inventories likely underestimate and misrepresent agricultural NH3 emissions across spatial and temporal scales . There are limited N2O observations in the SJV of California, where most N2O emissions is expected from the agriculture sector . These studies show that top-down observations of N2O are at least two times higher than bottom-up inventories . In addition, these studies use either short-term airborne or tower observations, which provide limited seasonal and spatial information on N2O emission trends. The dairy sector is an important source of GHG and NH3 emissions in the SJV. Methane emissions from dairy farms is primarily emitted by enteric fermentation from ruminant gut microbes and anaerobic decomposition of dairy manure in storage ponds . Dairy manure management contributes a substantial fraction ofCH4, N2O, NH3 emissions and the relative magnitudes depends on manure management practices . Solid manure management includes storing manure in piles, deep pits, open lots, and daily spreading of dairy waste. In contrast, in a liquid manure management system, waste from barns and other dairy infrastructure, such as milking parlors, are washed and collected in slurry ponds or anaerobic lagoons . Anaerobic conditions, such as found in anaerobic manure lagoons, promote the production of CH4, and to a lesser extent N2O and NH3 emissions . Solid manure storage systems have reportedly higher N2O emissions than CH4 and NH3 emissions relative to manure lagoons. Nitrous oxide is generated from denitrification and nitrification reactions in manure-amended soils, manure storage, and direct N deposition by animals . In general, denitrification accounts for most of N2O emissions under anaerobic conditions. Nitrous oxide, along with NH3 and NO, is indirectly emitted through volatilization of manure N from nitrification and denitrification in soil after redeposition . Ammonia emissions, on the other hand, are primarily a byproduct of urea hydrolysis during the decomposition of urine and feces, which is mostly found in animal housing . Ammonia volatilization at liquid-surface interface occurs under high pH conditions since the pKa of NH4 + /NH3 is 9.25 . Storage of animal feed, such as silage piles, also emit NH3 and N2O As California moves towards meeting GHG and air pollution reduction goals, it is critical to gain a better understanding of the magnitude, temporal patterns, and source of emissions from dairy farms in the SJV region.Ground-based mobile lab measurements were collected in autumn of 2018 , spring , summer , and autumn of 2019 , and winter of 2020 . Table 3.2 shows a summary of these measurements and associated environmental conditions. Atmospheric measurements were performed with a mobile platform outfitted with multiple trace gas analyzers based on cavity ring down spectroscopy and an isotopic N2O analyzer based on off-axis integrated cavity output spectroscopy . In addition, a global satellite positioning unit recorded geolocation and vehicle speed and a weather station measured wind direction, wind speed, air temperature and relative humidity. A stationary 3 m meteorological tower with a 3-D sonic anemometer mounted was used to collect ambient temperature, wind speed, and wind direction. Atmospheric measurements of CH4, NH3, and N2O were collected from an inlet height of 2.87 m above ground level. Greenhouse gas measurements were corrected using high and low gas mixtures before and after each measurement period. The gas mixtures were tied to the NOAA Global Monitoring Division scale.The highest ΔNH3:ΔCH4 maxima were observed during the summer and autumn seasons, when air temperatures were high, for free stall barns, corrals, manure lagoons, and silage. In animal housing, NH3 emissions are a byproduct of urea hydrolysis from the decomposition of urine and feces. In general, NH3 volatilization increases with higher concentrations of NH4 + /NH3, substrate temperature, pH, wind speed and turbulence . When temperatures are high, this dairy farm increases the ventilation and moisture of free stall barns with ceiling fans and cools milking cows with periodic cooling water mist. Increased wind speed and ventilation rates tend to decrease CH4 emissions in animal housing . Increased turbulence and moisture conditions during the summer months potentially promoted more NH3 emissions in the free stall barns and decreased CH4 emissions. Methane emissions from animal housing are impacted by weather conditions and management practices. The quantity and quality of manure deposited onto the housing floor affects whether methanogenesis is promoted.