The other by-products, acetate and butyrate, help induce methanogenesis. Manure management systems vary among dairy farms but generally consist of dry and wet management practices . Dry manure management consists of deep pits, solid manure storage, dry lots, and daily spread . In a wet manure management system, manure waste from animal housing areas are washed and typically collected in manure lagoons, where anaerobic conditions produce CH4 . Dry manure handling practices reduce anaerobic conditions since they do not flush waste with water. So far, however, there are only two studies on seasonal CH4 emissions from anaerobic lagoons in California, but none have studied emissions from all four seasons . Measuring and modeling emissions from dairy manure management are challenging given the variability of practices . It is only recently that mobile measurement campaigns measured CH4 emissions from a small number of dairies with anaerobic lagoons . Given that field data is still variable, the majority of N2O emissions is estimated to originate from barns, unlike dairy CH4 emissions, which are mostly expected from anaerobic lagoons and slurry systems . The next largest emitter of N2O from dairy manure management is estimated to come from corrals and solid manure piles . Corrals include loafing pens, hard standings, and dry lots. Studies have also measured N2O emissions from anaerobic lagoons and slurry stores, which was unexpected since anaerobic conditions are dominant in wet manure storage . In anaerobic wet manure, nitrogen is mostly found in the form of ammonium and organic nitrogen, cannabis curing but denitrification is possible at inlets from wet manure storage systems if aerobic conditions are present . Nitrous oxide can also form through the denitrification of nitrate generated by Feammox, Mnammox, or anammox in the cases where NO3 – is present .
Nitrification can also occur under aerobic conditions, where N2O is emitted as a by-produced when NH4 + is first oxidized to nitrite and then converted to NO3 – . Ammonia is formed and volatilized from dairy manure almost immediately after urine and feces are excreted. Ammonia travels to the manure surface via diffusion and is released to the atmosphere via convective mass transfer . In general, NH3 volatilization increases with higher concentrations of NH4 + /NH3, substrate temperature, wind speed and turbulence . Ammonia emissions are highest between a pH of 7 to 10 and decrease with lower pH and is impacted by the pKa of the reaction .Methane is the second most important anthropogenic greenhouse gas after carbon dioxide and is increasingly becoming a critical priority for near-term climate action, given its relatively short lifetime and substantial potential for rapid mitigation . Over the last several decades, the growth rate of atmospheric CH4 has significantly changed, reaching stable zero growth from 1999 to 2006, followed by an increase beginning 2007 . This rise in the global mole fraction of atmospheric CH4 has been the subject of several studies that focus on explaining this phenomenon, without a definitive explanation. A rise in CH4 emissions could be indicative of changes in total emissions from various sources, including from biogenic, thermogenic, and pyrogenic CH4 and/or changes in the atmospheric sink of CH4 . The isotopic signature of CH4 is an important tool to diagnose the source of this increase in CH4 . The global stable carbon isotope ratio of atmospheric CH4, expressed as δ 13CCH4, has shifted towards more negative values simultaneously with the rise of the atmospheric mole fraction of CH4 . Recent isotopic evidence suggests that this rise in CH4 is likely dominated by increased emissions of biogenic CH4, which are more depleted in 13C relative to fossil and pyrogenic CH4 sources. Based on this explanation, possible biogenic sources responsible for the rise in atmospheric CH4 include ruminants, rice paddies, and wetlands, among others. Previous work have shown that isotopic signatures of CH4 emitted by enteric fermentation depend on the carbon isotopic ratio of diet composition, driven by the proportion of plants with C3 and C4 photosynthetic pathways, with estimates δ 13CCH4 of about -60‰ for C3-fed ruminants and about -50‰ for C4-fed ruminants . Other conflicting hypotheses about the CH4 budget include an underestimate of fossil-derived sources in CH4 inventories based on an isotope mass balance . Further studies, however, show that an increase in fossil-derived CH4 emissions is inconsistent with the observed trend in atmospheric δ13CCH4 . Additionally, there are large uncertainties in the magnitude and trends of atmospheric sinks of CH4 . Given that our understanding of the CH4 budget remains incomplete, there is a clear need for sufficient in situ isotopic characterization of CH4 at the local level to identify the location and type of sources that dominate the current rise in global CH4 emissions . Even at local to regional scales, the budgets of both CH4 and its stable carbon isotope remain uncertain . Improved knowledge is particularly important for ensuring effective mitigation of CH4 at scales where policies to reduce CH4 are being enacted . In California, there are statewide efforts underway to reduce CH4 emissions, but it remains challenging to accurately monitor progress given the large inconsistencies between atmospheric observations and greenhouse gas inventories . Atmospheric observations have inferred higher CH4 emissions than reported in GHG inventories at the statewide and regional levels and from individual sectors, including dairies . However, there is little information about the processes that produce this apparent discrepancy.
The California Air Resources Board GHG inventory estimates that dairies contribute about half of statewide CH4 emissions, with contributions from enteric fermentation by ruminant gut microbes and manure managed in anaerobic conditions. However, these estimates are based on emission factors derived from a few pilot and lab-scale studies conducted outside of California and thus likely not representative of California’s climate and unique bio-geography . Given that mitigation practices are targeted towards the bio-geochemical and management processes that produce CH4, new tools for source apportionment and process understanding are required . Stable isotopes of CH4 may be a promising way forward. The few studies that have measured isotopic signatures of CH4 from dairies in California were done in the Los Angeles Basin. Townsend-Small et al. investigated the isotopic signature of major sources of CH4 in the Los Angeles megacity and found that isotopic values of δ13CCH4 from fields applied with cow manure were characterized by values between -62.1 per mil to -59.2‰, whereas δ13CCH4 of manure bio-fuel from a manure digester facility ranged from -52.4‰ to -50.3‰. Cow breath, on the other hand, had more depleted δ13CCH4 source signatures between -64.6‰ and -60.2‰. A more recent study by Viatte et al. measured isotopic signatures of δ13CCH4 from the largest dairy farms in Southern California, and observed values between -65‰ to -45‰, attributing the most depleted observations to enteric fermentation. In Europe, grow room previous research has shown that δ 13CCH4 signatures vary dependent on the type of dairy manure storage. In Heidelberg, Germany, Levin et al., observed more enriched δ13CCH4 from manure piles and a biogas generator than liquid manure . Two recent studies used mobile surveys to measure δ 13CCH4 in Europe. In Germany, Hoheisel et al. conducted mobile measurements to determine δ 13CCH4 signatures around Heidelberg and in North RhineWestphalia. The δ13CCH4 signatures ranged from -66.0‰ to -40.3‰ for three dairy farms with biogas plants. More enriched δ13CCH4 signatures were observed from plumes downwind of the biogas plant relative to plumes downwind of the animal housing. In Northern England, Lowry et al., found that methane plumes downwind of dairy farms had δ 13CCH4 signatures from -67‰ to -58‰. Atmospheric measurements downwind of manure piles were more enriched in 13CCH4 with values close to -50‰ relative to cow breath, which were close to -70‰.
Isotopic endmembers were variable downwind of animal housing dependent on the cattle population and amount of manure waste present. In general, CH4 from barns with fewer cows and more manure waste were more enrichedin 13C. In comparison, beef cattle feedlots have isotopic signatures within the range of expected enteric fermentation, with δ13CCH4 signatures of -66.7 ± 2.4‰ in Alberta, Canada to -56.2‰ ± 1.2‰ in the Colorado Front Range, USA . Beef cattle are generally pasture raised until they are sent to feedlots, where their diet is primarily maize with varying proportions of wheat . In this study, we present seasonal atmospheric measurements of δ 13CCH4 from dairy farms located in the San Joaquin Valley, California, where 91% of the state’s dairy herd resides . Our primary objective was to measure δ 13CCH4 emitted from anaerobic manure lagoons and enteric fermentation source areas across seasons. Our second objective was to use δ13CCH4 source signatures from enteric fermentation and anaerobic lagoons to identify the dominant source responsible for CH4 hotspots detected from downwind plume sampling of other dairies in the region. We hypothesized that the δ 13CCH4 signatures from dairy anaerobic manure lagoons and enteric fermentation can be used to apportion CH4 emissions between these two dairy farm source processes. These isotopic signatures can help contribute to the body of knowledge that aims to resolve the CH4 budget in California and globally.Ground-based mobile measurements were collected at a dairy in Tulare County , California, in the fall, spring, summer, and winter seasons from 2018 to 2020. Hereafter, we will refer to this dairy as the reference test site farm. Figure 2.1shows a schematic of the reference test site farm layout. The reference test site has on average 3070 milking cows that spend most of their time in free stall barns, with an additional ~400 dry cows and ~3000 heifers that are primarily in open lots . Wet manure management is used for waste deposited in the free stall barns, where manure waste is flushed from barn floors and diverted to a processing pit. Wastewater from the milking parlor also enters the processing pit. Processing pit water is reused to flush lanes or is pumped over stationary inclined screen . A manure separator then removes coarser solids from liquid effluent, which gravity flows into cell 1. The liquid manure navigates from separation cell 1, cell 2, the primary lagoon, and finally into a holding pond via gravity, decreasing the content of suspended volatile solids through anaerobic decomposition and settling as it moves from one component to the next. Water waste from the holding pond is later used as irrigation water for cropland. Hereafter, manure lagoons refer to cell 1, cell 2, primary lagoon, and the holding pond. Dry manure management refers to the fraction of waste that is separated from the liquid waste stream, which is spread out on the ground and solar dried. Once dry, this manure is distributed into free stall beds or stacked and covered in the dry bedding. The primary forages are wheat and maize preserved as silage. Silage piles are covered with a double layer of plastic. The feed composition for different seasons was obtained by weighing each feed ingredient as it was included into the mixer wagon. All weights were transferred electronically to feed management software . FeedWatch data were retrieved once monthly for ingredient identification, quantity fed per pen, pen population and dry matter composition. Each ingredient was identified as C3 or C4 except for distiller’s grain, which could be a changing combination of C3 and C4 sources. Sum of dry weights by pen for C3, C4, distillers feeds were calculated. The feed composition by cattle production group is presented in .We also made measurements at other dairies within a 10 x 10 km region of agricultural land in the same county, which includes additional dairy farms, beef feedlots, poultry farms, and a landfill that are also emitting CH4 . Other potential sources of emissions surround the region, including a wetland, plugged and abandoned oil and gas wells that are permanently sealed, and a wastewater treatment plant. Residential land is primarily located south of the region and contains an extensive natural gas pipeline network. Globally, the δ 13CCH4 signatures from fossil fuel sources are typically around – 44‰ , with δ 13CCH4 signatures between −50‰ to −36‰ from fugitive natural gas in urban settings . Urban studies also use ethane to CH4 ratios as a tracer to distinguish between sources in mixed source regions .