In general, an influx and accumulation of fresh manure to a corral encourages methanogenesis and also enhances N2O and NH3 emissions with concentrated urine patches . In several studies, CH4 uptake occurred in corrals, specifically in late summer when soil was dry and in winter when soil was frozen or cold, thereby inhibiting methanogenesis . On the barn floor, aerobic and anaerobic conditions may also lead to relatively lower N2O emissions . In addition, CH4 emissions tend to be negatively correlated with heat stress in naturally ventilated dairy barns because of decreased animal activity . ΔN2O:ΔCH4 maxima were also highest during the summer for free stall barns, corrals, and crops. This may be explained by the higher air temperatures in animal housing areas and irrigation of croplands using the manure wastewater from the holding pond. Generally, the relative abundance of available N, whether as NH4 + or NO3 – , soil oxidation reduction potential, soil temperature, moisture, oxygen availability, pH, microbial communities, and degradable carbon sources impact N2O emissions. Manure application to cropland promotes N2O and NH3 losses . Direct N2O losses are generated from nitrification and denitrification reactions in the soil. NH3 volatilization is an indirect source of N2O when NH3 is volatilized from manure, for example, and re-deposited onto soil, where it is converted into N2O. Methane losses from manure application are relatively low because of carbon uptake by the soils under aerobic conditions . Higher N2O emissions generally occur in warmer and moist soils, drying weed which enhance denitrification and nitrification . Thus, during the summer, higher temperatures and moist conditions in the animal housing areas increased CH4 emissions and, to an even larger extent, N2O emissions.
We also observed higher N2O losses from manure effluent application during the summer, with relatively low CH4 emissions.Manure lagoons were characterized by considerably higher ΔN2O:ΔCH4 in autumn compared to winter and summer measurements. Aerobic conditions at the inlets of manure lagoons can lead to denitrification reactions performed by facultative anaerobes . Summer measurements are likely to have higher CH4 emissions relative to N2O emissions. Increasing air temperatures and wind speed commonly increase CH4 emissions since they affect microbial activity, diffusion, and convection of liquid manure storage . N2O emissions from denitrification are also impacted by similar factors, including warm temperatures, labile C, and anaerobic conditions . Other factors that influence N2O emissions from manure include redox potential, pH, and substrate concentration. Winter measurements were conducted a few days after a rainfall event, which may have increased CH4 emissions from the manure lagoons relative to N2O emissions. Methane emissions may increase after a rainfall event, given that it agitates the surfaces and increases ebullition rates of CH4 from super-saturated lagoon waters . Our study shows that manure lagoons had relatively higher CH4 emissions than N2O emissions during the summer, given warmer air temperatures, and winter months, following agitation of manure surface from rainfall events. The solid drying area and dry bedding of manure had higher ΔN2O:ΔCH4 values in winter relative to autumn measurements. Winter measurements were conducted only a few days after a rainfall event, which may have produced higher N2O emissions relative to CH4 emissions in the dry manure storage piles. In contrast, dry bedding had relatively highΔNH3:ΔCH4 values in the summer compared to autumn measurements. Higher air temperatures during the summer may have volatilized more NH3 relative to CH4 emissions. The solid drying area had the lowest ΔNH3:ΔCH4 values among all sources across seasons.
Higher ΔNH3:ΔCH4 values for the solid drying area were observed in the winter and spring relative to summer and autumn. Solid manure storage is heterogenous in aerobic and anaerobic composition depending on manure management practices. Nitrous oxide emissions from solid manure storage are positively related to total N content since it enhances nitrification and denitrification . N2O production is also positively related to the total carbon content because denitrifiers strongly rely on carbohydrates for energy . The heterogeneity of solid manure storage also affects the relative abundance of methanogens and methanotrophs in the substrate . Methane fluxes from solid manure systems are positively correlated with moisture, C/N ratio, NH4 + -N, and total organic carbon . High CH4 and NH3 emissions occur primarily at the early stage of decomposition of carbon and nitrogen sources from fresh manure . Methane fluxes increase with higher NH4 + since it inhibits CH4 oxidation via production of toxic hydroxylamine and nitrate from ammonium oxidation or competition for methane monooxygenase. Static solid manure piles are predominantly aerobic, but may form anaerobic areas if the proper moisture, density, and porosity is met. The anaerobic areas in the piles enhance CH4 emissions . In our study, N2O:CH4 enhancement ratios from dry bedding were primarily influenced by rainfall events that enhanced N2Oemissions during the winter measurements. In addition, NH3:CH4 enhancement ratios from dry bedding were primarily influenced by higher air temperatures that increased NH3 emissions during the summer. This study’s enhancement ratios were consistent with previous relevant studies . Our summer and autumn NH3:CH4 and N2O:CH4 enhancement ratios were higher than previously reported in literature and state inventories. Our winter NH3:CH4 enhancement ratios were lower compared to another California study conducted during the winter . This difference may be explained by the rainfall events presiding our measurements, which enhanced CH4 emissions more than NH3 emissions. Our work underscores the importance of seasonal measurements as enhancement ratios are greatly influenced by changes in environmental factors, such as temperature, rainfall, and wind speed. Enhancement ratios may be a useful tool to characterize and identify emission sources from dairy farms. As shown in this study, animal housing , wet manure management , dry manure management , silage piles, and cropland had distinct enhancement ratios. This tool could be particularly useful for source attribution of an emission plume in a region with multiple sources of CH4, NH3, and N2O emissions.
Seasonal information about enhancement ratios is also important as shown by the seasonal variability in enhancement ratios for different sources of emissions. Dairy management practices and physicochemical and meteorological factors greatly influenced the relative contributions of CH4, NH3, and N2O emissions.Manure lagoons contribute about 35% of California dairy farm CH4 emissions statewide . In these lagoons, organic-rich manure waste is stored as a liquid, creating anaerobic conditions that produce CH4 that is subsequently emitted to the atmosphere, much of it from the lagoon surface. However, our understanding of manure lagoon CH4 emissions is far from complete, complicating mitigation strategies for reducing or capturing CH4 . In addition, temporal and spatial variability complicate emission estimates, which depend on physicochemical and micrometeorological predictors. Physicochemical predictors include organic substrate availability, pH, oxidation-reduction potential , nutrients, electron acceptors, curing weed chemical oxygen demand . Micrometeorological factors include air and pond temperature, friction velocity, wind speed, and precipitation . As such, it is essential to quantify the magnitude and uncertainty associated with CH4 emissions from dairy manure lagoons specific to the location of interest. The processes that impact CH4 fluxes from manure lagoons are production, transport, and consumption. The large amounts of organic substrates found in liquid dairy manure under anaerobic conditions provide a conducive environment for methanogenesis and CH4 production. Acetoclastic methanogens and acetogenic and hydrolyzingmicroorganisms drive this methane fermentation process. Methanogenic substrates, such as H2, CO2, formate, and acetate, are generated as by-products by microorganisms in the dissolved and suspended solids found in the stored liquid manure . Total solids content in dairy slurry is an indicator of the volatile solids content, the biodegradable organic matter that may produce CH4 . Dairy slurry with high VS content tend to have higher CH4 production rates . Favorable conditions for methanogenesis include neutral pH, ORP below -200 mV, nutrients and depletion of electron acceptors such as NO3 – . The fraction of degradable organic matter greatly determines the amount of CH4 production in liquid manure and is expressed as biochemical or chemical oxygen demand . Higher BOD or COD tends to produce more CH4 . Methane oxidation can occur when there are low CH4 production rates under high oxygen conditions and a slow diffusion process . Slurry may form crusts as it contains more solids that can float to the lagoon’s surface. The crust layer may slow the diffusion of gases and provide a conducive environment for CH4 oxidation under aerobic conditions . The primary transport pathways for CH4 to reach the surface of manure lagoons are through diffusion, ebullition , and agitation events . Diffusion of CH4 occurs within the aqueous boundary layer orplant-mediated transport via aerenchymatous vegetation. Transport of dissolved gases through the aqueous boundary layer is generally a slow process that is dependent on the concentration gradient . Albeit uncommon in dairy manure lagoons, another potential CH4 pathway is through aerenchymatous vegetation, as is commonly found in wetlands and lakes .
Methane may also escape through ebullition when CH4 is produced at such a fast rate that it forms bubbles and passes through the substrate layer . Mechanical agitation, from such events as rainfall and high wind speed, may also release CH4 trapped in manure lagoons to the atmosphere . Wind speed and friction velocity affects near-surface turbulence, and subsequently influences ebullition and diffusion of gases . Increased turbulence of the lagoon surface emits more CH4 to the atmosphere . Temperature can influence diffusion and ebullition of CH4 fluxes from the lagoon surface at short time scales through changes in CH4 solubility, transfer of gas across the air–water interface, and thermal contraction and expansion of free-phase gas . Latent heat flux at diel scales serves as a proxy for CH4 volatilization as evaporation of water and CH4 emissions are driven by similar physical mechanisms and tend to positively covary . Methane production and oxidation rates are also impacted by the temperature effect on microbial metabolism and enzyme kinetics, with higher temperatures generally associated with higher CH4 production or oxidation rates. Furthermore, CH4 production is influenced indirectly by temperature through seasonal changes in substrate availability . These methods can be broadly separated into two categories: floating chambers and micrometeorological methods . Each of these approaches has its benefits and disadvantages. One of the main advantages of the eddy covariance method is its ability to measure long-term diurnal and temporal CH4 fluxes. It is relatively low-maintenance and time-efficient compared to other techniques. Like other micrometeorological methods, the eddy covariance technique also measures across large spatial scales without disturbing the ecosystem. However, there is an inherent uncertainty with CH4 emission estimates using micrometeorological methods since they are each based on unique assumptions about the micrometeorological transport of mass and energy and surface homogeneity . Another disadvantage of using the eddy covariance method is the inability to separate CH4 fluxes between different areas of the manure lagoon. Other micrometeorological methods, such as presented in Thiruvenkatachari et al., , where mobile atmospheric measurements were coupled with a dispersion model, and floating chambers could apportion CH4 emissions to different areas of the manure lagoon. Floating chambers are a cost-effective method to measure accurate direct CH4 emission rates from different regions of the manure lagoon. Some of the disadvantages of floating chambers include: it is labor-intensive; there is a risk of disturbing the observational environment; chambers capture only a snapshot of CH4 fluxes at a given point in time; and the sampling protocol needs to be carefully designed to avoid inaccurate estimates, such as large pressure differences between the inside of the chamber and ambient levels . In addition, floating chambers run the risk of over accumulation of CH4 within the chamber. This is especially a risk in manure lagoons where CH4 can reach high concentrations at fast rates. In California, there are 1,750,329 milk cows, of which 93% are in the Central Valley, wherein the predominant manure management includes storage of manure in lagoons . The California GHG inventory currently quantifies CH4 emissions from dairy manure management practices with emission factors based on several parameters, including cow population and demographics, average statewide manure management practices, and climate . However, these estimates are based on emission factors derived from few pilot and lab-scale studies outside of California . Consequently, current GHG inventory estimates are likely not representative of California’s climate and unique biogeography. In addition, the current inventory includes no temporal information on emissions at timescales shorter than 1 year. So far, there is not a clear consensus whether inventories are representative of emissions given a dearth of measurements. As such, a major obstacle to assessing emissions through field measurements and comparing them to inventories are the different timescales .