Methane fluxes from the manure ponds had a robust diurnal pattern, increasing with air and surface pond temperatures. As such, latent heat flux and CH4 flux also had a strong positive diurnal relationship, especially during the pre-sedimentation stage. Higher wind speed and friction velocity also increased diurnal maxima of CH4 fluxes. Rising lagoon temperatures and wind speeds commonly enhance CH4 emissions by promoting high microbial activity, diffusion, and convection of liquid manure storage . Another eddy covariance study also found that wind speed increased CH4 fluxes from manure lagoons in California, with a range between 20 to 65 kg m-2 yr-1 . Seasonal patterns, in contrast, were dominated by other factors such as precipitation events, seasonal variations in substrate availability, and manure management practices. Increased CH4 fluxes during this period was likely driven by perturbation of the pond surface, increased wastewater inputs to the manure pond system, and/or a combination of both. Previous research has shown an increase in CH4 emissions after a rainfall event as it agitates the surface of the manure pond, releasing an outburst of trapped gas bubbles . A similar case is observed after surface thawing events, cannabis drying racks wherein trapped CH4 is released . In addition, it is likely that during these precipitation events extra manure from the corral surface entered the manure pond system due to the engineering design of the manure pond system.
Specifically, sufficiently large rainfall events flush manure deposited in the corral area into the channel that conveys wastewater to the lagoon, increasing the amount of manure substrate deposited to the system Seasonal changes in substrate availability, sometimes termed the seasonal hysteresis effect, may also explain the rising CH4 fluxes observed during the spring season. Methanogenesis rates are slowest during the winter season when temperatures are low, leading to a buildup of volatile solids in the manure ponds. As temperatures rise during the spring season, manure ponds release more CH4 compared to other seasons with comparable temperatures because of greater availability of substrate due to winter buildups . Future modeling studies are needed to assess how much of an impact these factors contribute to increased seasonal CH4 fluxes. A stark decrease in monthly CH4 fluxes from 2019 to 2021 was likely driven by changes in CH4 production and consumption rates and transport pathways. The area covered by crust and vegetation substantially increased from June 2020 to June 2021, providing a favorable environment for CH4 oxidation under aerobic conditions and slower diffusion rates .Vegetation with aerenchyma formations may serve as both a conduit to transport CH4 from the lagoon to the atmosphere or provide an environment for CH4 oxidation and production . Given that CH4 fluxes drastically decreased after crust formation, there was likely an increase in CH4 oxidation and a slower diffusion of gases to the atmosphere. Future work is needed to investigate the role of vegetation and crust formation on CH4 emissions from manure ponds.
Based on manure sampling in August 2019, we assessed that CH4 was produced deeper in the water column or sludge layer of the manure pond at 0.3 m and 0.8 m, especially closer to the inlet at location 1 and location 2. Neutral pH conditions and low ORP levels less than -200 mV were found at 0.3 m and 0.8 m, providing a conducive environment for methanogenesis. The ORP levels were greater than 0 mV at the surface layer of manure pond, indicative of biomass accumulation, which likely increased occurred during the sedimentation stage . This coincides with the crust layer formation at the surface. The most alkaline conditions were found at the surface at location1 and location 2, which likely inhibits methanogenesis. In addition, higher TS and VS content was measured near the inlet, at location 1 and location 2 at 0.3 and 0.8 m, providing biodegradable organic matter needed to produce CH4 .We evaluated our eddy covariance CH4 fluxes by comparing it to other estimates using different techniques. A scaling factor was used to compare between this study’s CH4 flux estimate using the eddy covariance technique with the Thiruvenkatachari et al. study and the floating chamber pilot study discussed in Section 4.3.6. To best compare to these studies, we only selected daytime CH4 fluxes measured in August 2019, before there was significant crust formation, considering that the Thiruvenkatachari et al. study measured during the daytime hours on August 14, 2018, and the floating chamber pilot study was conducted during the daytime hours on August 15, 2019. It is also important to consider that the CH4 flux footprint of our eddy covariance measurements primarily captures Source Area 3 & Source Area 4 of manure pond 1 .
As such, we estimate that 24 kg CH4 d -1 is emitted from Source Area 3 & 4 during the daytime in August 2019. To estimate a scaling factor, we compared our eddy covariance’s CH4 fluxes with the Thiruvenkatachari et al. study’s CH4 emission estimate for Source Area 4. Thiruvenkatachari et al. ’s CH4 emission estimate is 3.7 times higher than our eddy covariance CH4 flux estimate for Source Area 4. As such, we estimated CH4 emissions for the entire manure pond complex by applying this scaling factor to Thiruvenkatachari et al. ’s CH4emission estimates for the rest of the manure pond sources. Thus, we estimate that 117 kg CH4 d -1 is emitted from the manure pond complex , or 81 g CH4 head-1 d -1 using the eddy covariance technique. In comparison, Thiruvenkatachari et al. ’s estimates a total of 387 kg CH4 d -1 is emitted from manure pond 1. Floating chamber measurements were also consistent with the concentration gradient observed in the Thiruvenkatachari et al., study with CH4 emission estimates of 172, 89, and 28 kg d-1 for Source Area 1, 3, and 4, respectively . All methods estimate CH4 emissions within the same order of magnitude for each of the corresponding areas in manure pond 1 .As shown in this study, there are both strengths and disadvantages of using the eddy covariance technique to estimate CH4 fluxes from dairy manure lagoons. An important strength of eddy covariance is that we can estimate CH4 fluxes at both diurnal and seasonal time-scales. This allows to better predict the most influential drivers of CH4 fluxes at different time scales and provides important information for manure management strategies. For example, weed dry rack dredging the manure pond during the winter season may substantially reduce CH4 emissions during the spring season, when CH4 fluxes were highest. The limitation of the eddy covariance technique is that can prove challenging to capture CH4 fluxes from the entire manure pond complex when there are changes in the predominant wind direction. In our study, we primarily captured portions of Source Area 3 and 4 based on the CH4 flux footprint. Moreover, there could be substantial loss of data based on changes in predominant wind direction. As such, we recommend that future studies carefully plan the best location of the eddy covariance tower to avoid significant data loss and consider using multiple eddy covariance towers to fully capture different areas of dairy manure lagoons under different predominant wind directions.In summary, CH4 fluxes from manure lagoons varied across different timescales, both diurnally and seasonally. The primary factors influencing diurnal CH4 fluxes were also different from those driving the seasonal pattern of CH4 fluxes. Temperature and wind speed affected diurnal CH4 fluxes the most. In contrast, seasonal CH4 fluxes were most likely impacted by precipitation events, changes in substrate availability, and manure management practices. Higher CH4 fluxes were observed during the spring seasons, when methanogenesis rates increase with warmer manure pond temperatures and after precipitation events. This suggests that manure management practices, such as dredging manure ponds, during the winter months could potentially have the greatest reduction of CH4 emissions. Understanding how CH4 fluxes change over time, and which factors most control CH4 emissions is important to develop methane reduction strategies in the agricultural sector.Globally, the average temperature has increased more than one degree Fahrenheit since the late 1800s, and most of this increase has occurred over just the past few decades. The world’s most renowned climate scientists have warned that only a dozen years remain that global warming can be kept at a maximum of 1.5 C, beyond which an increase of even half a degree will significantly worsen the risk of drought, floods, extreme heat, and poverty for hundreds of millions of people . The increase in temperature has resulted from the rising levels of greenhouse gases in the atmosphere: carbon dioxide , methane , nitrous oxide , and fluorinated gases .
These levels have risen by about 40% in the last 150 years, with half of that rise occurring in the last three decades. About half of cumulative anthropogenic CO2 emissions between 1750 and 2010 have occurred in the last 40 years . From 1750 to 1970, cumulative CO2 emissions from fossil fuel combustion were 420 35 Gt CO2, and these had tripled to 1300 110 Gt CO2 by 2010. Between 1750 and 2010, cumulative CO2 emissions from forestry and other land use changes increased from 490 180 Gt CO2 to 680 300 Gt CO2. Of the total GHG emission in 2014, CO2 accounted for 76%, CH4 for 16%, and N2O for 6% . At the end of 2019, annual CO2 emissions from industrial activities and the burning of fossil fuels have increased to 36.8 Gt, and total CO2 emissions from all human activities, including agriculture and land use, have increased to 43.1 Gt of CO2 . Today, the agricultural sector has a significant carbon footprint and accounts for >25% of worldwide anthropogenic GHG emissions. Besides fossil fuel burning, the decomposition of soil organic matter and crop residue burning are major sources of CO2 emissions. Methane emission in agriculture occurs from flooded soils under rice cultivation, enteric fermentation in the digestive systems of livestock, and the decomposition of manure and crop residues under wet conditions. Emissions of N2O in agriculture result predominantly from soils fertilized with nitrogen, manure, and compost that release inorganic nitrogen into the soil. Among the largest emitters in agriculture are enteric fermentation , manure left on pasture , synthetic fertilizer , paddy rice , manure management , and burning of savannahs . Carbon dioxide and other gases emitted from industrial and agricultural sources trap heat in the atmosphere, resulting in an increase in global average temperatures and thus global climate change . The increase in concentration of GHGs in the atmosphere has a wide range of effects: rising sea levels; the increasing frequency and intensity of wildfires; more extreme weather events such as changes in the amount, timing, and distribution of rain, snow, and runoff; deadly heat waves; severe droughts; and tropical storms; and is a threat to food production. Therefore, controlling the emission of GHGs into the atmosphere is considered as the greatest environmental challenges of this century . Globally, economic and population growth are the most important drivers of increasing GHG emissions and are projected to increase continuously in future. Therefore, any reduction in GHG emissions is uncertain, and further increase in emissions cannot be ruled out. Soils constitute the largest C pool both in organic and inorganic forms. The amount of C in SOM ranges from 40 to 60% by weight. Although SOM usually constitutes less than 5% of soil weight, it is one of the most important components of a field ecosystem . Globally, approximately 2300–2500 Gt of C is stored in the top 2 m of soil, of which about 70% is stored in the subsoil below 0.2 m . The amount of C in soils is more than three times that of C in terrestrial vegetation, and at least 230 times higher than the 2009 global CO2 emissions . From this amount, approximately 60 Gt of C is exchanged with the atmosphere annually . Because of large C pool, soils offer the potential for GHG mitigation through C sequestration in above ground biomass or soils. Additionally, the management of biophysiochemical properties of soil and vegetation mitigates climate change by reducing emissions. Globally, there has been a strong interest in capturing C in agricultural soils, not only to mitigate the risk of global warming, but also to improve the soil quality . This paper reviews general aspects of soil C sequestration, including its potential and associated challenges and risks, with a special reference to South Asia.