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Abstract
Trace-gas emissions from animal feeding operations (AFOs) can contribute to air quality and global change gases. Previous and current estimated gas emissions from AFOs vary widely and many do not consider all forms of carbon (C) and nitrogen (N) emissions. Studies have found that as methanogenesis in the lagoons increased, conversion of ammonium (NH4 +) to dinitrogen (N2) also increased. The purpose of this research was to measure N2 and CH4 emissions from swine AFOs in three locations of the U.S. and to evaluate the possible universal relationship between lagoon methanogenesis and the conversion of NH4 + to N2 gas. This relationship was tested by measuring N2 and CH4 emissions in two climates at 22 different farms. Methanogenesis was correlated with NH4 +-to-N2 conversion by a near-constant N2 to CH4 emissions ratio of 0.20, regardless of C loading and climatic effects. The process is shown to be thermodynamically favored when there is competition between NH4 + oxidizing reactions. Under methanogenic conditions (redox potentials of methanogenesis) N2 production is favorable and nitrification/denitrification is not. Thus, N2 production is stimulated in methanogenic conditions. Evaluation of NH3 gas emissions from AFOs must consider other N emissions than NH3. Finally, a statistical model was developed to estimate methane and N2 emissions (kg gas ha−1) given feed input per lagoon surface area (kg feed ha−1) and local air temperature. Further studies are needed to investigate the mechanisms involved in manure processing and isolate the favorable mechanisms into engineering improved manure processing.
Highlights
Ammonia (NH3) is a significant air pollutant, especially in combination with acid gas production from fossil fuel combustion, because the resulting acid– base reaction potentially leads to an air quality problem in the form of haze and respirable particulate matter (PM)
Gas emissions were measured in six anaerobic, manure-processing swine lagoons across North Carolina (NC), 15 in the Central Great Basin (CGB), and one in Georgia Coastal Plains (GA)
NH4? to N2 was observed in all lagoons and a correlation was found between methanogenesis (CH4 emissions) and conversion of ammoniacal N to benign N2 gas
Summary
Ammonia (NH3) is a significant air pollutant, especially in combination with acid gas production from fossil fuel combustion, because the resulting acid– base reaction potentially leads to an air quality problem in the form of haze and respirable particulate matter (PM). Much of the N estimated as NH3 gas emissions has been found to be converted to dinitrogen gas (N2) (Harper et al 2000, 2004b; Weaver et al 2012), representing an even larger discrepancy for the N balance of farm systems suggested by the USEPA This aspect of dinitrogen emissions, not considered in most of the estimates of NH3 emissions from animal feeding operations (AFOs), highlights the fact that the N cycle in lagoons is not fully understood. Benign N2 emission from lagoons is a pathway of N emissions is that is significant and must be considered in the total N balance of AFOs. When the National Emissions Inventory (USEPA 2004) NH3 emissions values are combined with published (measured) N2 emissions (Harper et al 2000, 2004a, b; Weaver et al 2012), in many cases more N as NH3 plus N2 is emitted than is excreted by the animals, suggesting the need to reevaluate emissions’ estimates
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