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Mechanisms of Nitrogen Oxide Formation During Ensiling
Citation:
Green, P. AND F. Mitloehner. Mechanisms of Nitrogen Oxide Formation During Ensiling. U.S. Environmental Protection Agency, Washington, DC, 2014.
Impact/Purpose:
The goal of this EPA task is to better understand the chemical and biological reactions that generate NOx emissions during the ensiling process. This could be particulary important in ozone and NOx nonattainment areas such the San Joaquin Valley and the South Coast where small emission sources will be critical to attainment. Based on this effort, it seems plausible that the mechanism that creates NOx emissions in the early phase of the ensiling process is caused by microbes. This effort’s results will inform the design of future research into the mechanisms of NOx generation during ensiling, the results of which could identify ways to reduce the formation of NOx during ensiling while maintaining the quality of the resulting silage (animal feed). As a result of this exploratory research, we have also confirmed that measurable amounts of NOx are emitted during the early, aerobic stages of the ensiling process. This finding informs future CARB-sponsored research (i.e., NOx emission measurements to quantify NOx generation during ensiling) that could identify a significant source of NOx contributing to seasonal ozone levels in the South Coast and San Joaquin Valley Air Districts.
Description:
Silage (ensiled feed), as a dairy’s greatest operational cost, is its most critical feed commodity. The continued use of silage is essential to a highly productive and economically viable industry. Our previous work has shown that silages are a major source of volatile organic compounds (VOCs) from dairies contributing to the San Joaquin Valley’s (SJV’s) emissions inventory. Other studies have found up to 2 parts-per-million (Maw et al, 2002) concentrations of oxides of nitrogen (NOx) arising from the ensiling process. In addition, there is a CARB funded project underway to measure the amount of NOx and VOC emissions from silage at dairy locations in the field in California. Both VOCs and NOx are precursors to ozone formation and PM 2.5, which are long-standing air quality challenge in many areas of the country, especially in regions with hot, sunny summers and cold winters with valley geography which traps air masses. The San Joaquin Valley is such a region. As a result, California has been diligently identifying, understanding and reducing all sources of VOC and NOx emissions.The goal of this EPA task is to better understand mechanisms that could be creating NOx emissions from silage. To understand the mechanisms, NOx emissions were compared by treatments either (a) sterilizing the microbes inherently present in chopped corn, or (b) using chemical inhibitors to limit the activity of the peroxidase enzyme, which plausibly produces NO2 from nitrate. This EPA project specifically tested the use of radiation sterilization (by electron beam) to discern whether NOx formation during corn ensiling is microbial or due to pre-existing plant enzymes. Second, this EPA project tested three possible chemical inhibitors (azide, cystine and vanadate) of the enzyme thought to be responsible for NOx formation. The sterilization had the strongest effect on NOx formation. While each of the chemical inhibitors showed an influence on the final ensiled material, variations with the NOx results of the chemical inhibitors prevent us drawing any specific conclusions about the influence of those treatments on NOx emissions. Based on this work, it seems plausible that the mechanism that creates NOx emissions in the early phase of the ensiling process is caused by microbes. These experiments were designed to elucidate the possible mechanism(s) of NOx formation, not to attempt quantification or control, of emissions. Further research would be needed before it could be determined if NOx controls for silage are merited and appropriate controls might be identified.