|BAKER, BETH - Mississippi State University|
|KROGER, ROBERT - Covington Civil And Environmental|
|PRINCE CZARNECKI, JOBY - Mississippi State University|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2016
Publication Date: N/A
Interpretive Summary: The control of fertilizer movement from agricultural land is of utmost importance to reduce nutrient losses and water pollution. The issue with nutrient losses, is not only an important economic topic for the farmer, but can be a large problem for polluted bodies of water, such as the Gulf of Mexico. Aside from best management practices at the agronomic level, one method a farmer can use is to employ catch systems such as low-grade weirs. These systems catch the runoff coming off an agriculture field, and the retention allows for biological nitrogen reduction. These systems work under some, but not all, environmental conditions. The purpose of this study was to identify the microbiological effects of low-grade weir implementation. Using DNA-based approaches, this study identified an effect associated with season as well as distance from the weir, indicating that the weirs do have an effect on the biological properties of the soil. Weirs do not have a negative effect on soil microbial populations, and in some cases, enhanced microbial populations.
Technical Abstract: Agricultural activities throughout the Mississippi River Basin have been identified as a major source of nutrient pollution, particularly nitrogen from fertilizer application, to downstream waters including the Gulf of Mexico. Utilizing best management practices, such as low-grade weirs have been identified as a potential solution to mitigate nutrient loads in agricultural runoff. This study assessed the impacts of weir implementation in four agricultural drainage ditches (three with weirs and one control site) in the Mississippi Delta region on 1) denitrifying microbe abundance utilizing functional genes (nosZ, nirS, and nirK) via quantitative polymerase-chain reaction (qPCR), 2) microbial communities via terminal-restriction fragment length polymorphism (T-RFLP) of 16s rRNA, and 3) soil physiochemical parameters (soil C, N, moisture, and vegetation presence). Quantification using qPCR of 16S rRNA genes and nosZ and nirS genes was successful, however, nirK, was found to be below detection limit of 5,000 gc/g soil. Weir proximity was negatively correlated with 16S rRNA and soil moisture, thus 16S rRNA and soil moisture decreased as distance from weir increased. Subsequently, soil moisture was positively correlated with 16s rRNA and nirS abundance. Results of empirical Bayesian kriging did not exhibit obvious patterns of microbial diversity in relation to weir proximity and weirs were not found to directly influence microbial diversity. Microbial community diversity was, however, found to be significantly greater during fall 2012 than at all sites with weirs in spring 2013 (p<0.0001) and the control site in spring 2013 (p=0.001). Seasonal trends in microbial abundance also showed 16s rRNA, nirS, and soil moisture to be higher in fall than spring. Results highlight that weirs were not found to have adverse impacts on microbial diversity, abundance, or soil chemical parameters. Results support the potential benefits of weirs to enhance drainage ditch environments for denitrifying microbes and potential N remediation from agricultural runoff, mitigating impacts to downstream water resources.