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Can short rotation woody bioenergy crops improve shallow groundwater quality in the Lower Mississippi Alluvial Valley?
Proceedings of the 2022 Mississippi Water Resources Conference

Year: 2022 Authors: Kyaw T.Y., Siegert C., Renninger H.

In the Lower Mississippi Alluvial Valley (LMAV), agricultural runoff is a critical environmental problem because it degrades the water quality. Although the LMAV is a pivotal region for agricultural production, there are also areas of marginalized land where row crop production is less profitable due to experiencing seasonal waterlogging and high water tables. Consequently, these marginal lands are usually left unmanaged. However, establishing short rotations woody crops (SRWCs) on these floodplains can be beneficial to land owners because the climatic, agricultural, and infrastructural systems of the LMAV have the potential for developing a biomass-based economy. Furthermore, SRWCs possibly improve water quality due to their capacity to take up dissolved nutrients coming from agricultural fertilizations. Therefore, this study evaluated the nutrient mitigation potential of flood-tolerant SRWCs established as a bioenergy plantation. In 2018, we established a riparian bioenergy plantation in Sidon, MS in the LMAV. Our plantation had two planted blocks, each containing 75 individuals of black willow (Salix nigra), eastern cottonwood (Populus deltoides), and American sycamore (Platanus occidentalis), and two unplanted blocks (control). To collect shallow groundwater samples, we installed 16 groundwater wells (2 m depth) along the elevational gradient of the plantation. We collected groundwater samples monthly in 2018, 2019, and 2021, and biweekly in 2020 when the plantation was not flooded. We analyzed dissolved organic carbon (DOC) and nutrient concentrations in the samples, such as nitrate, ammonia, orthophosphate, and total phosphorus. To test the hypothesis that our plantations could mitigate nutrient runoff, we used a linear mixed effects model by considering locations where water samples were collected as fixed effects, and blocks and date of data collection as random effects. Our preliminary results showed that with an average nitrate concentration of 1.77 mg/L from adjacent agriculture, our plantation was successful at mitigating 90% of nitrate in 2019 in planted blocks, while there was no significant mitigation of nitrate in unplanted blocks. In both planted and unplanted blocks, there was a significant increase of DOC concentrations in 2020. However, before its discharge into the river, the average DOC concentration detected in the unplanted blocks (27.35 mg/L) was about 3 times higher than that of planted blocks. No significant mitigation of ammonia and orthophosphate has been observed yet. Our results suggested that SRWCs could mitigate nitrate runoff. Therefore, planting flood-tolerant SRWCs for bioenergy along the marginal floodplains potentially improves the water quality of the LMAV.

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