Proceedings of the 2020 Mississippi Water Resources Conference

Year: 2020 Authors: Kyaw T.Y., Siegert C., Renninger H.

Agricultural runoff loaded with surplus nutrients contributes to degradation of water quality of the Lower Mississippi Alluvial Valley (LMAV). In the LMAV, marginal lands experiencing frequent floods and seasonally high water tables may be less suitable for conventional agriculture. In such riparian areas, planting short rotation woody crops (SRWCs) as feed stocks for bioenergy production and also as vegetation filter strips can meet complementary goals of income generation and nutrient mitigation. Considered as the nutrient concentration hotspot, the Mississippi Delta of the LMAV is both geographically and ecologically important for minimizing nutrient delivery to the Gulf of Mexico. Therefore, this study aims to quantify composition of nutrients (e.g., dissolved organic carbon, total phosphorus, ortho-phosphate, and inorganic nitrogen) in groundwater of a SRWC plantation and access survival during an exceptional flood year. In June 2018, 300 cottonwood (Populus deltoides) and 300 willow (Salix nigra) cuttings, and in November 2019, 300 sycamore (Platanus occidentalis) seedlings were planted in Sidon, MS adjacent to an oxbow of the Yazoo River. Groundwater samples were collected from 16 groundwater wells with a depth of approximately 2 m at the highest, lowest and midpoints in each of four replicated plantation blocks. Additionally, water level loggers were placed inside the groundwater wells to monitor water level changes. During the late growing season when water tends to be limiting in the region, groundwater levels were within 1 m of the surface in 2018 and 0.8 m in 2019, whereas the site was continuously flooded in the dormant season up to 1.2 m in 2018 and 3.9 m in 2019. Because of such exceptionally high flooding, survival of willows decreased from 98% in 2018 to 35% in 2019, and cottonwoods decreased from 62% in 2018 to 15% in 2019. Among the four replicates, no survival was found in the two blocks that were continuously underwater from January to August and had deeper floodwater (3.8 m) above them. Only the trees which had continuous but shorter floods (January to June) and less deep water (3.6 m in willows and 3 m in cottonwoods) survived. Besides flooding, beavers damaged 30% of the surviving willows, whereas no damage was found in cottonwoods. Analyses of water quality data are pending. Therefore, even though there were extreme floods in 2019, SRWCs could tolerate a continuous six-month flooding with a height of up to 3.6 m for willows and 3 m for cottonwoods.