Improving shallow groundwater quality by planting short rotation woody crops as riparian buffers in marginal agricultural land of the Lower Mississippi Alluvial Valley

Thu Ya Kyaw, Heidi J. Renninger, Courtney M. Siegert and Randall J. Rousseau, Department of Forestry, Mississippi State University, Starkville, MS

Background/Question/Methods
Riparian buffers can improve water quality by taking up excess nutrients. However, their effectiveness depends on nutrient loads, species composition, and other environmental factors. In agricultural settings of the Lower Mississippi Alluvial Valley (LMAV), unproductive seasonally flooded land may be put back into production with short rotation woody crops (SRWCs) for bioenergy, thereby potentially improving water quality while also providing a revenue source for farmers. Therefore, this study sought to quantify how much excess nutrients could be removed by planting SRWCs compared to unplanted buffers.
To investigate the nutrient mitigation potential of SRWCs, we established a plantation near the Yazoo River in Sidon, MS in the LMAV in 2018. There were two planted blocks (treatment), and each had 75 individuals of black willow (Salix nigra), eastern cottonwood (Populus deltoides), and American sycamore (Platanus occidentalis), whereas another two blocks (control) remained unplanted.
Along an elevational gradient of the plantation, we set up 16 groundwater wells (2 m depth). Before high floods each year, we collected groundwater samples monthly (in 2018 and 2019) and biweekly (in 2020). Concentrations of nitrate, ammonia, and total phosphorus in samples were analyzed using colorimetry methods to determine water quality.

Results/Conclusions
With an average concentration of 1.19 mg/L (SE = 0.40 mg/L) from adjacent agriculture, our plantation was successful at removing 69% of nitrate in groundwater passing through the plantation (p = 0.013) in the treatment blocks. In the control blocks, there was an increasing trend of nitrate concentrations along the transect to the Yazoo River although there was no statistical significance. This suggested that reduction in nitrate was due to uptake by SRWCs and not due to downslope movement of groundwater. Furthermore, we estimated that about 2.6 kg/ha/year of nitrate was removed in the upper 2 m of the rooting zone, and the removal capacity could increase with the growth of SRWCs.
Before moving through the plantation, average concentrations of ammonia and total phosphorus were 0.30 mg/L (SE = 0.10 mg/L) and 3.89 mg/L (SE = 0.71 mg/L), respectively, and no significant annualized reduction was observed. Thus, additional data are needed to provide more insights into these nutrients.
Overall, the results of this study support planting SRWCs as riparian buffers for mitigating nitrogen runoff. Therefore, extensive deployment of SRWCs in the marginal floodplains of the LMAV may provide a viable option to improve water quality.