Assessment of forest management effects on nitrate removal by riparian buffer systems

A study was conducted to determine the impact of different forest management techniques on shallow groundwater quality in coastal plain riparian zones. Considerable past research had shown that riparian zones are effective in removing or assimilating nitrates entering from upslope agricultural fields via shallow lateral flow, bur the impact of different forest management techniques on this process was unknown. The study was conducted at a site near Tifton, Georgia, on a second-order coastal plain stream. The riparian buffer system consisted of a grass buffer a managed forest zone, and a forest zone adjacent to the stream. Three forest treatments were studied: mature forest (MF), clearcut (CC), and selective thinning (ST). Following a nine-month pretreatment period trees were completely or selectively removed from the CC and ST treatments, respectively. Shallow groundwater quality was evaluated in networks of wells on transects extending downslope from the edge of the agricultural field to the stream. Results from the study showed that all three forest management treatments were effective in assimilating nitrate-nitrogen (NO3-N). Significant differences in NO3-N concentrations in the shallow groundwater between the three different treatments did not occur The only statistically significant effect that was observed on groundwater quality was under the CC treatment, where solute concentrations (both NO3-N and chloride [Cl]) decreased after the tree cutting. This was attributed to a combination of effects including possible increased NO3-N uptake by rapidly growing vegetation, dilution associated with less evapotranspiration by young vegetation as compared to mature forest, and more throughfall of rainfall under the CC than under the other two treatments. No treatment effects were observed on ammonium-nitrogen (NH4-N) concentrations. Overall the study showed that regardless of forest management techniques, coastal plain riparian forests are effective in assimilating NO3-N.