Denitrification and organic carbon in a series of riparian buffers in the Bear Creek National Demonstration Watershed

Johnson, Amber
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Riparian buffers have been shown to be effective at improving surface water quality. Vegetative uptake and denitrification are interrelated nitrogen removal services of riparian buffers. Surface denitrification rates are much higher than subsurface rates, however, subsurface denitrification is also essential to overall nutrient removal services. Soil quality can vary under different vegetation types in surface and subsurface soils. This study was conducted to determine if denitrification potential and its controlling factors were different under warm season grasses and introduced cool season grasses at different depths. Denitrification potential measured by the Denitrifier Enzyme Activity Assay (DEA), Total Organic Carbon (TOC), Total Nitrogen (TN), Dissolved Organic Carbon (DOC), Bioavailable DOC (%BDOC) and Inorganic Nitrogen (IN) were measured during three sampling periods (Summer 2001, Fall2001, and June 2002) at five depths up to the non-permeable aquitard (3-4m). Sampling was done following soil morphological features, from the surface, throughout the rest of the vadose zone, at the border with fluctuating groundwater conditions evidenced by mottling, throughout the mottled zone, and at the border with impermeable till. Surface samples were higher than all other samples. Vadose zone samples were significantly different from other subsurface depths during active plant growth, but were not significantly different from other subsurface depths when grasses were dormant. Multivariate analysis and amendment studies showed that after accounting for depth in the soil profile, soil water was the most important factor controlling denitrification followed by organic matter (% C and N) and C availability (%BDOC). The results were confounded by age of the buffers, i.e. all the warm season grasses were relatively recently established, and all cool season grasses had been established for more than 50 years. The lack of startling differences between vegetation types indicates that warm season grasses may rapidly restore organic C and soil processes after establishment. The results suggest that different plant communities affect denitrification potential in the surface and vadose zones, and imply that a diverse mixture of plants should be used in buffer establishment to maximize year-long denitrification potential.