Effect of stream bank erosion on microbial enzyme activity in hyporheic soils of Piedmont streams in the Southeastern United States
Streams and rivers in the southeastern United States have been profoundly altered since times of European settlement. Land use practices such as row crop agriculture, dams and impoundments, urbanization and development have caused extensive erosion and habitat loss, especially in the Piedmont region, with about 10% of NC streams designated impaired by the U.S.E.P.A. Hyporheic soils are the saturated zone beneath the stream bed and within the stream bank. This ecotone is characterized by mixing between the groundwater and channel water, reflecting traits of both. This can be a region of high dissolved and particulate organic matter with associated microbial activity. It is also the region regulating passage of materials from the terrestrial to the aquatic environments with the movement of groundwater.
The goal of this research was to evaluate effects of stream erosion, as reflected by stream bank height, on the activity of five extracellular microbial enzymes. Five major soil enzymes involved in nutrient cycling within soil were examined using methods modified for hyporheic soils. Enzymes studied included phenol oxidase, protease, acid phosphatase, β-glucosidase, and β-galactosidase. All assays were done on freshly collected soil samples. Air-dried soils were tested for active carbon content using the permanganate oxidation method.
All steams were first through third order streams, representing agricultural, exurban, and urban regions. Bank height ranged from 10 to 244 cm at the 39 sites studied. Hyporheic soils were taken just below the waterline from the stream bank, with soils analyzed from the surface and at 20 cm depth. There was a negative correlation between bank height and enzyme activity for phenol oxidase, acid phosphatase, β-glucosidase, and β-galactosidase (R values 0.3 - 0.6). Higher activity was measured in surface soils than at 20 cm, but there was a significant correlation at both depths. In contrast to the other soil enzymes, protease activity was positively correlated with bank height, increasing with bank height, in both surface and 20 cm soils. In preliminary analysis of soil from 11 sites, soil carbon in hyporheic soils decreased with bank height in both surface and 20 cm samples. Soil carbon was also negatively correlated with protease activity at both soil depths, suggesting a role for soil carbon in regulation of this extracellular enzyme.
These results suggest significant roles for the microbial community in nutrient cycling within hyporheic soils of Piedmont streams, with stream bank erosion influencing activity of these extracellular soil enzymes.