Effect of Riparian Buffers on Soil Redox Potentials

Abstract

Excess pollution in Eastern North Carolina has led to unprecedented regulations to reduce the nitrogen loading to the Neuse River by thirty-percent by the year 2003. The primary culprit of excess nutrients is nonpoint sources. Agricultural processes have been identified as the prime perpetrator for the nonpoint source pollution. Interception of this pollution must be accomplished before it reaches groundwater and eventually surface water supplies. Denitrification reduces the nitrate into nitrogen gas, thus atmospheric nitrogen. One indicator of denitrifying conditions is soil redox potential.

This study evaluates the soil redox potentials at depths greater than 1.5m. The effect and interaction of buffer width and vegetation type on groundwater nitrate concentration was evaluated. Lower concentrations of nitrate-nitrogen were correlated to soil redox potentials at depths greater than 1.5m.

The Center for Environmental Farming Systems in Goldsboro, North Carolina had an ongoing riparian buffers study implemented five channelized streams. Three of these riparian buffers were instrumented with soil redox electrodes. Each replicate had two widths of buffers, 8m and 15m. There were five different vegetations: fescue (cool season grass), switch grass (deep rooted grass), forest (pine and mixed hardwoods), native vegetation, and no buffer (i.e., row crop or pasture). Redox electrodes were constructed and installed along the stream and buffer/field edge at depths of 1.5m and 3.0m. These depths corresponded to the depths of the existing ground water quality monitoring wells on each buffer. These wells and redox electrodes were monitored for a period sixteen months.

Redox values were usually lower at the deeper depth, and lower near the ditch than at the field/buffer edge. The deep electrodes along the ditch exhibited reducing conditions in every replicate. Buffer width affected redox readings in one replicate, buffer width affected nitrate-nitrogen concentrations in all replicates. Vegetation affected the redox readings with switchgrass generally exhibiting the lowest redox values of all vegetation types. Vegetation had little effect on the nitrate-nitrogen concentrations.

Low nitrate-nitrogen concentrations were related to low soil redox potentials in specific area of a given well. Where soil redox was low, there was a presence of organic carbon and the soil was saturated it was concluded that denitrification was occurring. Buffer width, location relative to the stream, and soil redox potential had an affect on the concentrations of nitrate-nitrogen. After three years of evaluation vegetation type has not shown an impact on nitrate-nitrogen concentrations.