Title: Effects of Management and Ecosystem Type on Nutrient Retention in Riparian Zones

Cooperators: Robert L. Potter, Judith L. Meyer (University of Georgia), David D. Bosch, Robert K. Hubbard,

Problem:
USDA specifications for riparian forest buffers provide for a three zone system. Zone 1 is an area of permanent woody vegetation directly adjacent to the stream channel. Zone 2 is an area of managed forest upslope from Zone 1. Zone 3 is a grass buffer between Zone 2 and the upland use such as agriculture. Little is known about the effects of management or regional variation on the effectiveness of these types of buffers.

Approach:
A five year experiment examined the effects management of the Zone 2 forest on water quality and soil properties in a coastal plain riparian forest. Two forest management treatments (clear cutting or selective harvest) in the Zone 2 were monitored for three years at the Gibbs Farm site. Soil nitrogen and carbon pools and processes were measured at six coastal plain riparian forest sites on seven dates over two years. The six sites, all dominated by hardwood vegetation, varied in upland management, width of forest buffer, and tree species composition.

Results:
Both management treatments had little influence upon nitrogen dynamics when compared with control plots. A pulse of NO3-N following cutting was minor and transitory in nature. The managed forest remained an effective sediment trap. There were slight increases in total N in groundwater in the harvested areas but they were also transitory. In general, it appears that the Zone 2 of coastal plain riparian forests can be harvested with little effect on water quality. Sites with buffers of 40 m or more were effective in attenuation of NO3-N, whereas sites with buffers of less than 40 m were less effective. Narrow (20 to 24 m) buffers were less effective, with higher soil nitrate adjacent to streams regardless of season. One site with a narrow buffer but high tree density was more effective compared to two other sites with narrow buffers but low tree density. Wider buffers (53 to73 m) had lower soil nitrate near the stream. Denitrification in surface soil ranged from 0 to 2 kg N ha-1 d-1 and was positively correlated with soil extractable NO3-N, pH, and water filled pore space, and negatively correlated with soil bulk density. The relative pool sizes of labile N in the top 1 m of soil were microbial biomass > fine roots > dissolved organic N > inorganic N. In general, it appears that there is less accumulation of nitrate N in near stream areas in areas with wider riparian forest buffers. Information from these studies on the management and variability of riparian forest buffers will provide data for testing of models of riparian forest functions and will help refine guidelines for riparian buffers.