Shallow Groundwater Nitrate and Breeding Bird Habitat in a Riparian Buffer Restored by Volunteer Vegetation

Abstract

Research has indicated that riparian buffers can be effective sinks for nitrate (NO3-N) in agricultural environments, however there is uncertainty regarding specific characteristics necessary for optimal buffer performance such as buffer width and vegetation type. This research evaluated the effect of increasing buffer width from 9 to 30 m by allowing fallow vegetation to occupy the riparian zone in an attempt to reduce NO3-N discharge to stream water draining the swine effluent irrigated site located in the Middle Coastal Plain of North Carolina. Buffer functions were assessed by comparing groundwater NO3-N concentrations from the 30-m zone to the same area prior to buffer widening. Increases in percent NO3-N reduction from the buffer edge to stream bank of up to 95% were observed in both shallow (0.8 to 1.2 m deep; p=0.02) and deep (2.4 to 4.5 m deep; p=0.003) groundwater on the east side of the stream and also in shallow groundwater on the west side of stream, although not statistically significant (p=0.2). The increases in NO3-N reduction were most likely due to increases in dissolved organic C (DOC) supplemented by riparian vegetation. Reduction in deep groundwater on the east side of stream significantly decreased (19.5% to ?5.1%; p=0.05) following widening, suggesting that the deeper confining layer along this side minimized the effects of buffer widening on groundwater DOC and hence NO3-N reduction. Stream sampling indicated no significant difference in upstream and downstream NO3-N levels between the two buffer widths (p=0.61).

Riparian buffers can be valuable refuge areas for wildlife in otherwise homogeneous agricultural landscapes. Three different buffer management practices, i) planted woody vegetation, ii) volunteer shrub vegetation, and iii) volunteer three-zone grass, shrub, and woody vegetation, were evaluated for their effects on avian habitat using breeding bird counts and vegetation surveys. The bird community at each site was highly dependent on the types of vegetation present. The highest species richness and total detections were observed in the three-zone buffer during 2002 (30 species; 84 detections/census day/1000 m) and 2003 (22 species; 64 detections/census day/1000 m) sampling periods. Differences in site characteristics such as buffer width, length, and land management likely affected the results; however these data suggest that restoration of riparian areas by allowing fallow vegetation to populate the zone is at the very least equally beneficial to avian wildlife as is restoration by planting specific grass, shrub, and tree species.