A Farm-Scale Test of Nitrogen Assimilation by Vegetated Buffer Systems Receiving Swine Lagoon Effluent by Overland Flow

Abstract

A farm-scale study was conducted from 2000 to 2004 to determine the effectiveness of grass-forest vegetated buffers in assimilating nitrogen (N) from overland flow application of swine lagoon effluent. The rationale for the study was that replicated buffer plot studies had shown that vegetated buffers will effectively assimilate N, but it was not known whether or not they would work at a larger scale. The study was conducted on a commercial farm near Tifton, Georgia. Wastewater was pumped from a single-stage anaerobic lagoon to vegetated buffers composed of grass and mature or newly planted pines. The buffers approximated 60 m in length by 90 m in width. The upper 10 m of each buffer was in grass, while the downslope area was in mature or newly planted pines. Six buffers were instrumented for wastewater application and water quality monitoring. Two buffers received wastewater at a 1X rate (600 kg N ha-1 year-1), two at a 3/4X rate (450 kg N ha-1 year-1), and two served as controls. The wastewater was applied to the 10 m grassed portion of the buffers. Transects of shallow groundwater wells starting at the grass-forest interface and running downslope were used to monitor water quality N. The study showed mixed results concerning N assimilation by the buffers. Upslope land use changes by the producer during the study added significant N inputs to one set of buffers, and they were unable to assimilate sufficient N from both these inputs and the wastewater to protect shallow groundwater quality. In contrast, almost all samplings of shallow groundwater under the buffers receiving N only from the overland flow applied swine lagoon effluent showed nitrate (NO3-N) concentrations 20 and 30 m downslope to be lower than 10 mg L-1 (drinking water standard). On these buffers, NO3-N concentrations in shallow groundwater were near background levels five years after wastewater application commenced. The study indicated that the ratio of buffer area width to wastewater application area width on the landscape should be at least 1:1, and that buffers for protection of water quality should be continuous on the landscape. It was concluded from the study that buffers can be used at the farm scale to assimilate N from applied wastewater when they are sufficiently wide relative to waste application area, rate, and other N sources at the farm scale.