Evaluation of agricultural nutrient reductions in restored riparian buffers

Abstract

Efforts to restore the Chesapeake Bay have focused on reducing agricultural nutrient losses. In particular, riparian buffer restoration has been an important component of nutrient reduction strategies, and one program used extensively to restore riparian vegetation on agricultural land is the Conservation Reserve Enhancement Program (CREP). I evaluated the effect of CREP on water quality on the Delmarva Peninsula by measuring groundwater nutrients under restored buffers on two farms, monitoring stream baseflow in 30 small watersheds (or subbasins), and monitoring stream stormflow in two subbasins. On the farms, nitrate concentrations were lower in the restored buffers than in the non-buffered sites, suggesting that buffer restoration was successful in filtering groundwater nitrate. In groundwater under a 7 year old CREP buffer, dilution by infiltration of rainwater accounted for 56% of the total nitrogen reduction, and denitrification accounted for 15 to 30%. At the watershed scale, CREP restored 1 to 30% of total streamline in 15 agriculturally-dominated subbasins in the Choptank River. However, I did not detect differences in nitrogen concentrations between these subbasins based on the amount of buffer restoration. Nitrogen concentrations actually increased in most of the streams since previous monitoring before restoration; therefore, buffers may not be extensive enough to have measurable affects on baseflow water quality. However, comparison of stormflow between two subbasins revealed significant nutrient differences. Total buffered streamline was greater and more widely distributed in Blockston than in Norwich subbasin. The amount and distribution of CREP may have influenced the stormflow nutrient yields, which were 2 times higher in Norwich versus Blockston. Lastly, I reviewed 20 years of stream monitoring data from German Branch subbasin in the context of all agricultural management practices implemented in the basin. A decade after management, I detected a 33% decrease in phosphorus concentrations in stream baseflow, but no significant changes in nitrogen concentrations. However, the rate of increase of 0.14 mg N L-1 yr-1 prior to management did not continue to present-day baseflow conditions and may have been suppressed by management practices. While these results are somewhat encouraging, complete understanding of watershed-scale effects of riparian buffers will require further interdisciplinary study.