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TECHNICAL REPORTS

Surface Water Quality

Using Nitrogen-15 to Quantify Vegetative Buffer Effectiveness for Sequestering Nitrogen in Runoff

Department of Agronomy and Range Science, University of California, Davis, CA 95616

^* Corresponding author (bedardhaughn{at}ucdavis.edu)

Received for publication February 18, 2004. Previous studies have observed higher levels of soluble nutrients leaving vegetative buffers than entering them, suggesting that the buffers themselves are acting as a source rather than a sink by releasing previously stored nutrients. This study used 98 atom % ¹⁵N-labeled KNO₃ at a rate of 5 kg ha^–1 to quantify buffer efficiency for sequestering new inputs of NO^–₃–N in an extensively grazed irrigated pasture system. Buffer treatments consisted of an 8-m buffer, a 16-m buffer, and a nonbuffered control. Regardless of the form of runoff N (NO^–₃, NH⁺₄, or dissolved organic nitrogen [DON]), more ¹⁵N was lost from the nonbuffered treatments than from the buffered treatments. The majority of the N attenuation was by vegetative uptake. Over the course of the study, the 8-m buffer decreased NO^–₃–¹⁵N load by 28% and the 16-m buffer decreased load by 42%. For NH⁺₄–¹⁵N, the decrease was 34 and 48%, and for DON–¹⁵N, the decrease was 21 and 9%. Although the buffers were effective overall, the majority of the buffer impact occurred in the first four weeks after ¹⁵N application, with the buffered plots attenuating nearly twice as much ¹⁵N as the nonbuffered plots. For the remainder of the study, buffer effect was not as marked; there was a steady release of ¹⁵N, particularly NO^–₃– and DON–¹⁵N, from the buffers into the runoff. This suggests that for buffers to be sustainable for N sequestration there is a need to manage buffer vegetation to maximize N demand and retention.

Abbreviations: DON, dissolved organic nitrogen

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