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TECHNICAL REPORTS

Surface Water Quality

Quantifying the Impact of Regular Cutting on Vegetative Buffer Efficacy for Nitrogen-15 Sequestration

^a Department of Plant Sciences, University of California, Davis, CA 95616
^b Current address: Department of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada

^* Corresponding author (bedard.haughn{at}usask.ca)

Received for publication January 28, 2005. This study used the stable ¹⁵N isotope to quantitatively examine the effects of cutting on vegetative buffer uptake of NO₃^––N based on the theory that regular cutting would increase N demand and sequestration by encouraging new plant growth. During the summer of 2002, 10 buffer plots were established within a flood-irrigated pasture. In 2003, ¹⁵N-labeled KNO₃ was applied to the pasture area at a rate of 5 kg N ha^–1 and 99.7 atom % ¹⁵N. One-half of the buffer plots were trimmed monthly. In the buffers, the cutting effect was not significant in the first few weeks following ¹⁵N application, with both the cut and uncut buffers sequestering ¹⁵N. Over the irrigation season, however, cut buffers sequestered 2.3 times the ¹⁵N of uncut buffers, corresponding to an increase in aboveground biomass following cutting. Cutting and removing vegetation allowed the standing biomass to take advantage of soil ¹⁵N as it was released by microbial mineralization. In contrast, the uncut buffers showed very little change in ¹⁵N sequestration or biomass, suggesting senescence and a corresponding decrease in N demand. Overall, cutting significantly improved ¹⁵N attenuation from both surface and subsurface water. However, the effect was temporally related, and only became significant 21 to 42 d after ¹⁵N application. The dominant influence on runoff water quality from irrigated pasture remains irrigation rate, as reducing the rate by 75% relative to the typical rate resulted in a 50% decrease in total runoff losses and a sevenfold decrease in ¹⁵N concentration.

Abbreviations: DON, dissolved organic nitrogen • LME, linear mixed effects • SFREC, Sierra Foothill Research and Extension Center

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