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

This study used the stable 15 N isotope to quantitatively examine the effects of cutting on vegetative buffer uptake of NO 3 − –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, 15 N‐labeled KNO 3 was applied to the pasture area at a rate of 5 kg N ha −1 and 99.7 atom % 15 N. One‐half of the buffer plots were trimmed monthly. In the buffers, the cutting effect was not significant in the first few weeks following 15 N application, with both the cut and uncut buffers sequestering 15 N. Over the irrigation season, however, cut buffers sequestered 2.3 times the 15 N 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 15 N as it was released by microbial mineralization. In contrast, the uncut buffers showed very little change in 15 N sequestration or biomass, suggesting senescence and a corresponding decrease in N demand. Overall, cutting significantly improved 15 N attenuation from both surface and subsurface water. However, the effect was temporally related, and only became significant 21 to 42 d after 15 N 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 15 N concentration.