Abstract
The influence of ungulate grazers on nutrient cycling and ecosystem productivity in grasslands has been shown to differ with moisture, nutrient availability, and feedbacks between above- and belowground activities. We examined the movement of nitrogen (N), applied as (15NH4)2SO4, through both dry and mesic sites in the northern range of Yellowstone National Park to test the hypothesis that plants were more able to acquire added N in grazed relative to ungrazed sites. Previous studies showed enhanced N mineralization in grazed areas, and detritus removal by grazers was predicted to enhance early-season plant growth. Thirteen months after tracer addition, there were no differences in plant 15N as a function of grazing, but historically ungrazed sites retained more 15N in accumulated litter than at grazed sites. This result demonstrated the importance of detritus in regulating redistribution of incoming N and the role of grazers in this process. Site moisture status influenced 15N recovery in all pools—soils, microbial biomass, and plants—and greater plant 15N acquisition occurred in roots at dry relative to mesic sites. Understanding how grazers influence nutrient cycling at the landscape scale requires further investigation of interactions among soil moisture, plant production, litter accumulation, grazing intensity, and belowground processes.
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