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Effects of migratory grazers on spatial heterogeneity of soil nitrogen properties in a grassland ecosystem

Augustine, David J., Frank, Douglas A.
Ecology 2001 v.82 no.11 pp. 3149-3162
ecosystems, feces, forage, geostatistics, grasslands, herbivores, landscapes, migratory behavior, mineralization, national parks, nitrogen, nitrogen balance, plant communities, plant litter, prediction, spatial variation, topography, ungulates, urine, variance
Large herbivores are known to respond to spatial heterogeneity in the distribution of forage resources, but how important herbivores are in creating those spatial patterns, how their influence may be scale dependent, and how herbivore‐induced patterns affect ecosystem processes remain unclear. We examined the effects of native migratory ungulates on the spatial distribution of soil nitrogen and N‐mineralization potential at scales ranging from 0.1–30 m in grasslands of Yellowstone National Park using geostatistical analyses of soils collected inside and outside long‐term (36+‐yr) exclosures. At small spatial scales (0.1–2 m), ungrazed grasslands showed a high degree of patchiness in the distribution of soil N and N‐mineralization potential (94% and 77% of sample variation explained by small‐scale patchiness, respectively). For both variables, patches occurred at a consistent mean size of ∼40 cm. In contrast, grazed grassland exhibited minimal spatial structure in the distribution of soil N and N‐mineralization rates (<24% of variation for both variables spatially dependent) and no consistent patch size at a scale of 0.1–2 m. In grazed grassland, most variation was extremely fine grained, occurring across distances <10 cm. The high degree of fine‐grained heterogeneity in grazed grassland was associated with greater plant diversity in small (20 × 20 cm) patches. Recycling of nitrogen through dung and urine is clearly important for maintaining long‐term nitrogen balance of the system, but we estimated that only 2.5% of the grazed grassland sampled was recently affected by urine. Results conflict with predictions of increased heterogeneity in grazed communities based primarily on the deposition of dung and urine in discrete patches and suggest that the dominant mechanism(s) by which grazers alter N cycling in this ecosystem operates through the plant community. We hypothesize that grazers promote fine‐scale heterogeneity by diversifying plant species effects on soils and/or increasing the spatial variability in plant litter inputs. At larger spatial scales (5–30 m), large herbivores altered the distribution of soil N across a topographic gradient. In a grazed community, soil N properties were associated with the topographic gradient and exhibited increasing variance among sampling points separated by increasing distances from 5 to 30 m. Ungrazed grassland exhibited no spatial structure in soil N distribution and no correlation with topography. Collectively, our data show that grazers influence the distribution of soil N properties at every spatial scale from individual plants to landscapes.