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

Large herbivores are known to respond to spatial heterogeneity in the distr ibution of forage resources, but how important herbivores are in creating t hose spatial patterns, how their influence may be scale dependent, and how herbivore-induced patterns affect ecosystem processes remain unclear. We ex amined the effects of native migratory ungulates on the spatial distributio n of soil nitrogen and N-mineralization potential at scales ranging from 0. 1-30 m in grasslands of Yellowstone National Park using geostatistical anal yses 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, resp ectively). For both variables, patches occurred at a consistent mean size o f similar to 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 patc h size at a scale of 0.1-2 m. In grazed grassland, most variation was extre mely fine grained, occurring across distances < 10 cm. The high degree of f ine-grained heterogeneity in grazed grassland was associated with greater p lant diversity in small (20 x 20 cm) patches. Recycling of nitrogen through dung and urine is clearly important for maintaining long-term nitrogen bal ance 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 dep osition of dung and urine in discrete patches and suggest that the dominant mechanism(s) by which grazers alter N cycling in this ecosystem operates t hrough the plant community. We hypothesize that grazers promote fine-scale heterogeneity by diversifying plant species effects on soils and/or increas ing the spatial variability in plant litter inputs. At larger spatial scales (5-30 m), large herbivores altered the distributio n of soil N across a topographic gradient. In a grazed community, soil N pr operties were associated with the topographic gradient and exhibited increa sing variance among sampling points separated by increasing distances from 5 to 30 m. Ungrazed grassland exhibited no spatial structure in soil N dist ribution and no correlation with topography. Collectively, our data show th at grazers influence the distribution of soil N properties at every spatial scale from individual plants to landscapes.