Trampling and spatial heterogeneity explain decomposer abundances in a sub-arctic grassland subjected to simulated reindeer grazing

Mammal grazing is composed of three mechanisms-removal of foliar tissue (defoliation), return of nutrients via dung and urine (fertilization), and trampling. To evaluate the relative role of these mechanisms in the effect of reindeer grazing on soil biota in northern grasslands, we subjected experimental plots in a sub-arctic alpine meadow to defoliation, fertilization (using NPK-solution), simulated trampling, and their factorial combinations once a year from 2002 to 2004 and measured the response of plants and decomposers (including microbes, nematodes, collembolans, and enchytraeids) in 2004. Trampling affected both plant and decomposer communities: the coverage of the moss Pleurozium schreberi and the sedge Carex vaginata, as well as the abundance of collembolans and enchytraeids were reduced in trampled plots. Trampling and fertilization also interacted significantly, with fertilization increasing the abundance of bacteria and bacterial-feeding and omnivorous nematodes in trampled plots only, and trampling decreasing fungal biomass in non-fertilized plots only. Defoliation had no overall effects on plants or decomposers. Nematode genera were not affected by the experimental treatments, but nematode and plant communities were significantly associated, and all decomposer biota, except collembolans, were strongly affected by the spatial heterogeneity of the study site. Our results indicate that trampling may have larger and defoliation and fertilization smaller roles than anticipated in explaining reindeer grazing effects in sub-arctic grasslands. However, even the effects of trampling seem to be outweighed by the spatial heterogeneity of decomposer abundances. This suggests that in sub-arctic grasslands spatial variation in abiotic factors can be a more important factor than grazing in controlling soil biota abundances.