The effect of reindeer grazing on decomposition, mineralization and soil biota in a dry oligotrophic Scots pine forest

Reindeer grazing in the Fennoscandian area has a considerable influence on the ground vegetation, and this is likely in turn to have important consequ ences For the soil biota and decomposition processes. The effects of reinde er grazing on soil biota, decomposition and mineralization processes, and e cosystem properties in a lichen-dominated forest in Finnish Lapland were st udied inside and outside a large long term fenced reindeer exclosure area. Decomposition rates of Vaccinium myrtillus leaves in litter bags were retar ded in the grazed area relative to the ungrazed area, as well as in subplot s from which lichens had been artificially removed to simulate grazing. The effect of reindeer grazing on soil respiration and microbial C was positiv e in the lichen and lifter layers of the soil profile, but retarded in the humus layer. There was no effect of grazing on gross N mineralization and m icrobial biomass N in the humus and upper mineral soil layer, but net N min eralization was increased by grazing. In these layers soil respiration was reduced by grazing, indicating that reindeer effects reduce the ratio of C to N mineralized by soil microorganisms. Grazing stimulated populations of all trophic groupings of nematodes in the lichen layer and microbe feeding nematodes in the litter layer, indicating that grazing by reindeer has mult itrophic effects on the decomposer food-web. Grazing decreased lichen and d warf shrub biomasses and increased the mass of litter present in the litter layer on an areal basis, but did not significantly alter total C storage p er unit area in the humus and mineral soil layers. The N concentration of l ichens was increased by grazing, but the N concentrations of both living an d dead Pinus sylvestris needles and Empetrum hermaphroditum leaves were not affected. There was some evidence for each of three mechanisms which could account fo r the grazing effects that we observed in our study. Firstly, reindeer may have changed the composition and quality of litter input by affecting plant species composition and through addition of N from urine and faeces, resul ting in a lack of available C relative to N for decomposer organisms. Secon dly, the organic matter in the soil may be older in the grazed area, becaus e of reduction of recent production of lichen litter relative to the ungraz ed area. The organic matter in the grazed area may have been in a different phase of decomposition from that in the exclosure. Thirdly, the soil micro climate is likely to be affected by reindeer grazing through physical remov al of lichen cover on the ground, and this can have a significant influence on soil microbial processes. This is supported by the strong observed effe cts of experimental removal of lichens on decomposer processes. The impact of reindeer grazing on soil processes may be a result of complex interactio ns between different mechanisms, and this could help to explain why the bel ow-ground effects of reindeer grazing have different consequences to those which have been observed in recent investigations on other grazing systems.