The effect of reindeer grazing on decomposition, mineralization and soil biota in a dry oligotrophic Scots pine forest
Authors: Stark Sari , Wardle David A. , Ohtonen Rauni , Helle Timo , Yeates Gregor W. 
Published: December 2004

Reindeer grazing in the Fennoscandian area has a considerable influence on the ground vegetation, and this is likely in turn to have important consequences for the soil biota and decomposition processes. The effects of reindeer grazing on soil biota, decomposition and mineralization processes, and ecosystem properties in a lichen‐dominated forest in Finnish Lapland were studied inside and outside a large long term fenced reindeer exclosure area. Decomposition rates of Vaccinium
myrtillus leaves in litter bags were retarded in the grazed area
relative to the ungrazed area, as well as in subplots from which lichens
had been artificially removed to simulate grazing. The effect of reindeer
grazing on soil respiration and microbial C was positive in the lichen
and litter layers of the soil profile, but retarded in the humus layer.
There was no effect of grazing on gross N mineralization and microbial
biomass N in the humus and upper mineral soil layer, but net N
mineralization 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 multitrophic effects on the decomposer food‐web. Grazing
decreased lichen and dwarf 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 per unit area in the humus and
mineral soil layers. The N concentration of lichens was increased by
grazing, but the N concentrations of both living and dead Pinus
sylvestris needles and Empetrum hermaphroditum leaves were not affected.  There was some evidence for each of three mechanisms which could account for 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, resulting in a lack of available C relative to N for decomposer organisms. Secondly, the organic matter in the soil may be older in the grazed area, because of reduction of recent production of lichen litter relative to the ungrazed 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 microclimate is likely to be affected by reindeer grazing through physical removal of lichen cover on the ground, and this can have a significant influence on soil microbial processes. This is supported by the strong observed effects of experimental removal of lichens on decomposer processes. The impact of reindeer grazing on soil processes may be a result of complex interactions between different mechanisms, and this could help to explain why the below‐ground effects of reindeer grazing have different consequences to those which have been observed in recent investigations on other grazing systems.