S. Stark et al., The effect of reindeer grazing on decomposition, mineralization and soil biota in a dry oligotrophic Scots pine forest, OIKOS, 90(2), 2000, pp. 301-310
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.