Non-parallel changes in soil microbial carbon and nitrogen dynamics due to reindeer grazing in northern boreal forests

Reindeer grazing has a considerable influence on mineralization processes in northern Fennoscandian boreal forests, but the mechanisms underlying the observed differences between grazed and ungrazed areas are not well understood. We studied the below-ground impacts of reindeer grazing by comparing the carbon and nitrogen mineralization rates inside and outside long-term fenced reindeer exclosure areas in five oligotrophic, lichen-dominated and five mesotrophic, dwarf-shrub dominated forests. The soil C mineralization rates and microbial metabolic activity (qCO₂) were significantly lower in the grazed than the ungrazed areas in both oligotrophic and mesotrophic forests. The reductions occurred irrespective of the impact on soil moisture. We conclude that reindeer grazing causes a reduction in the supply of labile C substrates to microbes, resulting in reduced organic matter decomposition rates through changes in the activity of the microbial biomass. Simultaneously, grazing had no consistent effect on the microbial N dynamics, but the impact ranged from no change to increased or decreased in N mineralization rates at the different study sites. The impact of grazing on the N mineralization potential thus seems to be site-specific and uncoupled from the impact of grazing on soil C mineralization. Reciprocal transplant incubations showed no interactions between N mineralization rates and the reindeer-mediated impact on the soil microclimate. We suggest that plant root damage due to trampling by reindeer may be an important mechanism for the deceleration of soil C cycling. In some cases, however, the impact of grazing on the soil active N pool may be strong enough to outweigh the reduction in soil organic matter decomposition, and by these means uncouple soil N dynamics from soil C quality.