Contrasting vegetation states do not diverge in soil organic matter storage: evidence from historical sites in tundra.

Ecosystems where severe disturbance has induced permanent shifts in vegetation and soil processes may represent alternative stable states. To date, little is known on how long‐lasting changes in soil processes are following such disturbances, and how the changes in plant and soil processes between the alternative states eventually manifest themselves in soil organic matter (SOM) storage. Here, we analyzed plant density, the shrub :  forbratio, microbial respiration, extracellular enzyme activities and SOM stocks in soils of subarctic tundraand historical milking grounds, where reindeer herding induced a vegetation transition from deciduous shrubsto graminoidsseveral centuries earlier but were abandoned a century ago. This provides the possibility to compare sites with similar topography, but highly contrasting vegetation for centuries. We found that enzymatic activities and N:P stoichiometry differed between control and disturbed sites, confirming that culturally induced vegetation shifts exert lasting impacts on tundrasoil processes. Transition zones, where shrubshad encroached into the historical milking grounds during the past 50 yr, indicated that microbial activities for N and P acquisition changed more rapidly along a vegetation shift than those for microbial C acquisition. Although plant and soil processes differed between control and disturbed sites, we found no effect of historical vegetation transition on SOM stock. Across the study sites, soil SOM stocks were correlated with total plant density but not with the shrub :  forbratio. Our finding that SOM stock was insensitive to a centennialdifference in plant community composition suggests that, as such, grazing‐induced alternative vegetation states might not necessarily differ in SOM sequestration.