Removal of grazers alters the response of tundra soil carbon to warming and enhanced nitrogen availability

The circumpolar Arctic is currently facing multiple global changes that have the potential to alter the capacity of tundra soils to store carbon. Yet, predicting changes in soil carbon is hindered by the fact that multiple factors simultaneously control processes sustaining carbon storage and we do not understand how they act in concert. Here, we investigated the effects of warmer temperatures, enhanced soil nitrogen availability, and the combination of these on tundra carbon stocks at three different grazing regimes: on areas with over 50‐yr history of either light or heavy reindeer grazing and in 5‐yr‐old exlosures in the heavily grazed area. In line with earlier reports, warming generally decreased soil carbon stocks. However, our results suggest that the mechanisms by which warming decreases carbon storage depend on grazing intensity: under long‐term light grazing soil carbon losses were linked to higher shrub abundance and higher enzymatic activities, whereas under long‐term heavy grazing, carbon losses were linked to drier soils and higher enzymatic activities. Importantly, under enhanced soil nitrogen availability, warming did not induce soil carbon losses under either of the long‐term grazing regimes, whereas inside exclosures in the heavily grazed area, also the combination of warming and enhanced nutrient availability induced soil carbon loss. Grazing on its own did not influence the soil carbon stocks. These results reveal that accounting for the effect of warming or grazing alone is not sufficient to reliably predict future soil carbon storage in the tundra. Instead, the joint effects of multiple global changes need to be accounted for, with a special focus given to abrupt changes in grazing currently taking place in several parts of the Arctic. A multitude of global changes are currently occurring across the Arctic and are anticipated to become more common in the years to come. Not only is the ongoing anthropogenic warming inducing a rise in temperatures at twice the rate of the global average (Hoegh‐Guldberg et al. 2018), but also soil nitrogen availability is predicted to increase due to microbially mediated mineralization that is stimulated by warmer temperatures, drier soils (Jiang et al. 2016) and the expansion of deciduous shrubs (Myers‐Smith et al. 2011). Most drastically, soil nitrogen availability could change in response to stochastic atmospheric deposition events resulting from polluted air masses arising from industry (e.g., Kühnel et al. 2013). Such extreme nitrogen deposition events are predicted to become more common due to