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.

中文翻译：

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去除食草动物改变了苔原土壤碳对变暖的响应并增强了氮的利用率

极地北极目前正面临着多种全球变化，这些变化有可能改变冻原土壤储存碳的能力。但是，由于多个因素同时控制维持碳储存的过程，因此我们无法预测土壤碳的变化，我们不了解它们如何协同作用。在这里，我们研究了三种不同放牧方式下温度升高，土壤氮素利用率提高以及这些因素的组合对苔原碳储量的影响：在驯鹿轻度或重度放牧历史超过50年的地区以及5年内放牧严重地区的旧棚屋。与早先的报道一致，变暖普遍减少了土壤碳储量。但是，我们的结果表明，变暖减少碳储存的机制取决于放牧强度：在长期轻度放牧下，土壤碳损失与较高的灌木丰度和较高的酶活性有关，而在长期大量放牧下，碳损失与土壤干燥和较高的酶活性有关。重要的是，在增加土壤氮素利用率的情况下，在两种长期放牧方式下，变暖都不会导致土壤碳损失，而在重度放牧的区域内的排泄物中，变暖和增加的养分利用率会导致土壤碳损失。放牧本身不会影响土壤碳储量。这些结果表明，仅考虑变暖或放牧的影响不足以可靠地预测冻原中未来的土壤碳储量。相反，需要考虑多个全球变化的共同影响，