Isla H. Myers-Smith , Haydn J.D. Thomas, Eleanor R. Walker

High-latitude soils contain up to 60% of the world’s carbon stocks, but are vulnerable to carbon loss as climate change alters temperature and precipitation, litter quality, and soil biota. Tundra soils are thought to be particularly sensitive to warming due to accelerated permafrost thaw, but quantifying the response of decomposition to changing soil moisture remains a challenge. Understanding the interaction between temperature, soil moisture and decomposition rates is therefore critical to predicting how the global carbon cycle will be influenced by climate change. We combined an elevational gradient with a moisture and temperature manipulation experiment to investigate differences in decomposition (mass loss) across a diverse range of soil moisture conditions along an elevational gradient from the boreal forest to alpine tundra. We used two standardised substrates of contrasting quality: green and rooibos tea, using the Tea Bag Index to isolate the effect of litter quality. We found that litter quality was the primary control on decomposition, highlighting the importance of litter inputs on rates of decomposition. Contrary to expectations, we observed an increase in litter mass loss with elevation, corresponding with higher soil moisture at higher elevations. In the moisture manipulation experiment, we also found greater litter mass loss in watered treatments for recalcitrant litter, but greater decomposition in warmed treatments for labile litter. Across both experiments, the effect of soil moisture was greater than the effect of soil temperature on litter mass loss. Overall, our findings suggest that decomposition is highly sensitive to litter quality, and that the direct impact of warming on decomposition at high latitudes will be mediated by soil moisture availability, informing predictions of the fate of high-latitude soil carbon under changing climatic conditions.