A Space-For-Time approach to study the effects of increasing temperature on leaf litter decomposition under natural conditions

Decomposition processes influence the formation of soil organic carbon stocks, and it is necessary to understand how both will respond to climate change. A Space-For-Time (SFT) substitution allows the comparison of litter decomposition under current and future conditions in the field, using a spatial gradient of environmental conditions. Here we used a SFT approach to study the effects of a difference in temperature similar to the predicted increase of 3.2-3.5 °C in Central Europe until 2100. To simulate this difference, we setup a five-fold replicated SFT substitution along mountain slopes and compared decomposition in two sites differing in ∿3.6 °C (600 vs 1200 m a.s.l). With this setup we compared the decomposition of high-quality (nettle) and low-quality (hay) litter, with and without fauna access, during summer, in five mountains in the Austrian Alps (Salzburg). Temperature loggers placed in one of the mountains indicated that the actual difference between the two altitudes was only ∿1.8 °C during summer. Nonetheless, decomposition of low-quality litter was 12% faster at 600 m than at 1200 m; altitude alone explained 19% of total variance. On the contrary, decomposition of high-quality litter was 9% faster at 1200 m. Fauna was the main driver of the high-quality litter decomposition at both altitudes and explained 26% of total variance, whereas altitude explained only 10%. Decomposition rates of the high-quality litter (21.4 ± 2.9 mg g−1 d−1; ±s.d.) was much higher than that of the low-quality (7.9 ± 1.0 mg g−1 d−1). Overall, the decomposition of the low-quality, slow-decomposing litter was more sensitive to warming and less influenced by the activity of detritivores, compared to a litter of higher quality. Through the use of the SFT substitution, we detected that a large part of the variance explained by the models was due to the variability between blocks and mountains, highlighting the important effect of spatial heterogeneity and the need for more replicated, field-based studies, to estimate the responses of decomposition processes to climate change.