Contrasting mechanisms underlie short-and longer-term soil respiration 1 responses to experimental warming in a dryland ecosystem 2 3 Running head : Warming effects on soil respiration

24 Soil carbon losses to the atmosphere through soil respiration are expected to rise with 25 ongoing temperature increases, but available evidence from mesic biomes suggests that 26 such response disappears after a few years of experimental warming. However, there is 27 lack of empirical basis for these temporal dynamics in soil respiration responses, and for 28 the mechanisms underlying them, in drylands, which collectively form the largest biome 29 on Earth and store 32% of the global soil organic carbon pool. We coupled data from a 30 ten-year warming experiment in a biocrust-dominated dryland ecosystem with 31 laboratory incubations to confront 0-2 years (short-term hereafter) vs. 8-10 years 32 (longer-term hereafter) soil respiration responses to warming. Our results showed that 33 increased soil respiration rates with short-term warming observed in areas with high 34 biocrust cover returned to control levels in the longer-term. Warming-induced increases 35 in soil temperature were the main driver of the short-term soil respiration responses, 36 whereas longer-term soil respiration responses to warming were primarily driven by 37 thermal acclimation and warming-induced reductions in biocrust cover. Our results 38 highlight the importance of evaluating short and longer-term soil respiration responses 39 to warming as a mean to reduce the uncertainty in predicting the soil carbon–climate 40 feedback in drylands. 41 42