Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

Abstract

A positive soil carbon (C)￢ﾀﾐclimate feedback is embedded into the climatic models of the IPCC. However, recent global syntheses indicate that the temperature sensitivity of soil respiration (RS) in drylands, the largest biome on Earth, is actually lower in warmed than in control plots. Consequently, soil C losses with future warming are expected to be low compared with other biomes. Nevertheless, the empirical basis for these global extrapolations is still poor in drylands, due to the low number of field experiments testing the pathways behind the long￢ﾀﾐterm responses of soil respiration (RS) to warming. Importantly, global drylands are covered with biocrusts (communities formed by bryophytes, lichens, cyanobacteria, fungi, and bacteria), and thus,RSresponses to warming may be driven by both autotrophic and heterotrophic pathways. Here, we evaluated the effects of 8￢ﾀﾐyear experimental warming onRS, and the different pathways involved, in a biocrust￢ﾀﾐdominated dryland in southern Spain. We also assessed the overall impacts on soil organic C (SOC) accumulation over time. Across the years and biocrust cover levels, warming reducedRSby 0.30ￂﾠￎﾼmolￂﾠCO2ￂﾠm￢ﾈﾒ2ￂﾠs￢ﾈﾒ1(95% CIￂﾠ=ￂﾠ￢ﾈﾒ0.24 to 0.84), although the negative warming effects were only significant after 3ￂﾠyears of elevated temperatures in areas with low initial biocrust cover. We found support for different pathways regulating the warming￢ﾀﾐinduced reduction inRSat areas with low (microbial thermal acclimation via reduced soil mass￢ﾀﾐspecific respiration and ￎﾲ￢ﾀﾐglucosidase enzymatic activity) vs. high (microbial thermal acclimation jointly with a reduction in autotrophic respiration from decreased lichen cover) initial biocrust cover. Our 8￢ﾀﾐyear experimental study shows a reduction in soil respiration with warming and highlights that biocrusts should be explicitly included in modeling efforts aimed to quantify the soil C￢ﾀﾓclimate feedback in drylands.