Biocrusts alter the effects of long-term warming on soil respiration in a dryland ecosystem

Climate warming is expected to have contrasting impacts on soil respiration in dryland ecosystems, with responses ranging from positive to negative across short-, mid-, and long-term timescales. However, the long-term (>10 yr) effects of warming and their underlying mechanisms remain understudied in biocrust-dominated dryland ecosystems. In this study, we investigated the effects of 10-13 years of experimental warming on soil respiration and its underlying regulatory mechanisms at microsites with contrasting biocrust cover in a dryland ecosystem in southeastern Spain. We also examined how long-term warming and biocrust cover influenced the accumulation rate of soil organic carbon (SOC) in the surface layer (0-1 cm). Our results showed that initial and mid-term reductions in soil respiration induced by warming were transient at microsites with low biocrust cover, where respiration rates eventually returned to control levels. In contrast, the suppressive effect of warming on soil respiration persisted over the long term at microsites with high biocrust cover. At low biocrust cover microsites, soil respiration dynamics were primarily regulated by changes in SOC stocks and the activity of carbon-degrading enzymes such as β-glucosidase and β-D-cellobiosidase. Conversely, at high biocrust cover microsites, the long-term response of soil respiration appeared to be more closely associated with shifts in biocrust cover rather than enzymatic activity. Notably, SOC accumulation rates were not significantly affected by either long-term warming or biocrust cover. Overall, our findings underscore the value of long-term experimental studies for capturing delayed or persistent ecosystem responses and reducing uncertainties in projections of soil respiration and carbon-climate feedbacks under global warming.