Abiotic and biotic dryland mechanisms controlling litter decomposition in rainless periods

Decomposition of plant litter is a key process in the carbon cycle, controlled mainly by environmental factors, litter quality and the decomposer community, at least in mesic and humid regions. In drylands representing ~40% of the global terrestrial area, water as a crucial environmental factor is scarce, thus limiting the classic pathway of microbial degradation of plant litter. In the past two decades, there has been an effort to study litter decompositions under dry conditions, focusing on different decay mechanisms that operate without rain or snow as water sources. These mechanisms include abiotic processes, mainly photodegradation driven by solar radiation and thermal degradation driven by heat, and humidity-enhanced microbial degradation enabled by non-rainfall water sources, such as fog, dew and atmospheric water vapor. However, the involvement of these dryland decay mechanisms in litter decomposition is not well constrained. The objective of this study was to quantify the relative contribution of dryland decay mechanisms to mass loss and CO₂ flux under rainless conditions. In a full-factorial semi-controlled study with quartz tubes, we exposed six litter species from tropical, temperate, and Mediterranean regions to all combinations of high and low radiation, heat and humidity for 90 days. Our results suggest that in the early stage of decomposition, CO₂ fluxes are more affected by climate factors than by litter traits, with the combination of high solar radiation, low heat and high air humidity resulting in the highest fluxes for all species. Interactions between two or three decay drivers contribute to additional mass loss compared to the effect of each separate decay mechanism. The results from this experiment indicate the importance of the interaction between decay mechanisms to achieve significant mass loss and CO₂ flux. To assess the contribution of different dryland decay mechanisms on litter decomposition under field conditions, we conducted a litterbag experiment in 12 sites and two microsites per site (under and between shrubs), which widely diverged in microclimatic conditions during the rainless summer season. Decay of a more recalcitrant litter (oak leaves) was primarily related to solar radiation, while decay of a more labile litter (wheat straw) was related to both humidity and solar radiation. The shrub microsite was characterized by less heat and lower solar radiation, higher humidity, and generally lower mass loss than the intershrub microsite. The results from both experiments indicate that a combination of solar radiation and humidity is essential for litter decomposition in rainless periods, and that heat alone is insufficient to induce significant litter decay. Moreover, the more extreme the abiotic climate factors (warmer and drier) in our experiments, the slower the decay process, suggesting that under climate change, we should expect a lower litter decomposition rate. Hence, carbon and nutrient cycles might slow down ecosystem productivity in a future climate.