Differential climate dependence of microbial, mesofaunal and macrofaunal litter decomposition across a Mediterranean to hyper-arid aridity gradient

Litter decomposition in most terrestrial ecosystems is regulated by moisture-dependent biological activity, leading to a positive association between precipitation and decomposition rates. In drylands, decomposition is often higher than predicted by climate conditions and weakly associated with annual precipitation, a discrepancy known as the dryland decomposition conundrum. One possible resolution may be that low microbial decomposition is compensated by litter consuming macro-arthropods that are better adapted for activity under arid conditions. In this study we quantified the contribution of organisms of different sizes to litter decomposition across an aridity/precipitation gradient from Mediterranean (mean annual precipitation (MAP) of 526 mm) to hyper-arid climate (MAP = 22 mm). We performed a litter box experiment in seven sites along the gradient during two different seasons – a dry summer and a wetter winter. We manipulated access to litter by organism size and monitored the activity of macro-detritivorous fauna in each site during both periods. We found that microbial decomposition rate increased with MAP. However, litter mass loss induced by mesofauna and macrofauna followed a unimodal pattern, with mesofaunal and macrofaunal decomposition peaking under semi-arid (MAP = 367 mm) and arid (84-148 mm) climate conditions, respectively. This result corresponded to macro-detritivore abundance, species richness and biomass that similarly peaked in the arid sites. These patterns were consistent across seasons, but macrofaunal decomposition rates in the arid sites were 2.5- to 7-fold higher in summer than in winter. Whole-community decomposition was dictated by microbial decomposition in winter and by macrofaunal decomposition in summer. Whole community decomposition rates in arid sites during summer were as high as in the semi-arid and Mediterranean sites in winter, eliminating total differences across these climates at the annual scale. Our findings highlight the importance of macro-detritivores for litter decomposition under arid conditions, which compensates for low microbial and mesofaunal activity, advocating a possible resolution for the dryland decomposition conundrum. This is not the case under hyper-arid climate conditions, where macrofaunal activity is severely limited and cannot compensate for low microbial decomposition. We conclude that the relationship between climate conditions and decomposition is mediated by organism size. Moreover, differential adaptation of microorganisms, mesofauna and macrofauna to aridity may alleviate the dependence of decomposition on moisture availability. This new mechanistic understanding is essential for integrating faunal effects into biogeochemical models in the face of the global aridification trend.