Nutrient cycling in most terrestrial ecosystems is controlled by moisture-dependent decomposer activity. In arid ecosystems, plant litter cycling exceeds rates predicted based on precipitation amounts, suggesting that additional factors are involved. Attempts to reveal these factors have focused on abiotic degradation, soil–litter mixing and alternative moisture sources. Our aim was to explore an additional hypothesis that macro-detritivores control litter cycling in deserts. We quantified the role different organisms play in clearing plant detritus from the desert surface, using litter baskets with different mesh sizes that allow selective entry of micro-, meso- or macrofauna. We also measured soil nutrient concentrations in increasing distances from the burrows of a highly abundant macro-detritivore, the desert isopod Hemilepistus reaumuri. Macro-detritivores controlled the clearing of plant litter in our field site. The highest rates of litter removal were measured during the hot and dry summer when isopod activity peaks and microbial activity is minimal. We also found substantial enrichment of inorganic nitrogen and phosphorous near isopod burrows. We conclude that burrowing macro-detritivores are important regulators of litter cycling in this arid ecosystem, providing a plausible general mechanism that explains the unexpectedly high rates of plant litter cycling in deserts.
Nutrient dynamics in most terrestrial ecosystems are regulated by moisture-dependent processes. In drylands, nutrient dynamics are often weakly associated with annual precipitation, suggesting that other factors are involved. In recent years, the majority of research on this topic focused on abiotic factors. We provide an arthropod-centric framework that aims to refocus research attention back on the fundamental role that macro-arthropods may play in regulating dryland nutrient dynamics. Macro-arthropods are prevalent in drylands and include many detritivores and burrowing taxa that remain active during long dry periods. Macro-arthropods consume and process large quantities of plant detritus and transport these nutrients to the decomposer haven within their climatically buffered and nutritionally enriched burrows. Consequently, arthropods may accelerate mineralization rates and generate a vertical nutrient recycling loop (VRL) that may assist in explaining the dryland decomposition conundrum, and how desert plants receive their nutrients when the shallow soil is dry. The burrowing activity of arthropods and the transportation of subterranean soil to the surface may alter the desert microtopography and promote desalinization, reducing resource leakage and enhancing productivity and species diversity. We conclude that these fundamental roles and the arthropods’ contribution to nutrient transportation and nitrogen fixation makes them key regulators of nutrient dynamics in drylands.
<p>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 &#8211; 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.</p>
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