Biotic and abiotic drivers of litter decomposition in drylands : The role of UV and trophic interactions

Abstract

Decomposition returns to the soil more than 50% of primary productivity and provides the main source of energy and nutrients to complex detritus-based soil food webs. In terrestrial ecosystems, decomposition is hierarquically governed by climate, litter quality and soil biota. However, in drylands decomposition seem to be governed by complex abiotic/biotic interactions that are largely unknown. The main objective of this thesis was to study the interactions of these abiotic and biotic factors that control decomposition in a semi-arid ecosystem. As a first objective I assessed the effects of UV radiation and microbial activity in the early stage of decomposition, and explore the legacy effects of these two components on a later stage of decomposition involving detritivores (isopods) (chapter 2). In a laboratory experiment UV radiation accounted for a small proportion (<3%) of the total litter decomposition and no photopriming effects were observed. Contrary to UV radiation, microorganisms had a paramount role during decomposition through legacy effects on detritivores, as we found substantial cumulative effects of microbes and detritivores adding up to 42% of total decomposition (chapter 2). The latter promoted the expression of synergistic effects in litter mixtures (chapter 2 and 3) and worked as a link between predator and litter decomposition (chapter 3). In addition, the removal of microorganisms by biocides reverted litter mixture synergistic effects and gave rise to antagonisms among litter species (chapter 2). In summary, UV radiation had a minor effect as compared to microbes in their legacy effects on detritivore-driven decomposition. The aim of chapter 3 was to evaluate the multiple effects of predators on decomposition via indirect consumptive and non-consumptive effects, and via ecological engineering effects, and how they may generate synergies with litter mixtures (chapter 3). Despite the high impact of predators on the detritivores (both on mortality and on behavior), multiple synergistic predator engineering ecosystem effects with the identity and diversity of the litter, prevailed over predator top-down control, making predators to enhance rather than inhibit decomposition. These results suggest that the traces of predator activity (e.g. excreta) provide with some extra nutrients that can boost microbial activity and ultimately benefit secondary decomposers and accelerate decomposition. In chapter 4 I assessed the role of fertility islands on decomposition of leaf litter mixtures in drylands. Decomposition under shrub canopy consistently showed antagonistic effects and slower decomposition differently depending on the identity of the litter species. On the contrary, decomposition away from shrubs resulted in mostly additive effects, with only a mild synergy in one of the species. Unexpectedly, no diversity (richness) effect was found. As we also found far more mesofauna in decomposition mesocosms under shrubs, the results suggest that mainly biotic-driven decomposition operates under shrubs and abiotic-driven decomposition operates away from shrubs. The information provided by this thesis may help understanding how litter decomposes in drylands, where it may remain either on the plant or on the ground, and to understand the interaction between abiotic and biotic factors, which have effects difficult to disentangle when predicting decomposition in these ecosystems.