Inverse effects of soil moisture and litter quality on litter decomposition along a gradient from hyper-arid to temperate climate

Abstract

Litter decomposition, a key component of the global carbon cycle, is greatly affected by climate. Our current understanding of climate change effects on decomposition stems from (1) spatial observational studies along climate gradients, where direct and indirect effects of climate on litter decomposition are confounded; (2) reciprocal litter translocations and common garden experiments, where biotic interactions are disrupted; or (3) manipulation experiments, which can be less realistic than observational studies. Experimental studies combining all of the above mentioned methods can separate indirect from direct climate effects on decomposition, as the confounding effects of one method can be explained with another method. Additionally, this setup can directly test if observations along a spatial gradient can predict responses to climate change, that is the validity of the space-for-time approach. However, studies combining the different methods are still scarce. We combined a pronounced climate gradient, large- and small-scale reciprocal litter translocations (local litter and standard litter, i.e. tea), and in situ precipitation manipulation for studying soil moisture effects on litter decomposition. All our experiments (translocation of litter and tea across the gradient, slope comparisons within sites and rainfall exclusions) indicated positive direct effects of climate on litter decomposition. However, as local litter quality decreased with increasing precipitation, litter from species of the drier sites decomposed quicker than litter from species of the wetter sites across the gradient. In other words, while the direct effects of climate favoured litter decomposition in wetter sites, its indirect effect (i.e. litter quality) favoured the decomposition of litter from species of the drier sites within each site. Synthesis: Our results highlight the need for experimental evidence from reciprocal translocations and climate manipulations in litter decomposition studies as they indicate that space cannot always substitute for time. Such evidence would improve predictions of models of the global carbon cycle that include interactions between climate and vegetation.