Arthropods dominate plant litter decomposition in drylands

The research, published today as a final Version of Record after previously appearing as a Reviewed Preprint in eLife, was described by editors as a fundamental study that substantially advances our understanding of the role of different-sized soil invertebrates in shaping the rates of leaf litter decomposition. The authors provide compelling evidence that the summed effects of all decomposers on decomposition rates, with large-sized invertebrates being more active in summer and microorganisms in winter, result in similar levels of leaf litter decomposition across sites of different aridity levels. The research will be of interest to ecologists modelling carbon cycles to understand global warming.

Leaf litter decomposition is a key process that determines the cycling of elements such as carbon in land ecosystems. The rate of decomposition is influenced by climate, the quality of leaf litter and the identity and abundance of different decomposer organisms.

"Evidence suggests that decomposition is faster under warm and wet conditions, and this has led to an assumption that microorganisms dominate in the decomposition process, largely ignoring growing recognition that animals may also play an important role," says co-lead author Viraj Torsekar, at the time a postdoctoral scholar at The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem, Israel, and now an Assistant Professor at GITAM University, Visakhapatnam, India. "We speculated that the higher tolerance of macrofauna such as termites and beetles to arid climates may counterbalance the effect of smaller decomposers, leading to similar decomposition rates overall across different climatic extremes."

To investigate this, the team placed plant litter baskets of three different mesh sizes in seven sites across Israel, ranging from hyper-arid habitats with scarce rainfall (mean annual rainfall of 22mm) to sites with cooler, wetter Mediterranean climates (mean annual rainfall of 526mm). The three basket types were 'micro' (large enough for microorganisms only), 'meso' (microorganisms and invertebrates smaller than 2mm, such as springtails) and 'macro' (large enough to include larger invertebrates between 2mm and 2cm in size, such as termites, woodlice and beetles). The baskets were installed for periods in both hot and dry summer, and cold and wetter winter seasons, with pitfall traps used to study the composition and abundances of the macrofauna assemblages.

The team found that the litter removal rate differed across seasons, sites and mesh sizes. Overall, microbial decomposition was minimal over the summer season and, in winter, was higher in wetter conditions. By contrast, meso-faunal decomposition was moderate in both seasons, and highest in semi-arid sites. Decomposition by macro-fauna (termites, woodlice and beetles) contributed minimally to decomposition in winter but dominated decomposition in the summer months. Species richness and abundance data from pitfall traps revealed that macro-faunal assemblages were most abundant and species-rich in arid sites, where macro-faunal decomposition was the highest. The authors explain that larger invertebrates are better able to cope with hot and dry conditions by moving into moist and cooler areas when needed.

The puzzle of why plant litter decomposition in arid land is faster than expected has perplexed scientists for half a century and has been called 'the desert decomposition conundrum'. Previous attempts to resolve this have proposed that plant litter degradation in the desert might be facilitated by light, heat, fog, dew or humidity. But the findings of this study support a long-suggested but largely overlooked hypothesis -- that it is macro-fauna decomposers that dominate plant litter decomposition in deserts.

"Our findings show that the opposing climate dependencies of micro- and macro-fauna decomposers have led to similar or even higher annual decomposition rates in arid sites compared to those measured in wetter climates," says senior author Dror Hawlena, Professor at The Alexander Silberman Institute of Life Sciences, The Hebrew University of Jerusalem.

"This suggests that it is the different climate dependencies of different-sized decomposers rather than abiotic factors that explain the discrepancy between classic decomposition models and the observed decomposition rates in drylands, providing a plausible solution to the dryland decomposition conundrum," concludes co-lead author Nevo Sagi, at the time a PhD student at The Hebrew University of Jerusalem, and now a postdoctoral scholar at the University of Texas at Austin, US. "Understanding the mechanisms that regulate decomposition in drylands is key for conserving and restoring fundamental ecosystem processes in these ever-expanding areas, and in improving our understanding of global processes such as carbon cycling."