Rainfall frequency, not quantity, controls isopod effect on litter decomposition

Increasing climate variability is one of the dominant components of climate change, resulting particularly in altered rainfall patterns. Yet, the consequences of rainfall variability on biogeochemical processes that contribute to greenhouse gas emissions has received far less attention than have changes in long-term mean rainfall. In particular, it remains unclear how leaf litter decomposition responds to changes in rainfall frequency compared to changes in cumulative rainfall quantity, and if changes in rainfall patterns will differentially affect organisms in the decomposer food web (e.g., microbial decomposers that break down leaf litter through saprotrophic processes versus detritivores that directly ingest leaf litter). To address this knowledge gap, we disentangled the relative importance of cumulative rainfall quantity and rainfall frequency on both microbial and detritivore-driven litter decomposition, using the isopod Arrnadillidium vulgare as a model macro-detritivore species and simulating rainfall in a full-factorial microcosm experiment. We found that microbially-driven decomposition was positively related to cumulative rainfall quantity, but tended to saturate with increasing cumulative rainfall quantity when rainfall events were large and infrequent. This saturation appeared to result from two mechanisms. First, at high level of cumulative rainfall quantity, large and infrequent rainfall events induce lower litter moisture compared to smaller but more frequent ones. Second, microbial activity saturated with increasing litter moisture, suggesting that water was no longer limiting. In contrast, isopod-driven decomposition was unaffected by cumulative rainfall quantity, but was strongly controlled by the rainfall frequency, with higher isopod-driven decomposition at low rainfall frequency. We found that isopod-driven decomposition responded positively to an increase in the weekly range of soil moisture and not to mean soil or litter moisture, suggesting that an alternation of dry and moist conditions enhances detritivore activity. Collectively, our results suggest that A. vulgare morphological and behavioral characteristics may reduce its sensitivity to varying moisture conditions relative to microbial decomposers. We conclude that the activity of microorganisms and isopods are controlled by distinct aspects of rainfall patterns. Consequently, altered rainfall patterns may change the relative contribution of microbial decomposers and detritivores to litter decomposition.