Non-rainfall Moisture: A Key Driver of Microbial Respiration from Standing Litter in Arid, Semiarid, and Mesic Grasslands

Abstract

Models assume that rainfall is the major moisture source driving decomposition. Non-rainfall moisture (NRM: high humidity, dew, and fog) can also induce standing litter decomposition, but there have been few measurements of NRM-mediated decomposition across sites and no efforts to extrapolate the contribution of NRM to larger scales to assess whether this mechanism can improve model predictions. Here, we show that NRM is an important, year-round source of moisture in grassland sites with contrasting moisture regimes using field measurements and modeling. We first characterized NRM frequency and measured NRMmediated decomposition at two sites in the Namib Desert, Namibia (hyper-arid desert), and at one site in Iowa, USA (tallgrass prairie). NRM was frequent wetness is needed to accurately attribute decomposition to periods when NRM wets litter. Greater recognition of NRM-driven decomposition and its interaction with other processes like photodegradation is needed, especially since fog, dew, and humidity are likely to shift under future climates. HIGHLIGHTSNon-rainfall moisture (NRM; humidity, fog, dew) induces decomposition in grasslands. NRM decomposition depends on substrate type and occurs at colder times than rain. Including NRM (instead of rain alone) improved predictions of litter decomposition.

Fungal Communities on Standing Litter Are Structured by Moisture Type and Constrain Decomposition in a Hyper-Arid Grassland

“…Dry periods lasting for more than 100 hours were not uncommon at the Rain Site ( Supplementary Figure 1 ). Previous work has shown that wet events are longer at the NRM Site ( Evans et al, 2020 ); for example, at the NRM Site, only 5% of wet events lasted two hours or less while at the Rain Site 22% of wet events were this short. Mean temperature during wet events was only slightly lower at the NRM Site, which had a narrower temperature range (range 1.6–26.9°C) than did the Rain Site (range −0.7–39.1°C).…”

“…In many arid and semi-arid systems, organisms cope with water limitation by using non-rainfall moisture (fog, dew, and atmospheric water vapor; hereafter “NRM”) as a supplemental water source ( Dirks et al, 2010 ; Jacobson et al, 2015 ; Gliksman et al, 2017 ; Wang et al, 2017 ). In some grasslands, dew can occur as frequently as 95% of nights ( Ritter et al, 2019 ) and recent work has demonstrated that NRM is an important driver of decomposition in both arid and mesic grasslands alike ( Evans et al, 2020 ). NRM can be a particularly important moisture source for standing litter (senesced litter that has not fallen to the ground surface yet), where frequent wetting and subsequent microbial growth can “prime” litter for decomposition once it reaches the soil surface ( Wang et al, 2017 ).…”

“…First, in many systems fog and dew occur more regularly than rainfall ( Eckardt et al, 2013 ), so organisms that rely on NRM may not face such prolonged dry periods as those relying solely on rain, and may therefore be more sensitive to desiccation ( Jacobson et al, 2015 ). Second, since NRM typically forms at night and in the early morning when temperatures are low, the ability to remain active at colder temperatures may be more important for organisms relying on NRM as a primary moisture source than those that are solely rainfall-dependent ( Evans et al, 2020 ). Finally, fog can transport microbial communities that differ from those dispersing through rain and clear air ( Evans et al, 2019 ).…”

Quantifying microbial growth and carbon use efficiency in dry soil environments via 18O water vapor equilibration

“…We also measured respiration without any addition of water and found no significant difference in respiration rates between untreated soils and soils undergoing vapor equilibration (Figure 3a). However, recent studies have highlighted that in some ecosystems (e.g., arid ecosystems), non‐rainfall moisture (high humidity, dew, and fog) can significantly increase microbial activity (Evans, Todd‐Brown, Jacobson, & Jacobson, 2019). The increase in respiration rates following water addition to dry soils is a common phenomenon (Birch, 1958).…”

“…undergoing vapor equilibration (Figure 3a). However, recent studies have highlighted that in some ecosystems (e.g., arid ecosystems), nonrainfall moisture (high humidity, dew, and fog) can significantly increase microbial activity (Evans, Todd-Brown, Jacobson, & Jacobson, 2019).…”