1 INTRODUCTION

Plant litter decomposition is a critical component of the carbon cycle worldwide. While litter decay processes are fairly well understood in mesic systems (Adair et al., 2008), in drylands, litter decomposition involves the interaction of several less understood processes including photodegradation (Austin & Vivanco, 2006; Smith et al., 2010), biotic degradation by non-rainfall moisture (NRM; fog, dew and water vapour; Evans et al., 2020; Gliksman et al., 2017) and decomposer responses to intense abiotic stressors such as prolonged desiccation (Logan et al., 2021). This can lead to complex dynamics that do not fit neatly within classic paradigms. For example, while plant biomass production is tightly coupled with rainfall in drylands (Poulter et al., 2014; Seely & Louw, 1980), the decomposition of this biomass is rarely correlated with rainfall in these systems (Austin, 2011). Since arid and semi-arid lands cover 40% of Earth's land surface and can account for half of the interannual variability in global carbon storage (Poulter et al., 2014), this currently leaves a gap in our ability to describe terrestrial decomposition processes globally, especially as moisture regimes in drylands worldwide change (Dai, 2013; Forthun et al., 2006; Haensler et al., 2011; Kutty et al., 2019; Niu et al., 2010).

Compared to most mesic systems where rainfall-supported biotic activity is the primary driver of litter decomposition, in drylands, abiotic processes play a greater role in driving decomposition (Austin, 2011) and account for the majority of total litter decay in some ecosystems (Austin & Vivanco, 2006). Photodegradation—the direct or indirect decomposition of litter by solar radiation—is particularly important in drylands where ground-level solar irradiance is high and precipitation is low and erratic, reducing decomposer activity (Austin & Vivanco, 2006). Incorporating photodegradation into existing litter decay models can substantially improve model predictions (Adair et al., 2017; Chen et al., 2016), but more work is needed to understand the mechanisms by which photodegradation interacts with other litter decay processes.

Solar radiation influences litter decomposition through multiple mechanisms. Photolysis of organic compounds such as lignin, cellulose and hemicellulose directly accelerates litter mass loss (Brandt et al., 2009; Day et al., 2019) and also produces intermediaries such as peroxides and reactive oxygen species that can further degrade organic components of litter through indirect pathways (King et al., 2012; Messenger et al., 2009). By cleaving double bonds in recalcitrant compounds like lignin, solar radiation can make litter more susceptible to subsequent microbial degradation (King et al., 2012; Wang et al., 2017). This process, known as photopriming or photofacilitation, accelerates mass loss more than either abiotic photodegradation or microbial decomposition alone (Gliksman et al., 2017; Wang et al., 2015). Since photopriming links two major decomposition processes in drylands (biotic degradation and photodegradation), understanding the mechanisms underlying photopriming is essential to accurately describe carbon turnover in these systems.

In many drylands, NRM-driven biotic decomposition and photodegradation interact with one another through photopriming mechanisms. By manipulating nighttime humidity and daytime solar irradiance in a Mediterranean shrubland, Gliksman et al. (2017) found synergistic effects of NRM-supported microbial activity and photodegradation on diel time-scales. Lin et al. (2018) found that CO₂ production and lignin degradation were significantly greater when microcosms experienced an alternating cycle of UV radiation during the day and dark wet conditions at night. Since NRM can occur as often as 95% of nights in some grasslands (Ritter et al., 2019) and account for the majority of litter mass loss (Evans et al., 2020), interactions between NRM-driven biotic decay and photodegradation may be critical to dryland litter decay.

While many studies have focused on the classical photopriming mechanism by which solar radiation makes lignin more susceptible to biotic decay (Austin & Ballare, 2010; Austin et al., 2016; King et al., 2012), structural lignin is usually located within plant tissues where it is not exposed to solar radiation until the outer surface is broken or removed. Instead, photodegradation of compounds present in plant cuticles may be more important in the early stages of decay (Bruhn et al., 2014). Physical traits such as cuticle thickness can slow litter decay by blocking decomposer fungi and water (Zukswert & Prescott, 2017) and plant cuticles contain many photo-reactive compounds that are susceptible to degradation by solar ultraviolet (UV; 290–400 nm) radiation (Bruhn et al., 2014; Day et al., 2019; Messenger et al., 2009). Since cuticles are effective water barriers in living plants (Shepherd & Griffiths, 2006), they may affect how well litter absorbs water during NRM events after senescence.

We set out to test a novel mechanism of photopriming by which solar radiation degrades the cuticle of plant litter, increasing moisture uptake during NRM events, subsequently enhancing biotic decomposition. Since litter moisture content controls biotic activity during NRM events (Jacobson et al., 2015) and moisture content depends in part on cuticle permeability, we hypothesized that as solar radiation degrades the cuticle, it becomes more permeable to moisture, which enhances microbial decomposition during NRM events.