Thermal abiotic emission of CO₂ and CH₄ from leaf litter and its significance in a photodegradation assessment

Abstract

Photodegradation has been recognized as a significant driver of plant litter decomposition in drylands. Another potential driver is the thermal emission of trace gases that occurs in the absence of solar radiation and microbial activity. Most field assessments documenting photodegradation have employed filters that absorb solar radiation, along with transparent filter controls; faster litter decay under transparent filters is taken as evidence of photodegradation. However, the temperature of litter under transparent filters is often higher, and its faster decay might conceivably stem from greater thermal emission, rather than photodegradation. If true, the growing consensus that photodegradation is a significant driver of litter decay needs rethinking. We assessed the contribution of thermal emission of CO2 and CH4 to the C loss of 12 litter types over a 34‐month photodegradation study in the Sonoran Desert by quantifying thermal emission responses and using field litter temperatures to estimate emissions. Emission of both gases from litter increased exponentially with temperature. Emission of CO2 was much greater than CH4, but their rates were strongly correlated. Concentrations of surface waxes and dissolved organic C in litter were strong predictors of emission of both gases. Emission declined from dried green leaves to naturally senesced litter, and as litter decayed. Diurnal litter temperature averaged 39.8°C under transparent filters over the field experiment and averaged 1.7°C higher than that of litter under filters that absorbed UV through blue solar wavelengths. Through all mechanisms, litter lost an average of 77.8% of its original C under transparent filters and 60.8% under filters that absorbed UV through blue wavelengths. However, thermal emission of these gases accounted for only 0.8% of the original C in litter under transparent filters and 1.0% under filters that absorbed UV through blue wavelengths, corresponding to only 1.2% and 2.0% of the total C lost from litter. While litter temperatures were higher under transparent filters, thermal emission losses from this litter were lower because emission from this litter declined faster with decay. We conclude that thermal abiotic emission was a minor C loss pathway and that photodegradation was responsible for the faster decay of litter in sunlight.

Contrasting responses to aridity by different-sized decomposers cause similar decomposition rates across a precipitation gradient

“…The puzzle of why plant litter decomposition in arid-lands is decoupled from annual precipitation and is faster than expected based on microbial decomposition models has bothered scientists for half a century 7,34,35 , and was later termed the desert decomposition conundrum 36 . Attempts to resolve this conundrum have focused predominantly on abiotic weathering agents, such as photodegradation 37,38 and thermal degradation 39 , alternative sources of moisture such as fog, dew and atmospheric water vapor 40 and soil–litter mixing 36,41 . We, in turn, hypothesized that the opposing climatic dependencies of macrofauna and that of microorganisms and mesofauna should lead to similar overall decomposition rates across precipitation gradients, except in hyperarid environments in which decomposers activity is predicted to be minimal regardless of organism size.…”

Arthropods as the Engine of Nutrient Cycling in Arid Ecosystems

“…In this important paper and many of the studies that followed, the researchers used litter bags or cages covered with mesh that exclude macrofauna and prevent litter redistribution, to assess litter mass loss in the presence or absence of focal abiotic factors. These studies offer strong experimental evidence that photodegradation, thermal degradation, and alternative sources of moisture, such as fog, dew, and atmospheric water vapor, can assist in explaining the unexpectedly high decomposition rate of exposed plant litter [14][15][16]. However, these studies also unintentionally narrow the original question of how "a considerable proportion of litter and standing dead material of ephemeral plants has been observed to disappear even in dry periods" [4] to the much-reduced question of what abiotic factors accelerate the decomposition of exposed surface litter that is neither being transported and ingested by macrofauna nor being transported and mixed with soil by wind or water flow.…”

Divergent Roles of UV Exposure and Microclimatic Conditions in the Decomposition of Standing and Soil Surface Litter in a Semi‐Arid Steppe

“…The magnitude of thermal degradation is minimal below 30°C but increases exponentially above 50°C (B. Wang et al., 2017; J. Wang et al., 2017). However, it has been estimated that thermal degradation alone contributes minimally to litter mass loss (1.2%–2%) (Day et al., 2019). At our experimental site, only a few days with transient temperatures above 50°C were recorded in midsummer, and the daily maximum temperature remained below 30°C for most of the time (Figure S1c in Supporting Information ), suggesting a limited influence of thermal degradation on litter decomposition in this study.…”