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

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.