Solar Photochemical Emission of CO₂ From Leaf Litter: Sources and Significance to C Loss

Although photodegradation can be a significant mechanism of plant litter decay in drylands, the abiotic photochemical emission component of this process is not well characterized. We measured the temperature response of abiotic photochemical emission of CO₂ from 12 litter types under midday sunlight in the Sonoran Desert, assessed what litter traits predicted emission rates, and estimated its significance to litter C loss. Emission rates increased exponentially with temperature (Q₁₀= 1.75) and declined as litter decayed. Rates varied substantially among litter types. Microbial respiration rates of litter explained the most variation in emission rates at lower temperature (78% at 45 °C), suggesting that compounds amenable to microbial consumption were major precursors of emission. In contrast, wax concentrations explained the most variation at high temperature (76% at 70 °C), implying that precursors shifted with temperature. Photochemical emission of initial litter at 55 °C was a strong predictor of the mass loss of litter in full sunlight over 34 months in the field, explaining 67% of the variation among litter types. We estimate that photochemical emission of CO₂ was responsible for 10% of all C lost from litter in sunlight over 34 months and accounted for 39% of the greater C lost from litter in sunlight compared to litter that did not receive sunlight in the UV to blue wavelength bands. Hence, abiotic photodegradation via photochemical emission appeared to be a significant pathway for C loss from sunlit litter. The substantial exponential increase in photochemical emission with temperature illustrates that temperature needs to be considered in assessments of abiotic photodegradation and implies this process will be greatest in systems where litter is exposed to high solar irradiance in conjunction with high temperatures.