A spectral weighting function for abiotic photodegradation based on photochemical emission of CO₂ from leaf litter in sunlight

Photodegradation can be a significant driver of leaf litter decomposition although the spectral effectiveness of sunlight in driving this process is not well characterized. We developed spectral weighting functions (WFs) for the photochemical emission of CO₂ from three leaf litter types using 10 cutoff filters that provided contrasting polychromatic sunlight under clear skies in Tempe, AZ, USA. An iterative nonlinear least-squares regression fitting procedure was used to estimate how effective sunlight at a given wavelength was in eliciting CO₂ emission. Although absolute CO₂ emission rates varied appreciably among litter types, their WFs were very similar. Using the average WF of all litter types, the effectiveness of sunlight declined from 300 nm by one and two orders of magnitude at 399 and 498 nm, respectively. The slope of the WF was most similar to WFs for CO emission from terrestrial leaf litter and photobleaching of dissolved organic matter in lakes, and was much more gradual than WFs addressing UV damage to biotic processes. Peak effectiveness of clear-sky noon sunlight with our WF occurred at 330 nm, with UV-B (280–320 nm), UV-A (320–400 nm) and visible (400–550 nm) wavebands responsible for 9, 61 and 30% of CO₂ emission, respectively. Results from past field studies suggest that solar UV is typically less effective in driving litter mass loss than our WF predicts; we discuss possible reasons for this discrepancy. The gradual slope of our WF implies that differences in UV-B irradiance associated with stratospheric ozone thickness or latitude are unlikely to significantly influence photochemical litter emission.