A UV dosimeter for estimating the influence of soil deposition and other factors on solar UV exposure of decomposing leaf litter in dryland ecosystems

Background/Question/Methods   Exposure to solar UV radiation can accelerate the decomposition of leaf litter in dryland ecosystems via photodegradation. However, under field conditions UV exposure of ground-level leaf litter is influenced by numerous factors including time of year, clouds, shading by overstory plants, and coverage by soil. Quantification of the temporal and spatial variation in UV exposure of litter is thus challenging but necessary to estimate and model the role of photodegradation and other (microbial) processes as drivers of litter decomposition in these systems. In this study, we tested whether polysulphone film, which has previously been used as human UV dosimeters, could be used to reliably measure plant-effective UV doses under different UV and soil cover conditions. Small dosimeters (window size = 2.3 x 3.5 cm) were constructed of polysulphone film (0.025, 0.076, or 0.127 mm-thickness) and changes in optical absorbance at 330 nm (DA330; relative to dark controls) were related to varying plant-effective UV doses under field and greenhouse conditions. These dosimeters were then dusted with different levels of native desert mineral soil (0, 0.32 and 0.64 g; simulating 0, 50 and 100% relative cover) and exposed to UV in a greenhouse.

Results/Conclusions   Calibrations against broad-band UV sensors (previously calibrated against a spectroradiometer) indicated that DA330 of the polysulphone film was related in a non-linear fashion to plant-effective UV doses (R2 = 0.90-0.95 for exponential regression models), though saturation occurred after only ca. 4-6 h of UV exposure. Different calibration relationships existed for the different film thicknesses as well as for ambient solar and greenhouse UV lamp conditions. After 1 h of exposure in a greenhouse, mean UV doses were 56 and 66% less in the 100 and 50% soil cover treatments, respectively relative to the 0% cover. These findings indicate, 1) that these inexpensive UV dosimeters may be of value in characterizing cumulative UV exposures of leaf litter occurring in various habitats at least over periods < 1 day, and 2) these dosimeters can detect attenuation effects of UV by soil, though it appears that complex relationships exist between the degree of soil cover of litter and UV dose.