Moisture availability influences the effect of ultraviolet-B radiation on leaf litter decomposition

Abstract

Altered surface ultraviolet-B (UV-B) radiation resulting from a combination of factors that include changes in stratospheric ozone concentrations, cloud cover, and aerosol conditions may affect litter decomposition and, thus, terrestrial nutrient cycling on a global scale. Although litter decomposition rates vary across biomes, patterns of decomposition suggest that UV-B radiation accelerates litter decay in xeric environments where precipitation is infrequent. However, under more frequent precipitation regimes where litter decay rates are characteristically high, the effect of UV-B radiation on litter decomposition has not been fully elucidated. To evaluate this association between moisture regime and UV-B exposure, a litter decomposition experiment was designed for aspen (Populus tremuloides) leaf litter, where conditions that influence both abiotic (photodegradation) and biotic (microbial) processes could be manipulated quantitatively. We found that experimentally increasing UV-B exposure (0, 7.4, and 11.2 kJ m-2 d-1, respectively) did not consistently increase litter decomposition rates across simulated precipitation frequencies of 4-, 12-, and 24-days. Instead, a UV-B exposure of 11.2 kJ m-2 d-1 resulted in a 13% decrease in decomposition rates under the 4-day precipitation frequency, but an increase of 80% under the 24-day frequency. Furthermore, the same UV-B dose increased litter decomposition rates under the 24-day precipitation frequency by 78% even in conditions where microbial activity was suppressed. Therefore, under more xeric conditions, greater exposure to UV-B radiation increased decomposition rates, presumably through photodegradation. In contrast, when decomposition was not moisture-limited, greater UV-B exposure slowed decomposition rates, most likely from the resulting inhibition of microbial activity. Ultimately, these experimental results highlight UV-B radiation as a potential driver of decomposition, as well as indicate that both the direction and magnitude of the UV-B effect is dependent on moisture availability, a factor that may change according to future patterns in global precipitation.

Effects of solar UV radiation and climate change on biogeochemical cycling: interactions and feedbacks

“…5 Extending this to the release of nutrients from decomposing litter is difficult, as there are very few studies that have directly evaluated the effects of UV radiation on litter nutrient release. However, several recent studies on photodegradation in semi-arid ecosystems have shown that litter exposed to UV radiation demonstrated reduced N immobilization 132,133 or no N immobilization in semi-arid ecosystems independent of direct UV effects. 134,135 It is currently not clear whether these effects are biotically or abiotically driven, but the effects do suggest that the lack of immobilization of nitrogen may result in increased N losses over the longer term, affecting carbon-nitrogen interactions in these ecosystems.…”

Solar ultraviolet radiation and ozone depletion-driven climate change: effects on terrestrial ecosystems

“…80,86 In addition, apparent photodegradation is enhanced in the presence of oxygen but can also occur under anoxic conditions, 4 indicative of the involvement of multiple chemical pathways. 80 Generally, rates of photodegradation tend to increase with increasing moisture content of litter 87,94 and air temperature. 4 The effects of UV radiation on biotic (microbial) and abiotic ( photodegradation) processes are not entirely independent and there is evidence that photodegradation can modify or partially degrade compounds in ways that enhance or retard subsequent microbial decomposition of litter (i.e.…”

Heterotrophic respiration in disturbed forests: A review with examples from North America

“…This process could cause a significant proportion of carbon loss from litter in arid ecosystems [ Brandt et al , 2009]. The effect of photodegradation in decomposition is reported to be influenced by moisture [ Smith et al , 2010] and lignin content of plant litter [ Austin and Ballare , 2010].…”