UVB exposure does not accelerate rates of litter decomposition in a semiarid riparian ecosystem

Authors: Uselman, Shauna; Snyder, Keirith; Blank, Robert - Bob; Jones, Timothy - Tim

Submitted to: Journal of Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2011
Publication Date: 3/5/2011
Citation: Uselman, S.M., Snyder, K.A., Blank, R.R., Jones, T.J. 2011. UVB exposure does not accelerate rates of litter decomposition in a semiarid riparian ecosystem. Soil Biology and Biochemistry. 43:1254-1265.

Interpretive Summary: Organic matter decomposition may be driven by biological, chemical, and/or physical breakdown processes. Traditional scientific models developed to describe litter decomposition have demonstrated that climate and litter quality (i.e. chemical composition) play a large role in explaining differences in rates of decomposition across different ecosystems via their influence on biological processes. Comparison of model estimates with observed data has suggested that additional mechanisms may be involved in decomposition in deserts, such as the chemical process of photodegradation and/or physical processes. Non-biological processes are likely to be of greater importance in explaining rates of decomposition in harsh environments such as deserts, where exposure to chemical and/or physical breakdown processes is greater. In ecosystems where the northern Tamarisk beetle has been released to control saltcedar, beetle herbivory has lead to large-scale defoliation and alterations of leaf litter quality, and loss of leaves has reduced shading from trees. As a result, leaf litter that falls from trees is exposed to increased levels of solar radiation during decomposition. In particular, high intensity short-wave ultraviolet-B radiation (UVB) has been found to breakdown organic molecules via photo-chemical reactions, and the importance of this process in desert ecosystems where exposure to radiation is high and biological processes are constrained by harsh environmental conditions is of growing interest. This study aimed to examine the interactive effects of litter quality and UVB exposure on litter decomposition in a saltcedar-invaded ecosystem during the establishment of an insect biological control agent in northern Nevada. Decomposition of leaf litter from natural saltcedar, beetle-affected saltcedar, and perennial pepperweed (an understory weed) was studied under conditions of reduced UVB and ambient UVB (using plastic filters to manipulate UVB levels) and under conditions with no plastic filter. Our findings suggested that the physical processes of frequent cycles of freezing and thawing, and possibly rainfall intensity, influenced decomposition and nutrient release at this site. However, contrary to our expectations, greater exposure to UVB did not result in faster rates of decomposition. Rates of decomposition and nutrient release by litter type followed this ranking: perennial pepperweed > beetle-affected saltcedar > natural saltcedar. Rapid litter decomposition rates exhibited by perennial pepperweed are consistent with vulnerability of this site to secondary invasion by this understory weed. In this desert ecosystem, decomposition and nutrient release were strongly affected by litter type and much less so by UVB exposure.

Technical Abstract: Aboveground litter decomposition is controlled mainly by substrate quality and climate factors across terrestrial ecosystems, but photodegradation from exposure to high-intensity ultraviolet-B (UVB) radiation may also be important in arid and semi-arid environments. We investigated the interactive effects of UVB exposure and litter quality on decomposition in a Tamarix-invaded riparian ecosystem during the establishment of an insect biological control agent in northern Nevada. Feeding by the northern tamarisk beetle (Diorhabda carinulata) on Tamarix spp. trees leads to altered leaf litter quality and increased exposure to solar UVB radiation from canopy opening. In addition, we examined the dynamics of litter decomposition of the invasive exotic Lepidium latifolium, because it is well-situated to invade beetle-infested Tamarix sites. Three leaf litter types (natural Tamarix, beetle-affected Tamarix, and L. latifolium) differing in substrate quality were decomposed in litterbags for one year in the field. Litterbags were subjected to one of three treatments: (1) Ambient UVB or (2) Reduced UVB (where UVB was manipulated by using clear plastic films that transmit or block UVB), and (3) No Cover (a control used to test for the effect of using the plastic films, i.e. a cover effect). Results showed a large cover effect on rates of decomposition and nutrient release, and our findings suggested that frequent cycles of freeze-thaw, and possibly rainfall intensity, influenced decomposition at this site. Contrary to our expectations, greater UVB exposure did not result in faster rates of decomposition. Greater UVB exposure resulted in decreased rates of decomposition and P release for the lower quality litter and no change in rates of decomposition and nutrient release for the two higher quality litter types, possibly due to a negative effect of UVB on soil microbes. Among litter types, rates of decomposition and net release of N and P followed this ranking: L. latifolium > beetle-affected Tamarix > natural Tamarix. Altered nutrient dynamics with beetle introduction as well as the rapid decomposition rates exhibited by L. latifolium are consistent with vulnerability to secondary invasion. In this desert ecosystem, decomposition and nutrient release were strongly affected by litter type and much less so by UVB exposure.