Collaborative Research: Photodegradation in Deserts: Litter Optical and Structural Considerations

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National Science Foundation (NSF)

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Division of Environmental Biology

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Litter decomposition is a key pathway in the global carbon (C) cycle, releasing more C into the atmosphere annually than fossil fuel combustion. While litter decay in mesic systems is reasonablywell predicted by empirical models based on climatic and litter chemistry factors, this is not the case in arid systems. Specifically, mass loss in arid systems is faster than predicted and decay patterns are near linear rather than exponential. Recent research has revealed that breakdown of litter by solar radiation (photodegradation) can be a significant driver of mass loss in arid systems, although the relative strength of this driver appears quite variable. The UV component of sunlight appears to be the most effective waveband in driving mass loss, and lignin appears to be the main target. We propose that the optical properties of leaf litter vary substantially among different plant growth forms, and that this has a large influence on the effectiveness of photodegradation. We suspect that the surface UV-screening of leaf litter is greatest in evergreens, intermediate in grasses, and least in forbs. Screening effectiveness is important because it dictates radiation fluxes inside litter, such as in vascular tissue where the majority of lignin resides. We also suspect that effective surface screening persists much longer in evergreen litter than in grasses or forbs, because of the high concentrations of wall-bound screening compounds in evergreens that are not be readily lost during decay. Hence, photodegradation may become a stronger driver of decay much earlier in the decomposition of forb and grass litter than evergreen litter. We contend that these changes in the potential effectiveness of photodegradation during decomposition may explain the relatively linear litter decay patterns, as well as the variable effectiveness of photodegradation, found in arid systems. We will test our ideas through a series of complimentary field experiments in the Sonoran Desert that will allow us to determine how surface screening varies among species, how persistent it is through decomposition, whether surface screening is the dominant controller of photodecay, and whether loss of litter surface screening coincides with faster rates of photodegradation and changes in microbial community composition. Specifically, our experiments will: (1) Evaluate the generalization that UV surface screening of leaf litter varies by plant growth form, being least effective in forbs and most effective and persistent in evergreen litter. (2) Assess the relative and potential effectiveness of photodegradation versus microbial drivers on litter mass loss by employing treatments that minimize or maximize photodegradation (filtering sunlight or abrading litter surfaces) in combination with treatments that minimize or maximize microbial drivers (biocide or water supplements). (3) Perform a proof of principle experiment under controlled environmental conditions to assess the potential for photodegradation in the absence of confounding temperature effects. Several optical approaches, including fiber-optic microprobes and UV-fluorescing films, will be used to assess litter screening properties, and will be complimented with assessments of litter structure, chemistry, and microbial community composition (high-throughput pyrosequencing of ribosomal genes).

The controls over litter decomposition in arid systems are not well understood, hindering the development of models that accurately predict litter mass and C loss rates. We will assess the optical and structural properties of litter, which have received surprisingly little attention in the context of photodegradation, but likely vary substantially among litter types, and in turn likely have a strong influence on the effectiveness of photodegradation, as well as microbial community composition. Our work has the potential to explain the puzzling observation that litter decay is linear in arid systems, and provide information for improved models of decomposition. Our findings will also be applicable to decomposition in other ecosystems that experience periods of high irradiance and periodic drought (e.g. many grasslands).

We will develop a website that serves as a hub to: (1) Publish a guide to litter decomposition aimed at K12 students that discusses the relevance of decomposition to the biosphere, drivers and controls of decomposition, and how desert decomposition contrasts with other biomes. (2) Publish a companion classroom activity that guides teachers and students through the steps to conduct a simple litter decomposition field experiment and analyze their findings. (3) Solicit K12 science teachers to take part in this experiment. (4) Disseminate scientific findings from our project to the general public. This project will provide research experience for high school, undergraduate and graduate students. Our findings will improve our understanding of the basic controls and drivers of decomposition in arid systems, providing valuable information for C modelers, and will also be applicable to other ecosystems that experience high irradiance and periodic drought. Our findings will contribute to society at large by improving the ability of the scientific community to predict how C fluxes and stocks will respond to global change.


Biological Sciences


Science, Engineering and Technology

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