Shedding light on plant litter decomposition: Advances, implications and new directions in understanding the role of photodegradation

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models. © 2012 US Government

Sunlight and Soil-Litter Mixing: Drivers of Litter Decomposition in Drylands

Decomposition of leaf litter is a key component of biogeochemical cycles but the mechanisms driving it in arid and semiarid ecosystems (drylands) remain unresolved. Here, we review recent findings that demonstrate dual roles of solar radiation (ultraviolet and photosynthetically active radiation) and soil–litter mixing as drivers of decomposition in drylands. We focus on the known and potential mechanisms by which these factors influence leaf litter decomposition, explore how the importance of these two drivers may shift over time, and propose possible avenues by which these factors may interact. Special attention is given to UV in sunlight, as this radiation is known to have multiple roles in influencing decomposition and has received considerable recent research attention. We also identify important uncertainties and challenges and offer a generalized conceptual model to guide future research aimed at enhancing our mechanistic understanding and quantitative modeling of the processes by which soil deposition and solar radiation together influence leaf litter decomposition rates in globally extensive dryland ecosystems