Decomposition of leaf litter is a key component of biogeochemical cycles but the mechanisms driving it in arid and semiarid ecosystems (drylands) remain unresolved. While a number of studies have demonstrated the importance of photodegradation (photochemical mineralization driven by ultraviolet and photosynthetically active radiation) and soil-litter mixing as determinants of decomposition in drylands, how these drivers shift over time, interact with one another, and are influenced by ongoing changes in climate and land use remain poorly understood. Here we summarize findings from recent field and laboratory studies probing interactive effects of sunlight and soil-litter mixing on woody and herbaceous leaf litter decomposition, 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 decomposition rates in globally extensive dryland ecosystems. We identify potentially important avenues by which changes in climate and land use could alter the importance of these interacting drivers and address some of the uncertainties and challenges associated with predicting future litter decomposition dynamics.
Findings from a number of field studies indicate that sunlight (UV+PAR) generally accelerates mass loss of litter in dryland plants but the magnitude and rate of photodegradation varies with species and conditions. Soil mixing of surface litter leads to the development of soil-litter-microbial complexes, which then enhances biotic decomposition processes while simultaneously shielding litter from photodegradation. Based on these findings a generalized conceptual model for sunlight-soil mixing effects in dryland decomposition is proposed whereby the mechanisms driving decomposition are predicted to shift from strongly abiotic (photodegradation of standing dead litter) to strongly biotic (microbial degradation of buried litter) over a continuum of soil coverage of litter from none (standing dead) to partial (recently detached) to fully buried litter. Ongoing shifts in dryland vegetation life-form composition (e.g., from grass to shrub domination), driven by changes in land use and climate, will likely increase soil movement in these environments such that role of soil deposition on litter decomposition may be magnified relative to present-day conditions. The importance of photodegradation may increase as standing dead material accumulates due to vegetation die-back, but these changes will be contingent on atmospheric (e.g., ozone levels and cloud cover) and vegetation conditions that influence the solar radiation exposure of standing and surface litter.