Soil-Litter Mixing Mediates Drivers of Dryland Decomposition along a Continuum of Biotic and Abiotic Factors

Litter decomposition is a key ecosystem process that determines rates of carbon and nutrient cycling. Photodegradation and soil-litter mixing have emerged as important drivers of dryland litter decomposition, but how these processes interact with decomposing microorganisms has received less attention. In this study, we examined the effects of ultraviolet-B radiation (UV-B; 280–315 nm) and soil-litter mixing on the decomposition of litter and its associated microbial community in an arid shrubland. We performed a full factorial litter decomposition experiment using leaf litter from a dominant shrub (Prosopis velutina) and a dominant grass (Eragrostis lehmanniana) that were exposed to solar radiation with near-ambient or attenuated UV-B, and were either soil-free or soil-covered; we then quantified litter decomposition and microbial community composition over a 12 month period. In general, shrub litter decomposed more rapidly than grass litter regardless of soil coverage, likely due to its lower C:N. Attenuation of UV-B had modest effects on decomposition but UV-B exposure did increase fungal biomass, perhaps reflecting facilitative aspects of photodegradation. Both bacteria and fungi emerged as important regulators of decomposition, and microbial decomposition was indirectly mediated by litter C:N, soil coverage, and UV-B effects on the microbial community. Bacterial colonization was inhibited in soil-free treatments, but was facilitated when litter was soil-covered. These findings suggest that UV-B may play an important role in facilitating fungal decomposition of litter, while soil-litter mixing is fundamental for promoting bacterial decomposition of litter. [Figure not available: see fulltext.]