In drylands with heterogeneous vegetative cover and large patches of bare ground, photodegradation may be an important driver of litter decomposition at certain times of the year or stages of decay. However, low plant cover in drylands also promotes soil movement by wind and water, causing soil-litter mixing that may counterbalance photodegradation via direct or indirect effects on decomposer microbial communities. We used a common-garden experiment to evaluate the outcome of interactions between UV-B radiation (280-320 nm) and soil-litter mixing on microbial communities associated with decomposition of shrub (mesquite, Proposis velutina) and grass (Lehmann lovegrass, Eragrostis lehmanniana) foliage. In a factorial experiment, we manipulated UV radiation levels (near-ambient and sub-ambient UV-B) and the degree of litter coverage by soil (none vs. 100%). We measured phospholipid fatty acid (PLFA) profiles to quantify how microbial and plant lipids in the decomposing material were affected by these treatments after zero, one, three, six and twelve months of decay. We hypothesized that UV effects would be strongest in the fully exposed litter and that soil coverage would negate UV effects and promote microbial activity and decomposition.
UV treatment and soil-litter mixing differentially affected fungal and bacterial biomarkers. Near-ambient UV stimulated fungal biomarkers by ~50% over the sub-ambient UV-B treatment at three and six months of litter decay. Soil coverage tended to negate this stimulatory UV effect on fungal biomarkers and strongly promoted bacterial dominance within the decomposer community (ordinations for both plant litter types differed significantly based on soil coverage). Soil coverage also reduced bacterial lipid stress ratios on mesquite litter, suggesting that ambient UV levels were inhibitory for bacteria. Whereas soil-induced changes in microbial communities were accompanied by significant reductions in rates of litter mass loss, ambient UV stimulated decomposition and promoted fungal abundance. The latter may reflect photo-priming effects and reductions in the ‘lignin-bottleneck’. These data suggest that soil-litter mixing retards litter decomposition in part by creating conditions that favor bacteria over fungi, thus attenuating the ‘fungal loop’ that tends to dominate arid ecosystems. Therefore, global change factors that increase soil movement in arid ecosystems may significantly alter decomposition dynamics by reducing photodegradation and altering the composition of dominant decomposers.