Print PageMechanistic models of decomposition dynamics have been successfully applied to mesic ecosystems, but predicting decomposition in arid and semiarid ecosystems (i.e., drylands) remains problematic due to a poor understanding of driving variables. Recent studies have shown that leaf litter decomposition in drylands can be influenced by both soil deposition and solar UV radiation, but how these factors interact to drive decomposition has received little attention. We hypothesized that UV and soil deposition interact, such that UV effects on litter decomposition via photodegradation would predominate up to a certain level of soil coverage and then be superceded by soil effects, which mediate microbial processes. To test this hypothesis, we conducted a greenhouse experiment using leaf litter from velvet mesquite (Prosopis velutina) and Lehmann’s lovegrass (Eragrostis lehmanniana), two common plants of desert shrublands in the southwestern USA. Leaf litter was exposed to two UV treatments (no UV-B (280-320 nm), and simulated clear-sky, ambient summer UV-B (UV-BBE = 8 kJ m-2 d-1)) and three soil coverage treatments (0%, 50%, and 100% leaf area covered) in a full factorial design (n=9 replicates/treatment). Collections were made over a 10-month period and analyzed for changes in litter mass, C, N and microbial biomass.
Results/Conclusions
When averaged over time, there was no significant main effect of UV-B on leaf dry mass loss for either E. lehmanniana (P=0.90) or P. velutina (P=0.74), however, mass loss in the 100% soil coverage treatment at the final collection was reduced by UV-B in E. lehmanniana (P=0.03) but not in P. velutina (P=0.24). By comparison, there was a significant soil treatment main effect on litter dry mass loss for both E. lehmanniana and P. velutina (P<0.001). For both species, the 100% soil coverage treatment had significantly (P<0.03) greater mass loss than either the 0% or 50% soil coverage treatments, and rates of decomposition (k, yr-1; determined from exponential models) increased with increasing soil coverage (k = 0.23, 0.24, 0.52 [E. lehmanniana]; 0.57, 0.63, 0.86 [P.velutina] for 0, 50 and 100% soil treatments, respectively). These results are consistent with field litter bag studies which have shown litter decomposition in this system is more closely linked to soil deposition than solar radiation environment and suggest that 1) when litter is in contact with soil (either above or below) soil-mediated microbial processes are more important drivers than direct UV-induced photodegradation,, and 2) UV-B may inhibit rather than accelerate decomposition in these cases.
