The role of ultraviolet radiation in litter decomposition in arid ecosystems

In arid ecosystems, as much as 75% of solar radiation that penetrates the atmosphere hits the surface of the soil. The combination of high irradiance, high temperature, and low moisture puts constraints on the activity and organization of microbial communities. To separate the direct effects of UV absorbance on litter decomposition from the indirect effects of microbial selection, we placed mixed cohorts of senescent piñon (Pinus edulis) and juniper (Juniperus monosperma) litter into triplicate microcosms assigned to four treatments: UV irradiated (0.6 mW/cm2 UV-A and UV-B for 12 h/day), with and without water additions, and non-irradiated with and without water additions. After 26 weeks, mass loss rates did not differ significantly among treatments with 90% organic matter remaining for piñon litter and 60% for juniper. The distribution and abundance of functional groups as assessed by FTIR spectra did not reveal any differences in relation to treatment, but differences were observed in the quantity and quality of dissolved organic carbon (DOC) extracted from the samples. The amount of DOC extracted increased by 56% for piñon litter and decreased by 32% for juniper in the UV-only treatment compared to initial values. Litter treated with UV and water had the lowest concentration of DOC with a decrease of 69% for juniper litter and 28% for piñon. The largest concentration of reactive phenols was found in the UV-only treatment with a 309% increase for piñon litter and a 10% reduction for juniper when compared to initial values. The treatments receiving water in the presence or absence of UV showed a similar response for both litter types, a reduction of 20–30% in phenolic concentration. Five extracellular enzyme activities, used, as indicators of microbial activity, were higher in the treatments that received water, but activities did not show an interaction with UV irradiation. The results suggest that UV radiation alone, or in combination with microbial activity, was as effective at decomposing litter as microbial activity alone. Thus, solar radiation can be an important contributor to litter degradation in arid systems.