Interactive effects of UV radiation and soil coverage on leaf litter decomposition in velvet mesquite (Prosopis velutina)

Background/Question/Methods Recent studies have shown that leaf litter decomposition in dryland ecosystems can be influenced by both soil deposition and solar UV radiation, but how these factors interact to drive decomposition has received little attention. We conducted a growth chamber study to test whether coverage with sterile soil could reduce the direct effect of UV on litter photodegradation in senescent leaflets of velvet mesquite (Prosopis velutina), a common woody plant of desert grasslands and shrublands in the southwestern USA. Leaf samples were exposed to two UV treatments (no UV and simulated clear-sky, ambient summer UV [added as UV-B and UV-A; 280-390 nm] for SE Arizona, USA) and three soil coverage treatments (no soil, low soil coverage [ca. 10-20% of leaf surface covered], and heavy soil coverage [ca. 80-90% coverage] in a factorial treatment design [n=5-6 replicates/treatment]). Results/Conclusions After nearly 32 weeks of UV exposure, leaf dry mass loss was significantly (p<0.01) greater in the +UV than the –UV treatments, when averaged over soil treatments, and there was a significant UV x soil treatment interaction (p<0.01). Specifically, mass loss was greater in the +UV than the –UV treatment for the no soil and low soil coverage treatments, but UV had no significant (p=0.12) effect on mass loss in the heavy soil treatment. Leaf C loss data revealed similar UV and soil coverage patterns to that of the mass loss data, while N loss data showed no consistencies. These findings suggest that soil coverage can potentially ameliorate the direct effects of UV on litter decomposition in desert ecosystems where soil erosion and deposition commonly occur together with high solar UV irradiances.