Background/Question/Methods In forested ecosystems with closed canopies, the decomposition of plant litter is an enzymatic process. In open canopy arid ecosystems, photodegradation and enzymatic degradation both contribute. In temperate forests, basiodiomycetes appear to be the dominate source of phenol oxidases. These taxa are not common in arid systems. On the soil surface, phenol oxidases from ascomycetes supplemented by photoreactions sustain litter decomposition rates comparable to those of mesic ecosystems. Below the surface, alkaline soils have high potential phenol oxidase activity. The origin of these enzymes is unclear, but aridity leads to enzyme stabilization on mineral surfaces creating a reserve that responds rapidly to precipitation. Results/Conclusions The combination of surface photodegradation and high soil phenol oxidase activity creates a decomposition potential that is less moisture dependent than primary production, which limits the capacity of the ecosystem to accumulate of soil organic matter.