Plant litter decomposition rates in mesic systems are well predicted by empirical models that use climatic factors and litter chemistry. These models consistently under predict rates of decomposition in arid and semi-arid systems. Solar radiation can increase decomposition rates through photodegradation, but can increase or decrease microbial activity (respiration). We examined the effect of exposure to sunlight on litter mass loss and microbial respiration in a 16-month experiment in the Sonoran Desert. Leaf litter of four winter annuals, Amsinckia menziesii, Lupinus sparsiflorus, Cryptantha angustifolia and Plantago patagonica, and an evergreen shrub, Larrea tridentata, were placed in envelopes with tops that either transmitted all solar radiation, filtered ultraviolet radiation (UV), or filtered UV and blue radiation. We sampled litter after 4, 9, and 16 months, and measured litter microbial abundance and respiration. Microbial respiration was assessed with a gas chromatograph by measuring CO2 emission from litter incubated at high relative humidity for 24 hours. Bacterial and fungal abundance was quantified by epifluorescence and scanning electron microscopy.
After 16 months, litter in full sunlight lost more mass than litter not exposed to UV or UV and blue radiation. In all radiation exposure treatments, mass loss was not correlated with common predictors such as initial lignin concentration or C:N ratios, but was positively correlated with ash concentration and microbial respiration. Litter in full sunlight lost 1.3 and 1.6 times more mass than litter not exposed to UV or UV and blue radiation, respectively. Microbial respiration of litter increased over time in all radiation exposure treatments. Litter exposed to full sunlight had higher rates of microbial respiration than litter that was not exposed to UV and blue radiation. However, microbial counts revealed that litter exposed to full sunlight had fewer bacteria and fungi. Microbial respiration was not correlated with fungal or bacterial abundance in all radiation exposure treatments.