The mechanisms controlling plant litter decay in arid systems are poorly understood, precluding our ability to predict decay rates and carbon fluxes in these systems. Traditionally, the chemical quality of litter for microbes (often approximated by C:N or lignin:N), in conjunction with a climatic index that approximates microbial activity, can predict litter decay rates in more mesic systems. The failure of these predictors in arid systems has generally been attributed to abiotic mechanisms including decay by sunlight (photodegradation). We assessed the decay rates and the magnitude of UV and UV/blue photodegradation of leaf litter of 12 Sonoran Desert species, comprising 4 species of 3 growth forms (woody dicots, suffrutescent dicots, grasses). Litter was placed in envelopes that transmitted (1) full sunlight, (2) full sunlight minus UV radiation, or (3) full sunlight minus UV through blue radiation, and placed on the surface of the Sonoran Desert. We assess litter mass loss over 21 months, and measured chemical, physical and optical characteristics of initial litter to identify what characteristics were useful predictors of total decay rates, as well as decay attributable to photodegradation.
Plant species or litter type had a large effect on mass loss and decay rates. Mass loss (oven-dried organic) over 21 months ranged from 18 to 82%, and decay constants (k) ranged from 0.18 to 0.85 per year, depending on species. Mass loss over 21 months was positively correlated with N and ash concentrations in initial litter in all 3 radiation-exposure treatments. Mass loss was positively correlated with specific litter area (SLA, surface area per dry mass). Mass loss was not correlated with traditional indices of litter quality such as C:N or lignin:N ratios of initial litter. Exposure to both UV and blue sunlight accelerated litter mass loss and decay rates. Litter exposed to full sunlight lost on average 1.3 times more mass than litter not exposed to UV sunlight (range 1.1 to 1.6) and 1.5 times more mass than litter not exposed to UV/blue sunlight (range 1.2 to 2.0). The magnitude of photodegradation was greatest in litter having inherently slow decay rates, which was in turn was associated with low initial N concentration and low SLA (i.e. dense leaves).