Ashes to Ashes, Dust to Dust: Decomposition in the Desert

Abstract

Decomposition, the process of breaking down organic material into its increasingly finer physical and chemical constituents, is an important component in the cycling of carbon and nutrients through an ecosystem. While ultraviolet (UV) radiation is known to be detrimental to human health, might it also play an important role in decomposition, and consequently soil fertility and land cover, in the arid southwestern US? To address this question, a 4-week field experiment was designed to quantify decomposition under contrasting radiant energy regimes at the Santa Rita Experimental Range near Tucson from July-August 2011. Velvet mesquite (Prosopis velutina) leaves were placed in litterbags constructed with fiberglass mesh and plastic that was either UV transparent or UV-B absorbing. The litterbags were deployed in open areas receiving full sun or in the shaded area beneath a shrub canopy. Leaf mass loss (an indicator of decomposition rates), soil-surface temperature, levels of photosynthetically active radiation, soil moisture, and precipitation were quantified over the 4-week period. UV (present vs. absent) and radiant energy environments (open areas vs. shaded) were compared using a mixed-effect model controlling for temporal autocorrelation. Soil-surface temperatures and decomposition rates in open areas were significantly higher (F1, 64 = 89.4, p < 0.0001; F1, 97 = 4.83, p = 0.0303, respectively) than those in shaded areas, but did not differ between UV treatments (F1, 97 = 0.064, p = 0.8012). These results suggest that over a short time period, radiant energy levels influence decomposition, but via temperature effects rather than via levels of UV.