Plant litter decomposition is a fundamental ecosystem service because it regulates soil fertility, the turnover and fate of carbon and nutrients, and ultimately plant growth and productivity. Photodegradation recently has been proposed as a significant driver of litter decomposition in arid and semi-arid ecosystems. However, the mechanisms through which photodegradation contributes to litter decomposition remain poorly understood. In this dissertation, I conducted both field and laboratory experiments to test the hypothesis that photodegradation has both direct (through abiotic photochemical reactions) and indirect (through interacting with microbial decomposition) contributions to litter decomposition.

First, I evaluated how photodegradation by ultraviolet (UV) radiation affected litter decomposition for two litter positions in a field experiment at Sedgwick Reserve in California. I found that photodegradation increased litter mass loss by up to 30% at positions with and without radiation exposure, suggesting UV radiation had both direct and indirect contributions to litter decay. Next, effects of UV photodegradation on litter biodegradability were examined in a laboratory experiment. Surprisingly, UV radiation in summer reduced litter biodegradability by 28% through reduction of litter N availability. This result suggests that UV radiation has both positive and negative impacts on microbial decomposition of litter. Finally, I examined impacts of UV radiation on litter chemical composition at a molecular level using two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Although conventional fiber analysis showed no changes in lignin concentration, 2D NMR spectroscopy data revealed that UV radiation degraded lignin p-hydroxyphenyl units and lignin beta-aryl linkages. Furthermore, lignin degradation likely led to loss of hemicellulose through enhancing microbial decomposition.

The combined results demonstrate that photodegradation is an important contributor to litter decomposition processes in arid ecosystems like California grasslands. More importantly, my work suggests that photodegradation is not purely an abiotic process, as it has significant indirect contributions to litter decay through effects on microbial decomposition. Thus future empirical and modeling studies of litter decomposition must examine the interaction of abiotic and biotic processes in arid ecosystems. This work identifies major pathways through which photodegradation increases microbial decomposition of litter. It also advances our understanding of chemical mechanisms underlying photodegradation.