COS 111-6
Indirect effects of photodegradation on litter microbial decomposition

Thursday, August 14, 2014: 3:20 PM
311/312, Sacramento Convention Center
Yang Lin, Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA
Rachel D. Scarlett, Department of Geography, University of California, Santa Barbara, CA
Jennifer Y. King, Department of Geography, University of California, Santa Barbara, Santa Barbara, CA

In arid and semi-arid ecosystems, photodegradation represents a significant pathway in organic matter decomposition and ecosystem C cycling. Photodegradation has been well documented to directly increase litter decomposition through photo-oxidation of organic matter. Increased exposure to solar radiation enhances litter mass loss rates by 23% on average. Exposure to radiation can also alter litter chemical composition and biodegradability, thus indirectly affecting litter mass loss rates. Yet, the extent of this indirect effect of photodegradation on litter decomposition remains unclear. Using a laboratory incubation, we investigated the effects of field UV exposure (UV pass and UV block) and the duration of exposure (2.5, 4, 6, and 12 months) on subsequent microbial decomposition of Bromus diandrus litter. We hypothesized that UV radiation exposure would increase loss of lignin and consequently increase subsequent microbial decomposition and that the effects of UV radiation exposure on lignin and microbial decomposition would be stronger with longer duration of UV exposure.


Surprisingly, UV radiation decreased the subsequent microbial decomposition rate by 28% when the exposure length was 2.5 months; UV radiation did not affect subsequent microbial decomposition for litter exposed for longer durations. Regardless of exposure duration, litter lignin concentration was not affected by UV treatments. Data from two dimensional nuclear magnetic resonance analyses showed that lignin chemical composition was only affected by UV treatments when the exposure lasted for 12 months. Our results suggest that a relatively long duration of UV exposure (12 months) is needed to induce measurable changes in lignin chemical composition of B. diandrus litter, but these changes did not necessarily affect litter biodegradability. Across all samples, we found a strong positive correlation between subsequent microbial decomposition rate and water extractable nitrogen (WEN), suggesting that microbial decomposition was limited by N availability. Interestingly, WEN was lower in UV pass treatments when the exposure lasted for 2.5 months, which corresponds to the reduced microbial decomposition rate observed for litter exposed to 2.5 months of UV radiation. The results suggest that UV radiation inhibits subsequent microbial decomposition through suppression of microbial N immobilization. Our study indicates that photodegradation has important indirect effects on decomposition processes through influencing microbial activities.