The effects of UV radiation, litter chemistry, and drought on desert litter decomposition

Background/Question/Methods

Recent studies suggest that UV radiation catalyzes litter photodegradation in dryland ecosystems.  Previous studies assessed the role of UV during decomposition of surface litter; however, exposure of standing dead tissue to UV radiation (photopriming) is understudied and can also mineralize carbon before it is transported into the soil matrix.  Other studies suggest that the importance of photodegradation declines with precipitation, suggesting photopriming importance could also respond to factors that affect microbially-mediated decomposition (e.g. litter chemistry and precipitation).  Our objectives were to test the relative importance of UV priming, litter chemistry, and precipitation on decomposition in a water-limited ecosystem. 

To test the effects of UV priming on decomposition, we photoprimed litter with three treatments: no radiation, UV-A+visible, and UV-A+UV-B+visible (equivalent to 3 months solar radiation in New Mexico).  We used three litter types with different chemical composition: filter paper (high cellulose), basswood sheets (high lignin), and mesquite (Prosopis glandulosa) leaflets (high nitrogen). Photoprimed litter was buried in mesh litterbags within a large scale precipitation manipulation experiment with ambient, ambient+30%, and ambient-45% rainfall in a semi-arid woodland at the Sevilleta Long Term Ecological Research site.  Mass remaining, carbon (%), and nitrogen (%) were measured at 1, 3, 6, and 12 months.  

Results/Conclusions

Initial C content was similar for all litter types (42-47%), while mesquite had a high initial nitrogen concentration (2.5%) relative to nearly undetectable amounts of nitrogen in basswood sheets and filter paper. Preliminary decomposition results show no influence of UV photopriming treatment on litter mass loss after 6 months for all litter types (P > 0.05). Conversely, precipitation treatment strongly, positively influenced mesquite mass loss (P < 0.001) with the lowest mass loss for ambient-45% rainfall.  In contrast, the other two litter types lost little mass throughout the experiment, and showed no mass loss responses to precipitation treatment (P > 0.05).  Our results suggest that precipitation and nitrogen content are more important drivers of short-term decomposition than is photopriming under the experimental conditions.  High nitrogen concentration and precipitation appear to positively influence biotic decomposition in early stages of decomposition after soil mixing.  However, it is possible that enhanced rates of decomposition from UV photopriming may occur in longer term decomposition in breaking down complex compounds.