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Aims: Photodegradation acts as a direct contributor to litter decomposition in arid and semi-arid ecosystems. However, its indirect effects are unclear. Does photodegradation condition litter for subsequent microbial decomposition? Methods: We conditioned litter of Bromus diandrus with ambient or reduced ultraviolet (UV) radiation and three periods of exposure (summer, summer-winter, and 1 year) in a California annual grassland. We then investigated how field UV exposure affected subsequent microbial decomposition of litter using a controlled laboratory incubation. Results: Surprisingly, microbial decomposition was decreased by UV radiation when the exposure occurred during summer but was unaffected by UV treatment for exposure longer than summer. Litter lignin concentrations did not explain these results, as they were not affected by UV radiation for any of the exposure periods. However, for the summer period exposure, UV radiation was associated with decreased litter N concentration, which corresponded with lowered subsequent microbial activity. Conclusions: Our results suggest a new mechanism through which photodegradation interacts with litter microbial decomposition: photodegradation may decrease microbial decomposition through inhibition of microbial N immobilization. Our results imply that solar radiation can interact with litter N cycling dynamics to influence litter decomposition processes.

The importance of photodegradation in surface litter decomposition has recently been recognized in arid and semi-arid terrestrial ecosystems, yet its importance in decomposing dense litter and the mechanisms through which it acts remain unclear. We investigated how ultraviolet (UV) radiation exposure and litter position affected decomposition processes in a California annual grassland. In a split-plot design, we exposed Bromus diandrus litter to two levels of UV radiation (UV pass and UV block) at two aboveground locations (at the top, suspended above the litter layer, and at the bottom of the litter layer) for 1 year. We found that UV radiation increased the litter decay constant by 23% at the top location over 1 year, consistent with the occurrence of photodegradation. Surprisingly, UV radiation also increased the litter decay constant by 30% at the bottom location over 1 year. We speculate that photodegradation indirectly increased microbial decomposition through priming effects. Overall, litter in the top location had a 29% higher decay constant than litter in the bottom location. In terms of litter chemistry, exposure to UV radiation increased loss of hemicellulose by 26%, but not loss of lignin. Litter in the bottom location exhibited greater loss of the cell solubles fraction and greater nitrogen immobilization, but lower loss of hemicellulose than litter in the top location. Our results demonstrate that litter position significantly regulates the contribution of photodegradation to overall decomposition, both through direct (top location) and indirect (bottom location) effects. Therefore, better quantification of both direct and indirect effects of photodegradation can greatly improve understanding of biogeochemical cycling in grasslands. © 2013 Springer Science+Business Media New York.

We conducted a 14C pulse-labeling experiment under field conditions to estimate the contribution of recent photosynthates to methane (CH4) emission in arctic wet sedge tundra dominated by Carex aquatilis and Eriophorum angustifolium. The average CH4 emission rate from plant-soil mesocosms in this study was 0.45 g C m-2 d-1. Carbon assimilated by plants via photosynthesis during pulse-labeling turned over rapidly and appeared as emitted CH4 within 24 h. Integration of flux measurements made over a 2-week period shows that the contribution of recent photosynthates to mid-season CH4 emission is relatively low. Less than 1% of the 14C-labeled carbon dioxide taken up through photosynthesis was emitted as 14CH4 during this study. © 2002 Elsevier Science Ltd. All rights reserved.

The role of Na+-K+-ATPase in increasing the aerobic glycolysis in skeletal muscles has been studied. The effects of epinephrine and amylin on the Na+-K+ pump activity has also been examined. Glycolysis was correlated with the Na+-K+-ATPase activity in resting rat digitorum longus and soleus muscles incubated at 30 degrees C. Results indicate that increasing amounts of epinephrine and amylin increased lactate production three to four times but decreased the intracellular Na+-to-K+ ratio. Glycolysis was suppressed by as much as 70% by ouabain in muscles exposed to epinephrine at high concentrations.