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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.

Litter chemistry is one of the most studied controls on decomposition in terrestrial ecosystems. Solar radiation has been shown to increase litter decomposition rates in arid ecosystems through the process of photodegradation. However, it remains unclear how photodegradation affects litter chemistry, especially the abundance and composition of lignin, which is thought to play a key role in photodegradation. Using two-dimensional nuclear magnetic resonance (2D NMR) spectroscopic methods, we quantified the molecular-level changes in litter chemistry associated with photodegradation. Litter of Bromus diandrus was exposed in the field to two levels of radiation [with and without ultraviolet (UV) wavelengths] and two durations of exposure (2.5 months during summer, and 1 year). Through fiber analysis by sequential digestion, we found that the litter hemicellulose fraction decreased significantly from 31.6 to 24.9 % after 1 year of decomposition. In litter exposed for 1 year, the hemicellulose fraction was significantly lower in litter with UV exposure compared to litter without UV exposure (23.8 vs. 25.9 %). These results indicate that UV photodegradation has a small but significant effect on litter chemistry compared to other decomposition processes. Even though fiber analysis showed no loss of total lignin, 2D NMR analysis demonstrated that UV exposure reduced the major lignin structural units containing β-aryl ether inter-unit linkages by 9 % and decreased the relative abundance of lignin p-hydroxyphenyl units by 20 %. The 2D NMR analysis also revealed that lignin guaiacyl units were preferentially lost after 1 year of decomposition relative to the reference material, but no effects of UV exposure on guaiacyl units were observed. These results suggest that photodegradation causes partial degradation, not necessarily complete breakdown, of lignin structures. Our data also demonstrate that applications of 2D NMR methods are valuable for acquiring detailed information on lignin and polysaccharide chemistry during both biotic and abiotic 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.

Litter decomposition contributes to one of the largest fluxes of carbon (C) in the terrestrial biosphere and is a primary control on nutrient cycling. The inability of models using climate and litter chemistry to predict decomposition in dry environments has stimulated investigation of non-traditional drivers of decomposition, including photodegradation, the abiotic decomposition of organic matter via exposure to solar radiation. Recent work in this developing field shows that photodegradation may substantially influence terrestrial C fluxes, including abiotic production of carbon dioxide, carbon monoxide and methane, especially in arid and semi-arid regions. Research has also produced contradictory results regarding controls on photodegradation. Here we summarize the state of knowledge about the role of photodegradation in litter decomposition and C cycling and investigate drivers of photodegradation across experiments using a meta-analysis. Overall, increasing litter exposure to solar radiation increased mass loss by 23% with large variation in photodegradation rates among and within ecosystems. This variation was tied to both litter and environmental characteristics. Photodegradation increased with litter C to nitrogen (N) ratio, but not with lignin content, suggesting that we do not yet fully understand the underlying mechanisms. Photodegradation also increased with factors that increased solar radiation exposure (latitude and litter area to mass ratio) and decreased with mean annual precipitation. The impact of photodegradation on C (and potentially N) cycling fundamentally reshapes our thinking of decomposition as a solely biological process and requires that we define the mechanisms driving photodegradation before we can accurately represent photodegradation in global C and N models. © 2012 US Government.

To increase our understanding of carbon (C) cycling and storage in soils, we used 14C to trace C from roots into four soil organic matter (SOM) fractions and the movement of soil microbes in arctic wet sedge and tussock tundra. For both tundra types, the proportion of 14C activity in the soil was 6% of the total 14C-CO2 taken up by plants at each of the four harvests conducted 1, 7, 21, and 68 days after labeling. In tussock tundra, we observed rapid microbial transformation of labile C from root exudates into more stable SOM. In wet sedge tundra, there appears to be delayed or indirect microbial use of root exudates. The net amount of 14C label transfered to SOM by the end of the season in both tundra types was approximately equal to the amount transferred to soils 1 day after labeling, suggesting that transfer of 14C tracer from roots to soils continued through the growing season. Overall, C inputs from living roots contributes 24 g C m-2 yr-1 in tussock tundra and 8.8 g C m-2 yr-1 in wet sedge tundra. These results suggest rapid belowground allocation of C by plants and subsequent incorporation of much of this C into storage in the SOM.

Methane flux and below-ground methane profile studies were conducted in a wet meadow vegetation manipulation site at the Toolik Lake Long-Term Ecological Research (LTER) site during the summers of 1995 and 1996. Control plots, moss-removal plots, and sedge-removal plots were studied to determine the role of these vegetation types in wetland methane emission and to study the gas transport mechanism. Methane emission was greatest from plots with intact sedges. Depth distributions of root density collected in 1995 showed a strong inverse relationship to pore water methane concentration. Results on insertion of arrays of gas-permeable silicone rubber tubing into the soil indicate that they are reasonable analogs for the physical process of gaseous diffusion through plants. The observed differences in flux between plots with and without sedges cannot be fully explained by differences in methane production or dissolved organic carbon concentrations in our measurements. Copyright 1998 by the American Geophysical Union.