No UV enhancement of litter decomposition observed on dry samples under controlled laboratory conditions
Highlights
► We exposed dry grass and pine litter samples to UV radiation for 60 days. ► We found no direct weight loss under UV exposure. ► Litter samples did not become chemically more labile through UV exposure. ► There was no enhancement of subsequent microbial decomposition. ► Our findings conflict with field observations of UV effect on litter degradation.
Introduction
It has long been known that microbial decomposition of organic matter responds strongly to temperature (e.g. Kirschbaum, 2000, Kirschbaum, 2010) and soil or litter moisture contents (Borken and Matzner, 2009). Global warming is likely to stimulate organic matter decomposition and lead to a loss of soil carbon (e.g. Sitch et al., 2008), and the extent of that stimulation will critically affect the future natural biogenic contribution to net CO2 emissions to the atmosphere (Sitch et al., 2008, Kirschbaum, 2010).
Over recent years, a number of workers have shown, however, that organic matter decomposition can be affected not only by the known biological drivers but can also be enhanced through exposure to visible and UV (UV-A and UV-B) radiation (Moorhead and Reynolds, 1989, Anesio et al., 1999, Schade et al., 1999, Day et al., 2007, Austin and Vivanco, 2006, Rutledge et al., 2010, Brandt et al., 2010). To the extent that decomposition is controlled by abiotic processes such as photooxidation, it will reduce its dependence on biotic drivers. Systems would then be less responsive to changes in the key controllers of microbial decomposition, such as temperature.
Most notably, Austin and Vivanco (2006) found that the stimulation of decomposition by radiation occurred in the absence of microbial activity. Their observed increase of decomposition under radiation exposure must therefore have been due to direct photooxidation rather than through microbial facilitation, which is the breakdown of complex organic compounds into simpler ones that can be degraded more easily by microbial enzymes at some time after UV or light exposure. They observed the strongest decomposition rates when they allowed all wavelengths to reach their samples, including UV and photosynthetically active radiation.
UV-B is believed to be particularly effective at breaking down lignins (Gehrke et al., 1995, Lanzalunga and Bietti, 2000, Henry et al., 2008), which are resistant to breakdown by most micro-organisms. Photochemical degradation of cellulose may also be possible through visible light although photooxidation appears to increase sharply with decreasing wavelength below about 500 nm (Schade et al., 1999, Brandt et al., 2009). We are not aware of any other attempt at generating an action spectrum of litter decomposition effects.
Further compelling evidence for direct photooxidation to play a role in litter weight loss has come from a recent study by Rutledge et al. (2010) who showed that CO2 emissions from peat samples responded almost instantaneously to changes in radiation. Their exposed samples were air-dry during the experiment which effectively eliminated microbial CO2, and there was no CO2 release when samples were darkened. This work indicated not only that radiation played an important role for total CO2 release, but also that the mechanism at least included direct photooxidation rather than relying solely on microbial facilitation.
There are fewer reports of the effect of radiation on decomposition under controlled laboratory conditions. Such studies under controlled conditions are important to not only confirm the apparent observations from the field, but to also better characterise the relevant action spectra, determine dose responses and identify to what extent an overall radiation effect is caused by direct photooxidation or microbial facilitation. Foereid et al. (2010) kept dried litter samples under broad-spectrum radiation sources including UV radiation for up to 289 days, and found no apparent weight loss with time. They did observe, however, that samples exposed to radiation for longer periods showed faster subsequent microbial degradation when samples had been rewetted. They concluded that microbial facilitation rather than direct photo-oxidative mass loss must have been responsible for any weight loss observed in the field.
Brandt et al. (2009) exposed different litter samples to UV radiation in the laboratory and found a clear enhancement of CO2 efflux rates under UV exposure. However, their observed enhancement was very small, amounting to a weight loss of less than 0.5% over 70 days of continuous exposure, and there was no evidence for microbial facilitation by UV exposure. While Brandt et al. (2009) showed that litter degradation can be enhanced by UV exposure, their observed rates were too small to account for the large enhancement of decomposition observed in field experiments.
We conducted a laboratory experiment under controlled conditions to try and further quantify any effect of UV exposure on litter decomposition and separately assess any effects on direct photooxidation and microbial facilitation.
Section snippets
Materials and methods
In our experiment, we investigated the effect of UV radiation on Pinus radiata needles and perennial ryegrass (Lolium perenne cv Nui). First, we investigated the effect of intensity of UV exposure on grass and pine needle degradation by observing any direct weight loss. A range of UV intensities was generated either through a set of wire meshes or by using different amounts of grass litter through self-shading. After the end of UV exposure, samples were moistened and incubated in the dark to
Results
Before UV exposure, over-dried litter material was green. Following UV exposure for 60 days, there was strong bleaching of the grass samples (data not shown). Only the most heavily shaded samples (receiving only 1.4% of incident UV radiation) still retained a slight tinge of green. In the self-shading treatment, it was also apparent that litter at the bottom of the most heavily packed trays had also retained a slight green tinge, whereas blades higher up in the tray were completely bleached
Discussion
Numerous workers have convincingly demonstrated in field experiments that exposure to visible light and UV radiation could enhance litter or organic matter decomposition (Moorhead and Reynolds, 1989, Anesio et al., 1999, Schade et al., 1999, Day et al., 2007, Austin and Vivanco, 2006, Rutledge et al., 2010). We set out to confirm and quantify photooxidation through measuring the effect of UV exposure under tightly controlled conditions in the laboratory. We used two different litter samples,
Acknowledgments
We would like to thank Rainer Hofmann and Richard McKenzie for useful background information about the conduct of UV experiments, Stephen Stilwell for assistance with calibrating our radiation sources and UV sensor and Richard McKenzie for provision of a solar spectrum of typical New Zealand solar radiation. We would also like to thank Adrian Walcroft and Ted Pinkney for technical assistance in the set-up of the experiment, Des Ross for advice on the extraction method for preparing microbial
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