No UV enhancement of litter decomposition observed on dry samples under controlled laboratory conditions

Soil Biology & Biochemistry 43 (2011) 1300e1307

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No UV enhancement of litter decomposition observed on dry samples under controlled laboratory conditions Miko U.F. Kirschbaum a, *, Suzanne M. Lambie a, Hui Zhou b a b

Landcare Research, Private Bag 11052, Palmerston North 4442, New Zealand State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 December 2010 Received in revised form 28 February 2011 Accepted 2 March 2011

In field studies, various workers have observed a stimulation of organic matter breakdown by visible light and UV radiation. We aimed to confirm the involvement of UV radiation under controlled laboratory conditions and quantify the magnitude of any stimulation. Grass and pine foliage samples were ovendried and continuously exposed to UV radiation at room temperature for up to 60 days. A range of UV flux densities was established using shading to different levels. After UV exposure under air-dry conditions, samples were rewetted and incubated in the dark with microbial inoculums to investigate whether UV exposure had rendered samples more susceptible to subsequent microbial decomposition. However, we found no weight loss associated with different UV flux densities. The same finding held true for grass and pine litter samples. Similarly, microbial decomposition of either grass or pine litter was not enhanced by prior UV exposure. These findings suggest that UV-induced photooxidation of dry materials cannot be responsible for the observed apparent enhancement of weight loss of litter samples under UV exposure in the field. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Bleaching Decomposition Environmental drivers Microbial degradation Photooxidation UV radiation

1. Introduction It has long been known that microbial decomposition of organic matter responds strongly to temperature (e.g. Kirschbaum, 2000, 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.

* Corresponding author. Tel.: þ64 6 353 4902; fax: þ+64 6 353 4801. E-mail address: [email protected] (M.U.F. Kirschbaum). 0038-0717/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2011.03.001

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 microorganisms. 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

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2. 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 assess the extent of any microbial facilitation by prior UV exposure. 2.1. Litter UV exposure A metal frame was erected over a metal bench top to house 6 fluorescent UV lamps (Phillips TL 40W/12RS). The bench top was covered with black cloth to stop back-reflection of the UV radiation and ensure that samples received radiation only from the UV radiation sources. Radiation received by our samples was measured with a UV-B Biometer Model 501 Radiometer (Solar Light Company, Pennsylvania, USA). The biometer was calibrated, and the spectral output of the UV lamps was measured with a Bentham spectroradiometer with DM150BC double monochromator, cosine diffuser and end window PMT detector (Bentham, Reading, UK). Unshaded samples in our experiment received irradiance comparable to that received at noon in summer under typical New Zealand conditions, especially in the biologically more active lower wavelength range below about 310 nm (Fig. 1). The flux density of solar irradiance, on the other hand, was higher at wavelengths greater than 310 nm. Integrating over the whole UV-B range, solar noon irradiance in New Zealand is about 2.0 W m2 for wavelengths from 290 to 315 nm and 3.7 W m2 from 290 to 320 nm (McKenzie

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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 photooxidative 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.

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Wavelength (nm) Fig. 1. Irradiance received by unshaded samples in the experiment compared to irradiance received at noon on a typical New Zealand summer day. Data are shown on either a linear (a) or logarithmic (b) scale. The shaded area shows the wavelength range usually designated as UV-B. Solar spectrum after McKenzie et al. (2009).

et al., 2009). This compares with irradiance received by our samples of 1.9 W m2 up to 315 and 2.2 W m2 up to 320 nm. The UV lamps also emitted about 0.27 W m2 in the UV-C range below 290 nm (calculated from the data shown in Fig. 1). As samples were exposed to that radiation continuously for 60 days, the received UV radiation load was therefore comparable to that received by litter under field conditions over a whole summer. In addition, our experimental lamps produced a much greater proportion of shorter-wavelength radiation than solar radiation so that the radiation under our experimental conditions was likely to promote litter breakdown even more than natural sunlight. The top of the lighting frame was covered with fine-meshed material to prevent dust collecting on top of the litter samples, which could have affected their weight during the experiment. The sides of the lighting frame were covered with reflective aluminium foil to backscatter radiation from the walls and create a more even radiation field for our samples. It also prevented UV light from escaping the UV exposure area as a safety precaution for staff working in the area. Fresh pine needles were collected from 20-year-old P. radiata trees (Old West Road, Palmerston North, New Zealand; Roger Parfitt, pers. comm.) and oven-dried at 80  C. Basal sheaths were removed from each fascicle and discarded, and needles were cut into approximately 2 cm lengths. Nui Ryegrass was grown under controlled conditions in a shade house, and irrigated daily. The grass was harvested periodically to maintain a short and vigorous sward. The harvested grass was dried at 80  C and stored at room temperature until the start of the experiment. The grass was sorted to remove dead material and cut into approximately 2 cm lengths. Both pine and ryegrass litter were exposed to six UV radiation levels of 1.4%, 18%, 41%, 60%, 73%, and 100% of incident UV radiation, with 5 replicates each. The level of UV exposure was controlled by a range of metal screens placed over individual litter samples. Frames were constructed of medium density fibre board with metal screens of the appropriate aperture size attached. Screens for the 100% treatment consisted of a frame with no mesh attached.

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Pre-cut pine and grass material was heated to 80  C, weighed after cooling to room temperature and placed into 10  10 cm polystyrene petri dishes (Labserv, BioLab, Auckland, New Zealand). We used approximately 2.5 g of pine litter and 1.0 g of grass litter. These amounts covered the area of the petri dishes with minimal self-shading. Accurate weights of each sample were recorded before the start of the experiment. Direct weight loss during the incubation was assessed by periodic weighing of samples. A complication arose in that litter samples and their trays (weighing about 20 g) were found to rapidly adsorb atmospheric moisture after having been dried in an oven (Fig. 2). To minimise moisture effects, all weight measurements of litter material were made within 5 s of removal from the oven. Despite these precautions, measured weights appeared to change by about 1% between measurement periods in line with changes in atmospheric moisture (see also Dirks et al., 2010). Those differences did not affect the relativities between samples measured on the same day, as all samples were treated in the same way so that relativities between different treatments should reflect true treatment differences, but they added to the residual variability between measurements. The effect of self-shading in a ryegrass litter layer was investigated by using a series of litter weights of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, and 5.0 g, with 5 replicates. The largest amount of litter filled the petri dishes to their rim. 5 g of litter provided an effective screen to material at the bottom of the petri dishes, as evidenced by material at the bottom remaining slightly green even after 60 days of exposure, whereas less shaded material was strongly bleached. Litter samples from both the exposure and self-shading experiments were randomly assigned into 5 blocks of 20 samples each. A separate 6th block consisted of 10 pine and 10 grass samples, of which 2 replicates were removed and weighed every 10 days to follow weight changes over time. These samples showed no consistent weight changes over time and are not further reported in the following. The litter samples were exposed to UV radiation continuously over 60 days. Samples were shifted to a different block every ten days, and the position of each sample was randomised within each block to ensure that each sample (subject to their individual shading treatment) received the same total UV exposure over the length of the experiment. When samples were removed from exposure, their air-dry weight was recorded, they were then dried to 80  C to assess whether their moisture adsorption properties had been affected by UV exposure. Also, their oven-dry weights were recorded to assess whether there was any weight loss due to UV exposure. The percentage of moisture adsorption showed no trends with UV exposure and is not further reported in the following.

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Results reported in the following are based on the weight measurements of oven-dried samples. 2.2. Chemical analysis Samples were chemically analysed using the acid detergent fibreesulphuric acid procedure (Rowland and Roberts, 1994), involving sequential treatment of the sample with different reagents to destroy various fractions, followed by gravimetric determination of the residues. A sub-sample of 0.5 g of the exposed litter samples was used and ground to a fine powder. In a first step, protein was hydrolysed using boiling acid detergent (25 g cetyl trimethyl ammonium bromide dissolved in 2.5 L of 0.5 M H2SO4). This first fraction was designated as labile material. The fraction remaining was comprised of cellulose, lignin and ash. It was then treated with 72% sulphuric acid to remove cellulose, leaving a fraction comprising lignin and ash. The residue was ignited at 550  C to combust all remaining organic matter, leaving the inorganic ash fraction. The amount lost on combustion was defined as acidedetergent lignin. For the present work, we used the initial decanted fraction (after the initial acid detergent treatment) as a measure of the labile fraction and the lignin fraction as a measure of recalcitrant material. 2.3. Litter incubation Following the UV radiation exposure described above, litter samples were then moistened and incubated in the dark. Subsamples of the UV-exposed pine needle (2.0 g) and grass (0.5 g) material were weighed and placed into 1.8 L glass jars, inoculated with appropriate microbial extracts and incubated at 25  C for 39 days. Microbial extracts were prepared from naturally decomposing materials, decomposing pine needles from the fresh humus layer of a pine stand, and decomposing grass litter from recently cut lawn grass. Extracts were prepared from the media by shaking with distilled water in an orbital shaker (50 rpm) for 30 min at 20  C (Des Ross, pers. comm.). The decomposing grass was shaken at a 4:1 water to grass ratio, and the pine-litter sample was shaken at a 10:1 water to litter ratio both extracts were sieved through a 250 mm sieve to exclude larger litter fragments and undecomposed fresh litter material. The concentrated stock solution was tested at a range of dilutions to ensure that sufficient inoculant was added to allow unrestrained decomposition of the litter but without adding excessive extra carbon from the inoculants. The same CO2 efflux rates were obtained across a wide range of inoculant dilutions (data not shown) so that the mid-range dilutions of 100:1 for pine-litter extracts and 1000:1 for grass-litter extracts could be used with confidence. Between 5 and 7 mL of the microbial inoculant was sprayed onto the litter samples and the jars sealed with lids containing septa for gas sampling, and placed in a constant temperature room in random order. Gas samples were taken on days 3, 5, 7, 10, 12, 14, 17, 21, 28, and 39 of the incubation. A 25 mL gas sample of the headspace of each litter sample was taken through the septum in the lid of the jar. The gas sample was then pushed into an evacuated 12 mL glass vial (Labco Limited, Buckinghamshire, United Kingdom) and the carbon dioxide (CO2) content of the gas sample measured with a gas chromatograph by flame ionisation after conversion of CO2 to methane (Shimadzu 2010, Kyoto, Japan). Following each gas sampling, the incubating jars were left open for 30 min in a fume cupboard to allow oxygen to be replenished and prevent any buildup of other gases. The moisture content of samples was then

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restored to their original weights by adding distilled water. Jars were then resealed and incubated again at 25  C. After the end of the incubation period, the degraded litter samples, and as much as practical of the remaining moisture, were removed from the jars, dried at 80  C, and the dry weight of the remaining organic material recorded. Decomposition activity was assessed by both the cumulative amount of CO2 released and by the amount of organic matter remaining at the end of the incubation. Both those provided consistent results, thus confirming the methodology but providing no additional information. In the following, we therefore report only cumulative gas fluxes. Statistical analyses were undertaken using StudenteNewmane Kuells ANOVA to determine the effects of each treatment on the litter samples. Treatment effects were considered to be significant if p < 0.05.

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Fig. 3. Weight change in samples incubated under UV radiation for 60 days. Data for grass (a) and pine litter (b) exposed to different UV intensities. Data are expressed as a percentage of the UV irradiance received by unshaded samples, which received about 11.6 MJ m2 of UV-B radiation (290e320 nm) over the course of the experiment. Data are means  95% confidence intervals of 5 replicates. There were no significant differences with UV exposure for either pine or grass litter.

been so small as not to be apparent within the variability due to moisture interactions and other random effects. Following UV exposure, a sub-sample of exposed litter samples was used for chemical analysis. Grass litter showed no change in chemical properties with UV exposure. Under all UV exposure levels, about 68e70% of grass litter was classed as labile material (Fig. 5a) and only 2e3% as lignin (Fig. 5c). Pine litter had a lower percentage of only 45e55% labile material (Fig. 5b), which decreases slightly, but not significantly, with increasing UV exposure. Pine litter contained about 25e35% lignin which increased slightly with increasing UV exposure (Fig. 5d). There was no indication that lignin decreased with increasing UV exposure as had been hypothesised if UV exposure had rendered litter samples more decomposable.

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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 (data not shown). There was not much bleaching of pine samples, with little difference in the colour of samples exposed to different intensities of radiation (data not shown). Weight lost showed no significant relationship with UV exposure for either pine or grass litter samples (Fig. 3). Observed weight changes were also quantitatively very small and amounted to only a fraction of a percent over the full exposure period. Reversible weight changes of up to a few percent were also observed when samples were weighed throughout the exposure period (data not shown) and were likely to constituted variable water-vapour adsorption in response to changes in ambient water-vapour levels (see Fig. 2). Samples exposed to different UV exposure were, however, measured on the same days and should have been affected in the same way by atmospheric water vapour. Variable water vapour should not have confounded the responses to UV exposure shown here. In the self-shading treatment (Fig. 4), weight changes also showed no statistical relationship with litter weight, especially no greater weight loss for lower-weight samples. Instead, the least apparent weight loss was observed for the lowest litter weights, and equal greatest weight loss for the greatest and second-lowest litter weights. As greater litter weights would have reduced the average radiation received by samples, it would imply greater weight loss at reduced radiation exposure. Hence, this treatment also gave no indication of weight loss to increase with average UV exposure. It is likely that the observed apparent slight weight loss of samples (Figs. 3 and 4) was due to different moisture adsorption on the pre-incubation day relative to that on measurement days (see Fig. 2). Even though our experimental procedure had been designed to minimise any confounding effect of moisture adsorption, it appears that we were unable to eliminate the problems completely. Hence, the recorded changes of up to 1.5% of initial dry weight probably did not constitute a change in dry weight but a change in adsorbed moisture. If there had been changes in dry weight in response to UV exposure, it should have differentially affected samples exposed to different intensity (Fig. 3) or extent of selfshading (Fig. 4). Hence, it is apparent that UV exposure either did not lead to dry-weight losses at all, or that any changes would have

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The remaining material from the litter samples was inoculated with microbial extracts and incubated in the dark to assess any changes in litter degradability. Grass litter decomposition proceeded rapidly, with peak rates reached after about 6 days (Fig. 6a), when about 25 mg C g C1 d1 of the initially available carbon was respired each day. Rates then declined rapidly, and there was little activity remaining at the end of the 39-day incubation.

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In contrast, it took about 12 days for peak CO2-loss rates to be reached for pine litter, and maximum daily carbon loss reached only about 5 mg C g C1 d1 (Fig. 6b). The subsequent decrease was less sharp for pine than for grass litter, and rates at the end of the incubation were still about 1/3 of peak rates. For grass litter, the cumulative amount of carbon lost to respiration over the incubation period was lowest for litter that had been exposed to the highest UV intensity, and highest for litter exposed to the lowest intensity (Fig. 7a), but the differences was only about 15% (between highest and lowest cumulative CO2 efflux). It was also in the opposite direction from what had been expected. There was no apparent trend with UV exposure in carbon efflux of the pine samples (Fig. 7b). In particular, none of the samples of either grass or pine litter showed increasing carbon loss with increasing UV exposure as had been hypothesised. In the selfshading experiment, there was also no statistical correlation between carbon efflux and the amount of litter weight used during the exposure phase (Fig. 8). We also assessed whether microbial facilitation might have manifested itself by samples reaching peak decomposition rates earlier, but there was no relationship between UV exposure and the period of peak CO2 rates (data not shown). The amount of residue of samples remaining at the end of the incubation was weighed and provided an independent measure of the effect of UV incubation on microbial decomposition, with data being consistent with those obtained from measuring CO2 efflux (data not shown). This provided an independent check of the accuracy of the methodology, but provided no additional information and is therefore not reported here.

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Incubation time (d) Fig. 6. CO2 efflux rates as a function of time for grass (a) and pine (b) litter samples from exposure and self-shading experiments. Data show means  95% confidence intervals of generally 5 samples per treatment, for a total of 70 grass (from exposure and self-shading treatments) and 30 pine samples. Some error margins were so small to be obscured by the size of symbols. A small number of samples were lost during the incubation due to technical problems. Data are expressed in units of carbon loss per unit of original sample mass.

4. 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

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effect of UV exposure under tightly controlled conditions in the laboratory. We used two different litter samples, dried them and exposed them to UV radiation of an intensity (Fig. 1) and for a duration that tried to emulate the conditions that they might experience under typical field conditions over summer. Exposure of our litter samples clearly bleached grass samples in all but the most deeply shaded treatments. However, it led to no direct weight loss. Weight loss did not increase with radiation intensity, whether those different intensities were caused by differential shading through external screens (Fig. 3), or by selfshading in deeper litter samples (Fig. 4). The same conclusion was reached for grass as for pine-litter samples. Hence, despite the high and long exposure level, a direct weight loss due to photooxidation, was not discernable within the measurement error of the experiment. Changes were thus either

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totally absent or much smaller than the changes observed by several authors in field experiments. Under our experimental conditions, weight changes appeared to be dominated by apparent adsorption of atmospheric humidity (Fig. 2). While our experimental approach was designed to minimise any effects of watervapour adsorption, we were not able to completely eliminate it as a confounding factor. However, it would not have affected the relativities between samples exposed to different UV intensities as they were all measured under essentially the same atmospheric conditions. We also investigated whether UV exposure might have merely split large and recalcitrant organic materials into smaller, more labile compounds that would have been easier to decompose by subsequent microbial action (microbial facilitation). Lignin, in particular, is highly resistant to microbial attack but readily absorbs UV radiation (Gould, 1982; Lanzalunga and Bietti, 2000) so that its breakdown into more labile materials, even without associated CO2 loss, seemed possible (Henry et al., 2008). It is well known from wood and paper processing that light exposure can lead to bleaching (Lanzalunga and Bietti, 2000), which was also observed in the present study, but the interaction between bleaching and microbial facilitation is less clear. However, in the present study, we found no decrease in lignin concentrations under UV exposure, nor an increase in the concentration of labile material. If anything, there was an indication that lignin in pine samples had increased with UV exposure. We also observed no UV facilitation of microbial breakdown. CO2 efflux from pine and grass samples exposed to different UV levels through either shading by external screen or through self-shading showed no consistent relationship with prior UV exposure (Figs. 7 and 8). Hence, unlike Foereid et al. (2010), we found no consistent evidence for UV exposure to render litter samples more susceptible to subsequent microbial breakdown. The results reported here appear to be very clear and robust, but are difficult to reconcile with the observations from field studies that have shown enhancement of decomposition rates by direct visible light and UV radiation. It is even more difficult to understand given that under field conditions UV has two potentially competing effects. Direct photooxidation can enhance decomposition, but UV is also harmful to microbial populations and can thus reduce the rate of microbial decomposition. Gallo et al. (2006), for example, simultaneously manipulated the amount of UV radiation (A and B), litter type, and moisture availability. While exposure to UV radiation did not affect overall decomposition rates of the two litter types in their study, UV exposure interacted with moisture availability to affect the quantity and chemistry of dissolved organic matter leached from litter. In addition, Gallo et al. (2006) found litter that received high levels of moisture and was not exposed to UV radiation decomposed at the same rate as litter that received low moisture and was exposed to UV radiation. The authors concluded that decomposition via UV radiation is just as effective as microbial decomposition. In systems that are typically dry and receive high levels of solar radiation, higher UV exposure was generally shown to be associated with slightly higher decomposition rates (Verhoef et al., 2000; Austin and Vivanco, 2006; Gallo et al., 2006; Day et al., 2007). Others found increased UV-B radiation either did not affect decomposition or decreased it because of detrimental effects on the microbial community (Gehrke et al., 1995; Duguay and Klironomos, 2000; Moody et al., 2001; Pancotto et al., 2003). These experiments were generally conducted in high-latitude systems or systems with substantial canopy cover (e.g. forests), both of which normally experience relatively low levels of total solar radiation and that experience conditions that favour microbial decomposition.

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These studies indicate that the role of photooxidation is dependent on the balance between abiotic and biotic drivers in the decomposition process. This balance can shift between arid and more mesic ecosystems, but can also shift in the same ecosystem between wet and dry periods. UV effects on decomposition may be greater in arid and semiarid systems where the faunal component of decomposition is minimized by the harsh abiotic environment, where photooxidation is maximized by lack of canopy interception and where biotic decomposition is limited by water limitation so that abiotic photooxidation can potentially play a bigger role (Kochy and Wilson, 1997; Pancotto et al., 2005; Austin and Vivanco, 2006; Zepp et al., 2007; Brandt et al., 2007; Smith et al., 2010). Henry et al. (2008) found that grass litter exposed on the surface over summer lost approximately twice as much of its lignin fraction as its general mass. This proportionally high lignin loss is consistent with photooxidation being the primary driver of lignin breakdown over summer. The large effect of summer sun exposure on decomposition indicates that lignin degradation can make litter more susceptible to subsequent microbial degradation under wetter conditions. The effects of summer sun exposure on ensuing decomposition were particularly strong for grass leaves. Lignin photooxidation results in the formation of low molecular weight water-soluble degradation products that can be easily washed away (Fiest and Hon, 1984). Given that a large fraction of cellulose can be protected by lignin in lignocellulose complexes (Berg and Staaf, 1980), it follows that photooxidation of lignin can provide decomposers greater access to relatively labile compounds such as cellulose throughout the wet season (Henry et al., 2008). However, these various field observations are also at odds with the results of our experiment. As we conducted our UV exposure on air-dry samples, microbial decomposition should have been insignificant so that the relative importance of non-biotic photooxidation should have been at a maximum. Similarly, while microbial facilitation through the breakdown of lignin or other complex and recalcitrant constituents could potentially have played an important role under our experimental conditions, we did, in fact, not observe any enhancement of microbial decomposition at all. If anything, our chemical analyses indicated (with marginal statistical significance) that lignin levels in pine samples were higher under greater UV exposure. The final step of lignin formation involves the radical-mediated oxidative coupling of pre-cursor molecules (Hatfield and Vermerris, 2001), and radical formation could have possibly been caused by UV. However, it seems unlikely that it should have occurred to a significant extent in dry samples under our experimental conditions, or that there should have been a sizeable amount of pre-cursor molecules to lead to substantial lignin formation. We are thus not able to reconcile the apparent contradiction between our findings and that of other research reported in the literature. It is possible that moisture status could possibly play a role (Dirks et al., 2010). Our samples were kept air-dry throughout the UV exposure, whereas samples under field conditions would be wetted at least occasionally by due or rainfall or high relative humidity could at least allow some moisture adsorption (Dirks et al., 2010). As studies of aquatic ecosystems has shown that UV exposure of materials results in a reduction of the average molecular mass of organic compounds, alteration of the capacity to absorb light both in the ultraviolet and visible spectrum, and the formation of novel photoproducts (Lanzalunga and Bietti, 2000). Photochemical reactions change the quality of dissolved organic matter (DOM) and produce dissolved inorganic carbon and volatile CO2, CO and carbonyl sulphides. Photochemical reactions have been well-observed in aquatic systems (Kieber et al., 1989; Miller and Zepp, 1995; Mopper et al., 1991; Tarr et al., 1995; Mayer et al., 2006, 2009) where it is both possible for photochemical reactions

to interact with chemical reactions (Lanzalunga and Bietti, 2000) and for any reaction products to be dissolved and washed away from the original reaction source. The absence of that interaction between photochemical reactions and an aqueous medium might possibly be the factor that caused effects to be mooted, or absent, under our experimental conditions. The fact that our experimental set-up allowed for no removal of dissolved organic matter could possibly account for the absence of any weight losses whereas they could possibly occur under field condition in even dry environments. At the same time, if greater production of dissolved organic matter had occurred, it should have led to some increase in the proportion of labile material in our chemical analysis, or a stimulation of microbial decomposition, which we also did not observe. We also used live plant material, which we oven-dried before UV exposure, whereas most other work, including previous laboratory incubation (Brandt et al., 2009; Foereid et al., 2010), used naturally senesced material. It may be possible that live plant material contains screening compounds that protects the live plant from UV damage (e.g. Kumari et al., 2009). If those compounds are degraded as part of normal senescence processes, it might render naturally senesced plant material more susceptible to subsequent UV exposure than material taken from live plants. To account for the large difference between some of the observations it would, however, require a substantial removal of these compounds during senescence, but this possibility cannot be discounted without further specific work to compare live and senesced material. We conclude from our observations that under dry conditions, UV radiation does not lead to direct photooxidation of fresh litter samples, or only at such low rates that were no discernable with our experimental approach. Brandt et al. (2009) used a more sensitive methodology to detect changes in carbon loss, and while they did observe CO2 emissions, their measured rates were very small. Foereid et al. (2010) observed no direct weight loss as a result of radiation exposure, but substantial microbial facilitation. The key differences between their work and ours was that they exposed their samples to radiation containing both UV and visible radiation whereas we used only UV radiation, and we used fresh rather than senesced plant material. In our work we found no direct weight loss after UV exposure nor subsequent microbial facilitation. These findings are thus in direct contradiction to a large number of field observations. At this stage, we are not able to reconcile these different findings. The question of the overall importance of photooxidation or microbial facilitation by UV exposure therefore still remains unanswered. We had hoped to be able to quantify the effect of photooxidation as a step towards assessing its global importance. Instead, we found no effect of UV radiation under the conditions that had been designed to allow its direct quantification. This clear conflict between our findings and those of other studies calls for identification of the key processes, or experimental conditions, that are critically important in causing the difference in litter response to UV exposure in field vs laboratory studies. 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 inoculant, Greg Arnold and Guy Forrester for statistical advice, and Adrian Walcroft and Louis Schipper for useful comments and suggestions on the manuscript.

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