Plant Ecology

, Volume 193, Issue 2, pp 185–194

Exposure to solar UV-B radiation accelerates mass and lignin loss of Larrea tridentata litter in the Sonoran Desert

Authors

    • School of Life SciencesArizona State University
  • Elisa T. Zhang
    • School of Life SciencesArizona State University
  • Christopher T. Ruhland
    • TS-242 Trafton Sciences Center, Department of Biological SciencesMinnesota State University
Original Paper

DOI: 10.1007/s11258-006-9257-6

Cite this article as:
Day, T.A., Zhang, E.T. & Ruhland, C.T. Plant Ecol (2007) 193: 185. doi:10.1007/s11258-006-9257-6

Abstract

We assessed whether exposure to solar ultraviolet-B radiation (UV-B) affects the mass loss of Larrea tridentata (creosotebush) litter in the Sonoran Desert of central Arizona. We placed three types of litter (leaves, twigs, or a natural mixture of leaves, twigs, and seeds) in bags constructed of UV-B-transmitting or UV-B-absorbing filter material that allowed either 85% (near-ambient UV-B treatment) or 15% (reduced UV-B treatment) of the biologically effective solar UV-B to reach litter inside the bags. Bags were placed outdoors for 4–5 months during the winter at two sites: a balcony or on the soil surface of the desert. Mass loss of leaf litter was greater under near-ambient UV-B than reduced UV-B at both sites: 21 (near-ambient) vs. 18% (reduced) on the balcony, and 18 vs. 14% at the desert site. Mass loss of twig litter was also greater under near-ambient UV-B at the desert site. Mass loss of the natural mixture of litter was also greater when exposed to near-ambient UV-B on the balcony, and tended to be greater at the desert site. We estimate that about 14–22% of the total mass loss of leaf litter during our 4–5 month experiments was attributable to solar UV-B exposure. Leaf litter exposed to near-ambient UV-B had lower concentrations of lignin, and fats and lipids, and slightly higher concentrations of holocellulose. The greater mass loss of litter under near-ambient UV-B appeared mainly attributable to loss of lignin, although losses of fats and lipids were also appreciable. A primary reason for greater mass loss of litter under solar UV-B appeared to be photodegradation, particularly of lignin.

Keywords

CreosotebushDecompositionDesertPhotodegradationUV

Introduction

Litter chemistry and climatic factors such as precipitation and temperature can exert a strong influence on decomposition rates. However, researchers have been unable to adequately explain or model litter decomposition rates in some ecosystems using these factors. For example, predicting surface litter decomposition rates in North American deserts using these factors has proven difficult and actual losses can greatly exceed estimates (Whitford et al. 1981; Schaefer et al. 1985). Pauli (1964) speculated that high solar irradiance was one reason why soil organic matter content is typically low in deserts, and several authors have suggested that the high levels of solar ultraviolet (UV) radiation found in deserts may be responsible for significant decomposition of litter in these systems (Moorhead and Reynolds 1989; Moorhead and Callaghan 1994; Whitford 2002).

Surprisingly few studies have tested the idea that solar UV exposure can affect litter decomposition rates. MacKay et al. (1986) used nylon netting to reduce solar irradiance in plots containing litterbags in the northern Chihuahuan Desert, and found that mass loss was greater in unshaded plots, even though microarthropods (including detritivores) were more abundant in the shaded plots, suggesting abiotic processes associated with higher solar irradiance led to greater mass loss. However, what wavebands of solar radiation might have been responsible for this, as well as whether temperature differences between treatments were involved, were not assessed. Higher lignin concentrations or lignin:N ratios have often been associated with slower litter decomposition rates, particularly in more mesic systems (Mellilo et al. 1982). However, such relationships are often lacking in desert systems. For example, Schaefer et al. (1985) did not find strong correlations between mass loss rates of litter of Larrea tridentata and several other Chihuanhuan Desert species and litter chemical composition or climatic factors. In fact, the highest correlation they found was a positive relationship between mass loss rate and initial lignin concentration, which seemed counterintuitive based on findings in more mesic systems. Given that lignin absorbs appreciably in the UV waveband and UV is well known to degrade lignin compounds in wood and paper products (Heitner and Scaiano 1993), they suggested that the high UV irradiance in deserts may degrade lignin, such that litter with higher lignin concentrations is prone to greater mass loss.

Further insights into the role of UV radiation in litter decomposition come from research over the past 20 years addressing the impacts of enhanced ultraviolet-B radiation (UV-B, 280–315 nm) in the context of stratospheric ozone depletion. Most of these studies have examined the effect of supplemental UV-B provided by lamps against a background of ambient solar radiation. Results from these studies have been variable. Rozema et al. (1997), studying litter of a dominant grass of dune grasslands of The Netherlands, found that exposure to supplemental UV-B accelerated mass loss rates, which they attributed to photodegradation of lignin. Gehrke et al. (1995), studying Vaccinium litter in a sub-arctic heath system in Sweden, found that exposure to supplemental UV-B had no effect on mass loss rates. In a parallel growth chamber experiment, they found that UV-B exposure reduced microbial respiration, possibly because of fewer fungal decomposers, and reduced lignin concentrations in litter, possibly through photodegradation. Studying oak litter decomposition in the UK, Newsham et al. (1997) found that supplemental UV-B resulted in slightly less initial mass loss (after 11 weeks), which may have been associated with less fungal colonization, and suggested that such direct effects of UV-B on litter decomposition may be negligible because of the low levels of UV-B irradiance found within oak woodlands for much of the year. Examining birch litter collected from the UK and placed under UV-B treatments at four sites across Europe, Moody et al. (2001) found that exposure to elevated UV-B caused a slight reduction in mass loss rates at two of the four sites.

While providing supplemental UV-B is a logical approach when attempting to simulate and test the effects of enhanced UV-B levels associated with ozone depletion, it does not test the question of whether solar UV-B exposure, in and of itself, affects litter decomposition. Another approach in studying UV-B effects involves removing most of the solar UV-B reaching samples through the use of filters that absorb most of the incident solar UV-B. This provides a relatively simple means to test the question of whether ambient solar UV-B exposure influences litter decomposition. Using this approach in Tierra del Fuego, Argentina, Pancotto et al. (2003) placed litterbags of a native perennial herb under these filters and found that mass loss was slower under the higher, near-ambient UV-B levels provided by the UV-B transmitting filters, possibly because the abundance of a fungal colonizer was reduced. In a subsequent study examining barley litter in Tierra del Fuego, Pancotto et al. (2005) found that mass loss tended to be faster under higher, near-ambient UV-B.

In this study, we assessed whether solar UV-B exposure accelerates the mass loss of desert surface litter. The significance of this mechanism has been suggested several times, although we are unaware of any experimental tests of this idea. We choose to test this idea using litter of Larrea tridentata (creosotebush), a dominant shrub of the Sonoran Desert. We suspected that if solar UV-B was effective in accelerating litter decomposition, this effect would be well pronounced in this system, where long-term solar UV fluxes are large and biotic decomposition may be limited much of the year by lack of moisture. We modified past approaches examining UV-B effects on decomposition by constructing litterbags out of UV-B absorbing or transmitting filters, thereby reducing the attenuating effects of the litterbags themselves on UV-B doses received by litter. In order to elucidate possible causes for differences in mass loss rates we also assessed how the chemical composition of litter changed following UV-B treatments.

Materials and methods

Study sites

The experiment was conducted in the Phoenix metropolitan area of central Arizona, USA (33.5° N, 111.8° W). The climate of the region is hot and arid with a mean annual temperature of 22.5°C and total precipitation of 19.5 cm (Cervany 1996). Visible and UV irradiance are relatively high in central Arizona because of its low latitude and the prevalence of clear, dry skies for much of the year. For example, Phoenix receives on average 85% of the annual total possible sunshine (Cervany 1996).

The first study site was an open, unshaded, second-story balcony (3.6 m × 2.0 m) on the southeast corner of a house in Scottsdale, AZ, a suburb of Phoenix, referred to as the ‘balcony’ site. In order to better mimic the natural conditions under which L. tridentata litter decomposes, we repeated the experiment at a second site in native L. tridentata desert at the Desert Botanical Gardens, Phoenix, AZ, 21 km from the first site. We refer to this as the ‘desert’ site. This site is in a native desert conservation area and consists of scattered L. tridentata shrubs (0.05 individuals/m2, 18% foliar cover) with bare ground between shrubs (Day et al. 2002). Our rationale for conducting the first experiment at the balcony site was: (1) This site was easily accessed on a daily basis which allowed us to work out treatment protocols in preliminary trials. An optimal design for litterbags was developed during initial trials here. (2) We were concerned about dust/soil contamination that we suspected could make subsequent chemical analyses of litter difficult. The balcony site provided a cleaner environment for litter where contamination might not be as pronounced as on the desert floor. (3) A comparison of results from the balcony with the desert site would provide an indirect means to assess how important contact with the soil surface (and subsequent inoculation with microbes) is in litter decomposition in our system.

Litter source and preparation

Recently abscised current-year L. tridentata litter was collected in November 2002 from beneath the canopy of several L. tridentata bushes growing in native desert adjacent to Arizona State University’s main campus in Tempe, AZ (within 25 km of both study sites). Rocks and other non-L. tridentata material, as well as large twigs (>2 mm in diameter), were removed through sieving and hand sorting. The litter was further sorted into three litter types: (1) an unaltered mixture of the leaves, twigs, and seeds, (2) twigs (<2 mm diameter) only, and (3) leaves only. Sorted litter was air dried and stored in sealed plastic bags in the laboratory until placement in litterbags. Just prior to placement in litterbags, litter was air dried again for 2 weeks, oven dried at 30°C for 3 days, and then weighed and placed in litterbags.

Litterbags

Litter was placed in 10 × 10 cm bags or envelopes constructed of either 125 μm-thick UVB-transparent film (Aclar Type 22A film, Proplastics, Linden, NJ; transmission >90% through the UV-B and UV-A (315–400 nm) wavebands) or 125 μm-thick UV-B-opaque film (Mylar-type Cadco clear polyester film, Cadillac Plastic and Chemical, Phoenix, AZ; sharp transmission cut-off below 325 nm). The Aclar envelopes represented a ‘near-ambient UV-B’ treatment while the Mylar-type envelopes represented a ‘reduced UV-B’ treatment. To our knowledge, neither of these film materials have the unintended and potentially toxic effect that cellulose acetate film has on some organisms (Krizek and Mirecki 2004). Each envelope was comprised of a top and bottom piece of the appropriate film, with the edges of the envelopes sealed with UV-B transparent tape (Scotch Multitask tape, transmission >90% through the UVB and UVA wavebands, 3M, St. Paul, MN). To allow water and microbes to reach the litter, we drilled 2-mm diameter holes, spaced 1 cm apart, throughout the top and bottom pieces of the film. We measured UV-B irradiance directly under the top film of envelopes under clear skies at midday in January with a UV-scanning spectroradiometer (OL 754, Optronic Laboratories, Orlando, FL). Biologically effective solar UV-B dose (based on the generalized plant damage action spectrum normalized to 300 nm, Caldwell 1971), was 85% of ambient in Aclar envelopes (near-ambient treatment) and 15% of ambient in Mylar-type envelopes (reduced treatment). To prevent smaller pieces of litter from falling through the holes on the bottom side of the envelope, a 10 cm × 10 cm piece of white nylon fabric (athletic jersey mesh) was taped on the inside bottom. We placed 1.80 (±0.05) g of oven dried litter of a particular type in each envelope on top of the nylon fabric.

Experiments

At the balcony site, envelopes were placed in square, black plastic greenhouse trays (42 cm × 43.5 cm × 5.5 cm high sidewalls) having a lattice bottom that allowed water to drain freely. The trays were anchored on top of wooden frames (8.9 cm high), which secured the trays to the balcony floor but allowed water drainage. Clear monofilament fishing line spaced 5 cm apart was threaded across the trays to secure litter envelopes in the bottom of the plastic trays. A total of 60 envelopes were randomly assigned to seven trays on the balcony. This represented a 3 × 2 factorial design consisting of three litter types (full mixture, twigs, or leaves) and two UV-B levels (reduced or near-ambient) for each litter type. There were 10 replicate envelopes of each treatment/litter type combination. Envelopes were placed at this site on 16 September, 2003 and retrieved on 22 February, 2004 after 21 weeks. Average air temperature and total precipitation over this period were 18.9°C and 5.11 cm, respectively, based on National Weather Service measurements at nearby Sky Harbor International Airport.

At the desert site, envelopes were placed directly on the ground in a level interstice between L. tridentata shrubs so as to receive full sun for most of the daytime. The interstice was divided into a grid and envelopes were randomly assigned to grid locations. Envelopes were 8 cm apart, and adjacent shrubs were at least 1.5 m from the edges of the grid. The envelopes at the desert site were modified by adding 2-cm long flaps of Aclar that extended from each corner. Nails (13 cm long) were driven through the flaps into the soil to secure the envelopes to the ground. As in the first experiment, 60 envelopes were used, representing 10 replicates of each treatment/litter type combination. Envelopes were placed in the desert on 22 December, 2003 and retrieved on 16 April, 2004 after 16 weeks. Average air temperature and total precipitation over this period were 17.2°C and 10.44 cm, respectively.

After collection, litter was sieved to remove inorganic material, air dried for 2 weeks, oven dried at 30°C for 3 days and weighed. We measured the total ash content of litter samples by placing subsamples collected before and after field treatments in a muffle furnace at 550°C for 8 h. The ash proportion of samples was used to correct mass values for soil and dust contamination. Decomposition rate was expressed as mass loss and expressed as percent loss over the treatment period.

Chemical composition

We assessed the chemical composition of the leaf litter at the balcony site prior to placing litterbags outdoors and at the end of the treatment period. Specifically, we measured concentrations of holocellulose, lignin, soluble fats and lipids, and total organic C and N. We did not assess the composition of the other litter types from the balcony site. We attempted to assess the composition of litter from the desert site, but suspect that slight contamination with dust prevented us from obtaining reliable results. Hence, only chemical composition of the balcony leaf litter is reported.

We modified the techniques of Allen (1989) for isolation and quantification of fats and lipids, lignin and holocellulose. Dried litter was ground to a fine powder using a ball-mill grinder and weighed with an analytical balance and samples were further divided for analysis of fats and lipids, lignin, holocellulose, total organic C and N, and ash. For fats and lipids, and lignin, litter samples (≈0.75 g) were placed on glass–fiber filter paper and tied into bundles using nylon thread, and extracted in diethyl ether for 6 h using a Soxhlet apparatus. Samples were dried (30°C) overnight, cooled and weighed for fat and lipid content. Soluble carbohydrates were removed by boiling the samples in water for 3 h. Proteins were removed by adding 16.5 ml of 10% H2SO4 and boiling for an additional hour. Samples were allowed to settle and the supernatant was removed. Hydrolysis of lignin was accomplished by adding 12 ml of 72% H2SO4 for 2 h with occasional stirring. Acid strength was reduced to 3% by the addition of water, and the samples were boiled for another 4 h. Samples were first filtered and washed repeatedly with hot water, then dried (30°C) overnight, cooled and weighed on an analytical balance for lignin determination.

For holocellulose, samples (≈0.25 g) were placed in 50-ml Erlenmeyer flasks containing 15 ml of water. Samples were delignified by the addition of 0.15 g of sodium chlorite (NaClO2) and 0.5 ml of 10% acetic acid. Flasks were covered with a glass marble and heated at 75°C for 4 h. Additional NaClO2 and acetic acid were added at hourly intervals during heating. Samples were filtered and washed repeatedly with chilled water, acetone, and diethyl ether. Samples were then dried (30°) overnight, cooled, and weighed on an analytical balance. Concentrations were corrected for total ash content by placing subsamples in a muffle furnace at 550°C for 8 h. Total organic C and N were measured with a dynamic flash combustion analyzer (FlashEA 1112, Thermo Finnigan, Waltham, MA).

Statistical analyses

We examined the effect of UV-B treatment on mass loss and chemical composition of litter using ANOVA, followed by comparison of initial, and reduced and near-ambient treatment means using the least significant difference test. All data were transformed using an arc-sine transformation to meet the assumption of homogeneity of variances.

Results

At both sites, leaf litter exposed to near-ambient solar UV-B had greater mass loss than leaf litter exposed to reduced UV-B (Table 1). For example, leaf litter on the balcony lost 21% of its mass under near-ambient UV-B, compared to 18% under reduced UV-B. Leaf litter in the desert lost 18% of its mass under near-ambient UV-B compared to 14% under reduced UV-B. At the desert site, twig litter exposed to near-ambient UV-B had greater mass loss than twig litter exposed to reduced UV-B. UV-B exposure did not affect twig litter mass loss at the balcony site. At the balcony site, the litter mixture exposed to near-ambient UV-B had greater mass loss, and this litter tended to have greater mass loss under near-ambient UV-B at the desert site (= 0.16, Table 1). At both sites, leaf litter lost more mass than twig litter within a given UV-B treatment (< 0.01).
Table 1

Mass loss of Larrea tridentata litter under reduced or near-ambient solar UV-B at the balcony and desert sites

Site

Litter type

Reduced UV-B

Near-ambient UV-B

P-value

Balcony

Leaves

18.11 ± 0.81

21.09 ± 0.84

0.02

Desert

Leaves

14.11 ± 0.99

18.18 ± 1.74

0.04

Balcony

Twigs

10.73 ± 0.52

10.09 ± 1.01

0.58

Desert

Twigs

10.18 ± 1.18

12.60 ± 0.63

0.05

Balcony

Mixture

17.34 ± 0.82

19.95 ± 1.10

0.04

Desert

Mixture

16.24 ± 1.58

18.56 ± 1.52

0.16

Three types of litter were assessed: leaves only, twigs only, or a full mixture of leaves, twigs, and seeds. Values are means ± SE, expressed as a percent loss of initial mass. P-values refer to the UV-B treatment effect within a site and litter type.

To access whether site had an effect on mass loss, we compared rates of mass loss at the balcony and desert within each litter type and treatment. Rates were used for comparison because the length of treatment at the two sites differed (21 vs. 16 weeks), and were calculated by dividing loss by time (and assuming a linear loss rate). Mass loss rates at the balcony and desert were similar, with the exception of twigs under near-ambient UV-B, which decomposed significantly faster at the desert site (data not shown). Hence, this suggests that litterbag contact with soil at the desert site might have led to faster decomposition in one case. However, in the other five litter type/treatment combinations, rates were similar at the desert and balcony, suggesting that soil contact or continuous inoculation was not critical to decomposition. More research is needed to address the importance of soil microbes on litter decomposition in this system.

We also assessed the chemical composition of the leaf litter at the balcony site. Compared to initial concentrations, the concentration of lignin increased following exposure to reduced UV-B, while concentrations of fats and lipids, as well as holocellulose, declined in both UV-B field treatments (Table 2). The C concentration increased regardless of UV-B treatment such that the C:N ratio also increased. Comparing UV-B treatments, litter exposed to near-ambient UV-B had lower concentrations of lignin (33.05 vs. 34.65% under reduced UV-B) and fats and lipids (1.33 vs. 2.41%), and higher concentrations of holocellulose (49.30 vs. 47.83%), than litter exposed to reduced UV-B (Table 2). Exposure to UV-B had no effect on the concentrations of total organic C, N or C:N ratio.
Table 2

Chemical composition of Larrea tridentata leaf litter prior to treatments (initial), and after exposure to reduced or near-ambient solar UV-B at the balcony site at the end of the treatment period

Parameter

Initial (%)

Reduced UV-B (%)

Near-ambient UV-B (%)

Lignin

32.48 ± 0.47a

34.65 ± 0.54b

33.05 ± 0.51a

Fats and lipids

5.96 ± 0.27a

2.41 ± 0.33b

1.33 ± 0.31c

Holocellulose

51.33 ± 0.57a

47.83 ± 0.37b

49.30 ± 0.53c

TOC

33.28 ± 0.43a

39.87 ± 1.21b

40.09 ± 0.33b

TON

1.85 ± 0.03a

1.86 ± 0.04a

1.88 ± 0.02a

C:N

18.01 ± 0.51a

21.49 ± 0.66b

21.35 ± 0.19b

Values are means ± SE, expressed on a percent dry-mass basis. Means followed by different letters within a row are significantly different (P < 0.05)

To examine what compounds might be responsible for differences in mass loss between treatments, we calculated the mass of these by multiplying the proportion of each compound by the ash-corrected litter mass. Not surprisingly, the amount or mass of all three classes of compounds that we examined was lower at the end of both UV-B treatments, compared to their initial mass (Table 3). On an absolute mass basis, the largest decline over the experiment involved holocellulose, which declined from an initial mass of about 0.71 g to <0.55 g in both treatments. While not as great of a decline on an absolute mass basis, at least one-third of the fats and lipids initially present in litter were gone at the end of each treatment (0.0826 vs. 0.0271 or 0.0143 g). Comparing UV-B treatments, litter exposed to near-ambient UV-B contained less lignin (0.3567 vs. 0.3930 g under reduced UV-B) and fats and lipids (0.0143 vs. 0.0271 g) than litter exposed to reduced UV-B (Table 3). Lignin accounted for the largest difference in mass between treatments, with litter under near-ambient UV-B containing 0.0363 g less lignin than litter under reduced UV-B (Table 3). Litter under near-ambient UV-B also contained appreciably less fats and lipids (0.0128 g lower).
Table 3

Chemical content of Larrea tridentata leaf litter prior to treatment (initial), or after exposure to reduced or near-ambient solar UV-B at the balcony site

Parameter

Initial (g)

Reduced UV-B (g)

Near-ambient UV-B (g)

Difference (g)

Lignin

0.4512 ± 0.0103a

0.3930 ± 0.0066b

0.3567 ± 0.0087c

0.0363

Fats and lipids

0.0826 ± 0.0038a

0.0271 ± 0.0036b

0.0143 ± 0.0023c

0.0128

Holocellulose

0.7103 ± 0.0292a

0.5427 ± 0.0065b

0.5319 ± 0.0106b

0.0108

TOC

0.4615 ± 0.0069a

0.4525 ± 0.0049b

0.4321 ± 0.0045c

0.0204

TON

0.0256 ± 0.0004a

0.0211 ± 0.0005b

0.0203 ± 0.0003b

0.0008

Values are means ± SE, expressed as dry mass (g) in remaining litter. Means followed by different letters within a row are significantly different (P < 0.05). ‘Difference’ refers to the difference in the mass of each compound between the reduced and near-ambient treatments

Discussion

At both of the sites, exposure to solar UV-B accelerated the mass loss of L. tridentata litter. This effect was most consistent and greatest in magnitude in the case of the leaf litter, in which exposure for 4–5 months increased mass loss by 3% on the balcony (from 18% to 21%) and 4% at the desert site (from 14% to 18%; Table 1). The effects on the full litter mixture and on the twig litter were not as pronounced, but were significant or marginally significant in the case of the full mixture at each site, and in the case of the twigs at the desert site. The mass losses we found were generally within the range reported for L. tridentata; Moorhead and Reynolds (1989) found 20% mass loss over 3 months in the Chihuahuan Desert, Santos et al. (1984) found 18% mass loss over 3 months in the Sonoran Desert, and Weatherly et al. (2003) found 11% mass loss over 4 months in the Mohave Desert.

Results from previous field studies examining UV-B effects on litter mass loss have been quite variable and generally subtle. For example, Rozema et al. (1997) found that supplemental UV-B accelerated litter mass loss. In contrast, Gehrke et al. (1995) found supplemental UV-B had no effect, while Newsham et al. (1997) and Moody et al. (2001) found that supplemental UV-B led to slightly slower mass loss. We suspect several reasons as to why UV-B did not accelerate mass loss in some of these studies. One reason may relate to the treatment protocols used: these former studies were designed to examine the effects of supplements of UV-B against a background of solar UV-B, and differences in UV-B doses between treatments were relatively small, compared to those found between UV-B-absorbing and UV-B-transparent filter treatments such as ours. Using the filter approach, solar UV-B exposure in Tierra del Fuego slowed mass loss in one case (Pancotto et al. 2003) but tended to accelerate it in another (Pancotto et al. 2005). Another reason for some of these negative or subtle effects of UV-B on mass loss may have stemmed from use of litterbags which themselves absorbed UV-B, thereby reducing doses to litter and underestimating UV-B effectiveness.

While differences in treatment protocols may explain some of the differences in findings on UV-B effects, we suspect the main reason that we found relatively strong UV-B effects on litter loss relates to the relatively high long-term UV-B doses received at our sites. This stems from the infrequent cloud cover, low latitude and little shading because of sparse vegetative cover, typical of the Sonoran Desert. We also suspect that the effectiveness of UV-B in accelerating mass loss may be even greater at other times of the year when solar irradiance is even higher and skies are clearer. We conducted our experiments over the winter, when solar irradiance is not particularly high and occasional cloud cover further reduces incident UV-B. For example, the monthly average percent of possible sunshine in Phoenix is lowest during the month of January (74%, Cervany 1996), and there were 15 and 20 precipitation events during our balcony and desert experiments, respectively. During late spring and early summer, doses of UV-B would be considerably higher at our sites because of higher solar angles, longer day length and clearer skies. For example, during the month of June, on average Phoenix receives 94% of the total possible sunshine that is available over this month (Cervany 1996).

While high UV-B doses would promote UV-B photodegradation, they could also impact loss rates through biotic effects on decomposers. Exposure to UV-B often reduces the abundance of certain microbes in litter, particularly fungi (Gehrke et al. 1995; Newsham et al. 1997; Moody et al. 1999; Hughes et al. 2003; Pancotto et al. 2003; Johnson 2003), as well as microarthropods such as mites and Collembola (Verhoef et al. 2000; Convey et al. 2002). Hence, while accelerating litter breakdown through photodegradation, UV-B can also slow breakdown by reducing biotic decomposition. This biotic effect of UV-B appeared to be less important in our system, possibly because of water limitations, and was apparently overshadowed by photodegradation. Desert systems are limited by moisture for much of the year, and microbial decomposition may be inherently slow during these periods, favoring UV-B driven photodegradation. In contrast, in more mesic, cloudy systems, the effect of UV-B exposure on limiting microbial decomposer abundance might overshadow photodegradation effects, leading to an overall slowing of litter decomposition.

Differences in the effects of UV-B on decomposition may also be associated with litter characteristics inherent to different species and ecosystems. Larrea tridentata litter decomposes relatively slowly (Weatherly et al. 2003) and appears to be of relatively low quality because of its high content of lignin and other phenolics (Hyder et al. 2005). This low litter quality would suppress decomposition by microbes and further enhance the significance of photodegradation in the breakdown of this litter. As Pancotto et al. (2005) noted, in this way litter quality may modulate decomposition by favoring abiotic (or biotic) processes. Interestingly, the high concentrations of lignin and other phenolics that are presumed to lower the quality of litter for microbial breakdown may also make this litter more susceptible to UV photodegradation since these compounds are also strong UV absorbers and would be vulnerable to UV photodegradation. Hence, the high concentrations of phenolics found in much desert vegetation may further favor the process of UV photodegradation over biotic decomposition in these systems.

The UV-B effects on mass loss we observed likely involved photodegradation of compounds in litter. Since lignin is a complex of monomers of hydroxycinnamic acids, including ferulic acid, which strongly absorb in the UV-B, we would expect lignin to be one of the main litter components that would be prone to UV-B photodegradation. Indeed, we found that UV-B exposure resulted in leaf litter with a lower lignin concentration (Table 2) and content (Table 3), and lignin accounted for the biggest difference in mass between treatments (Table 3). This litter also had a substantially lower concentration and content of fats and lipids. Fats and lipids are not known to be strong absorbers of UV-B. Possibly, this loss may have resulted from cell wall breakdown (e.g., via lignin photodegradation) that released fats and lipids that would then be prone to microbial uptake or leaching. The slightly greater holocellulose concentration associated with greater UV-B exposure (Table 2) appears to be simply an indirect effect of the loss of other compounds such as lignin, fats and lipids, that in turn increased the proportion of remaining compounds such as holocellulose. Hence, while the concentration or proportion of holocellulose in litter under near-ambient UV-B was higher (Table 2), the mass of holocellulose was similar between treatments (Table 3).

We have shown that exposure to near-ambient solar UV-B caused leaf litter to lose 3–4% more of its mass over the 4–5 month period of our experiments. Considering that leaf litter on the balcony under near-ambient UV-B lost 21% of its mass during this period, and that this loss was 3% less under reduced UV-B (i.e., 18%), we estimate that solar UV-B was responsible for about 14% (3 divided by 21) of the total mass loss over the experiment. At the desert site, we estimate that solar UV-B exposure was responsible for about 22% (4/28) of the total mass loss over that experiment. Exposure to UV-B also resulted in litter with lower lignin concentration and content, and we suspect that a main reason for greater mass loss was photodegradation of lignin. We suspect that solar UV-B exposure may be most effective in accelerating litter mass loss in systems that receive high solar irradiance over extended periods, such as deserts, because: (1) High UV-B doses would promote greater photodegradation. (2) Low moisture often limits microbial decomposition in these systems for extended periods, such that microbial decomposition is inherently slow, minimizing the potentially inhibitory effect of UV-B on microbes. (3) Low litter quality associated with the high phenolic concentrations in desert vegetation would discourage microbial breakdown while favoring photodegradation because phenolics are strong UV absorbers and prone to UV photodegradation.

Acknowledgements

We thank Christopher Buyarski for assistance in sample preparation and chemical analyses, and Brock McMillan for assistance and use of the flash combustion analyzer. Gregory Johnson and Robert Coleman provided advice on approaches. We also thank the staff of the Desert Botanical Gardens, Phoenix, AZ for permission to conduct our experiment in their Larrea conservation area. Some of the research was conducted while Elisa T. Zhang was a high school intern in the laboratory of the senior author. Partial support for this research was provided by NSF OPP-0230579.

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© Springer Science+Business Media, Inc. 2007