- Top of page
- Materials and Methods
Ultraviolet-B (UV-B, 280–320 nm) radiation alters plant carbon (C) allocation in several ways, which could lead to changes in root exudation and thereby in the supply of C substrates to rhizospheric microorganisms. Although the possibility of alterations in the quality and quantity of root exudates has been raised in connection with studies that have reported UV-B-induced changes in soil microbial communities (Avery et al., 2003, 2004; Rinnan et al., 2005) or in methane emission from peatlands (Niemi et al., 2002b), we have no knowledge of the potential effects of increased UV-B radiation on root exudation. This study presents the first attempt to estimate the effects of enhanced UV-B radiation on root exudates in parallel with measurements of biomass allocation to different plant parts.
We focused on mire ecosystems, which cover a considerable area of boreal and arctic regions (Gorham, 1991) where stratospheric ozone depletion has led to increased fluxes of UV-B radiation (WMO, 2003). Northern peat-forming wetlands, i.e. mires, have accumulated 270–370 × 1015 g C in peat since the last glaciation (Turunen et al., 2002) and are significant sources of the important greenhouse gas methane (Cicerone & Oremland, 1988; Bartlett & Harriss, 1993), both of which are a result of restricted oxic decomposition caused by the high ground water table. Peat soils also contribute to the export of dissolved organic carbon (DOC) to freshwater systems and oceans (Urban et al., 1989; Aitkenhead et al., 1999; Freeman et al., 2001).
In mire ecosystems, the major part of the plant biomass is situated below ground (Wallén, 1986; Saarinen, 1996). In addition to living below-ground biomass, photosynthates are directed to mycorrhizal symbionts, root exudation and root respiration. In cereals and pasture plants, about half of the below-ground allocation goes into root biomass, one-third into root exudates and respiration and the remainder into soil organic matter and microorganisms (Kuzyakov & Domanski, 2000). Although root exudates comprise only 0.5–5% of the net fixed C (Farrar et al., 2003), they serve as significant sources of C and energy for soil microorganisms. As an example, C substrates for microbial methane production in the water-logged mires are mainly derived from recent photosynthates (King & Reeburgh, 2002). The soil water concentration of organic acids, which consist of root exudates and products of anaerobic degradation of organic matter, correlates highly with wetland methane emission rates (Christensen et al., 2003).
Increased UV-B radiation induces various adaptive morphological changes in plants. These include leaf thickening and shortening, shifts in root : shoot ratio and alterations in reproductive structures (Barnes et al., 1996; Day et al., 2001; Phoenix et al., 2001). Focus has largely been on the above-ground plant parts, because UV-B does not penetrate into the soil to a significant extent. However, recent observations of UV-B-induced alterations in the soil microbial community composition (Johnson et al., 2002; Avery et al., 2003; Rinnan et al., 2005) stress that plant roots as the interface between the plant and the soil should no longer be ignored.
Our aim was to assess whether increasing solar UV-B radiation changes C partitioning to different plant parts and to root exudation, estimated as DOC and organic acid concentrations in the rhizosphere. We did not attempt to determine whether UV-B stress increases root exudation per unit of root per se, but to estimate the net response in the amount of root exudates in the pore water as a result of either altered exudation, unchanged exudation from a changed amount of roots, or altered microbial consumption of exudates.
As UV-B radiation has been previously observed to reduce the root length growth of sedges on a minerotrophic mire (Zaller et al., 2002), we hypothesized that exposure to enhanced UV-B would reduce allocation to the below-ground biomass and root exudation. The experimental design included two vascular plant species representing different growth forms to enable comparison between the widespread deeply rooting sedge Eriophorum angustifolium Honck. and the oceanic herb Narthecium ossifragum (L.) Huds. (Liliaceae). In order to keep the experimental environment close to natural conditions, the plants were grown in mesocosms with Sphagnum peat containing natural peat pore water and microbial communities. Although this limits the possibilities in terms of determining the causes of changes in the pore water constituents resulting from the presence of both producers and consumers, it enables estimation of the effects of the treatments and the plant species under seminatural conditions.
- Top of page
- Materials and Methods
Exposure of two mire plant species to enhanced UV-B radiation for about 2 months yielded results that suggest that increased UV-B radiation can influence the net efflux of root exudates by altering below-ground biomass. Furthermore, responses appeared to differ between plant species. The weak nature of the responses was not surprising considering the general scarceness of quantitative changes in plant photosynthesis or above-ground biomass under realistically enhanced UV-B radiation in field studies (Searles et al., 2001). In the current experiment, part of the biomass present at the end of the experiment had been produced before the UV-B exposure, so the UV-B effects on the biomass accumulated during the experiment are more difficult to detect. Effects of UV-B on the growth of mire plants have previously been observed to show only nonsignificant tendencies in 1 year, while significant differences have been found with repeated measurements over several years (Robson et al., 2003).
In N. ossifragum, enhanced UV-B radiation had no effects on the green above-ground biomass, although it tended to increase the total C concentration per unit dry weight of the leaves. Higher total C concentration suggests that the UV-B-exposed leaves contained more biomolecules that are rich in C compared with the ambient control. Because leaf chemical analyses other than those of total C, N and P could not be conducted because of low sample mass, we cannot elucidate which compounds could have contributed to this increase. Enhanced UV-B increased biomass allocation to rhizomes and stem bases. The rhizome biomass was negatively correlated with ammonium concentration, which implies higher nutrient acquisition to the storage structures by the increased biomass. This result is in contrast with the reported higher rhizome elongation of the arrow-rush Tetroncium magellanicum under filters reducing ambient UV-B on a southern Argentinean Sphagnum bog (Robson et al., 2003). However, the biomass of the largest root fraction of N. ossifragum, consisting of fine roots (third-node roots), was nearly significantly decreased by enhanced UV-B. Concomitant UV-B-induced reductions in the DOC and organic acid concentrations of the pore water in the Narthecium mesocosms indicate that the lower fine root biomass resulted in less total root exudation. Furthermore, the negative relationship between the pore water organic acid concentration and the total C concentration in N. ossifragum leaves suggests that, when more C was allocated to leaves, less was available for root exudation.
Although the difference was not statistically significant, the sedge E. angustifolium tended to have a higher root : shoot ratio in the UV-B treatment, which suggests slightly increased allocation to below-ground biomass. This is in contrast with the previously observed higher root length production of Carex species under reduced UV-B radiation in southern Argentina (Zaller et al., 2002), although root biomass or root : shoot ratio was not reported in that study. However, more consistent with our results, root : shoot ratio of the Antarctic grass Deschampsia antarctica remained unaffected by UV-B enhancement (van de Staaij et al., 2002). Similar UV-B supplementation as in the present study did not affect the physiology and morphology of Eriophorum vaginatum (Niemi et al., 2002a), although the leaf cross-sectional area was reduced under enhanced UV-B during a growing season of higher UV intensity (Niemi et al., 2002b).
In the current experiment, there was a trend for a higher organic acid concentration in the pore water of the Eriophorum mesocosms at the depth of 15 cm under enhanced UV-B radiation. As the roots of E. angustifolium were abundant at this depth, and as approx. 10% of DOC is derived from recent plant photosynthates in a similar mire type (Olsrud, 2004), the observed increase in organic acid concentration may have resulted from a UV-B-induced increase in photosynthate allocation below ground. The related species Eriophorum scheuchzeri responds to experimental light attenuation by reducing exudation of acetate (Ström et al., 2003), and a similar trend has been observed in E. angustifolium (Joabsson et al., 1999). Saarnio et al. (2004) found predominantly acetate and lactate in percolate samples collected from the rhizosphere of E. vaginatum grown in pots with quartz sand. Citrate, formate and succinate were present in small amounts. A similar composition of organic acids was detected in our samples, which supports the hypothesis that they originated from the plants.
In the control mesocosms without transplanted vascular plants, enhanced UV-B radiation had a tendency to decrease organic acid (but not DOC) concentration in the pore water, although this response was less than in the Narthecium mesocosms and statistically not significant. This alteration was independent of any changes in vascular plant root exudation, and was therefore a result of effects of UV-B radiation on Sphagnum mosses, microbial communities or the peat water.
Input of labile C into soil by plant exudation is usually only discussed in connection with vascular plants, although increased Sphagnum exudation has recently been mentioned as a potential response to CO2 enrichment (Mitchell et al., 2003). Furthermore, Fenner et al. (2004) used 13CO2 tracer to show that, as early as 4 h after labeling, C fixed by Sphagnum mosses comprised 4% of the total DOC in pore water. Hence, we propose that the lower organic acid concentration under enhanced UV-B indicates that UV-B may have decreased exudation of the measured organic acids by Sphagnum. Another possibility is that increased membrane leakage of magnesium and calcium from Sphagnum moss tissue in response to enhanced UV-B radiation (Niemi et al., 2002a,b) would lead to chelation of organic compounds and thereby to a lower concentration in the pore water.
The potential direct effects of UV-B on peat water could only occur in the surface water, with limited influence to a depth of 15 cm via diffusion. Under laboratory conditions, UV-B exposure of Sphagnum bog water has been shown to decrease the DOC concentration and increase the bacterial abundance (De Lange et al., 2003). Similarly, exposure of humic acids to enhanced UV-B has been shown to significantly stimulate microbial utilization of DOC, as shown by increased CH4 production (Bianchi et al., 1996). However, UV photolysis of DOC should lead to a higher concentration of low-molecular-weight organic acids (Wetzel et al., 1995).
The control mesocosms differed from the mesocosms with vascular plants by having slightly less microbial C and higher concentrations of DOC and monocarboxylic acids, especially in the pore water from the 15-cm depth. As the rhizosphere maintains higher microbial biomass and activity than bulk soil, the lack of living vascular plants in the control mesocosms resulted in the lower microbial biomass. This decrease further led to accumulation of organic acids, because microorganisms rapidly use labile C compounds. An earlier study demonstrated that the DOC concentration of pore water was higher in the moss-dominated intertussock tundra than in the E. vaginatum-dominated tussocks or in the wet sedge tundra with Carex species and E. angustifolium (Judd & Kling, 2002). These vegetation effects on DOC concentration are consistent with the effect of vascular plants on the organic acid concentration in the current experiment.
In conclusion, increased UV-B radiation appears to alter the below-ground biomass of the mire plants, which in turn leads to a change in the net efflux of root exudates. The responses are species-dependent, which implies that the feedbacks of altered exudation on ecosystem functioning, such as methane emission or DOC discharge, depend on the plant species composition. Use of natural peat monoliths instead of sterile growth media lowers resolution in detecting differences in root exudation and thus the effects of UV-B radiation may be underestimated in this study.