Bangor, LL57 2UP, UK. (Received 7 April 1993; accepted 28 May 1993). ABSTRACT. The reduction in wetland water table height that could be anticipated from ...
Ecological Engineering, 2 (1993) 367-373 Elsevier Science Publishers B.V., Amsterdam
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Short Communication
Climatic change and the release of immobilized nutrients from Welsh riparian wetland soils C. F r e e m a n a,b, M.A. Lock b and B. Reynolds
a
a Institute of Terrestrial Ecology and b School of Biological Sciences, University of Wales, Bangor, LL57 2UP, UK
(Received 7 April 1993; accepted 28 May 1993)
ABSTRACT The reduction in wetland water table height that could be anticipated from current climate change models was simulated within the laboratory using cores of peat-soil from a riparian wetland. The manipulation increased the rate of release of many solutes including nitrate (1250%), sulphate (116%), dissolved organic carbon (37%), sodium (66%), chloride (65%), iron (168%) and magnesium (16%). Calcium was the only solute to show a lower rate of release followingthe simulation ( - 26%). These changes have major implications for the use of constructed wetlands in ameliorating water quality. The study suggests that without suitable design safeguards, wetlands may only represent a temporary solution to water quality problems. In the future, climatic change could reverse their beneficial effects. INTRODUCTION Changes in land-use practices and the intensification of agriculture have resulted in increased loadings of nutrients into aquatic systems. These nutrients can radically alter the ecology of streams and rivers (Hynes, 1969). However, riparian wetlands have been widely recognised as potentially valuable "filters" of these nutrients (Lowrence et al., 1985; Peterson et al., 1987), and many wetlands have now been created artificially, in order to improve water quality (e.g. W e i d e r et al., 1988; Pride et al., 1990; Mitsch, 1992).
Correspondence to: C. Freeman, Institute of Terrestrial Ecology, University of Wales, Bangor, LL57 2UP, UK. 0925-8574//93 /$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
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Mathematical models suggest that our climate is changing and that this may lead to an increased frequency of summer droughts ( a n d / o r increased evapotranspiration) over North America and western Europe (Manabe and Weatherald, 1986; Mitchell and Warrilow, 1987). These drier conditions have the potential to reduce wetland water table levels. And since high water tables and waterlogged conditions are the feature that distinguishes wetlands from other terrestrial ecosystems, it seems likely that properties unique to wetlands, such as their ability to act as solute sinks, could be impaired. The following study was designed to test the following hypothesis: increased aeration upon lowering the water table of a riparian wetland could increase microbial degradation of the wetland soil matrix, potentially releasing previously immobilized materials, and causing the riparian zone to change from a net sink, to a net source, of solutes. In order to test that hypothesis, we manipulated water table levels within cores of riparian zone soil and investigated the release of previously immobilized materials. MATERIALS AND METHODS Intact cores were collected from a riparian wetland at Cerrig-yr-Wyn, Plynlimon, mid-Wales (UK Nat. Grid Ref. SN 820 866). The site vegetation was dominated by Sphagnum and Juncus species and was underlain by an organic rich flushed-peat soil (pH 4.2-5.1), with a further layer of mineral deposits at the base. The soil was sampled using a length of PVC pipe, subsequently referred to as the central core chamber (Freeman et al., 1993a). The perfusion system was based around the chamber and its intact monolith of the riparian soil. It consisted of a 60-cm length of l l 0 - m m diameter plastic piping, with the base drained by 12.5 mm diameter tubing which rises again, parallel to the core chamber in a 'J-shaped' fitting. The outlet from the 'J' tube can be raised or lowered to manipulate the height of the water table inside the column, thereby simulating the water table reductions anticipated as a consequence of climatic change. In addition, a series of ports connect the 'J' drainage pipe to the central core at 10-cm intervals. These help to simulate natural conditions where waters entering the soil profile can travel both downwards and laterally, at a rate dependent upon the hydraulic conductivity at the various depths. Full details of the apparatus can be found in Freeman et al. (1993a). Ten replicate systems were set up in a constant temperature laboratory (ll°C) with a 12 h light:dark cycle. Deionized water was added to each of the cores using a peristaltic pump at a rate of 3 cm 3 core -1 h-1. Half of the cores were maintained with the water table near the surface (controls) while the water table in the remainder was gradually reduced over a 10-week period to a
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maximum depth of 20 cm. The water table was maintained at that depth for a further 6 weeks and then returned to the surface for the final 2 weeks of the study, to simulate the end of the drought. The hydrochemistry of the water leaving each core was monitored every second week of the simulation. A D I O N E X 2000i ion chromatograph was used to determine chloride, nitrate and sulphate using an AS4A anion column with 1.7 mM NaHCO3//1.8 mM Na2CO 3 eluent at 2 ml min -~ flow rate and 25 mN H2SO 4 regenerant. Sodium, ammonium, potassium, magnesium and calcium were determined using the same system, but with a CS10 column and an eluent of 40 mM HC1/4 mM dl-2,3-diaminopropionic acid (mono hydrochloride), at a flow rate of 1 ml min -1 and 0.1 M tetra butyl ammonium hydroxide regenerant. Iron was determined using atomic absorption spectroscopy using a Perkin Elmer 280 A_AS flame system at 248.3 nm with an air/acetylene mixture, while monomeric aluminium was determined colorimetrically using catechol violet as the chromogenic reagent (Driscoll, 1984). Dissolved organic carbon (DOC) was estimated using a Skalar auto-analyser with UV digestion/colorimetric detection system. The data were analysed statistically by testing for parallelism between the cumulative release of each solute from the control and treated cores. The five replicates per treatment were averaged to yield a single time point and the cumulative data regressed using GENSTAT 5, to test for parallelism by comparison of regression slopes. RESULTS AND DISCUSSION A simulated climatic change scenario (increased drought) induced an increased release of the majority of the solutes measured (Fig. 1). This largely supported our hypothesis that increased degradation of the organic matrix could promote the release of materials that were previously sequestered within the soil matrix, thus reversing the wetland's ability to act as a solute sink. Significant increases (P < 0.001) were observed in the release of sulphate (116%; Fig. la), iron (168%; Fig. lb), nitrate (1250%; Fig. lc), sodium (66%; Fig. ld), chloride (65%; Fig. le), DOC (37%; Fig. lf), and magnesium (16%, Fig. lg). Calcium was the only element to exhibit a significantly lower release following the simulated drought ( - 2 6 % , P < 0.001; Fig. lh). The release of three of the measured solutes, ammonium (Fig. li), potassium (Fig. lj) and aluminium (Fig. lk), was not significantly affected. The different solutes began to respond to the simulated drought at different points within the anaerobic ~ aerobic cycle. Iron (Fig. lb) was affected almost immediately, while sodium (Fig. ld), chloride (Fig. le) and magnesium (Fig. lg) diverged 4 weeks into the drought. However, sulphate (Fig. la), nitrate (Fig. lc), DOC (Fig. lf) and calcium
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Fig. 1. Effects of a simulated drought upon hydrochemical releases from a riparian wetland:
Cumulative release of (a) sulphate, (b) iron, (c) nitrate, (d) sodium, (e) chloride, (f) dissolved organic carbon, (g) magnesium,(h) calcium, (i) ammonium,(j) potassium and (k) aluminium. Solid lines represent controls; broken lines indicate cores subjected to the climate change simulation.
(Fig. lh) did not begin to show maximum deviations until approximately 8 weeks into the study. The 8-week "lag-period" may prove of value; it could give sufficient time for remedial measures to be taken before high nitrate/ sulphate releases threaten fish stocks and water supplies. The observed increased release of nitrate and sulphate supports earlier studies, which have noted that drier conditions could induce both increased mineralization of nitrogenous organic compounds (Williams and Wheatley, 1988) and enhanced aerobic catabolism of organic-sulphur compounds (Brown, 1985, 1986). Furthermore, under the more aerobic (low water table) conditions, sulphide, the major form of reduced sulphur in waterlogged soils, could be oxidized to sulphate (Ponnamperuma, 1972), while the high redox potential would also inhibit denitrification (an anaerobic process). These conditions would allow nitrate and sulphate to accumulate further within the soil, increasing their availability for release into the drainage waters. Previous studies have likewise shown redox/organic-sequestration processes, to regulate the ability of wetlands to act as a sink/source o f iron
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(Henrot and Wieder, 1990). The more-aerated conditions of the simulated drought would be more likely to reduce iron release (through precipitation as iron oxides) rather than increase it. Furthermore, such precipitation mechanisms could also be expected to re-immobilize any iron that had been released through catabolism of the peat matrix. Thus, our observation of increased iron release suggests that iron that had been released from the soil matrix must have avoided precipitation through another mechanism, possibly rapid complexation with dissolved organic carbon (Theis and Singer, 1974). The process of increased iron mobilization also has major implications for the release of several other trace metals. Many metals such as cadmium, copper, zinc and lead are believed to be immobilized within wetlands through adsorption onto precipitated iron (hydr)oxide complexes (Benjamin and Leckie, 1981; Sinicrope et al., 1992). If less iron is maintained in precipitated form, then less trace metals can be immobilized through the (hydr)oxide adsorption process; thus increased trace element releases would be predicted. The increased releases of sodium (66%) and chloride (65%) were unexpected, as these solutes are generally considered relatively unaffected by immobilization processes during their movement through soils (sensu Reynolds and Pomeroy, 1993). In contrast, the enhanced release of DOC supported similar findings from a field-based study of drought conditions in a nearby Welsh riparian wetland (Freeman et al., 1991; Emmett et al., 1993). The additional DOC released may have consisted of soluble low molecular weight moieties that had been released by increased aerobic biodegradative activity within the wetland soil profile. Interestingly though, these findings conflict with a study of a nearby flush-channel mire, where drier conditions were found to reduce the release of DOC (Freeman et al., 1993c). The release of magnesium was far less affected by the simulated drought (16%), while the releases of ammonium, aluminium and potassium showed no significant response to the manipulation. This suggested that the solubilization of those materials proceeds at relatively similar rates under both aerated and waterlogged conditions. This finding was expected for magnesium, aluminium and potassium. The release of ammonium would have been expected to decrease, however, since more aerobic conditions would be expected to enhance ammonium oxidation. This suggests that ammonium oxidation within the wetland may have been limited by factors other than the degree of aeration. The only solute to show decreased release during the drought simulation was calcium. This may be indicative of increased biological uptake. However, it may also have been related to another biologically mediated process; lowering of the water table within wetlands has been shown to
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p r o m o t e a dramatically increased carbon dioxide flux (Moore and Knowles, 1989; F r e e m a n et al., 1993b). Such increases in C O 2 partial pressure within waterlogged soils can lead to precipitation of calcium as C a C O 3 (Ponnamperuma, 1972). This suggests that wetlands subjected to regular drying are likely to be enriched in (retained) calcium, relative to other elements. The results of this study suggest that climate change could cause riparian wetlands to b e c o m e less effective "filters" of many deleterious solutes, and potentially even to b e c o m e significant sources of several environmentally important solutes (e.g. nitrate, sulphate and iron). Furthermore, this suggests that constructed wetlands may only represent a temporary solution for the improvement of water quality, since their beneficial effect could potentially be reversed by climatic change at a later date. It may prove wise for this to be taken into consideration during the design and construction of artificial wetlands. An engineering-based approach could be developed to artificially maintain high riparian zone water table levels, even under drought conditions. Such wetlands could then continue to retain immobilized pollutants, even following a change to a drier climate. ACKNOWLEDGEMENTS W e are grateful to the Natural Environment R e s e a r c h Council, the Royal Society, the University of Wales, and the Welsh Office (UK), for funding this project. W e would also like to thank A. Jones, C. Ashcroft, B. Tilley and E. Evans for help in construction of the perfusion apparatus, Tim Sparks for performing the statistical analysis, the Institute of Hydrology (Plynlimon) for logistical support in the field, and two anonymous referees for their useful comments. REFERENCES Benjamin, M.M. and J.O. Leckie, 1981. Multiple site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci., 79: 209-221. Brown, K.A., 1985. Sulphur distribution and metabolism in waterlogged peat. Soil Biol. Biochem., 17: 39-45. Brown, K.A., 1986. Formation of organic sulphur in anaerobic peat. Soil Biol. Biochem., 18: 131-140. Driscoll, C.T., 1984. A procedure for the fractionation of aqueous aluminium in dilute acidic waters. J Anal. Chem., 16: 267-283. Emmett, B.A., J.A. Hudson, P.A. Coward and B. Reynolds, 1993. The role of a wetland in ameliorating the effects of afforestation management practices on streamwater chemistry, mid Wales. Submitted. Freeman, C., B.A. Emmett and B. Reynolds, 1991. Wetland biogeochemistry and the consequences of global warming. In: ITE Annual Report 1990/91. HMSO, London, pp. 28-30.
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Freeman, C., J. Hawkins, M.A. Lock and B. Reynolds, 1993a. A laboratory perfusion system for the study of biogeochemical responses to wetlands to climatic change. In: B. Gopai, A. Hillbricht-Ilkowska, and R.G. Wetzel (Eds.), Wetlands and Ecotones: Studies on Land-Water Interactions. National Institute of Ecology, New Delhi, pp. 75-83. Freeman, C., M.A. Lock and B. Reynolds, 1993b. Fluxes of carbon dioxide, methane and nitrous oxide from a Welsh peatland following simulation of water table draw-down: Potential feed-back to climatic change. Biogeochemistry, 19: 51-60. Freeman, C., M.A. Lock and B. Reynolds, 1993c. Impacts of climatic change on peatland hydrochemistry; a laboratory based experiment. Chem. Ecol., 8: 49-59. Henrot, J. and R.K. Wieder, 1990. Processes of iron and manganese retention in laboratory peat microcosms subjected to acid mine drainage. J. Environ. Qual., 19: 312-320. Hynes, H.B.N., 1969. The enrichment of streams. In: Eutrophication; Causes, Consequences and Correctives. National Academy of Sciences, Washington, DC, pp. 188-196. Lowrence, R., R. Leonard and J. Sheridan, 1985. Managing riparian ecosystems to control non-point source pollution. J. Soil Water Conserv., 40: 87-91. Manabe, S. and R.T. Weatherald, 1986. Reduction in summer soil wetness induced by an increase in atmospheric carbon dioxide. Science, 232: 626-628. Mitchell, J.F.B. and D.A. Warrilow, 1987. Summer dryness in northern mid-latitudes due to increased carbon dioxide. Nature, 330: 238-240. Mitsch, W.J., 1992. Landscape design and the role of created restored and natural riparian wetlands in controlling non-point source pollution. Ecol. Eng., 1: 27-48. Moore, T.R. and R. Knowles, 1989. The influence of water table levels on methane and carbon dioxide emissions from peatland soils. Can. J. Soil Sci., 69: 33-38. Peterson, R.C., B.L. Madsen, M.A. Wilzbach, C.H.D. Magadza, A. Paarlberg, A. Kullberg and W.K. Cummins, 1987. Stream management: emerging global similarities. Ambio, 16: 166-179. Ponnamperuma, F.M., 1972. The chemistry of submerged soils. Adv. Agron., 24: 29-96. Pride, R.E., J.S. Nohrstedt and L.D. Benefield, 1990. Utilisation of created wetlands to upgrade small municipal wastewater treatment systems. Water Air Soil Pollut., 50: 371-385. Reynolds, B. and A.B. Pomeroy, 1993. Hydrogeochemistry of chloride in an upland catchment in mid-Wales. J. Hydrol., 99: 19-32. Sinicrope, T.L., R. Langis, R.M. Gersberg, M.J. Busnardo and J.B. Zedler, 1992. Metal removal by wetland mesocosms subjected to different hydroperiods. Ecol. Eng., 1: 309-322. Theis, T,L. and P.C. Singer, 1974. Complexation of iron(II) by organic matter and its effect on iron(II) oxygenation. Environ. Sci. Technol., 8: 569-573. Weider, R.K., K.P. Heston and E.M. O'Hara, 1988. Aluminium retention in a man-made Sphagnum wetland. Water Air Soil Pollut., 37: 177-191. Williams, B.K. and R.E. Wheatley, 1988. Nitrogen mineralisation and water-table height in oligotrophic deep peat. Biol. Fert. Soil., 6: 141-147.
