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DIVISION S-10-WETLAND SOILS

The Influence of Organic Carbon on Nitrogen Transformations in Five Wetland Soils

Dep. of Limnology, Ecology Building, Lund Univ., S-223 62 Lund, Sweden

torbjorn.davidsson{at}limnol.lu.se

	ABSTRACT

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Today we see an increased use of wetlands for N removal in agricultural catchments. Since the most important process for nitrate (NO^-₃) removal, denitrification, requires organic C, different soils could be expected to be differently suited for wetland construction. In this study, we evaluate the importance of soil organic C and the effects of added dissolved organic C on N transformations in existing and proposed wetlands. We used ¹⁵N-labeled NO^-₃ to study N transformations in soil columns from five locations (a forest peaty soil, a field peaty soil, a silt loam, a loam, and a sandy loam). All five soils removed NO^-₃ at substantial rates (13–73% of the load). The field peaty soil had highest denitrification rate (11 mmol m^-2 d^-1), while sandy loam soil had the lowest rate (2 mmol m^-2 d^-1). Dissolved organic C did not seem to limit N removal in the soils, as glucose additions affected N turnover only slightly. The forest peat soil differed from the others by exhibiting low nitrification, and relatively high production of nitrite (NO^-₂), probably a result of low pH. Nitrate removal in the field peat soil and the sandy loam soil was counteracted by production of ammonium (NH⁺₄) and dissolved organic N, causing net N release. Although there was a positive relationship between soil organic matter and NO^-₃ consumption, we conclude that all soils were suited for N removal. The lack of response to glucose additions indicate that there was no short-term lack of electron donor in any of the soils, including the sandy loam soil.

Abbreviations: Dn, coupled nitrification–denitrification • DNRA, dissimilatory nitrate reduction to ammonium • DON, dissolved organic nitrogen • Dtot, total denitrification • Dw, denitrification of infiltrated nitrate • Tot-N, total dissolved nitrogen

	INTRODUCTION

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CONSTRUCTION OF WETLANDS has been initiated in southern Sweden to reduce N transport in agricultural streams (Granéli et al., 1990; Jansson et al., 1994; Leonardson et al., 1994). Wetlands have proven to be effective in transforming and trapping dissolved and particulate N derived from agricultural fields, thereby protecting nearby aquatic environments from these contaminants. This occurs by (i) biological transformation of NO^-₃ to nitrogen (N₂) gas (denitrification), (ii) plant and microbial uptake of inorganic N, and (iii) sedimentation and filtration of particle-bound N. Denitrification is considered the most important process since it removes N on a long-term basis. On the other hand, plant uptake, sedimentation, and filtration remove N temporarily, since mineralization and resuspension of these materials may contribute relatively mobile dissolved nutrients to the water column, hence, perpetuating the eutrophication process. In the latter cases, plant biomass or accumulated sediment must be removed to achieve a permanent N removal effect.

Previous studies have shown that subsurface-flow wetlands have high N turnover potential including both release and removal processes (Leonardson et al., 1994; Pavel et al., 1996; Hill, 1996; Verchot et al., 1997a and 1997b; Davidsson et al., 1997; Davidsson, 1997). Recent results have shown that it is difficult to make predictions of denitrification and N removal based on organic matter and N content in wetland soils (Stepanauskas et al., 1996; Davidsson et al., 1997; Davidsson, 1997). Factors regulating the removal (denitrification, uptake, and sorption), and release (mineralization and desorption) of N respond differently to a variety of soil parameters. Soil chemistry, soil physics, soil structure, soil texture, vegetation, etc., are important features that may influence N turnover in wetlands to varying degrees. Presently, tools are needed for predicting N removal prior to the construction of new wetlands, in order to make an optimal choice of location and estimate the outcome of invested effort and money.

The objectives of this study were to investigate to what extent soil organic C regulates NO^-₃ removal and related microbial N transformations, and if additions of easily available organic C (glucose) induce higher N turnover rates. We use a laboratory set-up, employing recently developed ¹⁵N-isotope techniques to investigate N turnover in five soils from existing or former wetlands. The soils differ in texture, organic matter and N content, pH and land use. We hypothesize that organic soils have higher denitrification rates than soils with high mineral content, and that additions of glucose will increase NO^-₃ removal and N turnover rates to a greater extent in mineral than in organic soils.

	Materials

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Study Sites
Soils from five different wetlands in southern Sweden were chosen for the study (Table 1) . All of the wetlands have, or have had, a hydraulic regime where water infiltrates through the soil. Four of the wetlands have been flooded as a management practice, whereas one wetland is natural and floods during heavy rains.

Isgrannatorp: Field peat soil
This grazed field is situated in the northeastern part of the county of Scania. It is a recently constructed watermeadow, where water from an adjacent stream is applied with a pump and perforated hoses (Leonardson et al., 1994). The soil consists of marsh peat with high organic content and high hydraulic conductivity. Reed canarygrass (Phalaris arundinacea L.), tufted hair-grass [Deschampsia cespitosa (L.) P. Beauv.], and reed sweet-grass [Glyceria maxima (Hartm.) Holmb.] make up the predominant vegetation.

Lillehem: Silt loam
This field is located in the eastern part of the county of Scania. The wetland was previously used as a watermeadow, where water was diverted from a stream, and flooded the field at high water flow. The field is grazed and has never been fertilized. The vegetation consists of species indicating long periods with no fertilization.

Trobro: Loam
The wetland is situated in the northeastern part of the county of Scania. This is a newly constructed wetland on a formerly fertilized agricultural field. The field is flooded by pumping nutrient-rich water from a nearby stream. The field is planted with reed canary-grass (Phalaris arundinacea L.)

Vomb: Sandy loam
This grazed field is located in South Scania. It is an old watermeadow used for hay production (Leonardson et al., 1994). The soil is sandy with an organic top layer and the present vegetation consists of common bent (Agrostis capillaris L.), yarrow (Achillea millefolium L.), and tufted hair-grass [Deschampsia cespitosa (L.) P. Beauv.] The field is flooded with water diverted from the adjacent River Klingavälsn.

	Methods

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Soil was collected from the wetlands during the winter period in 1995–1996. At each site, several soil samples were collected from depths between 5 and 20 cm. The soil was thoroughly mixed, sieved (2-mm mesh size), and stored in sealed plastic bags at +4°C. For each soil type, three 90-mm-long replicate cores were prepared by transferring soil to Plexiglas tubes (diameter 70 mm, length 200 mm; Fig. 1) . The columns were sealed with gas-tight caps, and a constant infiltration water flow rate of 25 mL h^-1 was created by a peristaltic pump, using aerated artificial lake water containing 200 µM K¹⁵NO₃ (Lehman, 1980). The infiltration rate and NO^-₃ concentration were chosen to mimic conditions in previous field experiments conducted at the sites where the Isgrannatorp peat and the Vomb sandy loam soil were collected (Davidsson and Leonardson, 1998). The cores had an inflow connection at the top made of thin silicone tubing, and an outflow at the bottom consisting of chloroprene rubber tubing with low gas conductivity. At the lower end of the plexiglass tubes rough fiber filters and 100-µm mesh nets prevented any outflow of particulate matter (Fig. 1). Outflow water samples were taken from the chloroprene tubes with a gas-tight syringe.

View larger version (32K): [in this window] [in a new window]   Fig. 1 Design of the soil–water flow-through system

Chemical Analyses
Water samples for chemical analyses were collected in plastic vials and frozen immediately after sampling, except those used for analyses of nitrous oxide (N₂O, see below). Inflow and outflow water were analyzed for N fractions, NO^-₃, and NH⁺₄. Nitrate + NO^-₂, NO^-₂, NH⁺₄ and total dissolved N (Tot-N) were analyzed using standard methods (Wood et al., 1967; Swedish Commission for Standardization, 1995; Chaney and Marbach, 1962; Koroleff, 1976). Dissolved organic N (DON) was estimated as the difference between total dissolved N and the inorganic fractions of NO^-₃ + NO^-₂ and NH⁺₄. In order to analyze N₂O, 6 mL of sample water were injected into pre-evacuated 12-mL glass tubes with rubber membranes (Exetainer Labco High Wycombe, UK). After vigorously shaking the Exetainers for 1 min, a 1 mL head space sample was injected into a gas chromatograph equipped with an ECD detector. A Bunsen solubility coefficient of 0.66 (21°C) was used to account for N₂O dissolved in the water phase (Weiss and Price, 1980).

Light absorbance at 430 nm was measured on a Beckman DU 650 spectrophotometer, and used as an indicator of the total amount of humic substance (Gjessing, 1976). Absorbance at 665 and 750 was used as an indicator of particulate matter in sample water. As the absorbance of outflow water was very low, no sample filtration was performed before chemical analyses. Total dissolved N values were therefore considered to represent dissolved N fractions (cf. Stepanauskas et al., 1996).

Nitrate + NO^-₂, NO^-₂, NH⁺₄, denitrification of infiltrated NO^-₃ (Dw), coupled nitrification–denitrification (Dn), isotopic composition in NO^-₃, light absorbance and N₂O were analyzed for all 5 d. Total dissolved N and isotopic composition in NH⁺₄ were analyzed for samples collected on Days 3 and 5 for the unamended and glucose-amended treatments, respectively.

Nitrogen-15 Analyses
Exetainers (12 mL) were filled with effluent water collected in the sampling syringes, and ZnCl₂ was added to a final concentration of 50 mg L^-1, after which the Exetainers were stored at 4°C. Before analysis, 2 mL of sample water was replaced with helium, and the vials were shaken for 5 min. One sample (50 µL) of the headspace gas was injected into a Hewlett Packard MS Engine quadrupole masspectrometer to measure the production of ²⁹N₂ and ³⁰N₂ (Nielsen, 1992; Davidsson et al., 1997). The isotopic composition of NO^-₃ + NO^-₂ was determined after conversion to N₂ by a denitrifying bacterial culture, NCIMP 1967, using an assay described by Risgaard-Petersen et al. (1993) and Davidsson et al. (1997). The N₂ gas produced was analyzed by mass spectrometry as described above, and the ¹⁵N fraction in NO^-₃ + NO^-₂ was calculated according to Risgaard-Petersen et al. (1993). The isotopic composition of NH⁺₄ was determined according to Risgaard-Petersen and Rysgaard (1995).

Calculations
Rates of denitrification based on Dw and Dn were calculated according to Nielsen (1992) and compensated for N solubility in water (Weiss, 1970): , , where ²⁹N₂ and ³⁰N₂ are the net fluxes of these N₂ gas isotopes. Total denitrification (Dtot) rates were estimated as the sum of Dw and Dn. Net fluxes of different N forms (F) were calculated from the equation: , where Ce and Ci are effluent and influent concentrations, V is the water flow rate and A is the soil surface area in the column. Rates of the efflux of unlabelled NO^-₃ (R) were estimated from the isotope dilution of the labeled NO^-₃ as described by Nishio et al. (1983) and Rysgaard et al. (1993): , where V and A are as above, Ci is NO^-₃ inflow concentration, i and e are the ¹⁵N fractions of inflow and outflow NO^-₃, and 0.00366 is the ¹⁵N background fraction of the nitrified NH⁺₄. The rate of nitrification was calculated by adding the efflux of unlabelled NO^-₃ (R) and the coupled nitrification–denitrification. Dissimilatory NO^-₃ reduction to NH⁺₄ (DNRA) was calculated using equations described by Rysgaard et al. (1993): , where V and A are as above Ce and Ci are concentrations of NH⁺₄ in effluent and influent, e is the ¹⁵N-labeled fraction of NH⁺₄ in effluent, 0.00366 is the labeled fraction of soil NH⁺₄, and n is the labeled fraction of the NO^-₃ that is reduced to NH⁺₄.

Statistical Calculations
Prior to statistical analyses, data differing from normality were subjected to appropriate transformations (Sokal and Rohlf, 1994). Change of the parameters during sampling was analyzed by a paired t test between the first and third day of sampling of the unamended treatment and between the first and second day for the C-amended treatment. When data was available for all three unamended and the two C-amended sampling dates, some statistical differences were recorded (Table 2) . Due to the lack of steady state in N dynamics on the first and second day after glucose addition, each of these recordings was tested against the average unamended rate using a paired t test. The differences between soil types were tested using one-way ANOVA and Tukey's post hoc test (Zar, 1984). Correlations were made between soil chemistry data and the average N turnover rate for the unamended treatment.

	Results

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View larger version (31K): [in this window] [in a new window]   Fig. 2 Nitrogen transformation rates in five soils. Rates are expressed in mmol m-2 d-1, except humic content, which is measured as absorbance at 430 nm. Different letters denote significant differences between soils (P < 0.05)

View larger version (53K): [in this window] [in a new window]   Fig. 3 Nitrogen dynamics in the soils before (open bars) and after 1 d (shaded bars) and 2 d (black bars) of glucose additions. Dw denotes denitrification of infiltrated NO-3, and Dn the coupled nitrification–denitrification. Rates are expressed in mmol m-2 d-1, except humic content which is measured as absorbance at 430 nm. Asterisks indicate 5% (*), 1% (**) and 0.1% (***) levels of significance between the different glucose–time treatments

	Discussion

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In this reaction, 1 mol of C in organic matter plus 0.8 mol of NO^-₃, produces 1 mol of CO₂ and 0.15 mol of NH⁺₄. The Redfield C/N ratio of 6.6:1 implies that mineralization using NO^-₃ as electron acceptor would cause NH⁺₄ release corresponding to about 20% of the reduced NO^-₃. A higher C/N ratio would result in lower figures for N mineralization. Assuming that half of the organic matter in soil consist of C, the C/N ratio in the soils of this study should amount to between 10 and 16, which would give a release of NH⁺₄ corresponding to between 8 and 12% of the denitrified NO^-₃. This indicates that at least in the Isgrannatorp peat soil, some other NH⁺₄ producing process is occurring. One possibility is that NH⁺₄ bound to soil particles is exchanged with potassium ions in infiltrating water. The soil chemistry parameters could not explain the variation in released N (NH⁺₄ production, DON production, absorbance at 430 nm as a measure of humic content, Table 3). The silt loam and loam soils showed net N consumption, as a result of fairly high NO^-₃ consumption in relation to moderate NH⁺₄ and DON production. Studies on NO^-₃ removal in wetlands are frequently reported in literature, but release processes are usually not discussed. Production of NH⁺₄ is rarely found in surface-flow wetlands, likely due to oxidation of mineralized NH⁺₄ at the soil–sediment water interface. Peterjohn and Correll (1984) showed that both NH⁺₄ and DON increased in the subsurface flow during passage through a riparian forest. In surface flow, these N fractions decreased. Export of NH⁺₄ and DON from other ecosystems has also been reported (Valiela et al., 1978: salt marsh, Hedin et al., 1995: forest, and D'Angelo and Reddy, 1994a and 1994b: wetland, Martin et al., 1997: agricultural soil).

Nitrification was positively, and NO^-₂ production negatively, correlated to soil pH (Table 3). It is reasonable to suspect that the low nitrification rates at Amböke were caused by low pH, a relation that has been reported in previous studies (Sharma and Ahlert, 1977). Moreover, the production of NO^-₂ in this soil indicates that the denitrification or nitrification processes were incomplete. Intermediates in the denitrification–nitrification processes, for example NO^-₂ and N₂O, have been shown to be sensitive to pH (Firestone et al., 1980), as well as to changes in soil redox conditions (Bouwman, 1990; Bandibas et al., 1994). In field conditions, wetlands probably produce small amounts of NO^-₂ and N₂O compared to drained and fertilized fields (cf. Blicher-Mathiesen and Hoffman, 1999). This is attributed to reduced conditions and low NO^-₃ concentrations. Moreover, it has been demonstrated that waterlogged soils may act as sinks for N₂O (Blackmer and Bremner, 1976). Net production of intermediates at hot spots or in soil layers may not result in their release from the wetland, since the compounds can be consumed in adjacent zones or layers (Davidsson et al., 1997). On the other hand, disturbed systems (for example, intermittently flooded and dry wetlands or wetlands receiving NO^-₃ concentrations that greatly exceed the denitrification potential) can very well act as sources for NO₂.

The DNRA seemed to be an insignificant process in these soils, and there are reasons to question whether the ¹⁵N labeling of NH⁺₄ is a result of dissimilatory NO^-₃ reduction or an assimilatory reduction, followed by NH⁺₄ release (cf. Davidsson et al., 1997). The correlation with soil organic matter might indicate that DNRA is favored in organic soils. It has previously been reported that DNRA increases in strongly reduced environments (Buresh and Patrick, 1978), but the soils in this investigation probably had overly high redox potential, indicated by the presence of NO^-₃ in outlet water. Highly reduced sediments have been shown to have relatively high rates of DNRA compared to denitrification (Risgaard-Petersen et al., 1996). We have not found any studies that report high DNRA rates in wetlands.

	Conclusions

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Although there was a relationship between soil organic matter and NO^-₃ consumption, all soils were suited for N removal. High NO^-₃ removal rates can be counteracted by dissolved organic N and NH⁺₄ release, at least on a short-term basis. In a longer perspective NO^-₃ consumption will dominate. The lack of response to glucose additions indicate that there was no short-term lack of electron donor even in the sandy loam soil of Vomb. Long-term N removal, however, must rely on production of organic C within the wetland, or a substantial input of allochthonous organic material.Blicher-Mattiesen Hoffman 1999

	ACKNOWLEDGMENTS

 
This project was financed by the Swedish Environmental Protection Agency and by the EU TMR network project ERBFMRX-CT960051 "Wetland Ecology and Technology". Scandinaviska Enskilda Banken's foundation for economic and technical research financed the purchase of the mass spectrometer. Göran Bengtsson is greatly acknowledged for consultations concerning mass spectrometry and isotope techniques. Birgitta Devlin corrected the language, and Stefan Kokalj provided excellent technical expertise.

Received for publication January 4, 1999.

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TOP ABSTRACT INTRODUCTION Materials Methods Results Discussion Conclusions REFERENCES

 

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