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Published in Soil Sci. Soc. Am. J. 68:320-325 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-10—WETLAND SOILS

Denitrification and Organic Carbon Availability in Riparian Wetland Soils and Subsurface Sediments

Alan R. Hill* and Mia Cardaci

Dep. of Geography, York Univ., Toronto, ON, Canada M3J 1P3

* Corresponding author (hill{at}yorku.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Study Area
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The influence of organic C quantity and quality on denitrification in riparian environments is poorly understood. We measured denitrification potential (DNP), organic matter, and several fractions of organic C in surface soils and subsurface sediments in a river riparian zone. Surface soils in conifer forest peat, mixed forest, and marsh sites had similar DNP, although mean organic matter ranged from 9.4% (marsh) to 19.6% (mixed forest) and 36.6% (peat). These soils also differed widely in organic C, water-extractable C, and anaerobic mineralizable C. Mean DNP in peat at depths of 0.8 to 1.4 m was four times lower than in the surface peat. Mean organic matter and organic C were significantly greater in the deep peat than at the surface, whereas the other C fractions were similar. Mean organic matter content of buried channel sediments at depths of 2 to 3 m was 3.6%; however, mean DNP was 75 to 80 times lower than in the surface mixed forest and marsh soils. When the three surface soil sites were considered separately, anaerobic mineralizable C showed the highest correlation with DNP in the marsh soils (r = 0.87) and the conifer peat soil (r = 0.82). Water-extractable C was also highly correlated with DNP in the marsh soils (r = 0.81). Correlations between DNP and either organic matter or the three C fractions were not significant in the deep peat, whereas the former channel sediments showed a significant relationship between DNP and both organic matter (r = 0.81) and water-extractable C (r = 0.81). These results show that C quantity and quality influence DNP, but no single index was a good predictor for all soil types.

Abbreviations: DNP, denitrification potential • WEOC, water-extractable organic C


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Study Area
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
STREAM RIPARIAN ZONES have the potential to regulate energy and material fluxes between terrestrial and aquatic ecosystems (Gregory et al., 1991; Naiman and Decamps, 1997; Cirmo and McDonnell, 1997). The role of riparian zones in the removal of nitrates from subsurface flows contaminated by agriculture and other human activities has received particular attention (Gilliam, 1994; Hill, 1996). Many studies have measured large declines in NO–3 concentrations along shallow ground water flow paths beneath riparian zones (Lowrance et al., 1984; Peterjohn and Correll, 1984; Haycock and Burt, 1993; Hill, 1996). Denitrification, the process by which bacteria reduce NO–3 to N gases in the absence of O2, has been identified as the primary mechanism of NO–3 removal in stream riparian zones (Cooper, 1990; Pinay et al., 1993; Verchot et al., 1997; Martin et al., 1999; Hill et al., 2000).

Organic C is required as an energy source for heterotrophic denitrification (Beauchamp et al., 1989; Groffman, 1994), and various indexes of organic matter availability for denitrification have been studied in agricultural and forest soils (Stanford et al., 1975; Burford and Bremner, 1975; Davidson et al., 1987; Bijay-Singh et al., 1988). However, limited research has been done on the effects of organic matter quantity and quality on denitrification in stream riparian zones. High denitrification activity has been measured in incubations of soil slurries and intact soil incubations in the organic-rich surface soils of riparian areas (Cooper, 1990; Groffman et al., 1991, 1992; Pinay et al., 1993; Schipper et al., 1993). Groffman et al. (1991) and Schnabel et al. (1996) found that denitrification in surface riparian soils was more strongly stimulated by glucose on riparian forest plots in comparison with grass plots, indicating a C limitation, even though the forest soils had higher organic matter content. Groffman et al. (1991) recorded higher C/N ratios in the forest plots (20:1) in comparison with the grass plots (<10:1) and suggested that previous inputs of fertilizers and lime on the grass plots may have increased the quality of the soil organic matter. Schipper et al. (1994) evaluated the decomposition of different plant materials as C sources for denitrification in riparian soils and found that denitrification and CO2 production in water cress and fresh pine needle treatments were up to five times higher than in a treatment with senescent pine needles. These studies suggest that differences in organic matter quality in addition to quantity affect denitrification rates in riparian surface soils.

Several studies have found the potential for denitrification to be low or nonexistent beneath the water table in subsurface riparian sediments (Ambus and Lowrance, 1991; Lowrance, 1992; Groffman et al., 1992). Research has suggested that ground water denitrification is limited by the low organic C contents of subsoils (Lowrance, 1992; McCarty and Bremner, 1992; Starr and Gillham, 1993; Groffman et al., 1996). Recent evidence however indicates important interactions between denitrification and organic C at depth in riparian zones. Gold et al. (1998) and Jacinthe et al. (1998) found that small patches of organic matter representing <2% of the soil mass in the C horizon of poorly drained riparian soils function as "hotspots" of denitrification activity. Hill et al. (2000) measured high rates of denitrification in narrow zones near interfaces between aquifer sands and either peats or buried river channel sediments at depths of several meters in a floodplain. These recent studies underscore a critical need to study the organic C content and composition of subsurface sediments in stream riparian zones in relation to denitrification.

The objectives of this study were (i) to quantify selected forms of organic C that reflect C quality in surface riparian soils and subsurface sediments and (ii) to assess the relationship between potential denitrification rates and organic C quality in these riparian soils and deeper sediments.


   Study Area
TOP
 ABSTRACT
 INTRODUCTION
 Study Area
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The study was conducted on a 100- to 200-m wide floodplain on the north side of the Boyne River, a fifth-order river approximately 70 km north of Toronto, ON (Fig. 1) . Vegetation on the floodplain consists of a conifer forest dominated by northern white cedar (Thuja occidentalis L.) with some tamarack [Larix laricina (Du Roi) K. Koch] on the peat areas where saturated soil conditions persist (Fig. 1). Areas near the river in the upstream portion of the site support a mixed forest. The overstorey consists of patches of white cedar interspersed with deciduous species including box elder (Acer negundo L.), black ash (Fraxinus nigra Marshall), balsam poplar (Populus balsamifera L.), and white birch (Betula papyrifera Marshall). Dense patches of black ash saplings form the understorey in the mixed forest. The dominant vegetation on the areas of marsh is green bulrush (Scirpus atrovirens Willd.) with some cattails (Typha latifolia L.), mint (Mentha spp.), and grasses.



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  		Fig. 1. Boyne River floodplain showing location of piezometer transects and plant communities.

 
The lithology of the floodplain, which has been previously described by Devito et al. (2000) and Hill et al. (2000) consists of peat deposits, which vary in depth from 0.5 m near the valley side slope to 3.0 m at distances of 40 to 100 m from the river (Fig. 2) . Surface peat grades into organic-rich loams that become increasingly sandy and lower in organic matter near the river bank. The riparian peat layer is underlain by fine beige and gray sands containing fine gravel layers, which extend to depths of 4 to 6 m beneath the floodplain.



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  		Fig. 2. Vertical cross-section along (a) Transect A at the upstream site and (b) Transect B at the downstream site showing stratigraphy, piezometer slot zones, and direction of ground water flow. Dashed lines indicate the maximum and minimum locations of the water table in 1996-2000.

 
In a zone extending up to 50 m inland from the river bank thin clay lens are underlain by a 1- to 2-m thick sequence of interbedded curved layers of yellow-brown sand and olive-brown muds. The layers are commonly 0.5- to 2.0-cm thick and include considerable amounts of fine woody debris and larger tree branches and logs near the base. These sediments formed when channel bar deposits were buried during lateral migration of the river channel. Underlying this deposit is a channel lag gravel containing shell fragments mixed with small lens of mud and occasional pieces of wood, which indicates the base of the former channel.

Previous research has shown that ground-water discharges to the floodplain from a 9- to 12-m thick sand plain aquifer underlain by clays (Devito et al., 2000). Although hydraulic heads show some variation over time, the general ground-water flow pattern remained similar in the floodplain throughout the year. Ground water flows in a mainly horizontal direction from the upland perimeter of the floodplain to the river (Fig. 2). Near the river on Transect B, recharge of near-surface water to deeper sediments occurs. Hydraulic gradients show that ground water near the surface on Transect A recharges from the peats to areas of fine sand (Devito et al., 2000). The water table remains near the surface of the peat year round. Seasonal variations in water table position occur near the river with maximum water table elevations in spring varying from 0.5 to 1.0 m above late summer levels (Fig. 2).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 Study Area
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Sets of 10 surface (0–10 cm) soil samples were collected at random points along 25-m transects perpendicular to the A piezometer transect in the conifer and mixed forest areas and B piezometer transect in the marsh area near the river (Fig. 1). These samples were collected in late spring when the water table was adjacent to the ground surface. Sets of 10 samples were also collected using a bucket auger at depths of 0.8 to 1.4 m in the peat beneath the conifer forest site on Transect A and at depths of 1.6 to 2.7 m in the interbedded sediments within 50 m of the river bank on Transects A and B.

Potential denitrification rates were determined using the acetylene block technique, which inhibits the final conversion of N2O to N2 gas (Yoshinari and Knowles, 1976; Knowles, 1982). Samples of homogenized fresh soil consisting of 25 g (50 g for 0- to 10-cm surface and deeper peat) were placed in 250-mL serum bottles. Fifty milliliters of a solution treated with nitrate (5 mg of N as KNO3) was added to the samples and acetylene (C2H2) gas generated from calcium carbide was added to each bottle to achieve a final concentration of 10% (10 kPa) in the gas phase. Serum bottles were evacuated and flushed three times with Ar to ensure anaerobic conditions. The slurries were incubated at 20°C and headspace gas was sampled by syringe at 6, 24, and 48 h. Denitrification rates were calculated from the linear portion of N2O produced over time. Nitrous oxide was determined by using a Hewlett Packard gas chromatograph equipped with an electron capture detector (Hewlett Packard, Palo Alto, CA). Gases were separated with a Poropak Q column using a carrier gas of ultra-high purity UHP oxygen-free N2. A Varian 3800 gas chromatograph (Varian, Palo Alto, CA) equipped with a thermal conductivity detector (TCD) and a Poropak Q column was used to measure CO2. The N2O and CO2 produced were corrected for the amounts dissolved in the water. At the conclusion of the incubations, gas phase volumes were measured and wet and dry (at 105°C) weights of sediments were determined. The nitrate concentrations remaining in the slurries were analyzed colorimetrically by the cadmium reduction method on a Technicon AutoAnalyzer (Technicon, Tarrytown, NY). Final concentrations of nitrate were always >2 mg NO3–N indicating that nitrate was nonlimiting during the incubations.

Organic matter was determined by the loss-of-ignition method at 430°C (Davies, 1974). Organic C was estimated following the Walkley–Black method (Nelson and Sommers, 1982). This dichromate method uses the dilution of H2SO4 to measure the most active forms of organic C, but does not give complete oxidation of all organic compounds. These results are reported as oxidizable organic C rather than total organic C because a correction factor to account for unrecovered organic C was not used. The appropriate correction factor for individual soils may vary widely from 1.09 to 2.27 (Nelson and Sommers, 1982). The water-extractable organic C fraction (WEOC) was determined following the procedure of Waring and Gilliam (1983).

Data were analyzed using standard techniques contained in the Statistical Analysis System (SAS). Relationships between riparian surface soil variables, which were normally distributed, were examined with the Pearson product moment correlation, whereas Spearman's rank procedure was used for subsurface sediment variables that deviated from normal. Differences and correlations were considered statistically significant at the p < 0.05 level.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 Study Area
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Surface Soils
Denitrification potentials for the 0- to 10-cm depth soils in conifer peat, mixed forest, and marsh sites were similar, but organic matter content and the three fractions of organic C differed widely between the three sites (Table 1). Organic matter content ranged from a mean of 9.4% in the marsh site to 36.6% in the conifer peat. As with organic matter, organic C was highest in the conifer site and lowest in the marsh site. The amount of WEOC ranged from 164 to 399 µg C g–1. There were no significant differences in WEOC between the conifer and mixed forest sites, however amounts of WEOC were significantly lower in the marsh site. Mean anaerobic mineralizable C values were highest in the conifer peat soils, while in the mixed forest and marsh soils mineralizable C was not significantly different.


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  		Table 1. Denitrification potential (DNP), organic matter, and fractions of organic C in surface and subsurface riparian soils. Values are the mean (standard error) of 10 samples.

 
Subsurface Sediments
Denitrification potentials in the subsurface sediments were much lower than in the surface soils. Mean DNP rates at depths of 0.8 to 1.4 m in the peat were approximately four times lower than in the surface peat, whereas DNP values in the interbedded sediments were 75 to 80 times lower than in the surface mixed forest and marsh soils (Table 1). Mean organic matter and organic C amounts in the deeper peat were significantly greater than in the surface peat, whereas WEOC was significantly lower and mineralizable C values were similar. Mean organic matter and the three fractions of organic C in the interbedded sediments were significantly lower than in the surface soils or the deeper peat (Table 1).

Correlations between Denitrification Potential, Organic Matter, and Carbon Fractions
When the three groups of surface soils were considered separately, significant correlations were observed between DNP and anaerobic mineralizable C for the conifer peat and marsh soils, whereas the correlation was considerably lower but still significant for the mixed forest soils (Table 2, Fig. 3) . There was a significant positive correlation between DNP and WEOC for the marsh soils. In contrast, the correlation between DNP and WEOC was weaker in the mixed forest soils and not significant in the conifer soil. The relationship between DNP and either organic matter or organic C was nonsignificant in the conifer peat soils (Table 2). For the mixed forest soils, the correlation coefficients observed between both organic matter and organic C were similar to the other C fractions. When all the surface soils were combined together no significant correlations were observed between DNP and either organic matter or the various organic C fractions.


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  		Table 2. Correlation coefficients (Pearson) between denitrification potential and organic matter and fractions of organic C in surface riparian soils.

 


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  		Fig. 3. Relationship between denitrification potential and anaerobic mineralizable C in riparian surface soils. (a) conifer forest peat site; (b) mixed forest site; (c) marsh site.

 
Subsurface sediments showed a considerable range of relationships between DNP and the content of organic matter and the three C fractions (Table 3). Correlations were not significant in the deeper peat. The interbedded sediments showed a significant relationship between DNP and organic matter. The relationship with WEOC was also significant, whereas the correlation with the other C fractions was not significant.


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  		Table 3. Correlation coefficients (Spearman nonparametric) between denitrification potential, organic matter, and fractions of organic C in deep peat and buried channel sediments.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 Study Area
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The significant correlations between DNP and organic matter content and the various C fractions found in this study suggest that organic matter quantity and quality influence denitrification in stream riparian soils. However, O2 content and nitrate supply are also important controllers of denitrification (Groffman, 1994) and these factors may vary between different areas within a riparian zone. Soil samples in this study were collected in late spring when the water table was adjacent to the ground surface producing saturated soils with low O2 levels at the conifer peat, mixed forest, and marsh sites. Ground-water NO3–N concentrations were 5 to 15 mg L–1 in the three surface soil areas, but were often <1 mg L–1 at depth in the peat and buried channel sediments (Devito et al., 2000; Hill et al., 2000). Consequently, the DNP lab assays of the surface soils are unlikely to have been strongly affected by initial differences in O2 and nitrate supply on the ambient microbial community. In contrast, the low rates of N2O production, which were linear during the 48-h incubation of the deeper peat and buried channel sediments, may have increased rapidly during a longer incubation period. Time lags of up to 48 h in denitrification production have been reported, particularly in sediments from sites where there is low ambient nitrate (Aeolian and Shaw, 2000).

The three groups of surface soils have similar DNP despite the large differences in organic matter and organic C fractions in the 0- to 10-cm depth horizon. The mixed forest and conifer forest soils had organic C contents that were two and four times higher respectively than the marsh soils. These results suggest that the quality of organic C may be more important than the total amount in regulating denitrification. The quality of soil organic C is related to the composition of plant litter inputs. It is likely that the sedge dominated marsh vegetation produces higher quality litter than the conifer forest due to a lower C/N ratio and less recalcitrant compounds such as lignins (Melillo et al., 1982; McClaugherty et al., 1985; Webster and Benfield, 1986). The mineralizable C per gram of organic C in the marsh soil was almost two times higher than in the conifer peat. The WEOC fraction, which is particularly susceptible to decomposition also formed only 0.2% of the soil organic C pool in the peat in comparison to 0.34% in the marsh soils.

Although a higher proportion of organic C was water extractable in the marsh soils, the amount of WEOC extracted was two times greater in the surface conifer peat. This larger amount of WEOC did not stimulate higher DNP suggesting that the bioavailability of soluble C may vary among the three riparian surface soils. Previous studies of soluble C in soils and streams suggest that the quality of dissolved organic C varies widely and can be dominated by refractory forms depending on the origin and extent of degradation (Cook and Allan, 1992; Boyer and Groffman, 1996; Volk et al., 1997).

Denitrification potentials at depths of 0.8 to 1.4 m in the peat were only 25% of the rates in the surface peat, despite substantially higher organic matter and organic C content in the deeper peat (Table 1). Mean DNP in the buried channel sediments was 80 times lower than in the surface marsh soil, although mean organic C content was only two times lower and quality as indicated by mineralizable C or WEOC per gram organic C was similar. These results suggest that the low DNP in the deeper peat and buried channel sediments may reflect a lag in denitrification over short incubation periods and cannot be clearly linked to organic matter content and quality. Low ground water nitrate concentrations were associated with elevated ferrous Fe concentrations of 0.5 to >3 mg L–1 in some areas of the deep peat and buried channel sediments (Carlyle and Hill, 2001). This pattern suggests a shift from nitrate to Fe as an electron acceptor. As a consequence the denitrifier population may be low under these conditions, which may partly explain the low DNP.

Our data indicate that DNP generally showed the highest correlation with anaerobic mineralizable C in surface riparian soils and the relationship with organic matter and other organic C fractions was lower or absent. Bijay-Singh et al. (1988) also reported that DNP in field-moist surface soils showed the highest correlation with the amount of C mineralized in anaerobic incubations. Similarly, highly significant correlations have been observed between denitrification and mineralizable C assessed under aerobic conditions (Burford and Bremner, 1975; Davidson et al., 1987).

The relationship between DNP and WEOC was highly significant in the surface marsh soils at the Boyne River site. Several studies have also revealed that denitrification is more strongly related to a soluble fraction of organic C than to total organic C in mineral soils (Burford and Bremner, 1975; Stanford et al., 1975) and stream sediments (Hill and Sanmugadas, 1985). Although WEOC provides a good index of denitrification potential in the surface marsh soils, this relationship was less strong in the mixed forest soil and was not significant in the surface peat (Tables 2 and 3). This contrasting pattern suggests that the availability of WEOC for denitrification differs between surface organic-rich and mineral soil horizons in the Boyne River floodplain. Mean WEOC content in the peat and mixed forest surface soil ranged from 305 to 399 µg C g–1 in comparison with 164 µg C g–1 in the marsh soil. Although we do not know the composition of this water-soluble C, it appears that a large fraction is unavailable to denitrifiers in the organic-rich soils.

Establishing relationships between an index of available C and denitrification activity in riparian soils and subsurface sediments would be useful for assessment of the capacity of different riparian areas to support denitrification. However, surface soils in riparian zones are influenced by a mix of forested and herbaceous plant communities which differ in litter quality and the organic matter content of subsurface sediments also vary in origin and age. Our results suggest that it is difficult to determine a standard index of C availability that is a good predictor of denitrification across a wide range of riparian soils and sediments.

We suggest that further research is needed to evaluate denitrification in relation to the content and quality of organic matter in riparian surface soils and subsurface sediments. Organic matter at depth in riparian environments may often be patchy in distribution and less labile than in surface soils. However, the much greater volume and more continuous interaction of subsurface sediments with ground water may compensate for lower denitrification rates at depth. Consequently, nitrate removal from ground water in many riparian zones may be influenced more by the denitrification activity of subsurface sediments rather than organic-rich surface soil horizons.


   ACKNOWLEDGMENTS
 
We gratefully acknowledge the assistance of Mike Waddington, School of Geography and Geology, McMaster University and Rick Bourbonniere, Canada Centre for Inland Waters with the CO2 analysis. We thank K. Sanmugadas for lab assistance and C. Randall for constructing the figures. Two anonymous referees provided helpful comments on an earlier draft of this paper. This research was funded by grants from the Natural Sciences and Engineering Research Council of Canada.

Received for publication November 15, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Study Area
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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