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Published in J. Environ. Qual. 33:201-209 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Landscape and Watershed Processes

Dissolved Organic Nitrogen Regulation in Freshwaters

V. B. Willetta, B. A. Reynoldsb, P. A. Stevensb, S. J. Ormerodc and D. L. Jones*,a

a School of Agricultural and Forest Sciences, Deniol Road, University of Wales, Bangor, Gwynedd, LL57 2UW, UK
b Centre for Ecology and Hydrology, Bangor, Gwynedd, LL57 2UW, UK
c Cardiff School of Biosciences, Cardiff University, P.O. Box 915, Cardiff CF10 3TL, UK

* Corresponding author (d.jones{at}bangor.ac.uk).

Received for publication November 13, 2002.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Dissolved organic nitrogen (DON) has been hypothesized to play a major role in N cycling in a variety of ecosystems. Our aim was to assess the seasonal and concentration relationships between dissolved organic carbon (DOC), DON, and NO–3 within 102 streams and 16 lakes within catchments of differing complexity situated in Wales. Further, we aimed to assess whether patterns of land use, soil type, and vegetation gave consistent trends in DON and dissolved inorganic nitrogen (DIN) relationships over a diverse range of catchments. Our results reinforce that DON constitutes a significant component of the total dissolved N pool typically representing 40 to 50% of the total N in streams and lakes but sometimes representing greater than 85% of the total dissolved N. Generally, the levels of DON were inversely correlated with the concentration of DIN. In contrast to DIN concentrations, which showed distinct seasonality, DON showed no consistent seasonal trend. We hypothesize that this reflects differences in the bioavailability of these two N types. The amount of DON, DOC, and DIN was significantly related to soil type with higher DON export from Histosol-dominated catchments in comparison with Spodosol-dominated watersheds. Vegetation cover also had a significant effect on DON concentrations independent of soil type with a nearly twofold decrease in DON export from forested catchments in comparison with nonforested watersheds. Due to the diversity in catchment DON behavior, we speculate that this will limit the adoption of DON as a broad-scale indicator of catchment condition for use in monitoring and assessment programs.

Abbreviations: DIN, dissolved inorganic nitrogen • DOC, dissolved organic carbon • DON, dissolved organic nitrogen


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DISSOLVED ORGANIC C and N represent important components of ecosystem biogeochemical cycles (Perakis and Hedin, 2002). The characteristics and functional significance of DOC have drawn considerable interest in recent years due to the high concentrations frequently found in soils and freshwater and marine environments (Tate and Meyer, 1983; Dafher and Wangersky, 2002). In comparison, however, DON has received less interest due to difficulties in its quantification and consequently most research has tended to focus on the concentrations, fluxes, and effects of NO–3 in freshwaters (Solorzano and Sharp, 1980; Antia et al., 1991; Merriam et al., 1996). This focus on NO–3 also reflects an emphasis on N pollution, particularly acidification, ground water pollution, and eutrophication, rather than natural ecosystem science. It has been shown, however, that DON typically constitutes between 10 and 70% of the total dissolved N in freshwaters, similar to reports for soil and marine environments (Antia et al., 1991; Chapman et al., 1998; Jones and Kielland, 2002).

Dissolved organic N present within freshwater can originate from one of three main sources: (i) direct unaltered inputs from wet or dry deposition, (ii) terrestrial inputs, and (iii) within-stream release (INDITE, 1994). Wet deposition typically contains 25 to 50% of its N in the form of DON; however, direct inputs to freshwater ecosystems are very low (Antia et al., 1991; McHale et al., 2000). It is therefore probable that DON in freshwaters originates from material produced both in situ (sediments) and ex situ (soils). In support of this, DOC concentrations in rivers and streams appear to be controlled to a large extent not only by the hydrologic conditions of the stream but also by terrestrial factors such as vegetation cover and catchment soil processes (Tate and Meyer, 1983; McDowell and Wood, 1984). Recently, mounting evidence indicates that stream water chemistry, and in particular DON, can act as an indicator for the mechanisms operating within the soil system such as nitrogen limitation or saturation (Perakis and Hedin, 2002). For example, it has been found that the concentration of DON in streams flowing from heathland ecosystems is greater than the concentration of DON flowing from coniferous forested catchments (Chapman et al., 1999) and that wetland forests are a dominant source of organic matter for blackwater streams (Dosskey and Bertsch, 1994). It has also been suggested that simple stream measurements such as DOC to DON ratio may be used as an indicator of landscape N saturation (Campbell et al., 2000).

With increasing concern about the effects of enhanced N deposition on ecosystem functioning, it has become necessary to monitor large areas of land to evaluate its impact. Unfortunately, soil data often possess a high degree of variability and it remains difficult to obtain an accurate overview of soil N status in a complex landscape. It is therefore desirable to identify a simple indicator that could give good information of the whole ecosystem status in terms of N deposition. Streams and rivers provide a useful integrator for assessing the N status within the terrestrial environment, as they are ultimately the main pathway for terrestrial compounds and nutrients to drain from such systems. However, very little is currently known of flow path dynamics and it is therefore with caution that we investigate the assumption that complete mixing of nutrients from terrestrial systems will occur, especially during storm flow rates. To date, the majority of studies that have used stream variables to quantify the nutrient status of unmanaged watersheds have used only a handful of streams, or have concentrated on one system only (Campbell et al., 2000; Chapman et al., 1999; Dalva and Moore, 1991; Dosskey and Bertsch, 1994; Hagedorn et al., 2000b; McHale et al., 2000).

The aim of this study was to assess the seasonal and concentration relationships between DOC, DON, and NO–3 within 102 streams and 16 lakes within catchments of differing complexity situated in Wales. Further, we aimed to assess whether patterns of land use, soil type, and vegetation gave consistent trends in DON and DIN relationships over a diverse range of catchments. Due to the scale of this experiment it was not possible to conduct in-depth process-level studies to elucidate the actual types of N compounds present or their origination and dynamics. However, presented here are indicators of possible processes and discussions on DON dynamics in streams and lakes, to act as a stimulus for further process-level work.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sampling Sites
To establish the relationships between pH, DIN, DOC, and DON concentrations and other environmental variables in freshwaters, 102 streams and 16 lakes were selected within Wales (Fig. 1) . Each stream was within an independent catchment area. The 118 catchment areas varied significantly in size (20–5673 ha; mean ± SE = 653 ± 78 ha), altitude (8–440 m; mean ± SE = 247 ± 8 m), distance from the coast (2–312 km; mean ± SE = 80 ± 8 ha), and vegetation cover. The mean ± SE stream length was 4 ± 0.3 km (range 0.3–21 km) and the mean ± SE lake area was 263 ± 130 ha (range 39–2156 ha). The 118 catchment areas were selected to cover the major soil types (>95% of the total) and land uses in an acidic and acid-deposition-sensitive part of Wales. Land uses included improved and unimproved ovine and bovine grazed grassland, coniferous forest, and bog and heathland with small amounts of deciduous woodland and arable land also present (<5% of the total land area). Most catchment areas possessed multiple land uses within their boundary. The dominant soil types within individual catchment areas included Alfisols, Histosols, Spodosols, and Inceptisols. These soils had either an udic or aquic soil moisture regime and all possessed a mesic soil temperature regime. Further details of the soils and vegetation within individual catchment areas can be found in Stevens et al. (1997). Catchment characteristics were determined with ARC/INFO 7.0.4 (ESRI, 1996). The datasets employed included an Ordnance Survey land-form PANORAMA digital terrain model derived from 1:50000 topographical maps; Soil Survey of England and Wales 1:63000 soil maps; the Institute of Terrestrial Ecology Land Cover map; and an Institute of Hydrology hydrological network dataset captured from the Ordnance Survey 1:50000 maps.



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  		Fig. 1. Map of the individual watersheds used for stream and lake sampling in the study. (A) Map of the whole UK. (B) Map of Wales.

 
Sampling and Analysis
Water samples were collected on the second week of every month from each stream and lake from January to September in 1995. At all sites, samples (1 L) were taken with a stainless steel vessel, 0.45-µm filtered on site, and stored in plastic bottles at 4°C to await analysis, which occurred within 24 h. Collection of duplicate samples at some sampling dates (two months out of nine) revealed no significant intersample variation, and therefore only one sample was taken at each location each month (data not presented).

Nitrate and nitrite were determined on an American Monitor (Indianapolis, IN) segmented-flow autoanalyzer by the hydrazine–Cu–sulfaniliamide–N–1-napthylethylenediamine procedure of Downes (1978). Ammonium was determined on an American Monitor segmented-flow autoanalyzer by the hypochlorite–phenate–nitroprusside procedure of USEPA (1983). Dissolved organic C and total dissolved nitrogen (TDN) were determined with an Auto-Analyzer II segmented flow autoanalyzer (Technicon Instrument Corporation, Tarrytown, NY) with an on-line UV persulfate digestion system. The concentration of dissolved organic N was made by subtracting the NO–3 + NH+4 concentrations from TDN concentration. The limits of detection for each analytical procedure were 0.2 mg L–1 for DOC, 0.05 mg L–1 for TDN, and 0.01 mg L–1 for NO–3 and NH+4.

Statistical Analysis
At almost all sampling dates and in greater than 99% of the samples taken, the concentrations of NO–2 and NH+4 were below the limits of detection and therefore they have not been included in the statistical analysis. In a few instances the concentrations of DON and NO–3 were also below the detection limit and in such cases a value equal to one half the detection limit was used in subsequent statistical analysis. Differences on mean parameter values for streams and lakes were tested by using t tests (SigmaPlot 5.0; SPSS, 2002), while regression analyses were performed on freshwater chemistry (Minitab 13; Minitab, 2000). Seasonal variations in relationships were tested by finding the mean of the results for January to February (winter) and then July to August (summer) relationships were also tested with linear regressions (Minitab 13).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen Content of Freshwaters
Concentrations of DON and DIN in more than 1000 Welsh freshwater samples collected between January and September are summarized in Table 1. Concentrations of DIN in the 118 streams and lakes sampled were typically much larger than for DON. A significant negative correlation was observed between the concentration of DON and DIN in solution in both stream waters and lakes (Fig. 2) . On average, DON constituted 40 ± 2% of the total N in solution; however, this ranged widely between catchments from 4 to 87%. The DOC to DON ratio of the freshwaters (Table 1) was found to be not significantly different from the C to N ratio of topsoil (0–30 cm) within Wales (mean ± SE = 22 ± 3, n = 16; Jones, 1999).


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  		Table 1. Characteristics of the nitrogen and acidity status of 102 streams and 16 lakes within Wales from January to September 1995.

 


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  		Fig. 2. Relationship between dissolved organic nitrogen (DON) and dissolved inorganic nitrogen (DIN; NO3–) in (A) streams and (B) lakes within Wales. Symbols represent experimentally derived annual means while lines represent the linear regression analysis results.

 
Statistical analysis across all sampling dates indicated a significant positive correlation between the concentration of DOC and DON in stream water and lakes (Fig. 3) . Further analysis, however, indicated that the relationship in streams varied seasonally with a strong significant regression in summer (r2 = 0.33, P < 0.001) but with no significant relationship in the winter (r2 = 0.02, {alpha} = 0.1; data not presented). Accordingly, there was no significant correlation between the DOC to DON ratio of the water between summer and winter sampling periods (Fig. 4C) . Comparisons of samples collected in summer and winter from each site indicated that freshwater stream DIN concentration was seasonally dependent with higher concentrations in the winter in comparison with the summer (Fig. 4A). In contrast, DON concentrations in freshwater streams were not well correlated between summer and winter sampling dates (Fig. 4B) whereas a significant correlation was observed between DOC concentrations at summer and winter samplings (r2 = 0.44, P < 0.001; data not presented). No significant relationship ({alpha} = 0.001) either seasonally or annually was observed between stream DON, DIN, or DOC concentration with catchment area or distance of catchment from the coast (data not presented). Both stream DON and DOC concentrations were significantly and positively correlated with catchment altitude and gradient; however, these catchment variables also autocorrelate with soil type. Although DON concentrations correlated strongly with freshwater stream Fe concentration (r2 = 0.29, P < 0.001) they did not correlate significantly with Ca, Mg, K, Na, Cl, SO2–4, Al, or Mn concentration (data not presented).



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  		Fig. 3. Relationship between dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in (A) streams and (B) lakes within Wales. Symbols represent experimentally derived annual means while lines represent the linear regression analysis results.

 


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  		Fig. 4. Seasonal relationship between dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN), and the DOC to DON ratio in streams and lakes within Wales. Symbols represent experimentally derived winter and summer means while dashed lines represent a theoretical 1:1 relationship.

 
pH Relationships with Freshwater Nitrogen
The mean annual pH of streams and lakes is summarized in Table 1. Although there was a very significant relationship between the pH of all the water samples measured in the summer and winter (r2 = 0.51, P < 0.001; data not presented), the pH of the samples tended to be 0.8 ± 0.1 pH units higher in the summer than in the winter (data not presented). Significant positive annual relationships were found between the concentration of DIN and freshwater pH (Fig. 5A) . In contrast, however, no significant relationship was observed between freshwater DON concentration and pH (Fig. 5B). In addition, the DOC to DON ratio of the dissolved organic fraction was not significantly correlated with pH (Fig. 5C).



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  		Fig. 5. Relationship between dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), the DOC to DON ratio, and pH in streams and lakes within Wales. Symbols represent experimentally derived annual means while lines represent the linear regression analysis results.

 
Soil Type Relationships with Freshwater Nitrogen
Almost all of the catchments examined typically contained two or more soil types within their boundary. In total, 25% of all the streams examined drained from catchments containing more than 75% Histosol coverage. Statistical analysis revealed significant relationships existing between freshwater nitrogen concentration and Histosol and Spodosolic soil type coverage. Typically, the levels of inorganic N in stream waters were strongly inversely correlated with Histosol coverage (Fig. 6A, 6B) . In contrast, the levels of DON tended to increase with Histosol coverage but disproportionately with the rate of decline in inorganic N (Fig. 6A, 6B). A significant relationship was observed between DON and Histosol coverage in the summer (P < 0.01) but not in the winter ({alpha} = 0.1; data not presented). No such seasonal relationship was observed for DIN. The DOC to DON ratio was significantly correlated with Histosol coverage of the catchment areas (P < 0.001). Generally, the DOC to DON ratio of the stream waters increased from 20 ± 1 in catchments with low Histosol coverage (<1% of total area) to 35 ± 2 in catchment areas containing high levels of Histosol coverage (>50% of total area). The levels of DOC were also significantly positively correlated with Histosol catchment coverage (Fig. 6C, 6D). The amount of N exported as DON from the catchment areas increased with Histosol coverage and ranged from 30% of the total dissolved N in catchments with low Histosol coverage to 80% in catchment areas containing high levels of Histosol coverage (Fig. 6E, 6F). In contrast to the results for Histosols, the amount of DON and DIN in stream waters was less well correlated with Spodosol coverage of the catchment areas although there was a tendency for DIN to increase with increased catchment coverage by Spodosols (P < 0.01). Generally, the DOC levels decreased with Spodosol coverage and the DOC to DON ratio of the streamwater also declined in parallel with this.



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  		Fig. 6. Effect of soil type (Histosol and Spodosol) watershed coverage on the concentration of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), and dissolved organic carbon (DOC) in streams and lakes within Wales. Values represent means ± standard errors.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Dissolved Organic Nitrogen Concentrations
This study aimed to assess general trends in stream water DON concentrations and dynamics across a wide range of catchment types. The concentrations of both DON and DIN reported in this study (Table 1) are very low in comparison with soils and approximately an order of magnitude lower than reported for freshwaters draining from lowland, arable-dominated catchments (Arheimer and Liden, 2000) and other forested watersheds (Lepistö et al., 1995). This probably reflects the relatively low fertilizer inputs into these catchments and the sites' moderate atmospheric inputs. Atmospheric deposition (wet + dry) across almost all of the sites typically ranges from 15 to 25 kg N ha–1 yr–1 with 70% of the inputs as NH3 and NH+4 (Reynolds et al., 1997). Low DON values could also be attributed to DON being the resultant N after NO–3 and NH+4 have been subtracted from the total N. This calculation involves large uncertainties especially when concentrations are close to detection limits. Although total DON was measured in this study, no compound-specific measurements were performed to ascertain the extent to which the quality of the DON varied between catchments. As bioavailability is critically dependent upon substrate quality, further studies of a similar nature should focus on addressing this issue.

Land Use Effects on Dissolved Organic and Inorganic Nitrogen Concentrations
Dissolved organic N has often been reported to comprise an important cog of terrestrial and aquatic nutrient cycles, with land use hypothesized to have a significant effect on the amount present and its functional significance (Antia et al., 1991; Chapin et al., 1993; Jones and Kielland, 2002). In some streams and rivers it has been found that DON is the major form of dissolved N (Hagedorn et al., 2000b); however, in other studies, NO–3 is the dominant N form exported (Campbell et al., 2000; Chapman et al., 1999; Arheimer and Liden, 2000). This study supports the hypothesis that ecosystem type, and moreover soil type, is a major determinant of the quantity and chemical form of N exported from catchment areas. In particular, soil type has a major effect on the concentration of DIN in freshwater but disproportionately less effect on the concentration of DON. Interestingly, the concentration of DON remained relatively stable across a wide range of hydrologically and ecologically diverse catchments while large variations were observed in DIN concentrations. This is in agreement with the findings of Arheimer and Liden (2000) who reported that land use (forestry versus arable) had a significant effect on NO–3 concentrations but relatively little effect on DON, PO3–4, or dissolved organic phosphorus (DOP) concentrations in 35 Swedish catchments. A possible explanation is that much of the DOC and DON present in the streams and lakes is derived from relatively recalcitrant soil C pools with slow turnover times. This DOC and DON produced ex situ and flowing into rivers may therefore reflect previous land use conditions when many of the catchments may have been of a similar nature.

As land use incorporates differences in both vegetation and/or soil type, we pose the question as to whether vegetation or soil type is more important in regulating DON levels in streams. Thirty-six catchments with a predominantly Spodosolic soil type, but with differing vegetation type (forested versus nonforested), were found to possess significantly different N characteristics (Table 2). These findings in conjunction with the findings presented in Fig. 6 indicate that both soil type and vegetation are of significance in regulating DON and DIN concentrations in stream waters, although the proportional significance of each factor remains unknown.


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  		Table 2. Effect of conifer forest coverage on the concentration of dissolved inorganic (DIN) and organic (DON) nitrogen in stream water within Spodosol-dominated catchment areas (n = 18 for both datasets).

 
Soil Type Effects on Dissolved Organic Nitrogen and Carbon Concentrations
In similar studies, the processes regulating in-stream cycling of DOC have concluded that two main systems operate. In an abiotic system, stream sediments or the mineral soil adsorbs organic C, while in a biotic system, DOC removal is attributed to microorganisms within the soil or stream that mineralize DOC to CO2 (McDowell, 1985). In a study by McDowell (1985), it was concluded that much of the DOC removal from stream water was accounted for by abiotic processes with the amount of DOC present governed by the buffering capacity of the mineral soil. The results presented here indicate that soil type has a major influence on the amount of DOC and DON present. Spodosolic-dominated catchments that possess soils with a significant DON sorption capacity and microbial activity possessed lower stream DOC and DON contents (Kaiser and Zech, 1998). In contrast, the Histosol-dominated catchments, which have little DOC buffering capacity and low microbial activity, possessed relatively higher concentrations of DOC due to the high levels of humic substances that are present within and readily leached from the soil matrix (Dalva and Moore, 1991; Fenner et al., 2001). The high levels of DOC in these soils have been attributed to the poor vegetation quality and the waterlogged and anaerobic conditions that lead to an inhibition of phenol oxidase activity and incomplete organic matter decomposition (Avery, 1990). Although our study indicated a general tendency for DON to increase with DOC in stream water in agreement with Hagedorn et al. (2000a), the DOC to DON ratio was not constant (Fig. 4C) and seasonal relationships existed between DOC and DON. This may suggest that DON is not controlled by the same processes controlling DOC; however, this could also reflect different seasonal sources of material within each catchment (i.e., baseflow versus surface flow) and is discussed below.

The greatest variability in DOC to DON ratios between catchments in this study was observed in the winter months. Although we did not explicitly measure the contribution of storm runoff and baseflow within each catchment, similar studies in Wales have demonstrated that storm runoff is mainly a winter phenomenon and is likely to be more dilute with respect to DOC and DON in comparison with baseflow water (B.A. Reynolds and P.A. Stevens, unpublished data, 2003). We therefore hypothesize that the differing quantities and qualities of DON and DOC in storm runoff and baseflow may be the primary cause of poor relationships between DOC and DON in the winter. In addition, our results may also reflect differences in net DOC production in soils that are known to vary seasonally (Hughes et al., 1990). Lepistö et al. (1995) found that soil texture may be a primary regulator of DON concentrations in rivers. Further work is needed to elucidate the relative contribution of biotic and abiotic processes in regulating seasonal DOC and DON quality and quantity within catchments.

Seasonal Effects on Dissolved Organic and Inorganic Nitrogen Concentrations
Dissolved organic C and N concentrations within streams are known to be highly dependent upon flow rate, and in particular storm events (Buffman et al., 2001; McHale et al., 2000) both of which autocorrelate with season. Water discharge also has been shown to be positively correlated with DON in Nordic rivers (Arheimer et al., 1996). In this study, the samples were collected during the second week of every month; therefore, it is quite conceivable that the concentrations of DON and DOC will not distinguish between the trends operating under baseflow or stormflow conditions. This has important implications for studies examining terrestrial and aquatic links as it has been found that under stormflow conditions organic compounds (especially DOC) may originate from different sources due to changes in the hydrologic pathway (Buffman et al., 2001; Hagedorn et al., 2000b; McDowell and Likens, 1988).

The seasonal pattern of increased DIN in winter stream waters reported here has been frequently reported in the literature and can be explained by the lack of N leaching and increased biological immobilization during the growing season (Burt et al., 1988; Arheimer and Liden, 2000). The results presented here, however, suggest that in contrast to DIN, no consistent seasonal trend is apparent for DON in agreement with results for lowland agricultural catchments in Sweden (Arheimer and Brandt, 2000). Analysis of Fig. 6 indicates that approximately 50% of the catchments had higher DON in the winter than in the summer and vice versa. The direction of this seasonal effect was not significantly related to soil type or vegetation type ({alpha} = 0.2). While seasonal differences in hydrological flow between catchments could potentially account for this diversity, this could not be explicitly tested here and therefore merits further attention. However, when considering low-flow conditions only, other studies have shown that summer DON may be significantly higher than during dormant seasons (Arheimer et al., 1996).

Dissolved Organic Carbon and Nitrogen Relationships
Carbon to nitrogen ratios of substrates are an important regulator of microbial activity. In this study, although DOC and DON concentrations were positively correlated (Fig. 3), the relationship was highly dependent upon catchment type and season. Assuming that both the C and N are contained within the same compounds, the results presented here suggest that the compounds leaving Histosol-dominated catchments have a higher DOC to DON ratio and are probably more recalcitrant than those from Spodosol-dominated catchments. As the potential for biodegradation is greater in the summer months, either before entering the streams or occurring in situ, we hypothesized that the amount of DON in the streams would decrease in the summer, and that the DOC to DON ratio would increase as the N-rich compounds are selectively removed. The lack of seasonal trend in either DON concentrations or DOC to DON ratio, however, suggests that very little of the DON is being processed and can therefore be viewed as recalcitrant. This is in agreement with studies of DON composition in soil solutions and freshwaters, which indicate that most of the material is of a high molecular weight nature that is resistant to microbial attack (Jones and Kielland, 2002). It has been previously reported that elevated stream discharge rates increase not only bioavailable DOC but also bacterial populations (Buffman et al., 2001). The relevance of these findings are not yet known for DON; however, the DOC to DON ratio results presented here suggest that this may be a catchment-specific response.

pH Effects on Dissolved Organic and Inorganic Nitrogen
While pH significantly correlated with DIN concentrations in streams (Fig. 5A), it had little effect on either the amount of DON in solution or the DOC to DON ratio. While the relationship between DIN and pH has been described previously (Arheimer and Liden, 2000), and can be readily explained by the effect of pH on microbially mediated nitrification processes, no such explanation is apparent for DON. We hypothesized that in catchments of low pH, soil and stream organic inputs would be of poor quality and that soil organic matter decomposition would be slower and more inefficient leading to the greater production of humic substances. Consequently, this would lead to increases in DOC and DOC to DON ratio with decreasing pH. The lack of a clear relationship indicates that direct pH effects may be of minor importance in comparison with other factors such as catchment vegetation coverage.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study aimed to assess the effect of watershed characteristics and seasonality on the quantity and quality of DON in a wide range of catchments within Wales. This study confirms previous findings that DON constitutes an important component of the N cycle in freshwater environments and typically constitutes about 40% but sometimes represents greater than 85% of the total N exported in rivers within Wales. In contrast to NO–3, the amount of DON present showed no uniform seasonality patterns and was not correlated to freshwater pH. Both vegetation and soil type had strong independent influences on the amount of DON present. A strong relationship was observed between DOC and DON concentrations in the summer; however, during the winter a poor correlation was observed. We hypothesize that this reflects differences in the contributions of hydrological flow pathways (surface versus subsurface) between individual catchments in winter. Further work is required at the compound-specific level to further elucidate the cause of these seasonal changes in the quality and quantity of DON. Unlike the relatively robust models that exist for predicting DIN flow from catchments, the behavior of DON appears to be regulated by numerous factors that will make modeling at the landscape level difficult. Given the observed diversity in catchment DON behavior, we further speculate that this will also prevent the adoption of DON measurements being used as broad-scale indicators of watershed condition for use in monitoring and assessment programs.


   ACKNOWLEDGMENTS
 
We thank the Natural Environment Research Council, the National Assembly for Wales, the Environment Agency, the Countryside Council for Wales, and the Forestry Authority for financial support. We thank the landowners and organizations who allowed free access to their land for sampling purposes.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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