Seasonal changes in the character and nitrogen content of dissolved organic matter in an alpine/subalpine headwater catchment.

Eran W Hood,Mark W Williams

doi:10.1100/tsw.2001.273

Abstract

We are studying the chemical quality of dissolved organic nitrogen (DON) in a high-elevation watershed in the Colorado Front Range. Samples were collected over the 2000 snowmelt runoff season at two sites across an alpine/subalpine ecotone to understand how the transition between the lightly vegetated alpine and forested reaches of the catchment influences the chemical character of DON. Samples were analyzed approximately weekly for dissolved organic material (DOM) content and chemical character. A subset of samples was analyzed for the elemental content of fulvic and hydrophilic acids. Concentrations of DON at both sites were highest in the spring at the initiation of snowmelt, decreased during snowmelt, and increased again during the late summer and fall. In contrast, concentrations of dissolved organic carbon (DOC) peaked on the ascending limb of the hydrograph and declined to seasonal minima on the descending limb of the hydrograph. The ratio of DOC:DON showed a seasonal shift at both sites with high values (40 to 55) during peak runoff in early summer and lower values (15 to 25) during low flows late in the runoff season. These results indicate that there was a seasonal change in the relative N content of DOM at both sites. Chemical fractionation of DOC showed that there were temporal and longitudinal changes in the chemical character of DOC. At the alpine site, the fulvic acid content of DOC decreased from 57% in June to 35% in September. The change in fulvic acid was less pronounced at the forested site, from 66% in June to 54% in September. Elemental analysis of fulvic and hydrophilic acids indicated that hydrophilic acids were N rich compared to fulvic acids. Additionally, fulvic and hydrophilic acids isolated at the alpine site had a lower C:N ratio than those isolated at the forested site. Similarly, the C:N ratio of organic acids at both sites was lower in September than in June during peak runoff. These differences appear to be a result of changes in both DOM precursor material and hydrologic flowpaths. Using C:N ratios of fulvic and hydrophilic acids, we estimate that nonhumic material carried between 54 to 73% of the organic N in surface water at the alpine site and 44 to 58% of the organic N in surface water at the subalpine site.

Highlights

The global nitrogen (N) cycle is undergoing changes as a result of human activities[1,2]
The pattern for dissolved organic nitrogen (DON) was similar at the forested SLP site, concentrations were higher in the spring and late summer/fall periods and decreased below 5 μmoles L1 on only two sample dates during peak snowmelt runoff
A similar trend in soil solution at the Harvard Forest was attributed to hydrologic flushing of accumulated decomposition products[11]; this mechanism is unlikely to be important for DON production in the alpine reaches of our study catchment which is dominated by exposed bedrock and talus

Summary

Introduction

The global nitrogen (N) cycle is undergoing changes as a result of human activities[1,2]. In a similar high-elevation system, McKnight et al.[15] found that aquatic fulvic acids in the subalpine reach of the Loch Vale catchment in Rocky Mountain National Park had a high N content relative to the N content reported for fulvic acids at other forested sites. These findings suggest that DON cycling has the potential to be influenced by changes in the N content of terrestrial and aquatic DOM precursor material resulting from increased deposition of inorganic N. Little work has been done to identify the DOM fractions with which DON is associated in surface waters, at the watershed scale

Methods

Results

Conclusion