Environ. Sci. Technol. 2004, 38, 4723-4727
Effect of Storage on the Isotopic Composition of Nitrate in Bulk Precipitation J O H N S P O E L S T R A , * ,† S H E R R Y L . S C H I F F , † DEAN S. JEFFRIES,‡ AND RAY G. SEMKIN‡ Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 and Environment Canada, National Water Research Institute, Burlington, Ontario, Canada L7R 4A6
Stable isotopic analysis of atmospheric nitrate is increasingly employed to study nitrate sources and transformations in forested catchments. Large volumes have typically been required for δ18O and δ15N analysis of nitrate in precipitation due to relatively low nitrate concentrations. Having bulk collectors accumulate precipitation over an extended time period allows for collection of the required volume as well as reducing the total number of analyses needed to determine the isotopic composition of mean annual nitrate deposition. However, unfiltered precipitation left in collectors might be subject to microbial reactions that can alter the isotopic signature of nitrate in the sample. Precipitation obtained from the Turkey Lakes Watershed was incubated under conditions designed to mimic unfiltered storage in bulk precipitation collectors and monitored for changes in nitrate concentration, δ15N, and δ18O. Results of this experiment indicated that no detectable nitrate production or assimilation occurred in the samples during a two-week incubation period and that atmospheric nitrate isotopic ratios were preserved. The ability to collect unfiltered precipitation samples for an extended duration without alteration of nitrate isotope ratios is particularly useful at remote study sites where daily retrieval of samples may not be feasible.
Introduction Human activities such as the burning of fossil fuels and the excessive use of inorganic nitrogen (N) fertilizers by the agriculture sector have been implicated in recent increases in atmospheric nitrogen deposition across the globe (e.g., Galloway et al. (1)). While pristine areas experience nitrogen deposition levels of less than 5 kg N‚ha-1‚yr-1, the eastern United States receives about 28 kg N‚ha-1‚yr-1 (2). Heavily polluted areas in Europe have nitrogen deposition in excess of 75 kg N‚ha-1‚yr-1 (3). One of the consequences of these elevated deposition levels is that forested ecosystems that have historically been thought of as nitrogen-limited are becoming nitrogen-saturated (4, 5). Nitrogen saturation can decrease forest health through increased water stress, reduced frost tolerance, soil acidification, nutrient leaching, and decreased fine root biomass (5). * Corresponding author phone: (519)888-4567 ext. 7277; fax: (519)746-7484;e-mail:
[email protected]. † University of Waterloo. ‡ National Water Research Institute. 10.1021/es030584f CCC: $27.50 Published on Web 08/14/2004
2004 American Chemical Society
Concerns over the long-term effects of elevated nitrogen deposition have led to an increase in the number of studies using stable isotope analysis to trace the fate of atmospheric nitrate deposition in forested ecosystems. The two sources of nitrate in most forested watersheds are (1) nitrate from atmospheric deposition and (2) nitrate produced by nitrification in soils (microbial nitrate). Atmospheric and microbial nitrate are isotopically distinct and therefore isotopic ratios, particularly 18O/16O, can be used to study nitrate sources and cycling in forested catchments (6-12). Even at sites receiving elevated nitrate deposition, several liters of water are often required for isotopic analysis of atmospheric nitrate using the methods of Chang et al. (13) and Silva et al. (14). In pristine areas, 10 L or more of precipitation may be needed to determine both δ15N and δ18O values of nitrate. Typical bulk precipitation collectors designed to sample water for chemical analyses might not accumulate sufficient water from individual rain events to determine nitrate isotope ratios; therefore, multiple collectors are often required. However, recently developed nitrate isotope methods, which use denitrifying bacteria to convert nitrate to nitrous oxide (N2O) for determination of δ15N and δ18O values, reduce the required sample size by two orders of magnitude (15, 16). Therefore, these new techniques will significantly decrease the collection period needed to accumulate sufficient precipitation. For many long-term catchment studies, it would be impractical to analyze nitrate isotope ratios for each precipitation event. A significantly less expensive and less laborintensive method of determining the isotopic signature of annual nitrate deposition is to combine individual precipitation samples acquired over an extended time interval. Thus, a mass-weighted mean δ15N and δ18O value is determined for the period of collection. At remote sites, or where resources are limiting, bulk collectors may accumulate precipitation for several days or weeks before filtering and preservation occurs. With this approach, microbial reactions that consume or produce nitrate might alter the concentration and isotopic signature of nitrate in unfiltered samples. Maximum alteration of nitrate would be expected during summer months when elevated temperatures promote greater microbial activity. Microbial alteration would lead to the determination of an erroneous isotopic composition for atmospheric nitrate, thus affecting subsequent source-contribution calculations. Several studies have investigated the temporal stability of ion concentrations in unfiltered bulk precipitation, with varying conclusions with respect to nitrate. Galloway and Likens (17, 18) found no change in the ionic composition of precipitation samples stored at 25 °C for seven months and attributed the results to the preservation effect of low pH (
