Environ. Sci. Technol. 1998, 32, 1642-1647
Relation of Climate Change to the Acidification of Surface Waters by Nitrogen Deposition PETER S. MURDOCH,* DOUGLAS A. BURNS, AND GREGORY B. LAWRENCE Water Resources Division, U.S. Geological Survey, 425 Jordan Road, Troy, New York 12180
Abrupt increases and decreases in mean seasonal and annual stream NO3- concentrations during the period of record (1983-1995) at Biscuit Brook, a headwater stream in the Catskill Mountains of New York, have provided an opportunity to study the biogeochemical processes that control NO3- movement through forested watersheds. The Catskills receive the highest rate of NO3- deposition in the New York and New England region of the United States, and many streams have measurable NO3- concentrations throughout the growing season. Correlations between deposition and stream NO3- concentrations are not statistically significant. Stream NO3- concentrations are positively correlated with mean annual air temperature, suggesting that on a year-to-year basis rates of N mineralization and nitrification rather than deposition or vegetation uptake are the primary factors controlling nitrogen leaching from forests where nitrogen in excess of the biological demand is available. Results from nitrification and stable isotope studies are consistent with this conclusion. These data suggest that the release of NO3- to Catskill surface waters and the associated acidification would be enhanced in the short term through increases in mean annual air temperature.
Introduction Acid deposition and its effects remain a major concern in Europe and North America, and they are a developing problem in Asia, southern Africa, and South America (1). One cause of surface water acidification in areas of high nitrogen (N) deposition is the leaching of NO3- from soils (1, 2). While sulfate deposition has decreased significantly in the northeastern United States during the past 10 years, N deposition has shown little change during this period (3). The 1990 Clean Air Act Amendments are expected to result in a 20% reduction in NOx emissions by the year 2000, but that decrease may be largely reversed by the year 2010 (4). Furthermore, emissions of both NH4+ and NO3- are projected to increase globally, particularly in Asia (5). Acidification from nitrogen deposition, therefore, is a global concern. Comparisons of input-output budgets of N at several forested watersheds in the northeastern United States indicate that streamwater nitrate (NO3-) concentrations generally increase with increases in wet deposition of N above 300 equiv ha-1 yr-1 (6, 7). Studies of streamwater chemistry during rainstorms and snowmelt, however, have failed to identify a direct relation between rates of N deposition and * Corresponding author phone: (518)285-5663; fax: (518)285-5601; e-mail:
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stream NO3- concentrations (8), and other studies have demonstrated the complexity of determining a direct link between nitrogen deposition and increases in stream acidification (6, 9). The inability to directly link N deposition with stream acidification has, in part, been the result of the predominance of short data series that are subject to seasonal and climatic variability. Longer data sets that are averaged over time steps greater than seasonal periods are needed to separate the “signal from the noise” in the N depositionsurface water relation. As a result of the great uncertainty in the relation between N deposition and stream NO3concentrations, the U.S. Environmental Protection Agency has recently announced that it will not adopt new regulations for N emissions from U.S. industry (10). The term “nitrogen saturation” describes a condition in which the supply of atmospheric N to an ecosystem exceeds the combined plant and microbial nutritional demand to the extent that N leaches from the watershed into surface waters and groundwaters (11). As vegetation takes up a progressively smaller fraction of the available N in the soil, microbial processes should become an increasingly important control on the availability of NO3- for leaching (6). Rates of N mineralization and nitrification are sensitive to changes in temperature and moisture (12-14); therefore, the correlation between stream NO3- concentrations and climatic variables is expected to strengthen as N saturation increases. Long-term records of air temperature, stream discharge, stream NO3- concentration, and N deposition rates are used in this paper to assess the effect of N deposition and climate on stream NO3- concentrations in a headwater stream and to provide insight into processes controlling nitrogen movement in forested watersheds that are not discernible in short time-series data.
Site Description Data for this analysis were collected in the Biscuit Brook watershed, a steep, second-order drainage of 995 ha in the central Catskill Mountains (Figure 1). The watershed is 100% forested by northern hardwoods and has not been logged since the 1920s. Concentrations of NO3- are detectable in Biscuit Brook during summer baseflow conditions (2). Ammonium and dissolved organic N concentrations in streamwater are low or below detection limits except during peak spring runoff (Gary Lovett, Institute of Ecosystem Studies, written communication, 1996; 15). Soils are primarily acidic Inceptisols (pH range 3.3-4.2) with low availability of exchangeable bases (16, 17). The Catskill Mountains receive the highest rate of N deposition in the New York-New England region of the United States (10.5-12.5 kg ha-1 yr-1) (18). The 30-yr mean annual air temperature is 4.3 °C, and mean annual precipitation is 175 cm/yr at the National Weather Service Slide Mountain station 5 km east and at mid-elevation to the Biscuit Brook watershed (19). Detailed descriptions of the watershed are presented by Murdoch (15).
Methods Deposition volume and weekly wet deposition samples were collected at a National Trends Network (NTN) station near the mouth of Biscuit Brook, and streamwater samples were collected weekly by hand and by stage-activated automated samplers during 5-15 stormflows each year. Snowmelt was collected on a subevent basis in 122 cm × 91 cm PVC pan lysimeters positioned in typical hardwood stands in the Biscuit Brook Watershed. Wet precipitation samples were S0013-936X(97)00863-8 Not subject to U.S. copyright. Publ. 1998 Am. Chem.Soc. Published on Web 04/24/1998
FIGURE 1. Location of data collection sites in the Biscuit Brook Watershed, Ulster County, NY. analyzed at the Illinois State Water Survey Laboratory in Champaign, IL (20). Nitrate concentrations in snow and streamwater samples were measured by ion chromatography (21), and stream discharge was determined by standard USGS methods at a continuous gaging station (22). Laboratory QA/QC data and methods are presented in Lincoln et al. (23). Annual budget computations were based on a water year (WY), defined as the period from October 1 of the preceding year through September 30 of the designated water year. Each water year was divided into two broad seasons for computing budgetssa dormant season representing the period from leaf fall to the typical end of snowmelt (October 1-April 30) and a growing season (May 1-September 30). Additionally, stream NO3- concentrations at baseflow were determined using data collected from July 1 through September 30. Air temperature was measured at the National Weather Service station at Slide Mountain (19). Mean daily temper-
ature was computed as the mean of the maximum and minimum daily values, and mean seasonal and annual values were computed as the sum of mean daily temperatures divided by the number of days in the season or year. Deposition N loads for each period were computed as the sum of weekly N loads. Mean N concentrations in atmospheric deposition were computed as the N deposition load divided by the total volume of precipitation for the budget period. Daily NO3- concentrations in streamwater were estimated using a combination of concentration-discharge relations, and interpolation for periods when a significant relation between concentration and discharge could not be established. Stream NO3- yield was then computed as the sum of daily yields (daily NO3- concentration × daily discharge) for each budget period, and volume-weighted mean stream NO3- concentration was computed as the total yield divided by the total discharge. The net nitrification rate was estimated using two different in situ incubation methods in a mixed beech-maple stand about 2 km SE of the Biscuit Brook gage station. Soils were incubated during 4-week periods from April through November of 1993-1996. In 1993-1994, 15 cm2 of the combined Oe and Oa horizons were removed, placed in gas-permeable plastic bags, and then returned to the forest floor for incubation (24). In 1995-1996, soil samples were incubated in a 5 cm diameter PVC pipe that was hammered into the ground to a depth of about 20 cm. This method results in less disturbance of the soil than the buried bag method because the core remains intact. Leaching losses of NO3are believed to be minimal because the top of the pipe was capped during the incubation. A side-by-side comparison of the two techniques indicated no significant difference in results (p ) 0.07, two-tailed t test; n ) 32). Soil temperature was determined as the mean value of the pre- and postincubation measurements. The isotopic composition of NO3- in samples of streamwater, snow cores, and snowmelt during the spring of 1994 was determined by passing a sufficient volume of water through an anion-exchange resin (Cl- form) to collect 200 µmol of NO3-. The NO3- in the resin was then separated into N and O and analyzed by mass spectrometry according to methods described in Kendall et al. (25). Values of δ18O are reported here in per mil units relative to Vienna Standard Mean Ocean Water. δ15N values are reported in the same units relative to atmospheric N. The isotopic content of NO3derived from nitrification was determined by incubating combined Oa and Oe horizon soil for 3 weeks in a laboratory at room temperature (22 °C). The reported values represent the mean of extractions of five samples with 1 M KCl.
Results and Discussion Neither mean annual stream NO3- concentrations nor annual deposition rates showed temporal trends over the study period (Figure 2). The year with the highest mean annual air temperature of the record (1983-1994) was WY91, and the three years with the lowest temperatures were WY9294, which coincide with the low global temperatures associated with the eruption of Mt. Pinatubo (June 1991) (26) (Figure 2c). Relation of N Deposition and Temperature to Stream NO3- Concentrations. Recent studies have indicated a correlation between rates of nitrogen deposition and stream NO3- concentrations (6, 7) or total N concentrations in forest soils (27, 28). These studies were based on comparisons of sites that received differing rates of N deposition over several decades and are assumed to be a spatial representation of temporal changes at individual sites. Very few monitoring stations have a sufficient length and intensity of record to document changes in stream NO3- concentrations as a result of N deposition over several years. VOL. 32, NO. 11, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Annual (A) mean stream NO3- concentration, (B) atmospheric N deposition, and (C) mean air temperature at Biscuit Brook, water year (WY) 1984-1995. Correlations between volume-weighted stream NO3concentrations and annual and seasonal N deposition at Biscuit Brook are not statistically significant (r 2 < 0.1, p > 0.1). This relation was not improved by comparing stream concentrations to mean annual or seasonal deposition N concentrations (r 2 < 0.1, p > 0.3). Correlations between volume-weighted mean stream NO3- concentrations and annual and seasonal mean air temperature, however, indicate a positive relation that is statistically significant for annual mean values (r 2 ) 0.31, p ) 0.05) (Figure 3a-c). Multiple regression of mean annual stream NO3- concentrations and several predictor variables (N deposition, deposition N concentration, deposition volume, streamflow, air temperature, etc.) indicate that neither annual N deposition nor deposition N concentration are statistically significant predictor variables. Seasonal relations between air temperature and stream NO3- concentrations are not as strong as the annual relations (Figure 3b,c) but each indicates a positive slope. WY 90 stands out in each of these correlations as an outlier. N deposition rates were significantly higher during the dormant season of WY90 than during other dormant seasons of the record (Figure 4). Parallel patterns in snowmelt lysimeter and streamwater N concentrations indicate that high-concentration meltwater entered the soil and surface waters in early and mid-January of 1990 (Figure 5a,b). Additionally, record-low mean daily air temperatures that occurred throughout the Northeast during December 1989 (-12.7 °C) combined with a thin snow cover may have caused deep soil freezing (Figure 5c) (29). While no soil temperature data were being collected in the watershed at this time, the forest floor and O horizon were frozen during excavation of soil in December 1989. Frozen soil resulting in reduced permeability is a possible cause of the direct transport of NO3- from the snowpack to the stream and appears to have caused a late-spring release of labile nitrogen from the lysing of roots and microbes (29). Experiments in which Canadian 1644
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FIGURE 3. Relations between stream NO3- concentration and air temperature at Biscuit Brook, WY 1984-1995. (A) Annual, (B) dormant season, and (C) growing season.
FIGURE 4. Dormant season (A) load and (B) concentration of N deposition at Biscuit Brook, WY 1984-1995. soils were exposed to below-freezing temperatures showed that freezing of forest soils can induce NO3- release (30). Elevated stream NO3- concentrations were reported in WY90 at three other research watersheds in the northeastern United States that also had low early-winter air temperatures and indications of soil freezing and that received high dormant season deposition of N (29, 31). Therefore, substantial evidence exists that N-leaching conditions during WY90 were
FIGURE 6. Relation of soil temperature to soil net nitrification rates for monthly incubations from April through November in a deciduous forest site near Biscuit Brook in the Catskill Mountains, NY, July 1993-July 1996.
FIGURE 5. Nitrate concentrations in (A) snowmelt and (B) streamwater during the snowmelt period of water year 1990. (C) Mean December air temperature at Biscuit Brook, WY 1984-1995. unique relative to the other years of the record. If WY90 is excluded from the data, the relation between annual volumeweighted mean stream NO3- concentration and annual mean air temperature improves significantly (r 2 ) 0.66, p ) 0.001). A nonparametric test (Kendall’s τ) that includes WY90 results in a significant rank-correlation between volume-weighted mean annual stream NO3- concentration and mean annual air temperature (τ ) 0.76; p ) 0.011). Regardless of whether WY90 is included or excluded from the regression analysis, the results indicate that the year-to-year variability in NO3leaching to streams during the available period of record is more strongly affected by temperature-dependent rates of N processing in the watershed than by rates of N deposition. Supporting Evidence of a Temperature-Stream Nitrate Relation. Data from the in-situ soil nitrification experiments indicated a positive correlation between soil temperature and nitrification rates that are consistent with a positive correlation between air temperature and stream NO3concentration. Net nitrification rates in the forest floor are high relative to other forested study sites in the northeastern United States (32) and are positively related to soil temperature in a given year (r 2 ) 0.4, p < 0.001 for three combined years, r 2 ) 0.7-0.8 for individual years) (Figure 6). Soil moisture was not significantly correlated with soil nitrification rates during WY93-96 (p ) 0.30), nor was moisture a significant factor as part of a multiple regression model to predict soil nitrification rates. Stark (33) has recently shown that optimum temperatures for nitrification are higher than typically observed under field conditions. Therefore, increases in air temperatures above those ambient conditions could yield even greater rates of nitrification and leaching.
Our results are consistent with these findings and illustrate both the sensitivity of these microbial processes to changes in temperature and how that sensitivity influences rates of N leaching from forested ecosystems. If the turnover of N by soil microbes is a major processing step between N deposition and the leaching of NO3- to the stream, then the δ18O of stream NO3- should be similar to that of NO3- derived from nitrification in the soil. The mean δ18O value of NO3- in the lab soil incubations was 14.7‰, and the values at Biscuit Brook during the snowmelt of 1994 ranged narrowly from +13 to +15‰. In contrast, precipitation as measured in snow cores and a snow lysimeter had much greater δ18O values of NO3- that ranged from +25 to +38‰. The δ15N values of Biscuit Brook streamwater varied from +1 to +3‰ during this sampling period, whereas snowcores and snowmelt varied more widely from -1 to +9‰. Data collected more recently from a watershed about 2 km SE of Biscuit Brook confirms that streamwater maintains distinctive δ18O values of NO3- that typically differ from those of precipitation by more than 20‰. Even during peak snowmelt periods, when large amounts of NO3- from precipitation were being flushed through the watershed with melting snow, the isotopic data indicate that nearly all of the stream NO3- consisted of microbially processed N. Results of the isotope and nitrification studies are consistent with the observed seasonal and annual relations between air temperature and stream NO3- concentrations. Except during unique hydrologic conditions such as observed during WY90, most if not all of the N entering Biscuit Brook has been subject to microbial processing within the watershed. Effect of Temperature on Baseflow NO3- Concentrations. The accepted conceptual model for surface water response to N saturation predicts that increased NO3- will first be observed in streams during periods of increased stream discharge in the dormant season as excess NO3- is washed from the watershed (6). With time and increased saturation, these episodic releases of NO3- would become longer and coalesce, leading to detectable concentrations during base flow as the NO3- infiltrates into the groundwater system. The transition from vegetation to microbial regulation of N release, with microbial control slowly broadening from the dormant season into the growing season as N saturation increases, could be the process underlying these observed patterns. Dormant season stream N yield was used in this study as an indicator of the amount of N available for groundwater recharge each year. If only the late growing season months (July-September) are used in computing a mean summer stream NO3- concentration (when the relative groundwater contribution to baseflow is greatest), 65% (p < 0.01) of the variability in late-summer NO3- concentrations in the stream VOL. 32, NO. 11, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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deposition. Despite a 30-year decline in atmospheric SO42deposition, acidified surface waters in the northeastern United States have shown minimal recovery because replenishment of available soil pools is often impeded by slow weathering rates (38-40). In these base-cation-depleted landscapes, short-term increases in NO3- leaching as a result of warm temperatures could enhance the leaching of base cations and thus have effects on soil nutrient availability that extend beyond the warm period. Recent results from David et al. (41) clearly show that annual N deposition in any given year is a small fraction of the soil N pool within a forested ecosystem. The data presented here suggest that if atmospheric N deposition declines, the effect of that decline in N-saturated ecosystems might not be immediately detectable in streamwater. The rate of release of N from these large soil pools in a given year will be dependent on processes that are sensitive to temperature, not rates of N deposition. Acidification of streams and base-cation depletion in soils by N deposition could thus be enhanced in the short term by climate warming and diminished by cooling, barring changes in the cycling of N.
Acknowledgments
FIGURE 7. Relation of base flow stream NO3- concentration during the late summer to (A) dormant season stream yield and (B) mean annual air temperature (in °C). can be explained by dormant season stream yield (Figure 7a). The relation between dormant season stream NO3concentration and late-summer stream NO3- concentration is also significant (r 2 ) 0.56; p < 0.01). Stream NO3concentrations in late-summer baseflow are also positively correlated with mean annual air temperature (r 2 ) 0.51; p < 0.01) (Figure 7b). Correlations between annual and dormant season deposition and summer baseflow NO3concentrations are weak (p > 0.1). The positive relation between dormant season stream yield and late-summer base flow NO3- concentrations in the stream indicates that conditions described in Stoddard’s stage 2 of N saturation have developed in the Biscuit Brook Watershed (6). The strong correlation observed between annual air temperature and base flow stream NO3- concentrations, combined with the results of the nitrification and isotope studies presented earlier, suggest that temperature-dependent processes within the watershed are also controlling NO3- concentrations entering the groundwater system. Long-Term Effect of Temperature on N Saturation and Soil Base-Cation Depletion. The temperature-dependent processes described in this paper may not change the longterm trend of increasing NO3- leaching resulting from N deposition, but they may explain short-duration (up to several years) variability in a saturation-induced trend and could cause an acceleration of that trend over time if climate warming occurs. In light of the recent proclamation of the Intergovernmental Panel on Climate Change that the global climate is being altered by man’s activity, the scenario of increased nitrogen leaching with increased temperatures in the future merits research consideration (34). Even shortterm periods of enhanced NO3- leaching induced by warm temperatures can have significant long-term effects on the nutrient status of forest soils. Studies of temporal trends in base-cation export from forested watersheds in North America (35, 36) and Europe (37) have shown significant depletion of exchangeable Ca as a result of leaching by acidic 1646
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Funding for this research has been provided by the U.S. Geological Survey, the U.S. Environmental Protection Agency, and the New York City Department of Environmental Protection. The monitoring stations are operated at the consent of the Frost Valley YMCA Environmental Education Center on whose property they are located and who use the research facilities to teach students about the environment.
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Received for review September 29, 1997. Revised manuscript received March 6, 1998. Accepted March 23, 1998. ES9708631
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