The recent recognition of widespread and significant upward trends in dissolved organic carbon (D... more The recent recognition of widespread and significant upward trends in dissolved organic carbon (DOC) concentrations in surface waters of northeastern North America and Europe has stimulated research to understand the cause of these trends. Several factors have been offered to explain these DOC trends including climate warming, chronic atmospheric nitrogen deposition, decreasing atmospheric sulfur deposition, and increasing surface water pH. Changes in these factors have acted to either increase the solubility of DOC or increase the rates of biogeochemical processes that generate labile carbon in the soil. Additionally, it is well known that rain events and snowmelt increase DOC concentrations in many surface waters through flushing along shallow flow paths where most labile carbon is stored. Changes in hydrologic flushing rates have generally not been explored as a possible explanation of these widely reported upward trends in DOC concentrations. Biscuit Brook, a 9.9 km2 catchment in the Catskill Mountains of New York has shown a significant increasing trend in DOC concentrations since 1992, consistent with other streams in this region. Stream chemistry has been monitored at Biscuit Brook on a weekly basis supplemented with event samples since 1983, providing a detailed data set with which to examine the causes of changes in DOC concentrations. Here, we examine the relative roles of climate warming, decreasing sulfate (SO42-) and nitrate (NO3-) concentrations, and changes in the frequency and size of hydrologic events on the long-term temporal pattern (1992 to 2004) of DOC concentrations in Biscuit Brook. DOC concentrations increased significantly in weekly samples collected primarily during low flow conditions. No similar trend was apparent in the high flow samples. Mean annual SO42- plus NO3- concentrations showed a strong inverse relation (r2 = 0.91, p < 0.01) to DOC concentrations, but these concentrations were not related to stream pH nor to air temperature. These results suggest that the DOC trends largely result from biogeochemical processes associated with the decreasing SO42- and NO3- concentrations such as an increase in the solubility of DOC caused by the decreasing ionic strength of the stream water. The frequency of hydrologic events as determined by hydrograph separation has not changed during 1992 to 2004, so it seems unlikely that changes in hydrologic flushing can explain the upward DOC trends in this stream. Events of similar size in the latter part of the record (2000 to 2004) produced higher DOC concentrations than were evident in the early part of the record (1992 to 1996) suggesting that changes in biogeochemistry and not hydrology are largely responsible for the trends observed at this site, which is representative of upland forested catchments of northeastern North America.
Human activities have considerably altered nitrogen cycling in the environment, increasing reacti... more Human activities have considerably altered nitrogen cycling in the environment, increasing reactive nitrogen (N) inputs to terrestrial and aquatic ecosystems. In the northeastern USA, this enhanced supply of N has been linked to many environmental concerns such as eutrophication, violations of drinking water standards, and forest health decline. Our work concentrates on atmospheric N inputs to ecosystems throughout New York state, which experiences among the highest rates of atmospheric N deposition in the nation. We've focused on a set of measurements at Connecticut Hill, which participates in several national atmospheric deposition monitoring networks (National Atmospheric Deposition Program, Clean Air Status and Trends Network, and the Atmospheric Integrated Research Monitoring Network). There we've co-located a network of 30 passive collectors for sampling multiple N species in dry deposition (HNO3, NO2, NH3). Our preliminary data allow us to address questions of: 1) within-site spatial variability; 2) deployment times and temporal averaging with regard to the frequency of sample collection; 3) how passive sampler concentrations compare to the suite of constituents measured by the national network instrumentation; 4) how local meteorological conditions influence concentrations; and 5) whether the isotopic composition of nitrate-N (δ 18O and δ 15N) in passively sampled dry deposition can be used to understand the relative contributions of different sources of atmospheric N. Further, we are quantifying sources of N fluxes in surface waters. We make use of a variety of isotopic and chemical tracers in stream water samples to distinguish among atmospheric and other (e.g., fertilizer or wastewater) sources. Our collaborative study aims to characterize atmospheric N sources, which is necessary to develop sound strategies for understanding and managing the effects of these and other N inputs.
A 24 ha catchment in the Catskill Mountains of southeastern New York was clearcut during the wint... more A 24 ha catchment in the Catskill Mountains of southeastern New York was clearcut during the winter of 1996-97. Soil water from the O-, upper B-, and lower B horizons was examined for interactions between inorganic monomeric aluminum (Alim) and nitrate (NO3-), dissolved organic carbon (DOC), pH, and base cations to discern how Alim was released from soils to stream water after the disturbance. Alim at concentrations greater than 2 μ moles l-1 can be toxic to some fish species and can inhibit the uptake of calcium by tree roots thereby decreasing tree tolerance to stress. Alim was strongly correlated with NO3- in upper and lower B-horizon soil water (r2 = 0.67 and 0.68 respectively), but the relation was much weaker in O-horizon soil water (r2 = 0.40). O-horizon soil water had the lowest pH values despite having lower NO3- concentrations than were measured in the B-horizon; high DOC concentrations in O-horizon soil water suggest that the acidity was partly due to organic acids. The O-horizon also had higher exchangeable base cations than the B-horizon that buffered the inorganic acidity produced by NO3- after the clearcut. The high organic content of the O-horizon also allowed for organic complexation of Al as indicated by the strong correlation between DOC and organic monomeric Al (r2 = 0.67). Alim concentrations were much higher and DOC concentrations were much lower in B-horizon soil water than in the O-horizon and in the B-horizon the high Alim concentrations persisted for a year longer after the clearcut. Alim concentrations in groundwater seeps were consistently low because mineral dissolution of base cations provided a high buffering capacity; as a result water with high NO3- concentration was buffered by base cations rather than by Alim. In contrast, B-horizon soil water, which had low buffering capacity and low DOC concentration, contributed large amounts of Alim to stream water, especially at NO3- concentrations above 100 μ moles l-1, an apparent threshold above which Alim was needed to buffer acidity that exceeded the buffering capacity of base cations within the B-horizon. In conclusion, DOC concentration and base-cation availability were major controls on Alim release from soils after the clearcut, yet, Alim concentrations in stream water draining the catchment greatly exceeded the 2 μ moles l-1 toxicity threshold during the first 2 years after the clearcut. An increase in exchangeable Al and a decrease in exchangeable base cations in the soil after the clearcut caused a decrease in the soil Ca:Al ratio; this decrease has been shown to cause tree stress, inhibit regrowth, and result in long-term forest decline.
Models of the effects of atmosphericN deposition in forested watersheds have notadequately accoun... more Models of the effects of atmosphericN deposition in forested watersheds have notadequately accounted for the effects of aquatic andnear-stream processes on the concentrations and loadsof NO &amp;lt;img src=&quot;/fulltext-image.asp?format=htmlnonpaginated&amp;amp;src=P72327630Q4G5565_html\10533_2004_Article_150174_TeX2GIFIE2.gif&quot; border=&quot;0&quot; alt=&quot; $$_3^ - $$ &quot; /&amp;gt; in surface waters. This studycompared the relative effects of aquatic andnear-stream processes with those from the terrestrialecosystem on the retention and transport ofNO &amp;lt;img src=&quot;/fulltext-image.asp?format=htmlnonpaginated&amp;amp;src=P72327630Q4G5565_html\10533_2004_Article_150174_TeX2GIFIE3.gif&quot;
ABSTRACT Clearcutting of northern hardwood forests in mountainous landscapes of the northeastern ... more ABSTRACT Clearcutting of northern hardwood forests in mountainous landscapes of the northeastern U.S. has been shown to cause large increases in stream nitrate (NO3-) concentrations accompanied by increased stream acidity, elevated losses of nutrient base cations, and aluminum concentrations sufficient to be toxic to brook trout. An 18 ha clearcut in the Catskill Mountains of southeastern New York, USA in 1997 resulted in stream NO3- concentrations that peaked at &gt; 1,000 mumol L-1, and base cation (Ca2+, Mg2+, K+) concentrations that increased by more than three-fold during the first year after harvest. In contrast, previous timber-stand improvement harvests in 1995 and 1996 in which &lt; 10% of tree basal area was removed from watersheds, resulted in no measurable change in stream water chemistry. Based on an analysis of these previous data, we hypothesized that there is likely a harvest threshold for changes in stream-water chemistry below which only minimal and tolerable changes in water quality occur. We tested this hypothesis by completing four forest harvests during 2002 to 2006 in which varying amounts of basal area were removed from northern hardwood forest plots and watersheds. These results have shown that at a basal area removal of about 33%, stream NO3- and K+ concentrations increased, but less than proportionally to the concentration changes observed after the clearcut. Calcium and Mg2+ concentrations increased as well, but these changes were about proportional to the concentration changes observed after the clearcut. Additionally, stream NO3- concentrations returned to background values within two years at the 33% partial harvest compared to about eight years in the clearcut. Soil-water lysimeter data from two other harvests in which 30 and 50% of basal area were removed from hardwood forest plots are consistent with that of the previous partial harvest. Nitrate concentrations increased less than proportionally to the changes observed after the clearcut, and returned rapidly to background values. These data suggest that expansion of the crowns and root networks of remaining trees can partially compensate for the removal of up to 50% of tree basal area resulting in minimal and short-lived changes in water quality associated with partial harvesting.
Inputs of reactive nitrogen (N) to watersheds in the northeastern USA are linked to environmental... more Inputs of reactive nitrogen (N) to watersheds in the northeastern USA are linked to environmental concerns such as acidification of upland ecosystems, accumulation of N in groundwater, and eutrophication of coastal waterways. Nitrogen sources include atmospheric deposition, fertilizer, and human and animal waste, and the relative strength of these sources has been shown to vary with watershed land use. Attribution of N sources is important in developing effective policy strategies for reducing N loads in streams and rivers. Here, we use dual (δ15N, δ18O) isotope analysis of nitrate as well as chemistry data from six streams in New York to determine dominant sources of N and to learn more about how N cycles through these watersheds. Land use in these watersheds included 100% forested land in each of two streams in the Catskill and Adirondack Mountains, 34 to 70% agricultural land (primarily dairy farms) in three streams, and 56% suburban land in one stream. Nitrate in atmospheric deposition had δ18O values that ranged from about +65 to +85‰ in precipitation and a similar range in dry deposition, and δ15N values that ranged from about -5 to +5‰ in precipitation and about -2 to +15‰ in dry deposition. Nitrate in stream water from the dominantly forested watersheds had δ18O values that ranged from +5 to +15‰, indicating that the dominant immediate source was from nitrification in soils, but with a tendency towards higher direct contributions of atmospheric nitrate during high flow. The two streams with the highest percent of agricultural land use had isotope values that indicated a mixture of nitrate sources from soil nitrification and animal waste. These data fall along a line with a δ18O:δ15N slope of 1:2 indicating a varying source that was denitrified. Nitrate in the watershed dominated by suburban land use had δ18O values as high as +30‰, indicating a large direct contribution of atmospheric nitrate transported over impervious surfaces and discharged to the stream via storm drains. These data show an inverse relation between δ18O and δ15N values suggesting a mixture of human waste and direct atmospheric sources of nitrate. Together, these chemistry and isotope data indicate that in streams with widely varying land uses, nitrate concentrations can be quite similar, despite widely varying sources. Stream nitrate reflects the mixing of N sources as attenuated and transformed by watershed biogeochemical cycling processes.
Water residence times and nutrient budgets in 3 small watersheds in the Croton water supply area,... more Water residence times and nutrient budgets in 3 small watersheds in the Croton water supply area, NY, were examined. The watersheds (less than 1km 2) have different level of urbanization (natural, semi-developed and fully developed), different mechanisms of runoff generation (quick flow on roads and slow flow through subsurface) and different watershed landscape characteristics (wet zones, hillslopes) . Measurements of the comprehensive chemical suite incl. components of nitrogen budget in the throughfall, stream water, soil water and groundwater in the saturated zone were performed bi-weekly over a period up to 2 years. Mean water residence times of the stream water were estimated using Oxygen-18 and Helium-3/Tritium isotopes. There are significant differences in the chemical composition and decrease of nitrification intensity and of mean streamwater residence time along the increasing watershed development. Within each watershed, longer water residence times (up to over 2 years) were estimated in the wetland zones without direct communication with streams in comparison to hillslope areas (up to over 1 year). The results can be used in watershed management and planning of the further urbanization of this water supply area.
The 24-ha Dry Creek watershed in the Catskill Mountains of New York State was clear-cut during 19... more The 24-ha Dry Creek watershed in the Catskill Mountains of New York State was clear-cut during 1997 to evaluate nutrient release to New York City reservoirs due to forest harvesting. The Dry Creek watershed is in the headwaters of the Neversink watershed, which is part of the New York City Reservoir system that supplies drinking water to over 20 million people. Soil water, groundwater seeps, and stream water chemistry were monitored to trace the transport of solutes before and after the timber harvest. Automated sequential zero-tension lysimeters and standard zero-tension lysimeters were installed at depths of 70, 300, and 500 mm to sample soil water in the O, B, and C-horizons, respectively. Pre-cut (water years 1993-1996) mean soil water concentrations from zero tension lysimeters indicate that O-horizon soil water (70 mm depth) had the highest nitrate (NO3-) and monomeric aluminum (Alm) concentrations (73 and 18 μmoles l-1, respectively). During that same time period water from ground-water seeps had lower NO3- and Alm concentrations (22 and 0.88 μmoles l-1, respectively) than any soil waters sampled. During the two years following the clear-cut, groundwater seep NO3- concentrations were 138-123 μmoles l-1 and Alm concentrations were 50-30 μmoles l-1 lower than that measured in soil water. Throughout the same time period, B-horizon soil water had the highest mean NO3- concentration (345 μmoles l-1) while C-horizon soil water had the highest mean Alm concentrations (51 μmoles l-1). But during storms in the first year after the clear-cut O-horizon soil water NO3- and Alm concentrations often peaked at more than twice those measured in the B-horizon. During the second year after the clear-cut, B-horizon storm NO3- concentrations were consistently greater than O-horizon concentrations. During the fourth and fifth years following the clear-cut, soil water NO3- concentrations had dropped below pre-cut concentrations however NO3- in groundwater seeps remained elevated. The NO3- concentration at the watershed outlet also remained above pre-cut levels. During the first years following the clear-cut, in the absence of watershed vegetation, soil NO3- was leached to watershed streams and to deeper groundwater. As the forest has regenerated soil NO3- has been immobilized while groundwater continues as a source of NO3- to watershed streams 4-5 years after the cut. Four to five years after the clear-cut Alm concentrations were below pre-cut levels for all waters sampled. The elevated stream water NO3- concentrations that continue to be measured at the stream outlet, are not accompanied by elevated Alm concentrations since the groundwater seeps that are the source of the NO3- have never been a significant source of Alm.
Catskill Mountain streams in New York often receive pulses of NO3 during storms and snowmelt from... more Catskill Mountain streams in New York often receive pulses of NO3 during storms and snowmelt from watershed soils and acid deposition. This "flushing effect" of nutrients and acids was documented in forest-soil water through use of sequential lysimetry, in which soil-water was collected in equal-volume increments from zero-tension lysimeters placed within the soil profile. Stormflow in previously unsaturated soils began with percolation of water through the upper soil to the B horizon. NO3 pulses in soilwater during storms and snowmelt were typically delayed in the B-horizon relative to the O-horizon, indicating a percolation process for soil water movement between the lysimeters, but during the fall rains and spring snowmelt periods water-table measurements indicated that the B-horizon lysimeter was overtopped by the water table. Net flux was similar from throughfall and 0-lysimeters for individual events, suggesting that throughfall could either percolate unchanged through the soil column or that microbial release of N was rapid and similar to throughfall inputs. Patterns of change in O-Lysimeter concentrations during individual storms indicate repeated backflushing periods in which percolating acidic deposition or snowmelt from an individual event rose back into the O-horizon with a rising water table. This back-flushing phenomena may be enhancing the leaching of soil calcium beyond what would occur if acidic deposition was only percolating once through the O horizon . The sequential lysimetry method allowed investigators a more detailed look at N dynamics in forest soils than has been possible through the typical monthly lysimeter sampling strategy.
High levels of mercury (Hg) in aquatic biota have been identified in surface waters of the Adiron... more High levels of mercury (Hg) in aquatic biota have been identified in surface waters of the Adirondack region of New York, and factors such as the prevalence of wetlands, extensive forest cover, and oligotrophic waters promote Hg bioaccumulation in this region. Past research in this region has focused on improved understanding of the Hg cycle in lake ecosystems. In the study described herein, the landscape controls on total Hg and methylmercury (MeHg) concentrations in riverine ecosystems were explored through synoptic surveys of 27 sites in the upper Hudson River basin of the Adirondack region. Stream samples were collected and analyzed for total Hg, MeHg, dissolved organic carbon (DOC), and ultraviolet absorbance at 254 nm (UV254) during spring and summer of 2006-08. Landscape indices including many common land cover, hydrographic, and topographic-based measures were explored as predictors of Hg through multivariate linear regression. Multivariate models that included a wetland or riparian area-based metric, an index for open water area, and in some cases a topographic metric such as the wetness index explained 55 to 65 percent of the variation in MeHg concentrations, and 55 to 80 percent of the variation in total Hg concentrations. An open water index (OWI) was developed that incorporated both the basin area drained by ponded water and the surface area of these ponds. This index was inversely related to concentrations of total Hg and MeHg. This OWI was also inversely related to specific ultra-violet absorbance, consistent with previous studies showing that open water increases the influence of algal-derived carbon on DOC, decreasing aromaticity, which should decrease the ability of the dissolved carbon pool to bind Hg. The OWI was not significant in models for total Hg that also included UV254 as a predictive variable, but the index did remain significant in similar models for MeHg suggesting that biogeochemical factors in addition to decreasing carbon aromaticity limit MeHg concentrations in this river basin. These data are consistent with removal of MeHg by photo-reduction and volatilization in open waters. Study results, in addition to confirming the importance of riparian wetlands as sources of Hg to flowing waters, also highlight that open water bodies can limit the downstream transport of Hg in river networks. These results may be broadly applicable in northeastern North America and other settings where rivers consist of linked open water bodies.
We quantified the response of vegetation and nutrient uptake in a northern hardwood forest in sou... more We quantified the response of vegetation and nutrient uptake in a northern hardwood forest in southeastern New York for three to four years after three intensities of harvesting: clearcutting, heavy timber stand improvement (TSI), light TSI (97, 29, and 10% basal area reductions, respectively). We also quantified effects of white-tailed deer (Odocoileus virginianus) herbivory on nutrient retention by vegetation. Total biomass and nutrient accumulation in vegetation was higher after TSI than clearcutting in the first two years but was highest in the fenced clearcut in subsequent years, indicating that TSI or partial harvesting is a viable management tool for harvesting timber while consistently maintaining high rates of nutrient retention. After clearcutting, biomass and nutrient retention were initially dominated by woody stems <1.4 m tall and herbaceous vegetation, but saplings 0.1-5.0 cm DBH became the most important contributors to biomass and nutrient accumulation within four years. However, after both intensities of TSI, trees >5.0 cm DBH continued to account for most biomass and nutrient accumulation whereas understory vegetation accumulated little biomass or nutrients. Heavy TSI resulted in increased regeneration of only two tree species (Acer pensylvanicum, Fagus grandifolia), but clearcutting allowed these two species, mature forest species (A. saccharum, Betula alleghaniensis), and the early successional Prunus pensylvanica to regenerate. Several early successional shrub and herbaceous species were also important to nutrient retention after clearcutting, including Polygonum cilinode, Rubus spp., and Sambucus racemosa. Herbivory by white-tailed deer dramatically reduced biomass and nutrient accumulation by woody stems <5 cm DBH after clearcutting (5.5 vs. 0.7 Mg biomass/ha and 30.4 vs. 6.3 kg N/ha on fenced and unfenced clearcut sites, respectively, after four years), indicating the important influence this herbivore can have on nutrient retention in recently disturbed forests.
The Catskill Mountains of southeastern New York receive atmospheric nitrogen (N) deposition of 10... more The Catskill Mountains of southeastern New York receive atmospheric nitrogen (N) deposition of 10 to 15 kg-1ha-1yr-1, and streams in the region show a wide range of nitrate (NO3^{-}) concentrations from near 0 to 100 \mumolL^{-1}. Past research indicates that most NO_{3}- in streams in this region originates through nitrification in the soil. Past forest and soil disturbance is one of the major factors that affects nitrification. A 23-ha tributary watershed of the Neversink River was clearcut in 1997 to understand how the resulting disturbance affects the N cycle. Stream NO3^{-} concentrations briefly increased above 1000 \mumolL^{-1} about 5 months after the cut, and remained above 200 \mumolL^{-1} for almost 2 years. Nitrogen budgets indicate that most of the stream NO_{3}- originated through nitrification in the soil, from which increased amounts of NO3^{-}$ leached to surface water in the absence of competition with uptake by vegetation. Past research in a variety of settings across North America has shown that clearcutting increases N-mineralization and nitrification rates in forest soil, but we found no increase in the net rates of either of these processes during monthly in-situ incubations. This seeming paradox is attributed to limitations of the in-situ incubation method, which measures only the net rates of these processes in the absence of N uptake by vegetation. The clearcut in this Catskill setting (in which high net nitrification rates were measured prior to harvesting) did not cause large enough changes in physical factors that affect this microbial process, such as soil temperature and moisture, to produce a measurable change in the net rate of nitrification. Consequently, nitrate concentrations in stream water decreased sharply during the third and fourth years after the clearcut in response to a second growth forest dominated by pin cherry (Prunus pennsylvanica).
The recent recognition of widespread and significant upward trends in dissolved organic carbon (D... more The recent recognition of widespread and significant upward trends in dissolved organic carbon (DOC) concentrations in surface waters of northeastern North America and Europe has stimulated research to understand the cause of these trends. Several factors have been offered to explain these DOC trends including climate warming, chronic atmospheric nitrogen deposition, decreasing atmospheric sulfur deposition, and increasing surface water pH. Changes in these factors have acted to either increase the solubility of DOC or increase the rates of biogeochemical processes that generate labile carbon in the soil. Additionally, it is well known that rain events and snowmelt increase DOC concentrations in many surface waters through flushing along shallow flow paths where most labile carbon is stored. Changes in hydrologic flushing rates have generally not been explored as a possible explanation of these widely reported upward trends in DOC concentrations. Biscuit Brook, a 9.9 km2 catchment in the Catskill Mountains of New York has shown a significant increasing trend in DOC concentrations since 1992, consistent with other streams in this region. Stream chemistry has been monitored at Biscuit Brook on a weekly basis supplemented with event samples since 1983, providing a detailed data set with which to examine the causes of changes in DOC concentrations. Here, we examine the relative roles of climate warming, decreasing sulfate (SO42-) and nitrate (NO3-) concentrations, and changes in the frequency and size of hydrologic events on the long-term temporal pattern (1992 to 2004) of DOC concentrations in Biscuit Brook. DOC concentrations increased significantly in weekly samples collected primarily during low flow conditions. No similar trend was apparent in the high flow samples. Mean annual SO42- plus NO3- concentrations showed a strong inverse relation (r2 = 0.91, p < 0.01) to DOC concentrations, but these concentrations were not related to stream pH nor to air temperature. These results suggest that the DOC trends largely result from biogeochemical processes associated with the decreasing SO42- and NO3- concentrations such as an increase in the solubility of DOC caused by the decreasing ionic strength of the stream water. The frequency of hydrologic events as determined by hydrograph separation has not changed during 1992 to 2004, so it seems unlikely that changes in hydrologic flushing can explain the upward DOC trends in this stream. Events of similar size in the latter part of the record (2000 to 2004) produced higher DOC concentrations than were evident in the early part of the record (1992 to 1996) suggesting that changes in biogeochemistry and not hydrology are largely responsible for the trends observed at this site, which is representative of upland forested catchments of northeastern North America.
Human activities have considerably altered nitrogen cycling in the environment, increasing reacti... more Human activities have considerably altered nitrogen cycling in the environment, increasing reactive nitrogen (N) inputs to terrestrial and aquatic ecosystems. In the northeastern USA, this enhanced supply of N has been linked to many environmental concerns such as eutrophication, violations of drinking water standards, and forest health decline. Our work concentrates on atmospheric N inputs to ecosystems throughout New York state, which experiences among the highest rates of atmospheric N deposition in the nation. We've focused on a set of measurements at Connecticut Hill, which participates in several national atmospheric deposition monitoring networks (National Atmospheric Deposition Program, Clean Air Status and Trends Network, and the Atmospheric Integrated Research Monitoring Network). There we've co-located a network of 30 passive collectors for sampling multiple N species in dry deposition (HNO3, NO2, NH3). Our preliminary data allow us to address questions of: 1) within-site spatial variability; 2) deployment times and temporal averaging with regard to the frequency of sample collection; 3) how passive sampler concentrations compare to the suite of constituents measured by the national network instrumentation; 4) how local meteorological conditions influence concentrations; and 5) whether the isotopic composition of nitrate-N (δ 18O and δ 15N) in passively sampled dry deposition can be used to understand the relative contributions of different sources of atmospheric N. Further, we are quantifying sources of N fluxes in surface waters. We make use of a variety of isotopic and chemical tracers in stream water samples to distinguish among atmospheric and other (e.g., fertilizer or wastewater) sources. Our collaborative study aims to characterize atmospheric N sources, which is necessary to develop sound strategies for understanding and managing the effects of these and other N inputs.
A 24 ha catchment in the Catskill Mountains of southeastern New York was clearcut during the wint... more A 24 ha catchment in the Catskill Mountains of southeastern New York was clearcut during the winter of 1996-97. Soil water from the O-, upper B-, and lower B horizons was examined for interactions between inorganic monomeric aluminum (Alim) and nitrate (NO3-), dissolved organic carbon (DOC), pH, and base cations to discern how Alim was released from soils to stream water after the disturbance. Alim at concentrations greater than 2 μ moles l-1 can be toxic to some fish species and can inhibit the uptake of calcium by tree roots thereby decreasing tree tolerance to stress. Alim was strongly correlated with NO3- in upper and lower B-horizon soil water (r2 = 0.67 and 0.68 respectively), but the relation was much weaker in O-horizon soil water (r2 = 0.40). O-horizon soil water had the lowest pH values despite having lower NO3- concentrations than were measured in the B-horizon; high DOC concentrations in O-horizon soil water suggest that the acidity was partly due to organic acids. The O-horizon also had higher exchangeable base cations than the B-horizon that buffered the inorganic acidity produced by NO3- after the clearcut. The high organic content of the O-horizon also allowed for organic complexation of Al as indicated by the strong correlation between DOC and organic monomeric Al (r2 = 0.67). Alim concentrations were much higher and DOC concentrations were much lower in B-horizon soil water than in the O-horizon and in the B-horizon the high Alim concentrations persisted for a year longer after the clearcut. Alim concentrations in groundwater seeps were consistently low because mineral dissolution of base cations provided a high buffering capacity; as a result water with high NO3- concentration was buffered by base cations rather than by Alim. In contrast, B-horizon soil water, which had low buffering capacity and low DOC concentration, contributed large amounts of Alim to stream water, especially at NO3- concentrations above 100 μ moles l-1, an apparent threshold above which Alim was needed to buffer acidity that exceeded the buffering capacity of base cations within the B-horizon. In conclusion, DOC concentration and base-cation availability were major controls on Alim release from soils after the clearcut, yet, Alim concentrations in stream water draining the catchment greatly exceeded the 2 μ moles l-1 toxicity threshold during the first 2 years after the clearcut. An increase in exchangeable Al and a decrease in exchangeable base cations in the soil after the clearcut caused a decrease in the soil Ca:Al ratio; this decrease has been shown to cause tree stress, inhibit regrowth, and result in long-term forest decline.
Models of the effects of atmosphericN deposition in forested watersheds have notadequately accoun... more Models of the effects of atmosphericN deposition in forested watersheds have notadequately accounted for the effects of aquatic andnear-stream processes on the concentrations and loadsof NO &amp;lt;img src=&quot;/fulltext-image.asp?format=htmlnonpaginated&amp;amp;src=P72327630Q4G5565_html\10533_2004_Article_150174_TeX2GIFIE2.gif&quot; border=&quot;0&quot; alt=&quot; $$_3^ - $$ &quot; /&amp;gt; in surface waters. This studycompared the relative effects of aquatic andnear-stream processes with those from the terrestrialecosystem on the retention and transport ofNO &amp;lt;img src=&quot;/fulltext-image.asp?format=htmlnonpaginated&amp;amp;src=P72327630Q4G5565_html\10533_2004_Article_150174_TeX2GIFIE3.gif&quot;
ABSTRACT Clearcutting of northern hardwood forests in mountainous landscapes of the northeastern ... more ABSTRACT Clearcutting of northern hardwood forests in mountainous landscapes of the northeastern U.S. has been shown to cause large increases in stream nitrate (NO3-) concentrations accompanied by increased stream acidity, elevated losses of nutrient base cations, and aluminum concentrations sufficient to be toxic to brook trout. An 18 ha clearcut in the Catskill Mountains of southeastern New York, USA in 1997 resulted in stream NO3- concentrations that peaked at &gt; 1,000 mumol L-1, and base cation (Ca2+, Mg2+, K+) concentrations that increased by more than three-fold during the first year after harvest. In contrast, previous timber-stand improvement harvests in 1995 and 1996 in which &lt; 10% of tree basal area was removed from watersheds, resulted in no measurable change in stream water chemistry. Based on an analysis of these previous data, we hypothesized that there is likely a harvest threshold for changes in stream-water chemistry below which only minimal and tolerable changes in water quality occur. We tested this hypothesis by completing four forest harvests during 2002 to 2006 in which varying amounts of basal area were removed from northern hardwood forest plots and watersheds. These results have shown that at a basal area removal of about 33%, stream NO3- and K+ concentrations increased, but less than proportionally to the concentration changes observed after the clearcut. Calcium and Mg2+ concentrations increased as well, but these changes were about proportional to the concentration changes observed after the clearcut. Additionally, stream NO3- concentrations returned to background values within two years at the 33% partial harvest compared to about eight years in the clearcut. Soil-water lysimeter data from two other harvests in which 30 and 50% of basal area were removed from hardwood forest plots are consistent with that of the previous partial harvest. Nitrate concentrations increased less than proportionally to the changes observed after the clearcut, and returned rapidly to background values. These data suggest that expansion of the crowns and root networks of remaining trees can partially compensate for the removal of up to 50% of tree basal area resulting in minimal and short-lived changes in water quality associated with partial harvesting.
Inputs of reactive nitrogen (N) to watersheds in the northeastern USA are linked to environmental... more Inputs of reactive nitrogen (N) to watersheds in the northeastern USA are linked to environmental concerns such as acidification of upland ecosystems, accumulation of N in groundwater, and eutrophication of coastal waterways. Nitrogen sources include atmospheric deposition, fertilizer, and human and animal waste, and the relative strength of these sources has been shown to vary with watershed land use. Attribution of N sources is important in developing effective policy strategies for reducing N loads in streams and rivers. Here, we use dual (δ15N, δ18O) isotope analysis of nitrate as well as chemistry data from six streams in New York to determine dominant sources of N and to learn more about how N cycles through these watersheds. Land use in these watersheds included 100% forested land in each of two streams in the Catskill and Adirondack Mountains, 34 to 70% agricultural land (primarily dairy farms) in three streams, and 56% suburban land in one stream. Nitrate in atmospheric deposition had δ18O values that ranged from about +65 to +85‰ in precipitation and a similar range in dry deposition, and δ15N values that ranged from about -5 to +5‰ in precipitation and about -2 to +15‰ in dry deposition. Nitrate in stream water from the dominantly forested watersheds had δ18O values that ranged from +5 to +15‰, indicating that the dominant immediate source was from nitrification in soils, but with a tendency towards higher direct contributions of atmospheric nitrate during high flow. The two streams with the highest percent of agricultural land use had isotope values that indicated a mixture of nitrate sources from soil nitrification and animal waste. These data fall along a line with a δ18O:δ15N slope of 1:2 indicating a varying source that was denitrified. Nitrate in the watershed dominated by suburban land use had δ18O values as high as +30‰, indicating a large direct contribution of atmospheric nitrate transported over impervious surfaces and discharged to the stream via storm drains. These data show an inverse relation between δ18O and δ15N values suggesting a mixture of human waste and direct atmospheric sources of nitrate. Together, these chemistry and isotope data indicate that in streams with widely varying land uses, nitrate concentrations can be quite similar, despite widely varying sources. Stream nitrate reflects the mixing of N sources as attenuated and transformed by watershed biogeochemical cycling processes.
Water residence times and nutrient budgets in 3 small watersheds in the Croton water supply area,... more Water residence times and nutrient budgets in 3 small watersheds in the Croton water supply area, NY, were examined. The watersheds (less than 1km 2) have different level of urbanization (natural, semi-developed and fully developed), different mechanisms of runoff generation (quick flow on roads and slow flow through subsurface) and different watershed landscape characteristics (wet zones, hillslopes) . Measurements of the comprehensive chemical suite incl. components of nitrogen budget in the throughfall, stream water, soil water and groundwater in the saturated zone were performed bi-weekly over a period up to 2 years. Mean water residence times of the stream water were estimated using Oxygen-18 and Helium-3/Tritium isotopes. There are significant differences in the chemical composition and decrease of nitrification intensity and of mean streamwater residence time along the increasing watershed development. Within each watershed, longer water residence times (up to over 2 years) were estimated in the wetland zones without direct communication with streams in comparison to hillslope areas (up to over 1 year). The results can be used in watershed management and planning of the further urbanization of this water supply area.
The 24-ha Dry Creek watershed in the Catskill Mountains of New York State was clear-cut during 19... more The 24-ha Dry Creek watershed in the Catskill Mountains of New York State was clear-cut during 1997 to evaluate nutrient release to New York City reservoirs due to forest harvesting. The Dry Creek watershed is in the headwaters of the Neversink watershed, which is part of the New York City Reservoir system that supplies drinking water to over 20 million people. Soil water, groundwater seeps, and stream water chemistry were monitored to trace the transport of solutes before and after the timber harvest. Automated sequential zero-tension lysimeters and standard zero-tension lysimeters were installed at depths of 70, 300, and 500 mm to sample soil water in the O, B, and C-horizons, respectively. Pre-cut (water years 1993-1996) mean soil water concentrations from zero tension lysimeters indicate that O-horizon soil water (70 mm depth) had the highest nitrate (NO3-) and monomeric aluminum (Alm) concentrations (73 and 18 μmoles l-1, respectively). During that same time period water from ground-water seeps had lower NO3- and Alm concentrations (22 and 0.88 μmoles l-1, respectively) than any soil waters sampled. During the two years following the clear-cut, groundwater seep NO3- concentrations were 138-123 μmoles l-1 and Alm concentrations were 50-30 μmoles l-1 lower than that measured in soil water. Throughout the same time period, B-horizon soil water had the highest mean NO3- concentration (345 μmoles l-1) while C-horizon soil water had the highest mean Alm concentrations (51 μmoles l-1). But during storms in the first year after the clear-cut O-horizon soil water NO3- and Alm concentrations often peaked at more than twice those measured in the B-horizon. During the second year after the clear-cut, B-horizon storm NO3- concentrations were consistently greater than O-horizon concentrations. During the fourth and fifth years following the clear-cut, soil water NO3- concentrations had dropped below pre-cut concentrations however NO3- in groundwater seeps remained elevated. The NO3- concentration at the watershed outlet also remained above pre-cut levels. During the first years following the clear-cut, in the absence of watershed vegetation, soil NO3- was leached to watershed streams and to deeper groundwater. As the forest has regenerated soil NO3- has been immobilized while groundwater continues as a source of NO3- to watershed streams 4-5 years after the cut. Four to five years after the clear-cut Alm concentrations were below pre-cut levels for all waters sampled. The elevated stream water NO3- concentrations that continue to be measured at the stream outlet, are not accompanied by elevated Alm concentrations since the groundwater seeps that are the source of the NO3- have never been a significant source of Alm.
Catskill Mountain streams in New York often receive pulses of NO3 during storms and snowmelt from... more Catskill Mountain streams in New York often receive pulses of NO3 during storms and snowmelt from watershed soils and acid deposition. This "flushing effect" of nutrients and acids was documented in forest-soil water through use of sequential lysimetry, in which soil-water was collected in equal-volume increments from zero-tension lysimeters placed within the soil profile. Stormflow in previously unsaturated soils began with percolation of water through the upper soil to the B horizon. NO3 pulses in soilwater during storms and snowmelt were typically delayed in the B-horizon relative to the O-horizon, indicating a percolation process for soil water movement between the lysimeters, but during the fall rains and spring snowmelt periods water-table measurements indicated that the B-horizon lysimeter was overtopped by the water table. Net flux was similar from throughfall and 0-lysimeters for individual events, suggesting that throughfall could either percolate unchanged through the soil column or that microbial release of N was rapid and similar to throughfall inputs. Patterns of change in O-Lysimeter concentrations during individual storms indicate repeated backflushing periods in which percolating acidic deposition or snowmelt from an individual event rose back into the O-horizon with a rising water table. This back-flushing phenomena may be enhancing the leaching of soil calcium beyond what would occur if acidic deposition was only percolating once through the O horizon . The sequential lysimetry method allowed investigators a more detailed look at N dynamics in forest soils than has been possible through the typical monthly lysimeter sampling strategy.
High levels of mercury (Hg) in aquatic biota have been identified in surface waters of the Adiron... more High levels of mercury (Hg) in aquatic biota have been identified in surface waters of the Adirondack region of New York, and factors such as the prevalence of wetlands, extensive forest cover, and oligotrophic waters promote Hg bioaccumulation in this region. Past research in this region has focused on improved understanding of the Hg cycle in lake ecosystems. In the study described herein, the landscape controls on total Hg and methylmercury (MeHg) concentrations in riverine ecosystems were explored through synoptic surveys of 27 sites in the upper Hudson River basin of the Adirondack region. Stream samples were collected and analyzed for total Hg, MeHg, dissolved organic carbon (DOC), and ultraviolet absorbance at 254 nm (UV254) during spring and summer of 2006-08. Landscape indices including many common land cover, hydrographic, and topographic-based measures were explored as predictors of Hg through multivariate linear regression. Multivariate models that included a wetland or riparian area-based metric, an index for open water area, and in some cases a topographic metric such as the wetness index explained 55 to 65 percent of the variation in MeHg concentrations, and 55 to 80 percent of the variation in total Hg concentrations. An open water index (OWI) was developed that incorporated both the basin area drained by ponded water and the surface area of these ponds. This index was inversely related to concentrations of total Hg and MeHg. This OWI was also inversely related to specific ultra-violet absorbance, consistent with previous studies showing that open water increases the influence of algal-derived carbon on DOC, decreasing aromaticity, which should decrease the ability of the dissolved carbon pool to bind Hg. The OWI was not significant in models for total Hg that also included UV254 as a predictive variable, but the index did remain significant in similar models for MeHg suggesting that biogeochemical factors in addition to decreasing carbon aromaticity limit MeHg concentrations in this river basin. These data are consistent with removal of MeHg by photo-reduction and volatilization in open waters. Study results, in addition to confirming the importance of riparian wetlands as sources of Hg to flowing waters, also highlight that open water bodies can limit the downstream transport of Hg in river networks. These results may be broadly applicable in northeastern North America and other settings where rivers consist of linked open water bodies.
We quantified the response of vegetation and nutrient uptake in a northern hardwood forest in sou... more We quantified the response of vegetation and nutrient uptake in a northern hardwood forest in southeastern New York for three to four years after three intensities of harvesting: clearcutting, heavy timber stand improvement (TSI), light TSI (97, 29, and 10% basal area reductions, respectively). We also quantified effects of white-tailed deer (Odocoileus virginianus) herbivory on nutrient retention by vegetation. Total biomass and nutrient accumulation in vegetation was higher after TSI than clearcutting in the first two years but was highest in the fenced clearcut in subsequent years, indicating that TSI or partial harvesting is a viable management tool for harvesting timber while consistently maintaining high rates of nutrient retention. After clearcutting, biomass and nutrient retention were initially dominated by woody stems <1.4 m tall and herbaceous vegetation, but saplings 0.1-5.0 cm DBH became the most important contributors to biomass and nutrient accumulation within four years. However, after both intensities of TSI, trees >5.0 cm DBH continued to account for most biomass and nutrient accumulation whereas understory vegetation accumulated little biomass or nutrients. Heavy TSI resulted in increased regeneration of only two tree species (Acer pensylvanicum, Fagus grandifolia), but clearcutting allowed these two species, mature forest species (A. saccharum, Betula alleghaniensis), and the early successional Prunus pensylvanica to regenerate. Several early successional shrub and herbaceous species were also important to nutrient retention after clearcutting, including Polygonum cilinode, Rubus spp., and Sambucus racemosa. Herbivory by white-tailed deer dramatically reduced biomass and nutrient accumulation by woody stems <5 cm DBH after clearcutting (5.5 vs. 0.7 Mg biomass/ha and 30.4 vs. 6.3 kg N/ha on fenced and unfenced clearcut sites, respectively, after four years), indicating the important influence this herbivore can have on nutrient retention in recently disturbed forests.
The Catskill Mountains of southeastern New York receive atmospheric nitrogen (N) deposition of 10... more The Catskill Mountains of southeastern New York receive atmospheric nitrogen (N) deposition of 10 to 15 kg-1ha-1yr-1, and streams in the region show a wide range of nitrate (NO3^{-}) concentrations from near 0 to 100 \mumolL^{-1}. Past research indicates that most NO_{3}- in streams in this region originates through nitrification in the soil. Past forest and soil disturbance is one of the major factors that affects nitrification. A 23-ha tributary watershed of the Neversink River was clearcut in 1997 to understand how the resulting disturbance affects the N cycle. Stream NO3^{-} concentrations briefly increased above 1000 \mumolL^{-1} about 5 months after the cut, and remained above 200 \mumolL^{-1} for almost 2 years. Nitrogen budgets indicate that most of the stream NO_{3}- originated through nitrification in the soil, from which increased amounts of NO3^{-}$ leached to surface water in the absence of competition with uptake by vegetation. Past research in a variety of settings across North America has shown that clearcutting increases N-mineralization and nitrification rates in forest soil, but we found no increase in the net rates of either of these processes during monthly in-situ incubations. This seeming paradox is attributed to limitations of the in-situ incubation method, which measures only the net rates of these processes in the absence of N uptake by vegetation. The clearcut in this Catskill setting (in which high net nitrification rates were measured prior to harvesting) did not cause large enough changes in physical factors that affect this microbial process, such as soil temperature and moisture, to produce a measurable change in the net rate of nitrification. Consequently, nitrate concentrations in stream water decreased sharply during the third and fourth years after the clearcut in response to a second growth forest dominated by pin cherry (Prunus pennsylvanica).
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