High Nitrate Retention during Winter in Soils of the Hubbard Brook Experimental Forest

Stream export of nitrogen (N) as nitrate (NO₃⁻; the most mobile form of N) from forest ecosystems is thought to be controlled largely by plant uptake of inorganic N, such that reduced demand for plant N during the non-growing season and following disturbances results in increased stream NO₃⁻ export. The roles of microbes and soils in ecosystem N retention are less clear, but are the dominant controls on N export when plant uptake is low. We used a mass balance approach to investigate soil N retention during winter (December through March) at the Hubbard Brook Experimental Forest by comparing NO₃⁻ inputs (atmospheric deposition), internal production (soil microbial nitrification), and stream output. We focused on months when plant N uptake is nearly zero and the potential for N export is high. Although winter months accounted for only 10–15% of annual net nitrification, soil NO₃⁻ production (0.8–1.0 g N m⁻² winter⁻¹) was much greater than stream export (0.03–0.19 N m⁻² winter⁻¹). Soil NO₃⁻ retention in two consecutive winters was high (96% of combined NO₃⁻ deposition and soil production; year 1) even following severe plant disturbance caused by an ice-storm (84%; year 2) We show that soil NO₃⁻ retention is surprisingly high even when N demand by plants is low. Our study highlights the need to better understand mechanisms of N retention during the non-growing season to predict how ecosystems will respond to high inputs of atmospheric N, disturbance, and climate change.     

Long-Term Trends in Stream Nitrate Concentrations and Losses Across Watersheds Undergoing Recovery from Acidification in the Czech Republic

Stream nitrogen (N) export and nitrate concentration were measured at 14 forested watersheds (GEOMON network) in the Czech Republic between 1994 and 2005. In the last several decades, emissions of sulfur (S) and N compounds have decreased throughout much of Europe. In the Czech Republic, atmospheric deposition of S has decreased substantially since the beginning of 1990s, whereas N deposition remains largely unchanged at most sites. The mean dissolved inorganic nitrogen (DIN) streamwater export ranged from 0.2 to 12.2 kg ha⁻¹ y⁻¹ at the GEOMON sites. Despite decades of elevated N deposition, 44–98% of DIN inputs to these watersheds were retained or denitrified, and many watersheds showed seasonal variation in nitrate concentrations. Dissolved organic N export was quantified in 1 year only and ranged from 0.05 to 3.5 kg ha⁻¹ y⁻¹. Spatial variability in DIN export among watersheds was best explained by spatial variability in average acidic deposition, particularly S deposition (R ² = 0.81, P < 0.001); DIN input and forest floor carbon:nitrogen (C/N) also provided significant explanatory power. DIN export was strongly influenced by the forest floor C/N ratio and depth of the forest floor soils (R ² = 0.72, P < 0.001). The only variable that predicted variations in forest floor C/N (R ² = 0.32, P < 0.05) among watersheds was S deposition. Forest floor depth was also related to deposition variables, with S deposition providing the most explanatory power (R ² = 0.50, P < 0.01). Variation in forest floor depth was also associated with climatic factors (precipitation and temperature). Temporal variability in DIN export was primarily associated with changes in acidic deposition over time; S deposition explained 41% of variability in DIN exports among all watersheds and years. Extensive acidification of forested watersheds was associated with the extraordinarily high S inputs to much of the Czech Republic during earlier decades. We hypothesize that recovery from acidification has led to improved tree health as well as enhanced microbial activity in the forest floor. As these watersheds move into a new regime with dramatically lower sulfur inputs, we expect continued declines in nitrate output.     

An Unexpected Nitrate Decline in New Hampshire Streams

Theories of forest nitrogen (N) cycling suggest that stream N losses should increase in response to chronic elevated N deposition and as forest nutrient requirements decline with age. The latter theory was supported initially by measurements of stream NO₃ ⁻ concentration in old-growth and successional stands on Mount Moosilauke, New Hampshire (Vitousek and Reiners 1975; Bioscience 25:376–381). We resampled 28 of these and related streams to evaluate their response to 23 years of forest aggradation and chronic N deposition. Between 1973–74 and 1996–97, mean NO₃ ⁻ concentration in quarterly samples from Mount Moosilauke decreased by 71% (25 μmol/L), Ca²⁺ decreased by 24% (8 μmol/L), and Mg²⁺ decreased by 22% (5 μmol/L). Nitrate concentrations decreased in every stream in every season, but spatial patterns among streams persisted: Streams draining old-growth stands maintained higher NO₃ ⁻ concentrations than those draining successional stands. The cause of the NO₃ ⁻ decline is not evident. Nitrogen deposition has changed little, and although mechanisms such as insect defoliation and soil frost may contribute to the temporal patterns of nitrate loss, they do not appear to fully explain the NO₃ ⁻ decline across the region. Although the role of climate remains uncertain, interannual climate variation and its effects on biotic N retention may be responsible for the synchronous decrease in NO₃ ⁻ across all streams, overriding expected increases due to chronic N deposition and forest aging. Received 4 December 2001; accepted 30 May 2002.     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams

The influence of land use on potential fates of nitrate (NO₃ ⁻) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here, we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use setting influences fates of NO₃ ⁻ in streams. We used 24 h releases of a stable isotope tracer (¹⁵NO₃-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO₃ ⁻ uptake processes. NO₃ ⁻ uptake lengths varied two orders of magnitude (24–4247 m), with uptake rates (6.5–158.1 mg NO₃-N m⁻² day⁻¹) and uptake velocities (0.1–2.3 mm min⁻¹) falling within the ranges measured in other LINX II regions. Denitrification removed 0–7% of added tracer from our streams. In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO₃ ⁻, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range of NO₃ ⁻ uptake processes in human-altered streams.     

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO 3 − Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO₃⁻ deposition. Experimental NO₃⁻ deposition (3 g NO₃⁻ N m⁻² y⁻¹) has significantly reduced soil respiration and increased the export of DOC/DON and NO₃⁻ across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO₃⁻ deposition. To test this alternative, we measured microbial biomass, respiration, and N transformations in the mineral soil of four northern hardwood stands that have received 9 years of experimental NO₃⁻ deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO₃⁻ deposition. We also observed no effect of NO₃⁻ deposition on annual rates of net N mineralization. However, NO₃⁻ deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO₃⁻ assimilation, the subsequent turnover of NH₄⁺, and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO₃⁻ export, suggesting that much of the exported NO₃⁻ resulted directly from the NO₃⁻ deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO₃⁻-induced physiological changes in mineral soil microbial activity. Given the lack of response we have observed in mineral soil, our results point to the potential importance of microbial communities in forest floor, including both saprotrophs and mycorrhizae, in mediating ecosystem-level responses to chronic NO₃⁻ deposition in Lake States northern hardwood forests.     

Whole-system Estimates of Nitrification and Nitrate Uptake in Streams of the Hubbard Brook Experimental Forest

Although they drain remarkably similar forest types, streams of the Hubbard Brook Experimental Forest (HBEF) vary widely in their NO₃ ⁻ concentrations during the growing season. This variation may be caused by differences in the terrestrial systems they drain (for example, varying forest age or composition, hydrology, soil organic matter content, and so on) and/or by differences between the streams themselves (for example, contrasting geomorphology, biotic nitrogen [N] demand, rates of instream nitrogen transformations). We examined interstream variation in N processing by measuring NH₄ ⁺ and NO₃ ⁻ uptake and estimating nitrification rates for 13 stream reaches in the HBEF during the summers of 1998 and 1999. We modeled nitrification rates using a best-fit model of the downstream change in NO₃ ⁻ concentrations following short-term NH₄ ⁺ enrichments. Among the surveyed streams, the fraction of NH₄ ⁺ uptake that was subsequently nitrified varied, and this variation was positively correlated with ambient streamwater NO₃ ⁻ concentrations. We examined whether this variation in instream nitrification rates contributed significantly to the observed variation in NO₃ ⁻ concentrations across streams. In some cases, instream nitrification provided a substantial portion of instream NO₃ ⁻ demand. However, because there was also substantial instream NO₃ ⁻ uptake, the net effect of instream processing was to reduce rather than supplement the total amount of NO₃ ⁻ exported from a watershed. Thus, instream rates of nitrification in conjunction with instream NO₃ ⁻ uptake were too low to account for the wide range of streamwater NO₃ ⁻. The relationship between streamwater NO₃ ⁻ concentration and rates of instream nitrification may instead be due to a shift in the competitive balance between heterotrophic N uptake and nitrification when external inputs of NO₃ ⁻ are relatively high. Received 11 October 2000; accepted 14 December 2001.     

Biogeochemical Budgets in a Mediterranean Catchment with High Rates of Atmospheric N Deposition – Importance of Scale and Temporal Asynchrony

In this study biogeochemical export in a set of catchments that vary from 6 ha to almost 1500 ha is investigated. Studying catchments across this large range of scales enables us to investigate the scale dependence and fundamental processes controlling catchment biogeochemical export that would not have been possible with a more limited data set. The Devil Canyon catchment, in the San Bernardino Mountains, California, has some of the highest atmospheric N deposition rates in the world (40–90 kg ha⁻¹year⁻¹ at the crest of the catchment). These high rates of deposition have translated into consistently high levels of NO⁻in 3 some streams of the San Bernardino Mountains. However, the streams of the Devil Canyon catchment have widely varying dissolved inorganic nitrogen (DIN) concentrations and export. These differences are also, to a more limited extent, present for dissolved organic carbon (DOC) but not in other dissolved species (Cl⁻, SO²⁻₄,Ca²⁺ and other weathering products). As catchment size increases DIN and DOC concentrations first increase until catchment area is ∼150 ha but then decrease as catchment scale increases beyond that size. The scale dependence of DIN export implies that catchments at different spatial scales are at different degrees of N saturation. The reason for this scale effect appears to be the dominance of flushing of DIN out of soil at small scales due to the temporal asynchrony between nutrient availability and biological N demand, the groundwater exfiltration of this flushed DIN at intermediate scales and the removal of this DIN from streamflow through in-stream processes and groundwater–surface water interaction at larger scales. While the particular scale effect observed here may not occur over the same range in catchment area in other ecosystems, it is likely that other ecosystems have similar scale dependant export for DIN and DOC.     

Minimal response in watershed nitrate export to severe soil frost raises questions about nutrient dynamics in the Hubbard Brook experimental forest

Experimental and theoretical work emphasize the role of plant nutrient uptake in regulating ecosystem nutrient losses and predict that forest succession, ecosystem disturbance, and continued inputs of atmospheric nitrogen (N) will increase watershed N export. In ecosystems where snowpack insulates soils, soil-frost disturbances resulting from low or absent snowpack are thought to increase watershed N export and may become more common under climate-change scenarios. This study monitored watershed N export from the Hubbard Brook Experimental Forest (HBEF) in response to a widespread, severe soil-frost event in the winter of 2006. We predicted that nitrate (NO₃ ⁻) export following the disturbance would be high compared to low background streamwater NO₃ ⁻ export in recent years. However, post-disturbance annual NO₃ ⁻ export was the lowest on record from both reference (undisturbed) and treated experimental harvest or CaSiO₃ addition watersheds. These results are consistent with other studies finding greater than expected forest NO₃ ⁻ retention throughout the northeastern US and suggest that changes over the last five decades have reduced impacts of frost events on watershed NO₃ ⁻ export. While it is difficult to parse out causes from a complicated array of potential factors, based on long-term records and watershed-scale experiments conducted at the HBEF, we propose that reduced N losses in response to frost are due to a combination of factors including the long-term legacies of land use, process-level alterations in N pathways, climate-driven hydrologic changes, and depletion of base cations and/or reduced soil pH due to cumulative effects of acid deposition.     

Dual isotope analyses indicate efficient processing of atmospheric nitrate by forested watersheds in the northeastern U.S.

Nitrogen from atmospheric deposition serves as the dominant source of new nitrogen to forested ecosystems in the northeastern U.S. By combining isotopic data obtained using the denitrifier method, with chemical and hydrologic measurements we determined the relative importance of sources and control mechanisms on nitrate (NO₃ ⁻) export from five forested watersheds in the Connecticut River watershed. Microbially produced NO₃ ⁻ was the dominant source (82–100%) of NO₃ ⁻ to the sampled streams as indicated by the δ¹⁵N and δ¹⁸O of NO₃ ⁻. Seasonal variations in the δ¹⁸O–NO₃ ⁻ in streamwater are controlled by shifting hydrologic and temperature affects on biotic processing, resulting in a relative increase in unprocessed NO₃ ⁻ export during winter months. Mass balance estimates find that the unprocessed atmospherically derived NO₃ ⁻ stream flux represents less than 3% of the atmospherically delivered wet NO₃ ⁻ flux to the region. This suggests that despite chronically elevated nitrogen deposition these forests are not nitrogen saturated and are retaining, removing, and reprocessing the vast majority of NO₃ ⁻ delivered to them throughout the year. These results confirm previous work within Northeastern U.S. forests and extend observations to watersheds not dominated by a snow-melt driven hydrology. In contrast to previous work, unprocessed atmospherically derived NO₃ ⁻ export is associated with the period of high recharge and low biotic activity as opposed to spring snowmelt and other large runoff events.     

Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest

Abstract Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous forest canopy in central Maine (18–20 kg N ha⁻¹ y⁻¹ as NH₄NO₃ applied as a mist using a helicopter). In 2003 and 2004 we measured NO₃ ⁻, NH₄ ⁺, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications. Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO₃ ⁻ accounted for more than 50% of TF N in the treated plots, followed by NH₄ ⁺ (35%) and DON (15%). Although NO₃ ⁻ was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH₄ ⁺as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested that the forest canopy retained more than 70% of the applied N (>80% when corrected for N which bypassed tree surfaces at the time of fertilizer addition). Results from plots receiving ¹⁵N enriched NO₃ ⁻ and NH₄ ⁺ confirmed bulk N estimations that more NO₃ ⁻ than NH₄ ⁺ was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for both ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in ¹⁵N, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely influenced by forest canopy processes, which should be considered in N addition studies. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Riverine nitrogen export in Swedish catchments dominated by atmospheric inputs

We present the first estimates of net anthropogenic nitrogen input (NANI) in European boreal catchments. In Swedish catchments, nitrogen (N) deposition is a major N input (31–94%). Hence, we used two different N deposition inputs to calculate NANI for 36 major Swedish catchments. The relationship between riverine N export and NANI was strongest when using only oxidized deposition (NO_y) as atmospheric input (r² = 0.70) rather than total deposition (i.e., both oxidized and reduced nitrogen, NO_y + NH_x deposition, r² = 0.62). The y-intercept (NANI = 0) for the NANI calculated with NO_y is significantly different from zero (p = 0.0042*) and indicates a background flux from the catchment of some 100 kg N km^?2 year^?1 in addition to anthropogenic inputs. This agrees with similar results from North American boreal catchments. The slope of the linear regressions was 0.25 for both N deposition inputs (NO_y and NO_y + NH_x), suggesting that on average, 25% of the anthropogenic N inputs is exported by rivers to the Baltic Sea. Agricultural catchments in central and southern Sweden have increased their riverine N export up to tenfold compared to the inferred background flux. Although the relatively unperturbed northernmost catchments receive significant N loads from atmospheric deposition, these catchments do not show significantly elevated riverine N export. The fact that nitrogen export in Swedish catchments appears to be higher in proportion to NANI at higher loads suggests that N retention may be saturating as loading rates increase. In northern and western Sweden the export of nitrogen is largely controlled by the hydraulic load, i.e., the riverine discharge normalized by water surface area, which has units of distance time^?1. Besides hydraulic load the percent total forest cover also affects the nitrogen export primarily in the northern and western catchments.     

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K⁺, Na⁺, Ca²⁺, Mg²⁺, NO₃ ⁻-N, NH₄ ⁺-N, PO₄ ³⁻-P, and SO₄ ²⁻ were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO₃ ⁻-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO₃ ⁻-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO₃ ⁻-N increased four fold, and streamwater NO₃ ⁻-N doubled. Significant changes also occurred in soilwater pH, DOC, SO₄ ²⁻, Ca²⁺, and Mg²⁺. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO₃ ⁻-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams. Received 30 January 2001; accepted 25 July 2002.     

Rapid Nitrate Loss and Denitrification in a Temperate River Floodplain

Nitrogen (N) pollution is a problem in many large temperate zone rivers, and N retention in river channels is often small in these systems. To determine the potential for floodplains to act as N sinks during overbank flooding, we combined monitoring, denitrification assays, and experimental nitrate (NO₃⁻ -N) additions to determine how the amount and form of N changed during flooding and the processes responsible for these changes in the Wisconsin River floodplain (USA). Spring flooding increased N concentrations in the floodplain to levels equal to the river. As discharge declined and connectivity between the river and floodplain was disrupted, total dissolved N decreased over 75% from 1.41 mg l⁻¹, equivalent to source water in the Wisconsin River on 14 April 2001, to 0.34 mg l⁻¹ on 22 April 2001. Simultaneously NO₃⁻ -N was attenuated almost 100% from 1.09 to <0.002 mg l⁻¹. Unamended sediment denitrification rates were moderate (0–483 μg m⁻² h⁻¹) and seasonally variable, and activity was limited by the availability of NO ₃⁻ -N on all dates. Two experimental NO₃⁻ -N pulse additions to floodplain water bodies confirmed rapid NO₃⁻ -N depletion. Over 80% of the observed NO ₃⁻ -N decline was caused by hydrologic export for addition #1 but only 22% in addition #2. During the second addition, a significant fraction (>60%) of NO₃⁻ -N mass loss was not attributable to hydrologic losses or conversion to other forms of N, suggesting that denitrification was likely responsible for most of the NO₃⁻ -N disappearance. Floodplain capacity to decrease the dominant fraction of river borne N within days of inundation demonstrates that the Wisconsin River floodplain was an active N sink, that denitrification often drives N losses, and that enhancing connections between rivers and their floodplains may enhance overall retention and reduce N exports from large basins.     

Land-Use Change and Stream Water Fluxes: Decadal Dynamics in Watershed Nitrate Exports

Stream water exports of nutrients and pollutants to water bodies integrate internal and external watershed processes that vary in both space and time. In this paper, we explore nitrate (NO₃) fluxes for the 326 km² mixed-land use Fall Creek watershed in central New York for 1972–2005, and consider internal factors such as changes in land use/land cover, dynamics in agricultural production and fertilizer use, and external factors such as atmospheric deposition. Segmented regression analysis was applied independently to dormant and growing seasons for three portions of the period of record, which indicated that stream water NO₃ concentrations increased in both dormant and growing seasons from the 1970s to the early 1990s at all volumes of streamflow discharge. Dormant season NO₃ concentrations then decreased at all flow conditions between the periods 1987–1993 and 1994–2005. Results from a regression-based stream water loading model (LOADEST) normalized to mean annual concentrations showed annual modeled NO₃ concentration in stream water increased by 34% during the 1970s and 1980s (from 1.15 to 1.54 mg l⁻¹), peaked in about 1989, and then decreased by 29% through 2005 (to 1.09 mg l⁻¹). Annual precipitation had the strongest correlation with stream water NO₃ concentrations (r = −0.62, P = 0.01). Among land use factors, corn production for grain was the variable most highly correlated to stream water NO₃ concentrations (r = 0.53, P = 0.01). The strongest associative trend determined using Chi-squared Automatic Interaction Detection (CHAID) was found between stream water NO₃ concentrations and N-equivalence of dairy production (Bonferroni adjusted P value = 0.0003). Large increases in dairy production were coincident with declining nitrate concentrations over the past decade, which suggest that dairy management practices may have improved in the watershed. However, because dairy production in the Fall Creek watershed has been fueled by large increases in feed imports, the environmental costs of feed production have likely been externalized to other watersheds.     

The saturation of N cycling in Central Plains streams: <Superscript>15</Superscript>N experiments across a broad gradient of nitrate concentrations

We conducted ¹⁵NO₃⁻ stable isotope tracer releases in nine streams with varied intensities and types of human impacts in the upstream watershed to measure nitrate (NO₃⁻) cycling dynamics. Mean ambient NO₃⁻ concentrations of the streams ranged from 0.9 to 21,000 μg l⁻¹ NO₃⁻–N. Major N-transforming processes, including uptake, nitrification, and denitrification, all increased approximately two to three orders of magnitude along the same gradient. Despite increases in transformation rates, the efficiency with which stream biota utilized available NO₃⁻-decreased along the gradient of increasing NO₃⁻. Observed functional relationships of biological N transformations (uptake and nitrification) with NO₃⁻ concentration did not support a 1st order model and did not show signs of Michaelis–Menten type saturation. The empirical relationship was best described by a Efficiency Loss model, in which log-transformed rates (uptake and nitrification) increase with log-transformed nitrate concentration with a slope less than one. Denitrification increased linearly across the gradient of NO₃⁻ concentrations, but only accounted for ∼1% of total NO₃⁻ uptake. On average, 20% of stream water NO₃⁻ was lost to denitrification per km, but the percentage removed in most streams was <5% km⁻¹. Although the rate of cycling was greater in streams with larger NO₃⁻ concentrations, the relative proportion of NO₃⁻ retained per unit length of stream decreased as NO₃⁻ concentration increased. Due to the rapid rate of NO₃⁻ turnover, these streams have a great potential for short-term retention of N from the landscape, but the ability to remove N through denitrification is highly variable.     

Effects of the Hemlock Woolly Adelgid on Nitrogen Losses from Urban and Rural Northern Forest Ecosystems

The objectives of this study were to quantify rates of nitrogen inputs to the forest floor, determine rates of nitrogen losses via leaching and to partition the sources of NO₃ ⁻ from healthy, declining, and salvage or preemptively cut eastern hemlock (Tsuga canadensis) stands in both an urban forest at the Arnold Arboretum in Boston, MA and a rural forest at Harvard Forest in Petersham, MA. Rates of nitrogen inputs (NH₄ ⁺ and NO₃ ⁻) to the forest floor were 4–5 times greater, and rates of nitrogen losses via leachate were more than ten times greater, at the Arnold Arboretum compared to Harvard Forest. Nitrate that was lost via leachate at Harvard Forest came predominantly from atmospheric deposition inputs, whereas NO₃ ⁻ losses at the Arnold Arboretum came predominantly from nitrification. Although our study was limited to one urban and one rural site, our results suggest that current management regimes used to control the hemlock woolly adelgid (Adelges tsugae), such as salvage cutting, may be reducing nitrogen losses in urban areas due to rapid regrowth of vegetation and uptake of nitrogen by those plants. In contrast, preemptive cutting of trees in rural areas may be leading to proportionately greater losses of nitrogen in those sites, though the total magnitude of nitrogen lost is still smaller than in urban sites. Results of our study suggest that the combination of the hemlock woolly adelgid, nitrogen inputs, and management practices lead to changes in the movement and source of NO₃ ⁻ losses from eastern hemlock forest ecosystems.     

Characteristics of DOC Exported from Northern Hardwood Forests Receiving Chronic Experimental NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition

Abstract Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests of the Great Lakes Region commonly receive elevated levels of atmospheric nitrate (NO₃⁻) deposition, which can alter belowground carbon (C) cycling. Past research has demonstrated that chronic experimental NO₃⁻ deposition (3 g N m⁻² y⁻¹ above ambient) elicits a threefold increase in the leaching loss of dissolved organic carbon (DOC). Here, we used DOC collected from tension-cup lysimeters to test whether increased DOC export under experimental NO₃⁻ deposition originated from forest floor or mineral soil organic matter (SOM). We used DOC radiocarbon dating to quantify C sources and colorimetric assays to measure DOC aromaticity and soluble polyphenolic content. Our results demonstrated that DOC exports are primarily derived from new C (<50-years-old) in the forest floor under both ambient and experimental NO₃⁻ deposition. Experimental NO₃⁻ deposition increased soluble polyphenolic content from 25.03 ± 4.26 to 49.19 ± 4.23 μg phenolic C mg DOC⁻¹, and increased total aromatic content as measured by specific UV absorbance. However, increased aromatic compounds represented a small fraction (<10%) of the total observed increased DOC leaching. In combination, these findings suggest that experimental NO₃⁻ deposition has altered the production or retention as well as phenolic content of DOC formed in forest floor, however exact mechanisms are uncertain. Further elucidation of the mechanism(s) controlling enhanced DOC leaching is important for understanding long-term responses of Great Lakes forests to anthropogenic N deposition and the consequences of those responses for aquatic ecosystems.     

Nitrate and nitrite inhibition of methanogenesis during denitrification in granular biofilms and digested domestic sludges

Anaerobic bioreactors that can support simultaneous microbial processes of denitrification and methanogenesis are of interest to nutrient nitrogen removal. However, an important concern is the potential toxicity of nitrate (NO₃ ⁻) and nitrite (NO₂ ⁻) to methanogenesis. The methanogenic toxicity of the NO_x⁻ compounds to anaerobic granular biofilms and municipal anaerobic digested sludge with two types of substrates, acetate and hydrogen, was studied. The inhibition was the severest when the NO_x⁻ compounds were still present in the media (exposure period). During this period, 95% or greater inhibition of methanogenesis was evident at the lowest concentrations of added NO₂ ⁻ tested (7.6–10.2 mg NO₂ ⁻-N l⁻¹) or 8.3–121 mg NO₃ ⁻-N l⁻¹ of added NO₃ ⁻, depending on substrate and inoculum source. The inhibition imparted by NO₃ ⁻ was not due directly to NO₃ ⁻ itself, but instead due to reduced intermediates (e.g., NO₂ ⁻) formed during the denitrification process. The toxicity of NO_x⁻ was found to be reversible after the exposure period. The recovery of activity was nearly complete at low added NO_x⁻ concentrations; whereas the recovery was only partial at high added NO_x⁻ concentrations. The recovery is attributed to the metabolism of the NO_x⁻ compounds. The assay substrate had a large impact on the rate of NO₂ ⁻ metabolism. Hydrogen reduced NO₂ ⁻ slowly such that NO₂ ⁻ accumulated more and as a result, the toxicity was greater compared to acetate as a substrate. The final methane yield was inversely proportional to the amount of NO_x⁻ compounds added indicating that they were the preferred electron acceptors compared to methanogenesis.     

Chronic Atmospheric NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Deposition Does Not Induce NO<Stack><Subscript>3</Subscript><Superscript>−</Superscript></Stack> Use by <Emphasis Type="Italic">Acer saccharum</Emphasis> Marsh.

The ability of an ecosystem to retain anthropogenic nitrogen (N) deposition is dependent upon plant and soil sinks for N, the strengths of which may be altered by chronic atmospheric N deposition. Sugar maple (Acer saccharum Marsh.), the dominant overstory tree in northern hardwood forests of the Lake States region, has a limited capacity to take up and assimilate NO₃⁻. However, it is uncertain whether long-term exposure to NO₃⁻ deposition might induce NO₃⁻ uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO₃⁻ deposition (30 kg N ha⁻¹ y⁻¹) could induce NO₃⁻ uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO₃⁻ deposition. Kinetic parameters for NH₄⁺ and NO₃⁻ uptake in fine roots, as well as leaf and root NO₃⁻ reductase activity, were measured under conditions of ambient and experimental NO₃⁻ deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO₃⁻ deposition did not alter the V _max or K _m for NO₃⁻ and NH₄⁺ uptake nor did it influence NO₃⁻ reductase activity in leaves and fine roots. Moreover, the mean V _max for NH₄⁺ uptake (5.15 μmol ¹⁵N g⁻¹ h⁻¹) was eight times greater than the V _max for NO₃⁻ uptake (0.63 μmol ¹⁵N g⁻¹ h⁻¹), indicating a much greater physiological capacity for NH₄⁺ uptake in this species. Additionally, NO₃⁻ reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO₃⁻ assimilation in sugar maple. Our results demonstrate that chronic NO₃⁻ deposition has not induced the physiological capacity for NO₃⁻ uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO₃⁻ deposition.