Stream Water NO3 Research Articles

Nitrate dynamics in the streamwater-groundwater interaction system: Sources, fate, and controls

Nitrate (NO3−) pollution has attracted widespread attention as a threat to human health and aquatic ecosystems; however, elucidating the controlling factors behind nitrate dynamics under the context of changeable hydrological processes, particularly the interactions between streamwater and groundwater (SW-GW), presents significant challenges. A multi-tracer approach, integrating physicochemical and isotopic tracers (Cl−, δ2H-H2O, δ18O-H2O, δ15N-NO3− and δ18O-NO3−), was employed to identify the response of nitrate dynamics to SW-GW interaction in the Fen River Basin. The streamwater and groundwater NO3− concentrations varied greatly with space and time. Sewage and manure (28 %–73 %), fertilizer (14 %–36 %) and soil organic nitrogen (12 %–28 %) were the main NO3− sources in water bodies. Despite the control of land use type on streamwater nitrate dynamics in losing sections, SW-GW interactions drove NO3− dynamics in both streamwater and groundwater under most circumstances. In gaining streams, streamwater nitrate dynamics were influenced either by groundwater dilution or microbial nitrification, depending on whether groundwater discharge ratios exceeded or fell below 25 %, respectively. In losing streams, groundwater nitrate content increased with streamwater infiltration time, but the influence was mainly limited within 3 km from the river channel. This study provides a scientific basis for the effective management of water nitrate pollution at the watershed scale.

Assessing the Linkages between Tree Species Composition and Stream Water Nitrate in a Reference Watershed in Central Appalachia

Many factors govern the flow of deposited nitrogen (N) through forest ecosystems and into stream water. At the Fernow Experimental Forest in WV, stream water nitrate (NO3−) export from a long-term reference watershed (WS 4) increased in approximately 1980 and has remained elevated despite more recent reductions in chronic N deposition. Long-term changes in species composition may have altered forest N demand and the retention of deposited N. In particular, the abundance and importance value of Acer saccharum have increased since the 1950s, and this species is thought to have a low affinity for NO3−. We measured the relative uptake of NO3− and ammonium (NH4+) by six important temperate broadleaf tree species and estimated stand uptake of total N, NO3−, and NH4+. We then used records of stream water NO3− and stand composition to evaluate the potential impact of changes in species composition on NO3− export. Surprisingly, the tree species we examined all used both mineral N forms approximately equally. Overall, the total N taken up by the stand into aboveground tissues increased from 1959 through 2001 (30.9 to 35.2 kg N ha−1 yr−1). However, changes in species composition may have altered the net supply of NO3− in the soil since A. saccharum is associated with high nitrification rates. Increases in A. saccharum importance value could result in an increase of 3.9 kg NO3−-N ha−1 yr−1 produced via nitrification. Thus, shifting forest species composition resulted in partially offsetting changes in NO3− supply and demand, with a small net increase of 1.2 kg N ha−1 yr−1 in NO3− available for leaching. Given the persistence of high stream water NO3− export and relatively abrupt (~9 year) change in stream water NO3− concentration circa 1980, patterns of NO3− export appear to be driven by long-term deposition with a lag in the recovery of stream water NO3− after more recent declines in atmospheric N input.

Long-term changes in atmospheric nitrogen deposition and stream water nitrate leaching from forested watersheds in western Japan

Japan receives nitrogenous air pollutants via long-range transport from China. However, emissions of nitrogenous air pollutants in China have stabilized or decreased in recent years. This study examined both the long-term trends in atmospheric nitrogen (N) deposition from the 1990s to the 2010s and the response of stream water nitrate (NO3−) leaching from forested areas in western Japan. A long-term (1992–2018) temporal analysis of atmospheric N deposition in Fukuoka (western Japan) was conducted. Atmospheric bulk N deposition was collected at forested sites in a suburban forest (Swest) and a rural forest (Rwest) in western Japan during 2009–2018. Stream water samples were also collected from four locations at sites Swest and Rwest during the same period. Results showed that atmospheric N deposition in Fukuoka started to decrease from the mid-2000s at an annual rate of −2.5% yr−1. The decrease in atmospheric N deposition was attributable mainly to decreased atmospheric ammonium (NH4+) deposition, which caused greater contribution of NO3− deposition to atmospheric N deposition. Concentrations of NO3− in the stream water samples from three of the four locations decreased significantly at an annual rate of −3.7 to −0.7% yr−1. However, stream water NO3− concentrations increased in one watershed where understory vegetation has been deteriorating owing to the increased deer population. This might weaken the recovery of N leaching from forested areas.

Stream water chemistry changes in response to deforestation of variable origin (case study from the Carpathians, southern Poland)

The purpose of the study was to examine the effects of deforestation of variable origin on the stream water chemistry, with a special focus on the NO3– ion. The research was conducted in 8 small catchments in the Tatra Mountains and 7 small catchments in the Beskid Śląski Mountains (the Carpathians, Poland). Relevant deforestation events occurred in the Beskid Śląski Mountains in the 1980s and 1990s, and were caused by intense acidic atmospheric deposition in the 1970s and 1980s, while tree stands in the Tatra Mountains were damaged abruptly by hurricane-force winds in 2013 and gradually by the bark beetle since the late 1990s.We show the key role of deforestation in the concentration of base cations (Ca2+, Mg2+), base anions (SO42-, HCO3–), and NO3– in stream water in catchments affected by acidic deposition. The lack of forest reduces the release of Ca2+, Mg2+ and HCO3– due to lower weathering rates in deforested areas than in forested areas. The absence of trees capable of absorbing atmospheric SO42- and NO3– in deforested catchments leads to a higher proportion of stream water SO42- and NO3– in the anion total than in forested catchments.The time passed since deforestation and degree of deforestation play a key role in NO3– concentration in stream water. The highest concentrations of NO3– were noted in stream water in freshly and fully hurricane-deforested catchments in the Tatra Mountains. The stream water NO3– concentration was markedly lower in a beetle-deforested catchment where only 40% of the forest had become damaged while the lowest NO3– concentrations were noted in more than 60% forested catchments. The most distinct seasonal changes in NO3– concentrations in stream water and in soils were noted in freshly and fully hurricane-deforested catchments.

Nitrous oxide emissions in proportion to nitrification in moist temperate forests

Chronic elevated nitrogen deposition has increased nitrogen availability in many forest ecosystems globally, and this phenomenon has been suggested to increase soil nitrification. Although it is believed that increased nitrogen availability would also increase nitrous oxide (N2O) emissions from forest ecosystems, its impact on N2O flux is poorly known. In this study, 3-years monitoring of N2O emissions was performed in a forested watershed receiving elevated nitrogen deposition and located in the suburbs of Tokyo, Japan. In addition, a comparative field survey was carried out in nine temperate forest sites with varying nitrogen availabilities. In the intensively studied forest site showing typical nitrogen saturation, the average annual N2O emissions from the whole watershed were estimated to be 0.88 kg N ha−1 year−1, comparable to the highest observed levels for temperate forests except for some very high emission sites in Europe. Although no correlation was found for humid spots with WFPS > 60%, a clear positive correlation was noted between N2O flux and net nitrification rate in situ for plots with water-filled pore space (WFPS) < 60%. The N2O flux varied across nine forest sites almost in proportional to the stream water NO3− concentration in the watershed that ranged from 0.14 to 1.64 mg N/L. We conclude that N2O emissions are related to nitrification in moist temperate forest, which may be associated with the magnitude of nitrogen saturation.

Excess Nitrate Export in Mountaintop Removal Coal Mining Watersheds

AbstractThroughout the Central Appalachian ecoregion, mountaintop removal coal mining (MTM) is the predominant form of land use change. The streams draining MTM impacted watersheds have been reported to contain high stream nitrate (NO3−) concentrations, yet the source and persistence of elevated NO3− remains unknown. Here we compiled data from multiple sources to conduct a regional evaluation of the impact of mining on stream NO3−, examine NO3− persistence after mining cessation, and identify potential N sources. Using water quality data from six large watersheds, we observe elevated NO3− in watersheds with the highest active mining density. At four small MTM watersheds with repeat sampling, we found that high levels of NO3− concentrations declined after mining cessation but remain elevated above reference after several decades. At MTM watersheds, we found annual mass flux of NO3− was 3.68 to 26.4 kg N ha−1 year−1, which is 1 to 2 orders of magnitude higher than a nearby forested reference watershed. Stream water NO3− isotopic ratios at these sites did not match previously suggested NO3− sources such as explosives used during mining and fertilizer applied during reclamation but were highly enriched in both δ15N and δ18O compared to the reference watershed suggesting high rates of NO3− retention. Explosive residue could account for the bulk of watershed NO3− export during active mining phases but other mining related N sources including fertilizer and rock‐derived N, the construction of valley fills, and alterations in watershed NO3‐ cycling likely contribute to the persistence of elevated NO3− export observed in this study.

Assessing tree ring δ15N of four temperate deciduous species as an indicator of N availability using independent long-term records at the Fernow Experimental Forest, WV.

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood δ15N in 2014, and tested the strength of correlation between tree ring δ15N of four species and stream water NO3- loss from 1971 to 2000. Higher soil NO3- was weakly associated with higher wood δ15N across species, and higher soil net nitrification rates were associated with higher δ15N for Quercus rubra only. The δ15N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO3-. L. tulipifera tree ring δ15N had a stronger association with stream water NO3- than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring δ15N, with a strong correlation between the wood δ15N and NO3- leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring δ15N, and given the weak relationships between contemporary wood δ15N and soil N cycle measurements, caution is warranted when using wood δ15N to infer changes in the N cycle.

Increase in stream water nitrate nitrogen concentrations caused by a disturbance to a forested catchment by Japanese oak wilt

ABSTRACTIn recent years, Japanese oak wilt (JOW) has become prevalent in Japan and has spread into almost all prefectures. To understand the effects of a JOW disturbance on nitrate nitrogen (NO3–) concentrations in a forest stream, we analyzed the changes in NO3– concentrations over a period of 7 years in a mixed broadleaved-conifer secondary forest in Japan. The JOW disturbance arose in the fifth year and lasted until the seventh year of the study period, resulting in a 4.7%, 3.0%, and 2.5% loss in the total basal area of the forest in the fifth, sixth, and seventh years, respectively, eventually resulting in an estimated canopy cover loss of 9.7%. The mean NO3– concentration after the appearance of JOW (3.99 μmol L–1) was significantly higher than before the start of the disturbance (1.99 μmol L–1). This study demonstrated that even the modest canopy cover loss by JOW-induced dieback of Quercus serrata trees could cause an increase in stream water NO3– concentration.

The role of seasonal and hydrological conditions in regulating dissolved inorganic nitrogen budgets in a forested catchment in SW Slovenia

During two consecutive years the monitoring of rainfall nitrate (NO3−) and ammonium (NH4+) concentrations, combined with high-frequency measurements of streamwater NO3− concentrations, provided insight into the mechanisms controlling the dissolved inorganic nitrogen (DIN) budgets of the forested Padež stream catchment in SW Slovenia. During both years, the catchment's annual wet atmospheric DIN deposition (12.6 and 13.8kg-N/ha) exceeded the annual DIN export (10.7kg-N/ha and 8.8kg-N/ha). The analysis of the monthly DIN budgets discloses seasonal patterns. In winter and early spring, the catchment behaves as a net source of DIN, whereas during late spring, summer, and early fall the catchment generally behaves as a net sink of the atmospheric DIN. Due to large evapotranspiration in the growth period, most of the DIN remains in the catchment and is flushed during the dormant season. Despite high evapotranspiration and consequently low runoff coefficients characteristic for the Mediterranean climate, large rainfall events in the growth period can cause intensive washout of DIN from the catchment. At a monthly basis, the DIN flushing might exceed the catchment's DIN retention capacity and the catchment might behave as a net source of DIN. Therefore, the hydrological conditions have the ability to shift the catchment's role in regulating the DIN budgets. The high responsiveness of the streamwater NO3− concentration in the growth period indicates a large pool of DIN in the forest soils which can become mobile by runoff formation. Wet atmospheric DIN deposition at the Padež catchment is considerable; highest DIN inputs can be expected when air masses approach the catchment from the southwest to southeast where there are extensive urban areas along the Italian, Slovenian and Croatian coast. However, the Padež catchment does not appear to be approaching N saturation, presumably due to strong internal N cycle in the forest soils.

Testing associations between tree species and nitrate availability: Do consistent patterns exist across spatial scales?

In forest ecosystems there are numerous factors that influence nitrate (NO3) availability and retention in ways that can significantly affect receiving waters. Unfortunately these factors often co-exist and interact making it difficult to establish the importance of each individually. Three reference watersheds at the Fernow Experimental Forest (FEF) provide a unique opportunity to evaluate the influence of tree species on soil NO3 availability across spatial scales because they differ in stream-water NO3 concentrations despite sharing the same macro-climate, geology, hydrology, dominant soils series, stand age, pollution regime, and land-use history.To test the strength of plant/soil associations inferred from previous work in two watersheds in the FEF, we measured NO3 availability and the composition of tree species in a third watershed that has intermediate stream-water NO3 concentrations. We also examined plant/soil relationships in these watersheds at the scale of individual trees.Across the spatial scales examined – from individual trees to small watersheds – there were consistent associations between tree species and soil NO3 availability. For individual trees, species with high underlying soil NO3 availability included Acer saccharum and Lireodendron tulipifera, whereas species with low underlying soil NO3 availability included Acer rubrum, Nyssa sylvatica, and Quercus prinus. For small plots we found plant/soil relationships that were consistent with those made apparent by sampling under individual trees of A. saccharum, A. rubrum, N. sylvatica, and Q. prinus. And at the scale of entire watersheds, those with higher stream-water NO3 had a greater importance of tree species associated with high NO3 availability at finer spatial scales than did the watershed with low stream-water NO3.The spatially-robust relationships between tree species and soil NO3 availability at the FEF suggest that well-characterized relationships between dominant tree species and soil properties should improve our understanding of how changes in the species composition of forest trees due to human activities, or natural causes, will alter rates of NO3 production and loss.

Assessing Nitrogen-Saturation in a Seasonally Dry Chaparral Watershed: Limitations of Traditional Indicators of N-Saturation

To evaluate nitrogen (N) saturation in xeric environments, we measured hydrologic N losses, soil N pools, and microbial processes, and developed an N-budget for a chaparral catchment (Sierra Nevada, California) exposed to atmospheric N inputs of approximately 8.5 kg N ha−1 y−1. Dual-isotopic techniques were used to trace the sources and processes controlling nitrate (NO3 −) losses. The majority of N inputs occurred as ammonium. At the onset of the wet season (November to April), we observed elevated streamwater NO3 − concentrations (up to 520 µmol l−1), concomitant with the period of highest gaseous N-loss (up to 500 ng N m−2 s−1) and suggesting N-saturation. Stream NO3 − δ15N and δ18O and soil N measurements indicate that nitrification controlled NO3 − losses and that less than 1% of the loss was of atmospheric origin. During the late wet season, stream NO3 − concentrations decreased (to <2 µmol l−1) as did gaseous N emissions, together suggesting conditions no longer indicative of N-saturation. We propose that chaparral catchments are temporarily N-saturated at ≤8.5 kg N ha−1 y−1, but that N-saturation may be difficult to reach in ecosystems that inherently leak N, thereby confounding the application of N-saturation indicators and annual N-budgets. We propose that activation of N sinks during the typically rainy winter growing season should be incorporated into the assessment of ecosystem response to N deposition. Specifically, the N-saturation status of chaparral may be better assessed by how rapidly catchments transition from N-loss to N-retention.

The Increased Contribution of Atmospheric Nitrogen Deposition to Nitrogen Cycling in a Rural Forested Area of Kyushu, Japan

Japan has been receiving increasing levels of atmospheric nitrogenous pollutants from the East Asian continent over the last few decades, so it is critical to evaluate the impact of this increased atmospheric nitro- gen (N) deposition on N cycling even in rural forests. Thisstudyevaluatedthecontributionofthecurrent level of atmospheric N deposition to N cycling in a rural forested area. Bulk precipitation and stream water were collected during 2007-2011 at the Shiiba Research Forest (SRF) located in the central Kyushu mountain range of southern Japan. Litterfall was also collected to investigate thecontributionofatmospheric N deposition to total N input (litterfall N+atmospheric N deposition). The results showed that atmospheric depositions of both nitrate (NO3 � ) and ammonium (NH4 + )w ere af ew times higher during 2009-2011 than in 1991. This could be the result of additional N deposition from the increased long-range transport of nitrogenous pollutants from the East Asian continent. The current level of annual N deposition (9.7 kg N ha �1 year �1 ) at the SRF was com- parable with that at many urban sites and was close to the reported threshold values causing N saturation in forest ecosystems. Although current atmospheric N deposition was an important component (23 %) of total N input (43 kg N ha �1 year �1 ) at the SRF, the concentrations of NO3 � in stream water were consistently low (<10 μmol L �1 ). These results indicate that atmospheric N deposition is currently largely incorporated into forest ecosystems without excess N export from forested watersheds.

Acid-base characteristics of the Grass Pond watershed in the Adirondack Mountains of New York State, USA: interactions among soil, vegetation and surface waters

Abstract. Grass Pond watershed is located within the southwestern Adirondack Mountain region of New York State, USA. This region receives some of the highest rates of acidic deposition in North America and is particularly sensitive to acidic inputs due to many of its soils having shallow depths and being generally base poor. Differences in soil chemistry and tree species between seven subwatersheds were examined in relation to acid-base characteristics of the seven major streams that drain into Grass Pond. Mineral soil pH, stream water BCS (base-cation surplus) and pH exhibited a positive correlation with sugar maple basal area (p = 0.055; 0.48 and 0.39, respectively). Black cherry basal area was inversely correlated with stream water BCS, ANC (acid neutralizing capacity)c and NO3- (p = 0.23; 0.24 and 0.20, respectively). Sugar maple basal areas were positively associated with watershed characteristics associated with the neutralization of atmospheric acidic inputs while in contrast, black cherry basal areas showed opposite relationships to these same watershed characteristics. Canonical correspondence analysis indicated that black cherry had a distinctive relationship with forest floor chemistry apart from the other tree species, specifically a strong positive association with forest floor NH4, while sugar maple had a distinctive relationship with stream chemistry variables, specifically a strong positive association with stream water ANCc, BCS and pH. Our results provide evidence that sugar maple is acid-intolerant or calciphilic tree species and also demonstrate that black cherry is likely an acid-tolerant tree species.

Streamside Management Zones Affect Movement of Silvicultural Nitrogen and Phosphorus Fertilizers to Piedmont Streams

Forestry best management practices (BMP) recommendations for streamside management zones (SMZs) are based on limited data regarding SMZ width, partial harvests, and nutrient movements after forest fertilization. Agricultural fertilization is commonly linked to increased stream nutrients. However, less is known about effectiveness of SMZ options for controlling nutrient movements after silvicultural fertilization. Diammonium phosphate and urea were applied to 12 subwatersheds in 3-year-old loblolly pine (Pinus taeda L.) plantations in the Virginia Piedmont. Three replicates of four SMZ treatments were superimposed on 12 subwatersheds in a previous SMZ harvest sediment study (7.6-m SMZ, 15.2-m SMZ thin, 15.2-m SMZ, and 30.5-m SMZ). Surface, near-surface, subsurface, and stream water samples were collected monthly for 1 year and analyzed for nitrate (NO3 ), ammonium (NH4 ), and orthophosphate (ortho-P). Transected measurements from streamside to fertilized plantations allowed interpretations of spatial nutrient measurements across SMZs. When compared with wider SMZs, 7.6-m SMZs had 3–10 surface water NO3 ,3 – 6 near-surface water NO3 , and 1–2 more stream water NO3 . No significant differences were detected for NH 4 for any SMZ treatment. The 15.2-m SMZ thin had small but significant increases (2– 8) in surface runoff for ortho-P relative to other SMZ treatments, perhaps because of increased surface water movement along thinning corridors. Across all SMZ treatments, comparisons of stream edges with fertilized stands indicated NO3 reductions of 33–98%, NH4 reductions of 68 –97%, and ortho-P reductions of 70 –98%. A 39% rainfall deficit during the study influenced results, but conventional SMZs 15.2 m protected streams from fertilization nutrient increases.

Spatial patterns of soil nitrification and nitrate export from forested headwaters in the northeastern United States

Nitrogen export from small forested watersheds is known to be affected by N deposition but with high regional variability. We studied 10 headwater catchments in the northeastern United States across a gradient of N deposition (5.4 − 9.4 kg ha−1 yr−1) to determine if soil nitrification rates could explain differences in stream water NO3− export. Average annual export of two years (October 2002 through September 2004) varied from 0.1 kg NO3−‐N ha−1 yr−1 at Cone Pond watershed in New Hampshire to 5.1 kg ha−1 yr−1 at Buck Creek South in the western Adirondack Mountains of New York. Potential net nitrification rates and relative nitrification (fraction of inorganic N as NO3−) were measured in Oa or A soil horizons at 21–130 sampling points throughout each watershed. Stream NO3− export was positively related to nitrification rates (r2 = 0.34, p = 0.04) and the relative nitrification (r2 = 0.37, p = 0.04). These relationships were much improved by restricting consideration to the 6 watersheds with a higher number of rate measurements (59–130) taken in transects parallel to the streams (r2 of 0.84 and 0.70 for the nitrification rate and relative nitrification, respectively). Potential nitrification rates were also a better predictor of NO3− export when data were limited to either the 6 sampling points closest to the watershed outlet (r2 = 0.75) or sampling points &lt;250 m from the watershed outlet (r2 = 0.68). The basal area of conifer species at the sampling plots was negatively related to NO3−export. These spatial relationships found here suggest a strong influence of near‐stream and near‐watershed‐outlet soils on measured stream NO3− export.

Effects of spring prescribed burning and wildfires on watershed nitrogen dynamics of central Idaho headwater areas

Fire is known for its potential to profoundly affect nitrogen (N) dynamics in both terrestrial and aquatic ecosystems. However, few studies have investigated fire effects on several important watershed N pools simultaneously or have directly compared effects of spring prescribed burns and wildfires that occurred in the same geographic area. We simultaneously measured N concentrations in soil, understory plant foliage, streamwater, and in-stream moss in paired (burned/unburned) headwater watersheds of replicated spring prescribed burns and wildfires in several growing seasons after fire. Watershed-scale burn severity, estimated via the satellite image-derived, reflectance-based delta Normalized Burn Ratio (dNBR), was significantly higher ( P < 0.05) after wildfires than prescribed burns. Fire effects were found in the terrestrial and aquatic ecosystems after wildfire but were limited to the terrestrial ecosystem after prescribed burns. Soil ammonium (NH 4 +) and nitrate (NO 3 −) concentrations were increased several-fold ( P < 0.05) in burned relative to corresponding unburned plots in prescribed burn and wildfire sites. In the first post-fire growing season, soil NO 3 − concentrations in wildfire-burned plots were one order of magnitude higher than in prescription-burned plots reflecting differences in burn severity. Plants resprouting after spring prescribed burns and wildfires sequestered post-fire available soil N, as indicated by higher foliar N concentrations. The relative increase in foliar N concentrations of four upland understory plant species or ecological groups averaged 47% in burned plots relative to unburned plots. Fire effects tended to be strongest and statistically significant in the first growing season after fire in the terrestrial ecosystem. In the headwater streams, effects of wildfire persisted, seemingly unattenuated, throughout the first three growing seasons after fire. Streamwater NO 3 − concentrations were one order of magnitude higher in wildfire-burned watersheds than in unburned watersheds and were positively correlated with dNBR during spring runoff. In-stream moss N concentrations were increased by about 40% throughout the study period, thus representing the aquatic analog to N retention via increased plant N uptake. The lack of impact on aquatic N dynamics after spring prescribed burning indicated N retention of post-fire available N within the terrestrial ecosystem. However, healthy forest ecosystems are adapted to periodic pulses of high post-fire availability of inorganic N in soil and streams. Therefore, from a N cycling perspective, rehabilitation measures are not required after wildfire and managers should target higher burn severities for prescribed burning at both the plot and watershed scale in order to stimulate N cycling and to reduce fuel loads more substantially.

Responses of soil and water chemistry to mountain pine beetle induced tree mortality in Grand County, Colorado, USA

Pine forest in northern Colorado and southern Wyoming, USA, are experiencing the most severe mountain pine beetle epidemic in recorded history, and possible degradation of drinking-water quality is a major concern. The objective of this study was to investigate possible changes in soil and water chemistry in Grand County, Colorado in response to the epidemic, and to identify major controlling influences on stream-water nutrients and C in areas affected by the mountain pine beetle. Soil moisture and soil N increased in soils beneath trees killed by the mountain pine beetle, reflecting reduced evapotranspiration and litter accumulation and decay. No significant changes in stream-water NO3- or dissolved organic C were observed; however, total N and total P increased, possibly due to litter breakdown or increased productivity related to warming air temperatures. Multiple-regression analyses indicated that % of basin affected by mountain pine beetles had minimal influence on stream-water NO3- and dissolved organic C; instead, other basin characteristics, such as percent of the basin classified as forest, were much more important.

NO3-の安定同位体比を用いた降雨流出時の渓流水NO3-流出起源の推定

NO3-の安定同位体比を用いた降雨流出時の渓流水NO3-流出起源の推定

Nitrous oxide emission from denitrification in stream and river networks

Nitrous oxide (N(2)O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N(2)O via microbial denitrification that converts N to N(2)O and dinitrogen (N(2)). The fraction of denitrified N that escapes as N(2)O rather than N(2) (i.e., the N(2)O yield) is an important determinant of how much N(2)O is produced by river networks, but little is known about the N(2)O yield in flowing waters. Here, we present the results of whole-stream (15)N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N(2)O at rates that increase with stream water nitrate (NO(3)(-)) concentrations, but that <1% of denitrified N is converted to N(2)O. Unlike some previous studies, we found no relationship between the N(2)O yield and stream water NO(3)(-). We suggest that increased stream NO(3)(-) loading stimulates denitrification and concomitant N(2)O production, but does not increase the N(2)O yield. In our study, most streams were sources of N(2)O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y(-1) of anthropogenic N inputs to N(2)O in river networks, equivalent to 10% of the global anthropogenic N(2)O emission rate. This estimate of stream and river N(2)O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.

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.