Streamwater Nitrate Concentrations Research Articles

Cryosphere and land cover influence on stream water quality in Central Asia's glacierized catchments

This work helps address recent calls for systematic water quality assessment in Central Asia and considers how nutrient and salinity sources, and transport, affect water quality along the continuum from the cryosphere to the lowland plains. Spatial and, for the first time, temporal variations in stream water pH, temperature, electrical conductivity, and nitrate and phosphate concentrations are presented for four catchments (485–13,500 km2), all with glaciers and major urban areas. The catchments studied were: Kaskelen (Kazakhstan), Ala-Archa (Kyrgyzstan), Chirchik (Uzbekistan) and the Kofarnihon (Tajikistan). Measurements were made in cryosphere, stream water, groundwater, reservoir and lake samples over a 22-month period at fortnightly intervals from 35 sites. The results highlight that glacier, permafrost and rock glacier outflows were primary and secondary nitrate sources (>1 mg N L−1) to the headwaters, and there were major increases in salinity and nitrate concentrations where rivers receive inputs from agriculture and settlements. Overall, the water quality complied with national and World Health Organization standards, however there were pollution hot-spots with shallow urban groundwaters contaminated with nitrate (>11 mg N L−1) and stream electrical conductivity above 800 μS cm−1 in some agricultural areas indicative of high salinity. Phosphate concentrations were generally low (<0.06 mg P L−1) throughout the catchments, though elevated (>0.2 mg P L−1) in urban areas due to effluent contamination. A melt water dilution effect along the main river channels was discernible, in the electrical conductivity and nitrate concentration seasonal dynamics, 100 s of km from the headwaters. Thus, the input of relatively clean water from the cryosphere is an important regulator of main channel water quality in the urban and farmed lowland plains adjacent to the Tien Shan and Pamir. Improved sewage treatment is needed in urban areas.

Assessing impacts of cemeteries on water quality in an urban headwater watershed with mixed human‐built infrastructure

AbstractCemeteries are understudied integral components to urban watersheds, which provide ecosystem services but can also export nutrients, trace elements, and other contaminants to nearby water bodies. In this study, we focus on Meadowbrook Creek, an urban headwater stream in Syracuse, New York (USA), which has shown significant nitrate contributions from a local cemetery. We collected biweekly surface water samples over the course of 1 year from 2022 to 2023 for analysis of major and trace elemental concentrations including Na, Ca, Mg, K, F, Cl, sulfate, and nitrate. Here, we aim to assess the impact of various human infrastructures on urban stream water quality with a particular focus on the cemetery and nitrate. A comparison between the new dataset in this study and previously reported water chemistry data in Meadowbrook in 2012 suggests a decade‐long impact of road salting and the cemetery on water quality particularly with respect to Na, Cl, and nitrate. Sulfate, Mg, Ca, and K are likely mainly geogenic. Stable nitrogen isotope data, the usage of concrete or steel vaults in the cemetery in the past 50 years, and the lack of correlation between nitrate and fluoride concentrations in stream water argue against burial decay products being a major source of nitrate to the stream. Instead, other nitrate sources that exist in the cemetery such as, fertilizer, decaying plant material, and wastewater, are more viable dominant nitrate sources. In addition, nitrate loading calculations indicate that the groundwater‐connected reach, including the cemetery, acts as an annual net sink for nitrate despite the seasonally varying sink‐source patterns.

Effects of climate and acidic deposition on interannual variations of stream water chemistry in forested watersheds in the Shimanto River Basin, southern Japan

AbstractAlthough the amount of acidic deposition has recently decreased in Japan, it has still deteriorated some forest ecosystems during the past several decades. Moreover, recent climate changes can affect stream water chemistry. We investigated the temporal trend and effects of climate conditions on stream water chemistry for more than 20 years in two areas (Yusuhara and Taisho) in the Shmanto River Basin, southern Japan, where the effects of acidic deposition are considered to be modest. Stream water samples were collected monthly from three forest watersheds selected at each site. The annual means of the stream chemistry were predicted by multiple regression analysis. The ammonium, nitrate, and sulfate concentrations in the bulk precipitation have decreased at Yusuhara, and the sodium, magnesium, calcium, chloride, nitrate, and sulfate concentrations in the stream water have decreased in both areas. The nitrate and sulfate concentrations apparently responded to the decreasing input of acidic deposition. The sunlight hours were positively related with the potassium, magnesium, calcium, nitrate, sulfate, and bicarbonate concentrations in stream water. The results suggest that long sunlight hours boost the photosynthetic activities, thus promoting soil respiration and decomposition of soil organic matter. Moreover, a higher carbonic acid concentration in the soil solution promotes cation weathering and carbonic acid dissociation to bicarbonate. Given the decreasing trends in magnesium and calcium concentration with no change in bicarbonate concentration, we inferred that previousinputs of acidic deposition enhanced the rate of rock weathering.

Controls on in-stream nitrogen loss in western Lake Erie tributaries

Management efforts to reduce cyanobacterial harmful algal blooms (cHABs) in the Great Lakes have focused on decreasing tributary inputs of phosphorus (P). Recent research has indicated that reduction of both P and nitrogen (N) can lessen cHABs severity. Microbially mediated N cycling in streambed sediment may reduce N riverine loads, yet little is known about in-stream N processing rates in the Maumee River Basin, a major source of nutrients to Lake Erie. During summer of 2019 and 2021, we sampled streambed sediment to measure potential nitrification and denitrification rates using the acetylene block method at 78 sites throughout the Maumee River network. We used structural equation models to identify indirect and direct drivers of denitrification. Precipitation was greater in 2019, resulting in a 67 % increase in mean discharge, 41 % of farm fields to be fallow, and a 50 % reduction in fertilizer use. During summer field surveys, median stream-water nitrate concentrations were not different between 2019 and 2021. Median denitrification rates were 13.3 mg N/m2/h and 31.2 mg N/m2/h, respectively, indicating high potential to remove N. Nitrate concentrations and nitrification rates were strong direct drivers of denitrification, especially in 2019 when coupled nitrification–denitrification sustained denitrification. Nitrate concentrations varied with land use. Notably, nitrate concentrations increased with the area of fallow land, which may indicate the presence of a legacy N source. These findings indicate that promoting streambed denitrification could reduce N loads to Lake Erie, but legacy N currently stored in the system may mask N reduction efforts.

Identifying Controls on Nitrate Sources and Flowpaths in a Forested Catchment Using a Hydropedological Framework

AbstractCatchment‐scale assessments of nitrogen retention and loss rarely account for soil and landscape heterogeneity and are, thus, unable to account for the suite of nitrogen cycling processes that ultimately affect the export of nitrate via stream water. Long‐term study at the Hubbard Brook Experimental Forest, NH has generated a unique data set that facilitates spatially explicit examination of interactions among hydrology, soil development, and nitrogen cycling processes. Using high‐frequency streamwater chemistry data with intensive subsurface hydrology and solute monitoring, we tracked areas of the catchment that are hydrologically active under different flow conditions to identify the source area of streamwater nitrate. We hypothesize that as the drainage network expands, increasing hydrologic connection to bedrock outcrop‐associated soils, streamwater nitrate concentration, and flux at the catchment outlet increase. Most nitrate export (&gt;80%) occurred during high flows when high nitrate, bedrock‐controlled areas of the catchment were most connected hydrologically to the drainage network (∼15% of the time). End‐member mixing analysis demonstrated that the bedrock‐controlled upper part of the catchment influences nitrate concentration at the outlet and contributes most to catchment nitrate export compared to the near‐stream soil units and seeps. Most of the time, nitrate at the catchment outlet comes from seeps and the near‐stream zone; under high flow conditions, bedrock‐controlled hotspots for nitrate production contribute more to export of nitrate. This analysis demonstrates how the source area of streamwater nitrate varies under different flow conditions, suggesting that long‐term nitrate dynamics may be driven primarily by a relatively small part of the catchment.

Groundwater‐surface water exchange as key control for instream and groundwater nitrate concentrations along a first‐order agricultural stream

AbstractLower‐order streams (first‐ and second‐order) define the initial, landscape‐related, chemical signature of stream water in catchments. To date, first‐order streams have been perceived as predominantly draining systems, which collect water and solutes from the surrounding groundwater and surface runoff and simply mirror the chemical composition of the inputs. In this study, the impact of stream‐groundwater exchange fluxes on water chemistry of a first order agricultural stream (Schönbrunnen) and its connected groundwater in south‐western Germany was assessed combining222Rn, dissolved ions (chloride, sulphate, nitrate), and salt tracer tests with investigations of stream discharge and groundwater hydraulic gradients. The findings suggest that stream‐water chemistry in lower‐order streams is governed by an intricate interplay between dynamic, bidirectional water and solute exchange between groundwater and the stream leading to a pronounced hydrologic turnover along the studied reaches. High nitrate concentrations (up to 79 mg/L as NO3−) in stream water were attenuated in downstream direction (a mean value of 39 mg/L) without an increase in discharge, suggesting that redox processes occurring during sediment passage in sequential infiltration and exfiltration zones affect stream water chemistry. Nitrate in stream water infiltrating into the aquifer at distinct losing spots was subject to denitrification within the first few decimetres of the streambed, while concurrent exfiltration of low‐nitrate groundwater into the stream at gaining spots compensated for flow losses and in turn diluted instream nitrate concentrations. In summary the findings imply that (1) instream mixing resulting from the bidirectional exchange of water between groundwater and the stream (hydrologic turnover) affects instream nitrate concentrations, (2) denitrification in the streambed of losing reaches and the near‐stream aquifer significantly contributes to reactive nitrate turnover and elimination, and (3) oxidation of ammonium could be a secondary source of nitrate inputs into the stream.

Forest disturbance and salvage logging have neutral long-term effects on drinking water quality but alter biodiversity

Forests host most terrestrial biodiversity and provide important ecosystem services, including the provision of drinking water. Increasing frequency and intensity of natural disturbances and subsequent salvage logging may impact both biodiversity and drinking-water quality. However, empirical evidence and particularly that generated from long-term studies, is scarce. Using data obtained from the monitoring of streamwater between 1985 and 2018 and mid-term data on biodiversity of twelve species groups, we quantified the combined effects of natural disturbances and salvage logging. We used generalized additive models to test the effects of cumulative disturbed and salvage-logged areas on annual maximum nitrate and dissolved organic carbon (DOC) concentrations. We used generalized mixed-effects models to test the effect of management (disturbed unlogged, disturbed logged and undisturbed, intact forest) on species numbers of studied taxa. We found that forest disturbances led to a temporal increase of nitrate concentration in streamwater, yet remaining far below 50 mg L−1, the limits recommended by the World Health Organization. Salvage logging did not exert any additional impact on nitrate and DOC concentrations, and hence did not affect streamwater quality. Natural disturbances increased the biodiversity in eight out of twelve species groups. Salvage logging additionally increased the biodiversity of five species groups related to open habitats, but decreased the biodiversity of three deadwood-dependent species groups. We conclude that neither natural forest disturbances in watersheds nor associated salvage logging have a harmful effect on the quality of the streamwater, which is used for drinking water. Setting aside naturally disturbed areas would promote the conservation of deadwood-dependent species.

山地渓流における硝酸態窒素流出の林相による違い－トドマツ林,カラマツ林,天然林流域の比較－

山地渓流における硝酸態窒素流出の林相による違い－トドマツ林,カラマツ林,天然林流域の比較－

Groundwater-surface water interactions and agricultural nutrient transport in a Great Lakes clay plain system

Nutrient export from agricultural land to surface waters is a significant environmental concern within the Great Lakes Basin (GLB). A field-based watershed-scale study was completed to investigate spatial and temporal variations of phosphorus and nitrate to assess nutrient transport pathways and groundwater-surface water interactions in an agriculturally dominated clay plain system. This was conducted in the 127 km2 Upper Parkhill Watershed, near Lake Huron in southwestern Ontario, Canada. Data collection occurred from June 2018 to May 2019 via continuous sensor deployment and discrete sampling of stream water, groundwater, hyporheic zone, and tile drainage water. Samples were analyzed for various nutrient species (total, total dissolved, soluble reactive, and particulate phosphorus, and nitrate-N) to examine the hydrological dynamics of principal transport pathways of agriculturally-derived nutrients. Total phosphorus and nitrate concentrations in stream water ranged from 0.007 to 0.324 mg/L and 0.32 to 13.13 mg NO3−-N/L, respectively. Tile drainage water total phosphorous concentrations varied from 0.006 to 0.066 mg/L. Groundwater total dissolved phosphorus concentrations ranged from <0.003 to 0.085 mg/L. Transport of phosphorus through tile drainage was observed to be greater than through groundwater over the study period. No distinct relationship was observed between nutrient concentrations in the hyporheic zone and the vertical hydraulic gradient within this zone in the studied stream reach. Preliminary correlations were discerned between water quality observations and recognized land management practices. Given the elevated stream nutrient concentrations, these results are consequential for the continual improvement of strategies and programs devised to conserve water resources within the GLB.

Reduction of stream nitrate concentrations by land management in contrasted landscapes

Optimizing management practices at the plot scale is sometimes not sufficient to reach water framework directive objectives for nitrate pollution. Land management measures involving targeted setting aside of croplands is a promising solution, but its efficiency depends on the local context. We used a distributed agro-hydrological model to compare management interventions intended to decrease vertical and lateral nitrate leaching from soil to groundwater and stream water in two contrasted agricultural catchments. The simulated scenarios combined two strategies: optimization of agricultural practices and land-use conversion from agricultural to natural land at different locations within the catchments. Long-term climate, discharge, and nitrate concentrations have been monitored for the two catchments and agricultural practices are well known over the 13-year simulation period (2002–2015). The Kervidy-Naizin site (KN) is subject to intense livestock pressure with mean nitrogen inputs of 257 kg ha−1 year−1, while the Aurade site (AU) is primarily cereal cultivation with nitrogen inputs of 109 kg ha−1 year−1. The results highlight a large nitrogen legacy in KN, resulting in a progressive and long lived (> 10 years) response to changes in management, while in AU, this response is perceptible after only 5–7 years. For both catchments, the most effective scenario involves wide riparian buffer strips in interception position covering about 15% of the catchment area. In KN, this land conversion scenario, simulated with the agro-hydrological model TNT2, created a decrease of nitrate concentration in stream water by 25% versus 15% in AU. Contrastingly, the implementation of best management practices decreased stream nitrate concentration only by 9% for KN and 4% for AU.

Stream Runoff and Nitrate Recovery Times After Forest Disturbance in the USA and Japan

AbstractTo understand mechanisms of long‐term hydrological and biogeochemical recovery after forest disturbance, it is important to evaluate recovery times (i.e., time scales associated with the return to baseline or predisturbance conditions) of stream runoff and nitrate concentration. Previous studies have focused on either the response of runoff or nitrate concentration, and some have specifically addressed recovery times following disturbance. However, controlling factors have not yet been elucidated. Knowing these relationships will advance our understanding of each recovery process. The objectives of this study were to explore the relationship between runoff and nitrate recovery times and identify potential factors controlling each. We acquired long‐term runoff and stream water nitrate concentration data from 20 sites in the USA and Japan. We then examined the relationship between runoff and nitrate recovery times at these multiple sites and use these relationships to discuss the ecosystem dynamics following forest disturbance. Nitrate response was detected at all study sites, while runoff responses were detected at all sites with disturbance intensities greater than 75% of the catchment area. The runoff recovery time was significantly correlated with the nitrate recovery time for catchments that had a runoff response. For these catchments, hydrological recovery times were slower than nitrate recovery times. The relationship between these two recovery times suggests that forest regeneration was a common control on both recovery times. However, the faster recovery time for nitrate suggests that nitrogen was less available or less mobile in these catchments than water.

Removal of Woody Riparian Vegetation Substantially Altered a Stream Ecosystem in an Otherwise Undisturbed Grassland Watershed

Riparian zones are key interfaces between stream and terrestrial ecosystems. Yet, we know of no whole-watershed experiments that cut only woody vegetation in the riparian zone in an otherwise intact watershed to isolate the role of riparian zones on stream ecology. We removed all of the woody riparian vegetation (from 10- and 30-m-wide buffers in headwaters and main channels, respectively) for 5 km of stream in a single watershed while leaving the remainder of the grassland watershed un-impacted. We assessed water chemistry changes 3 years before and 3 years after riparian wood removal and in two neighboring control watersheds with a before–after, control-impact design and analysis. Riparian woody removal caused 10–100-fold increases in mean stream water nitrate concentrations and pulses of high nitrate for 3 years thereafter. Other nutrients and total suspended solids increased 2–25 times for the 3 years of post-removal. In-stream rates of gross primary production, ecosystem respiration, and net ecosystem production had large treatment effect sizes but also high variance among samples. Past studies of whole-watershed deforestations showed similar water quality responses to our riparian deforestation. Riparian zones of grassland streams are sensitive to disturbance and likely impart relatively greater influence on stream structure and function than the upslope of the watershed. Our results further emphasize the role of riparian zones in biogeochemically linking aquatic and terrestrial habitats.

Nitrogen (N) Dynamics in the Mineral Soil of a Central Appalachian Hardwood Forest During a Quarter Century of Whole-Watershed N Additions

The structure and function of terrestrial ecosystems are maintained by processes that vary with temporal and spatial scale. This study examined temporal and spatial patterns of net nitrogen (N) mineralization and nitrification in mineral soil of three watersheds at the Fernow Experimental Forest, WV: 2 untreated watersheds and 1 watershed receiving aerial applications of N over a 25-year period. Soil was sampled to 5 cm from each of seven plots per watershed and placed in two polyethylene bags—one bag brought to the laboratory for extraction/analysis, and the other bag incubated in situ at a 5 cm depth monthly during growing seasons of 1993–1995, 2002, 2005, 2007–2014. Spatial patterns of net N mineralization and nitrification changed in all watersheds, but were especially evident in the treated watershed, with spatial variability changing non-monotonically, increasing then decreasing markedly. These results support a prediction of the N homogeneity hypothesis that increasing N loads will increase spatial homogeneity in N processing. Temporal patterns for net N mineralization and nitrification were similar for all watersheds, with rates increasing about 25–30% from 1993 to 1995, decreasing by more than 50% by 2005, and then increasing significantly to 2014. The best predictor of these synchronous temporal patterns across all watersheds was number of degree days below 19°C, a value similar to published temperature maxima for net rates of N mineralization and nitrification for these soils. The lack of persistent, detectable differences in net nitrification between watersheds is surprising because fertilization has maintained higher stream-water nitrate concentrations than in the reference watersheds. Lack of differences in net nitrification among watersheds suggests that N-enhanced stream-water nitrate following N fertilization may be the result of a reduced biotic demand for nitrate following fertilization with ammonium sulfate.

Differences in Stream Water Nitrate Concentrations between a Nitrogen-Saturated Upland Forest and a Downstream Mixed Land Use River Basin

Nitrogen (N) saturation of upland forests has been assumed to be a substantial N source downstream. However, removal processes of N, including assimilation and denitrification in the downstream area, have not been clarified. To evaluate the N removal processes, nitrate (NO3−) and organic N concentrations, as well as nitrogen isotope ratio (δ15N) and oxygen isotope ratio (δ18O) of NO3− were measured along three rivers of Tatara River Basin, Japan where upland forests have already been N-saturated. Geographic information system (GIS) based topographical analysis was also conducted to evaluate the land use as urban area in relation to topography. In two of the three rivers, NO3− concentrations did not increase from upstream to downstream, despite the potential non-point N sources of urban areas. In another river, NO3− concentrations rather decreased. The values of δ15N and δ18O of NO3− and organic N concentrations suggested the presence of denitrification and assimilation over N pollutants in the river whose watersheds have a lower percentage of urban area. The lower percentage of urban area could be explained by the lower topographic index. This study concluded that the NO3− leaching from upland N-saturated forests was substantially assimilated or denitrified in the downstream area.

The response of soil and stream chemistry to decreases in acid deposition in the Catskill Mountains, New York, USA

The Catskill Mountains have been adversely impacted by decades of acid deposition, however, since the early 1990s, levels have decreased sharply as a result of decreases in emissions of sulfur dioxide and nitrogen oxides. This study examines trends in acid deposition, stream-water chemistry, and soil chemistry in the southeastern Catskill Mountains. We measured significant reductions in acid deposition and improvement in stream-water quality in 5 streams included in this study from 1992 to 2014. The largest, most significant trends were for sulfate (SO42−) concentrations (mean trend of −2.5 μeq L−1 yr−1); hydrogen ion (H+) and inorganic monomeric aluminum (Alim) also decreased significantly (mean trends of −0.3 μeq L−1 yr−1 for H+ and −0.1 μeq L−1 yr−1 for Alim for the 3 most acidic sites). Acid neutralizing capacity (ANC) increased by a mean of 0.65 μeq L−1 yr−1 for all 5 sites, which was 4 fold less than the decrease in SO42− concentrations. These upward trends in ANC were limited by coincident decreases in base cations (−1.3 μeq L−1 yr−1 for calcium + magnesium). No significant trends were detected in stream-water nitrate (NO3−) concentrations despite significant decreasing trends in NO3− wet deposition. We measured no recovery in soil chemistry which we attributed to an initially low soil buffering capacity that has been further depleted by decades of acid deposition. Tightly coupled decreasing trends in stream-water silicon (Si) (−0.2 μeq L−1 yr−1) and base cations suggest a decrease in the soil mineral weathering rate. We hypothesize that a decrease in the ionic strength of soil water and shallow groundwater may be the principal driver of this apparent decrease in the weathering rate. A decreasing weathering rate would help to explain the slow recovery of stream pH and ANC as well as that of soil base cations.

Recovery from acidification alters concentrations and fluxes of solutes from Czech catchments

Changes in atmospheric deposition, stream water chemistry, and solute fluxes were assessed across 15 small forested catchments. Dramatic changes in atmospheric deposition have occurred over the last three decades, including a 70% reduction in sulphur (S) deposition. These changes in atmospheric inputs have been associated with expected changes in levels of acidity, sulphate and base cations in streams. Soil retention of S appeared to partially explain rates of chemical recovery. In addition to these changes in acid–base chemistry we also observed unexpected changes in nitrogen (N) biogeochemistry and nutrient stoichiometry of stream water, including decreased stream N concentrations. Among all catchments the average flux of dissolved inorganic nitrogen (DIN) was best predicted by average runoff, soil chemistry (forest floor C/N) and levels of acid deposition (both S and N). The rate of change in stream DIN flux, however, was much more closely correlated with reductions in rates of S deposition rather than those of DIN. Unlike DIN fluxes, the average concentrations as well as the rates of decline in streamwater nitrate (NO3) concentration over time were tightly linked to stream dissolved organic carbon/dissolved organic nitrogen ratios DOC/DON and DON/TP rather than catchment characteristics. Declines in phosphorus adsorption with increasing soil pH appear to contribute to the relationship between C, N, and P in our study catchments. Our observations suggest that catchment P availability and its alteration due to environmental changes (e.g. acidification) might have profound effects on N cycling and catchment N retention that have been largely unrecognized.

Exploring impacts of vegetated buffer strips on nitrogen cycling using a spatially explicit hydro-biogeochemical modeling approach

Agriculture has been recognized as a major anthropogenic source of surplus loads of nitrogen in the environment. Losses of nitrate via subsurface pathways are severely threatening groundwater and surface waters. This study explored the capability of a coupled hydro-biogeochemical spatially explicit model, simulating nitrogen cycling in agricultural soils and the associated fate of excess nitrate subjected to vertical and lateral displacement towards water bodies. Different vegetated buffer strips (VBS) were tested for their nitrate retention capability and impacts on N2O and N2 emissions. The effectiveness of a VBS to remove nitrate by denitrification strongly depends on soil characteristics and hydrological flow paths. Simulated N2 emissions from VBS with high soil moisture were up to twenty-fold compared to VBS where groundwater levels were low. Simulated streamwater nitrate concentrations without VBS were 3.7 mg NO3− l−1 and showed a decrease to 0.1 mg NO3− l−1 for a 20 m VBS.

A Decade of Streamwater Nitrogen and Forest Dynamics after a Mountain Pine Beetle Outbreak at the Fraser Experimental Forest, Colorado

Forests of western North America are currently experiencing extensive tree mortality from a variety of bark beetle species, and insect outbreaks are projected to increase under warmer, drier climates. Unlike the abrupt biogeochemical changes typical after wildfire and timber harvesting, the outcomes of insect outbreaks are poorly understood. The mountain pine bark beetle (Dendroctonus ponderosae) began to attack lodgepole pine (Pinus contorta) at the Fraser Experimental Forest in 2002 and spread throughout the research area by 2007. We compared streamwater nitrogen (N) from 2003 through 2012 with data from the previous two decades in four watersheds with distinct forest management histories, stand structures, and responses to the beetle outbreak. Watersheds dominated by old-growth had larger trees and lost 85% of overstory pine and 44% of total basal area to bark beetles. In contrast, managed watersheds containing a mixture of second-growth (30–60 year old) and old-growth (250- to 350-year old) had higher density of subcanopy trees, smaller mean tree diameter, and lower bark beetle-induced mortality (~26% of total basal area). Streamwater nitrate concentrations were significantly higher in old-growth watersheds during the outbreak than pre-outbreak levels during snowmelt and base flow seasons. In mixed-age stands, streamwater nitrate concentrations were unaffected by the outbreak. Beetle outbreak elevated inorganic N export 43 and 74% in two old-growth watersheds though the amounts of N released in streamwater were low (0.04 and 0.15 kg N ha−1) relative to atmospheric inputs (<2% of annual N deposition). Increased height, diameter, and foliar N of measured in residual live trees augmented demand for N, far in excess of the change in N export during the outbreak. Reallocation of soil resources released after pine mortality to overstory and understory vegetation helps explain high nutrient retention in watersheds affected by bark beetle outbreaks.

Dual nitrate isotopes clarify the role of biological processing and hydrologic flow paths on nitrogen cycling in subtropical low‐gradient watersheds

AbstractNitrogen (N) is an important nutrient as it often limits productivity but in excess can impair water quality. Most studies on watershed N cycling have occurred in upland forested catchments where snowmelt dominates N export; fewer studies have focused on low‐relief watersheds that lack snow. We examined watershed N cycling in three adjacent, low‐relief watersheds in the Upper Coastal Plain of the southeastern United States to better understand the role of hydrological flow paths and biological transformations of N at the watershed scale. Groundwater was the dominant source of nitrified N to stream water in two of the three watersheds, while atmospheric deposition comprised 28% of stream water nitrate in one watershed. The greater atmospheric contribution may have been due to the larger stream channel area relative to total watershed area or the dominance of shallow subsurface flow paths contributing to stream flow in this watershed. There was a positive relationship between temperature and stream water ammonium concentrations and a negative relationship between temperature and stream water nitrate concentrations in each watershed suggesting that N cycling processes (i.e., nitrification and denitrification) varied seasonally. However, there were no clear patterns in the importance of denitrification in different water pools possibly because a variety of factors (i.e., assimilatory uptake, dissimilatory uptake, and mixing) affected nitrate concentrations. Together, these results highlight the hydrological and biological controls on N cycling in low‐gradient watersheds and variability in N delivery flow paths among adjacent watersheds with similar physical characteristics.

Comparing the influence of wildfire and prescribed burns on watershed nitrogen biogeochemistry using 15N natural abundance in terrestrial and aquatic ecosystem components.

We evaluated differences in the effects of three low-severity spring prescribed burns and four wildfires on nitrogen (N) biogeochemistry in Rocky Mountain headwater watersheds. We compared paired (burned/unburned) watersheds of four wildfires and three spring prescribed burns for three growing seasons post-fire. To better understand fire effects on the entire watershed ecosystem, we measured N concentrations and δ15N in both the terrestrial and aquatic ecosystems components, i.e., soil, understory plants in upland and riparian areas, streamwater, and in-stream moss. In addition, we measured nitrate reductase activity in foliage of Spiraea betulifolia, a dominant understory species. We found increases of δ15N and N concentrations in both terrestrial and aquatic ecosystem N pools after wildfire, but responses were limited to terrestrial N pools after prescribed burns indicating that N transfer from terrestrial to aquatic ecosystem components did not occur in low-severity prescribed burns. Foliar δ15N differed between wildfire and prescribed burn sites; the δ15N of foliage of upland plants was enriched by 2.9 ‰ (difference between burned and unburned watersheds) in the first two years after wildfire, but only 1.3 ‰ after prescribed burns. In-stream moss δ15N in wildfire-burned watersheds was enriched by 1.3 ‰, but there was no response by moss in prescription-burned watersheds, mirroring patterns of streamwater nitrate concentrations. S. betulifolia showed significantly higher nitrate reductase activity two years after wildfires relative to corresponding unburned watersheds, but no such difference was found after prescribed burns. These responses are consistent with less altered N biogeochemistry after prescribed burns relative to wildfire. We concluded that δ15N values in terrestrial and aquatic plants and streamwater nitrate concentrations after fire can be useful indicators of the magnitude and duration of fire effects and the fate of post-fire available N.