Chronic NO3 Research Articles

Marsh sediments chronically exposed to nitrogen enrichment contain degraded organic matter that is less vulnerable to decomposition via nitrate reduction

Blue carbon habitats, including salt marshes, can sequester carbon at rates that are an order of magnitude greater than terrestrial forests. This ecosystem service may be under threat from nitrate (NO3−) enrichment, which can shift the microbial community and stimulate decomposition of organic matter. Despite efforts to mitigate nitrogen loading, salt marshes continue to experience chronic NO3− enrichment, however, the long-term consequence of this enrichment on carbon storage remains unclear. To investigate the effect of chronic NO3− exposure on salt marsh organic matter decomposition, we collected sediments from three sites across a range of prior NO3− exposure: a relatively pristine marsh, a marsh enriched to ~70 μmol L−1 NO3− in the flooding seawater for 13 years, and a marsh enriched between 100 and 1000 μmol L−1 for 40 years from wastewater treatment effluent. We collected sediments from 20 to 25 cm depth and determined that sediments from the most chronically enriched site had less bioavailable organic matter and a distinct assemblage of active microbial taxa compared to the other two sites. We also performed a controlled anaerobic decomposition experiment to test whether the legacy of NO3− exposure influenced the functional response to additional NO3−. We found significant changes to microbial community composition resulting from experimental NO3− addition. Experimental NO3− addition also increased microbial respiration in sediments collected from all sites. However, sediments from the most chronically enriched site exhibited the smallest increase, the lowest rates of total NO3− reduction by dissimilatory nitrate reduction to ammonium (DNRA), and the highest DNF:DNRA ratios. Our results suggest that chronic exposure to elevated NO3− may lead to residual pools of organic matter that are less biologically available for decomposition. Thus, it is important to consider the legacy of nutrient exposure when examining the carbon cycle of salt marsh sediments.

Effect of acute and chronic nitrate supplementation on the performance of endurance athletes: a systematic review

Evidence that dietary supplementation with nitrate-rich beet juice (NO3-) can improve endurance exercise performance. This paper aimed critically assess the effects of acute and chronic NO3- supplementation in the performance of trained healthy adult individuals and/or athletes. This systematic review analyzed 12 randomized, blinded, crossover clinical trials with control, which investigated the effect of acute and/or chronic supplementation of NO3- through juice or fresh beet on physical performance in endurance exercises. These studies involved 185 subjects. Four of the six acute studies in this review demonstrated improvements in sports performance in running and cycling. Among the eight chronic studies, only two showed no improvement in performance. Six studies with secondary supplementation yield favorable results. Of the six studies conducted with athletes, five demonstrated increases in VO2MAX. The current literature points to the positive effects of dietary supplementation of NO3- acute and chronic in trained and/or athletes. However, there are limitations regarding research on the results of NO3- supplementation in women and if there is an optimal time for observing effects on athletic performance.

Acute and Chronic Toxicity of Sodium Nitrate and Sodium Sulfate to Several Freshwater Organisms in Water-Only Exposures.

Elevated nitrate (NO3 ) and sulfate (SO4 ) in surface water are of global concern, and studies are needed to generate toxicity data to develop environmental guideline values for NO3 and SO4 . The present study was designed to fill existing gaps in toxicity databases by determining the acute and/or chronic toxicity of NO3 (tested as NaNO3 ) to a unionid mussel (Lampsilis siliquoidea), a midge (Chironomus dilutus), a fish (rainbow trout, Oncorhynchus mykiss), and 2 amphibians (Hyla versicolor and Lithobates sylvaticus), and to determine the acute and/or chronic toxicity of SO4 (tested as Na2 SO4 ) to 2 unionid mussels (L. siliquoidea and Villosa iris), an amphipod (Hyalella azteca), and 2 fish species (fathead minnow, Pimephales promelas and O. mykiss). Among the different test species, acute NO3 median effect concentrations (EC50s) ranged from 189 to >883 mg NO3 -N/L, and chronic NO3 20% effect concentrations (EC20s) based on the most sensitive endpoint ranged from 9.6 to 47 mg NO3 -N/L. The midge was the most sensitive species, and the trout was the least sensitive species in both acute and chronic NO3 exposures. Acute SO4 EC50s for the 2 mussel species (2071 and 2064 mg SO4 /L) were similar to the EC50 for the amphipod (2689 mg SO4 /L), whereas chronic EC20s for the 2 mussels (438 and 384 mg SO4 /L) were >2-fold lower than the EC20 of the amphipod (1111 mg SO4 /L), indicating the high sensitivity of mussels in chronic SO4 exposures. However, the fathead minnow, with an EC20 of 374 mg SO4 /L, was the most sensitive species in chronic SO4 exposures whereas the rainbow trout was the least sensitive species (EC20 > 3240 mg SO4 /L). The high sensitivity of fathead minnow was consistent with the finding in a previous chronic Na2 SO4 study. However, the EC20 values from the present study conducted in test water containing a higher potassium concentration (3 mg K/L) were >2-fold greater than those in the previous study at a lower potassium concentration (1 mg K/L), which confirmed the influence of potassium on chronic Na2 SO4 toxicity to the minnow. Environ Toxicol Chem 2020;39:1071-1085. © 2020 SETAC.

The Popeye mythology may be true: eating spinach can make you fatigue resistant!

The Popeye mythology may be true: eating spinach can make you fatigue resistant!

Scale dependence of controls on groundwater vulnerability in the water–energy–food nexus, California Coastal Basin aquifer system

Study regionThe Coastal California basin aquifer system (CCB), USA. Study focusUnderstanding the controls on nonpoint source (NPS) nitrate (NO3−) contamination in groundwater is motivated by the widespread detection of NO3−, implications for human health and aquatic ecosystems, groundwater sustainability, and a growing realization that such understanding across spatial scales promotes management and policy choices that optimize the Water–Energy–Food (WEF) Nexus. In the CCB, the conflicts in the WEF Nexus are apparent because of intensive agricultural practices that have contributed to chronic NO3− loading to groundwater. We evaluate the scale-dependent relations among source, transport, and attenuation (STA) factors that control NPS NO3− contamination in recently recharged CCB groundwater. Logistic regression models are presented for sub-regional and regional model domains. New hydrological insights for the regionDissolved oxygen (DO) (attenuation) in groundwater is often the most significant STA factor in all model domains, indicating that DO is an important, scale-invariant factor controlling NPS NO3− contamination. Farm fertilizer (source) is also a significant scale-invariant factor, while many of the transport factors are scale-dependent factors. We present vulnerability maps that illustrate the spatial patterns of predicted probability of detected elevated NO3−. Findings here improve knowledge about the scale dependence of STA factors, which help decision makers develop best management strategies and policies that advances groundwater sustainability and optimizes the WEF Nexus.

The Effects of Chronic Nitrate Supplementation and the Use of Strong and Weak Antibacterial Agents on Plasma Nitrite Concentration and Exercise Blood Pressure.

Chlorhexidine-containing mouthwash (STRONG), which disturbs oral microflora, has been shown to diminish the rise in plasma nitrite concentration ([NO2-]) and attenuate the reduction in resting blood pressure (BP) typically seen after acute nitrate (NO3-) ingestion. We aimed to determine whether STRONG and weaker antiseptic agents attenuate the physiological effects of chronic NO3- supplementation using beetroot juice (BR). 12 healthy volunteers mouth-rinsed with STRONG, non-chlorhexidine mouthwash (WEAK) and deionised water (CON) 3 times a day, and ingested 70 mL BR (6.2 mmol NO3-), twice a day, for 6 days. BP (at rest and during 10 min of treadmill walking) and plasma and salivary [NO3-] and [NO2-] were measured prior to and on day 6 of supplementation. The change in salivary [NO3-] 4 h post final ingestion was higher (P<0.05) in STRONG (8.7±3.0 mM) compared to CON (6.3±0.9 mM) and WEAK (6.0±3.0 mM). In addition, the rise in plasma [NO2-] at 2 h was lower in STRONG compared with WEAK (by 89±112 nM) and CON (by 200±174 nM) and in WEAK compared with CON (all P<0.05). Changes in resting BP were not different between conditions (P>0.05). However, during treadmill walking, the increase in systolic and mean arterial BP was higher 4 h after the final nitrate bolus in STRONG compared with CON (P<0.05) but not WEAK. The results indicate that both strong and weak antibacterial agents suppress the rise in plasma [NO2-] observed following the consumption of a high NO3- diet and the former can influence the BP response during low-intensity exercise.

Nitrogen processing following experimental sediment rewetting in isolated pools in an agricultural stream of a semiarid region

Water scarcity represents an important driver of stream microorganisms and N transformations rates. In semiarid regions many agricultural streams rich in NO3− undergo periods of flow cessation of severe duration and spatial extent. After drought, rainfalls tend to firstly rewet stream sediments by creating isolated pools that can remain in streambeds until the entire stream is reconnected. Sediment rewetting may lead the reactivation of nutrient processing, yet usually causes release of inorganic N. Thus, information about the role of sediments during this rewetting period on N processing and fluxes downstream is especially valuable when semiarid watercourses are affected by chronic NO3− exposure.We conducted a microcosm study to evaluate how a short-term sediment rewetting over a 7 days period in disconnected pools can control denitrification rates and N availability in an agriculturally impacted semiarid stream.We observed a significant stimulation of denitrification in dry sediments (<0.01μgNm−2h−1) upon rewetting with a maximum rate of 830μgNm−2h−1 after 24h. In parallel, NO3− in dry sediments (1170mgm−2) dropped considerably after 7 days of rewetting (72mgm−2), despite the fact that denitrification was restrained from 24h thereafter. However, from this time a notably increase of NH4+ concentration was detected until the end, which suggests that at this point, NO3− is transformed into NH4+ through dissimilatory reduction (DNRA) supported by low Eh (40mV). If compared with NO3−, NH4+ can be either more easily retained in sediments or assimilated preferably by organisms. We highlight that the diversification of N species provided by isolated pools creation just before flow resumption may progressively improve water quality in NO3−-rich streams, which is a key aspect in the face of future agriculture intensification and water scarcity scenario.

Nitrate ingestion: a review of the health and physical performance effects.

This paper provides an overview of the current literature and scientific evidence surrounding inorganic nitrate (NO3−) supplementation and its potential for improving human health and physical performance. As indicative of the ever-expanding organic and natural food consumer market, athletes and health enthusiasts alike are constantly searching for ingredient-specific “super foods” and dietary supplements capable of eliciting health and performance benefits. Evidence suggests that NO3− is the viable active component within beetroot juice (BRJ) and other vegetables, responsible for health-promoting and ergogenic effects. Indeed, multiple studies support NO3− supplementation as an effective method to improve exercise performance. NO3− supplementation (either as BRJ or sodium nitrate [NaNO3−]) has also demonstrated modest benefits pertaining to cardiovascular health, such as reducing blood pressure (BP), enhancing blood flow, and elevating the driving pressure of O2 in the microcirculation to areas of hypoxia or exercising tissue. These findings are important to cardiovascular medicine/exercise physiology and suggest a possible role for NO3− supplementation: (1) as a low-cost prevention and treatment intervention for patients suffering from blood flow disorders; and (2) an effective, natural ergogenic aid for athletes. Benefits have been noted following a single bolus, as well as daily supplementation of NO3−. While results are promising, additional research is needed to determine the impact of NO3− supplementation on anaerobic exercise performance, to identify principle relationships between isolated nitrate and other ingredients found in nitrate-rich vegetables (e.g., vitamin C, polyphenols, fatty acids, thiocyanate), to explore the specific dose-response relationships needed to elicit health and ergogenic benefits, to prolong the supplementation period beyond a relatively short period (i.e., >15 days), to determine if more robust effects can be observed with longer-term treatment, and to fully examine the safety of chronic NO3− supplementation, as this continues to be a concern of some.

Chronic N loading reduces N retention across varying base flows in a desert river

Stream ecosystems receive and transport nutrients from terrestrial ecosystems and are important sites of N retention and removal in catchments. Many streams experience high anthropogenic N loading, which can overwhelm N retention and removal mechanisms and cause large downstream fluxes. Small, headwater streams are important sites of N retention, but the role of streams in larger catchments or as discharge increases is less clear. We evaluated how NO3− uptake dynamics responded to chronic N loading at different sites in a river draining a large desert catchment (∼7600 km2). Based on nutrient saturation theory, we predicted that chronic N loading would result in decreased uptake efficiency. Previous research suggested that increasing stream discharge also is associated with decreasing N-uptake efficiency. We addressed these relationships for a desert river by examining NO3− uptake dynamics over variable stream discharge encompassing its long-term range in base flow. We used short-term nutrient-addition studies to estimate uptake parameters for NO3− in a reference reach and a reach subject to chronic NO3− input. NO3− uptake efficiency was lower in the N-enriched reach than in the reference reach. However, within a reach, temporal changes in discharge and N concentration did not always affect uptake efficiency as predicted; e.g., pulses of high N flux following monsoon-season flooding did not result in reduced uptake efficiency. Estimates of denitrification rates indicated that this N-removal process was only a small fraction of N uptake, a result suggesting that most N is temporarily retained and eventually is exported downstream. N concentration exerted the primary influence on NO3− uptake efficiency in this large desert stream. However, within reaches, other factors that influence N retention, including floods, biota, and variable flow paths, probably contributed to observed temporal variation.

Acute and chronic effects of dietary nitrate supplementation on blood pressure and the physiological responses to moderate-intensity and incremental exercise

Dietary nitrate (NO(3)(-)) supplementation with beetroot juice (BR) over 4-6 days has been shown to reduce the O(2) cost of submaximal exercise and to improve exercise tolerance. However, it is not known whether shorter (or longer) periods of supplementation have similar (or greater) effects. We therefore investigated the effects of acute and chronic NO(3)(-) supplementation on resting blood pressure (BP) and the physiological responses to moderate-intensity exercise and ramp incremental cycle exercise in eight healthy subjects. Following baseline tests, the subjects were assigned in a balanced crossover design to receive BR (0.5 l/day; 5.2 mmol of NO(3)(-)/day) and placebo (PL; 0.5 l/day low-calorie juice cordial) treatments. The exercise protocol (two moderate-intensity step tests followed by a ramp test) was repeated 2.5 h following first ingestion (0.5 liter) and after 5 and 15 days of BR and PL. Plasma nitrite concentration (baseline: 454 ± 81 nM) was significantly elevated (+39% at 2.5 h postingestion; +25% at 5 days; +46% at 15 days; P < 0.05) and systolic and diastolic BP (baseline: 127 ± 6 and 72 ± 5 mmHg, respectively) were reduced by ∼4% throughout the BR supplementation period (P < 0.05). Compared with PL, the steady-state Vo(2) during moderate exercise was reduced by ∼4% after 2.5 h and remained similarly reduced after 5 and 15 days of BR (P < 0.05). The ramp test peak power and the work rate at the gas exchange threshold (baseline: 322 ± 67 W and 89 ± 15 W, respectively) were elevated after 15 days of BR (331 ± 68 W and 105 ± 28 W; P < 0.05) but not PL (323 ± 68 W and 84 ± 18 W). These results indicate that dietary NO(3)(-) supplementation acutely reduces BP and the O(2) cost of submaximal exercise and that these effects are maintained for at least 15 days if supplementation is continued.

Chronic Atmospheric NO 3 − Deposition Does Not Induce NO 3 − Use by Acer saccharum 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 NO 3 − . However, it is uncertain whether long-term exposure to NO 3 − deposition might induce NO 3 − uptake by this ecologically important overstory tree. Here, we investigate whether 10 years of experimental NO 3 − deposition (30 kg N ha−1 y−1) could induce NO 3 − uptake and assimilation in overstory sugar maple (approximately 90 years old), which would enable this species to function as a direct sink for atmospheric NO 3 − deposition. Kinetic parameters for NH 4 + and NO 3 − uptake in fine roots, as well as leaf and root NO 3 − reductase activity, were measured under conditions of ambient and experimental NO 3 − deposition in four sugar maple-dominated stands spanning the geographic distribution of northern hardwood forests in the Upper Lake States. Chronic NO 3 − deposition did not alter the V max or K m for NO 3 − and NH 4 + uptake nor did it influence NO 3 − reductase activity in leaves and fine roots. Moreover, the mean V max for NH 4 + uptake (5.15 μmol 15N g−1 h−1) was eight times greater than the V max for NO 3 − uptake (0.63 μmol 15N g−1 h−1), indicating a much greater physiological capacity for NH 4 + uptake in this species. Additionally, NO 3 − reductase activity was lower than most values for woody plants previously reported in the literature, further indicating a low physiological potential for NO 3 − assimilation in sugar maple. Our results demonstrate that chronic NO 3 − deposition has not induced the physiological capacity for NO 3 − uptake and assimilation by sugar maple, making this dominant species an unlikely direct sink for anthropogenic NO 3 − deposition.

Microbial Cycling of C and N in Northern Hardwood Forests Receiving Chronic Atmospheric NO3− Deposition

Sugar maple (Acer saccharum Marsh.)-dominated northern hardwood forests in the upper Lakes States region appear to be particularly sensitive to chronic atmospheric NO 3 − deposition. Experimental NO 3 − deposition (3 g NO 3 − N m−2 y−1) has significantly reduced soil respiration and increased the export of DOC/DON and NO 3 − across the region. Here, we evaluate the possibility that diminished microbial activity in mineral soil was responsible for these ecosystem-level responses to NO 3 − 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 NO 3 − deposition. Microbial biomass, microbial respiration, and daily rates of gross and net N transformations were not changed by NO 3 − deposition. We also observed no effect of NO 3 − deposition on annual rates of net N mineralization. However, NO 3 − deposition significantly increased (27%) annual net nitrification, a response that resulted from rapid microbial NO 3 − assimilation, the subsequent turnover of NH 4 + , and increased substrate availability for this process. Nonetheless, greater rates of net nitrification were insufficient to produce the 10-fold observed increase in NO 3 − export, suggesting that much of the exported NO 3 − resulted directly from the NO 3 − deposition treatment. Results suggest that declines in soil respiration and increases in DOC/DON export cannot be attributed to NO 3 − -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 NO 3 − deposition in Lake States northern hardwood forests.

Anthropogenic N deposition and the fate of 15NO3- in a northern hardwood ecosystem

Human activity has substantially increased atmospheric NO 3 − deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO 3 − deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5–7 years). Although microbial immobilization is an initial sink (i.e., within 1–2 days) for anthropogenic NO 3 − in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO 3 − additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of 15NO 3 − in plots receiving chronic NO 3 − addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m−2 year−1) and three plots which receive ambient plus experimental N deposition (3.0 g NO3 −-N m−2 year−1). Chronic NO 3 − deposition significantly increased the N concentration and content (g N/m2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO 3 − deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO 3 − over a 1 year duration. Our results indicate that anthropogenic NO 3 − initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.

Simulated chronic NO3− deposition reduces soil respiration in northern hardwood forests

AbstractChronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO3− deposition (3 g N m−2 yr−1) to four sugar maple‐dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO3−‐amended plots. In all subsequent measurement years, soil respiration rates from NO3−‐amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO3− additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO3−‐amended plots are consistent with this explanation.