Decline In Nitrate Concentrations Research Articles

The latest status of nitrogen saturation on Kureha Hill, Toyama, Japan, based on 20‐year observations

AbstractExcessive anthropogenic nitrogen fixation alters the nitrogen cycle and increases nitrogen deposition, leading to nitrogen saturation, which in turns leads to forest decline and nitrate leaching into stream waters. Kureha Hill in Toyama Prefecture, Japan, is considered to be in nitrogen saturation, since many streams have contained high concentrations of nitrate for more than 20 years. In this study, the latest status of nitrogen saturation was verified by comparing most recent data with 20 years observational data, focusing on various indicators of nitrogen saturation, such as stream water quality, nitrogen budget of the watershed, and soil nitrification and mineralization potential. Both the nitrogen deposition on the Hyakumakidani watershed on Kureha Hill and the amount of nitrate leaching to the stream tended to decrease. The reduction in nitrate leaching could be attributed to the reduction in nitrogen deposition and other factors, such as nitrogen pools in the soil. Despite the recent decline in nitrate concentration of the Hyakumakidani stream, the pH level has remained unchanged. This may be due to decreased concentrations of sulfate and acid neutralizing capacity (ANC). The C/N ratio of the soil ranged from 12 to 18 from 2000 to 2023, which was lower than the nitrogen saturation threshold of 25. The net nitrification rate and net mineralization rate showed no significant changes between 2002 and 2023, indicating that the potential to produce nitrate has been unchanged. Similar values in net nitrification and net nitrogen mineralization rates reflected that the Hyakumakidani watershed remained in Stage‐3 nitrogen saturation.

Soil-specific outcomes in the OECD 216 Nitrogen Transformation Test.

The Organisation for Economic Co-operation and Development (OECD) 216 test guideline investigates the impact of agrochemicals on soil nitrogen transformation. After an evaluation of 465 OECD 216 studies, we describe two distinct yet contrasting outcomes in control nontreated samples that are possible in this testing framework, which we term the "rise" (consistent increases in nitrate concentrations throughout the test period) and "dip" (initial decline in nitrate concentration between Days 0-7, followed by a net-generation of nitrate across Days 7-28) responses. We raise significant concerns that control data from standardized, internationally recognized test guidelines can demonstrate such dissimilar patterns. We propose that, when present, the dip response undermines the intended functioning of the test system and removes the ability to draw appropriate ecotoxicological inferences from the data. In this work, we hypothesize the dip response is a product of conducting the study in low nitrogen content soils. Our results indicate that the dip response can be alleviated by using ammonium sulfate as an immediately available inorganic nitrogen source in place of the guideline-mandated complex, organic lucerne meal, demonstrating the influence of nitrogen availability and accessibility. However, not all low nitrogen soils exhibited the dip response, indicating the involvement of additional unidentified factors. Using our data and real-world regulatory examples, we advocate that datasets displaying the dip response should not be considered valid OECD 216 studies due to the influence of soil properties precluding an assessment of whether any impacts observed are driven solely by the test compound in question or are instead a product of the soil used. We propose methods to account for these soil-specific responses that could be integrated into the conduct and interpretation of OECD 216 studies. Such amendments will improve the reliability and robustness of the study system and enhance confidence in ecotoxicological conclusions derived from OECD 216 datasets. Integr Environ Assess Manag 2024;20:1611-1624. © 2024 SETAC.

Formation of Secondary Nitroaromatic Compounds in Polluted Urban Environments

AbstractNitroaromatic compounds (NACs) are important light absorption contributors to atmospheric brown carbon; however, their formation pathways and the key influential factors in polluted urban atmospheres are largely unknown. Herein, we present the molecular characterization of atmospheric particulate NACs in urban Shanghai by applying ultra‐high‐performance liquid chromatography coupled to Orbitrap mass spectrometry, and investigated their seasonal and diel variations and formation pathways. The total number and abundance of NACs increased significantly during air pollution episodes, and these NACs exhibited the greatest relative abundance among CHON compounds. The 12 quantified NACs exhibited higher levels in winter (52.3 ± 29.1 ng m−3) than summer (7.83 ± 3.30 ng m−3), largely owing to air masses mostly passing through northern heating regions. Among NACs, 4‐nitrophenol and 4‐nitrocatechol were the most abundant species. Most of NACs were enhanced via photooxidation during the day in summer, while the day‐night variations in winter were not as obvious. Further analysis of atmospheric formation relevance suggested that nitrophenol and its methylated derivatives were mainly formed through gas‐phase photochemical reactions, whereas aqueous‐phase oxidation processing could be an important pathway to form methyl‐nitrocatechols. Formation of NACs was relatively sensitive to NO2 under low‐NOx conditions, and became independent of NO2 under high‐NOx conditions. A larger fraction of NO2 was transformed into NACs than into inorganic nitrate products when nitrate was less abundant, implying that NACs would play more important roles in aerosol properties with the decline in nitrate concentrations.

Seasonal nitrogen uptake and regeneration in the western coastal Arctic

AbstractHere, we present the first study to investigate the seasonal importance of amino acid‐nitrogen (N) to Arctic near shore microbial communities. We measured primary productivity and the uptake of ammonium, nitrate, urea, and amino acids in two size fractions (> 3 μm and approximately 0.7–3 μm), as well as ammonium regeneration and nitrification using 15N and 13C tracer approaches in the near‐shore waters of the Chukchi Sea, during January, April, and August for two consecutive years. At discrete depths, nitrate comprised 46–78% of the total dissolved N pool during January and April but only 2–6% during August. Dissolved organic N (DON) concentrations increased between January and August though the carbon (C) : N (mol : mol) of the DON pool declined. Of the substrates tested, amino acids supported the bulk of both N and C nutrition in both size fractions during January and April (ice‐covered). Urea generally had the lowest uptake rate under ice‐covered conditions; uptake of urea‐C was only detectable in August. Though previous Arctic studies focused largely on nitrate, we found nitrate uptake was generally lower than other substrates tested. The sharp decline in nitrate concentration between April and August, however, indicates a drawdown of nitrate during that period. Rates of ammonium uptake were highest in August, when it was the dominant N substrate used. During all sample periods, rates of ammonium regeneration were sufficient to supply ammonium demand. Rates of nitrification varied between sample periods, however, with much higher rates seen in January and April.

Long-term nitrate removal in a stream riparian zone

The long-term capacity of riparian zones in regulating groundwater nitrate fluxes is not well understood. This study analyses patterns of nitrate removal for the period 1994–2012 at two sites in a river floodplain that have received high groundwater nitrate loading from a large upland aquifer for over 32 years. During the study, mean NO3 −–N concentrations entering the riparian zone varied between 20–30 and 30–42 mg/L at the upstream and downstream sites respectively, but did not show any clear inter-annual trend. A permeable sand layer in the riparian zone is underlain by a regional aquitard at a depth of 5–6 m and 4 m at the upstream site and downstream site respectively. Denitrification resulted in a decline in nitrate concentrations as lateral groundwater flow in the sand layer interacted with buried peat and channel bar deposits that range up to 3 m in depth at both riparian sites. This interaction was greater at the downstream site where the organic deposits extend down to <1 m from the aquitard in some locations. At the upstream site nitrate removal efficiency in the sand layer, at depths of 3–4 m ~20 m from the river bank, declined from 68 % in 1996–1998 to 42 % in 2009–2012. A smaller decline from 92 to 82 % occurred in the sand layer 10 m from the river bank during the study. In contrast, no clear pattern of change was evident at the downstream site where a nitrate removal efficiency of 98–100 % occurred at the river bank in most years between 1994 and 2012. These data suggest that the long-term nitrate removal performance of some riparian zones may decline if carbon availability for denitrification becomes limited as a result of variations in the quantity, quality and location of subsurface organic deposits that interact with deeper groundwater flowpaths.

Evaluating the implementation of the Nitrates Directive in Denmark and England using an actor‐orientated approach

AbstractThe Nitrates Directive (91/676/EEC) was adopted to address the issue of water pollution caused by nitrate derived from agricultural practices across the European Union (EU). This study evaluates the implementation of the Nitrates Directive by considering the perspectives of respondents from significant actor groups involved in the policy implementation process in Denmark and England. Structured field interviews were conducted and discovered that Nitrate Vulnerable Zone (NVZ) designation in England was a lengthy and inefficient process compared with that in Denmark. Generally, respondents from both countries considered that whole country designation was a more effective policy instrument than designation of discrete zones. Denmark's Action Programme measures reflected the severity of its water quality problems and go beyond the demands of the Nitrates Directive. The respondents recognized that Denmark's strict measures have led to a reduction in nitrate leaching and a decline in nitrate concentrations in streams from cultivated catchments. England's current Action Programme measures do not demand any substantial changes to farming practice and so have limited potential to reduce nitrate losses. This study concludes that a number of vague and ill prepared guidelines within the Nitrates Directive undermine its ability to protect water quality adequately, and hence current implementation of the Water Framework Directive (WFD) is an opportunity to re‐shape water policy in relation to nitrate. Copyright © 2007 John Wiley &amp; Sons, Ltd and ERP Environment.

Reducing Nitrate Leaching to Groundwater in an Intensive Dairy Farming System

Dairy farming is one of the main contributors to nitrate leaching to groundwater, particularly on soils that are susceptible to leaching, such as light well-drained sandy soils. In the Netherlands, as in many other European countries, these soils are predominantly used for dairy farming. A prototype dairy farming system that has been implemented in practice in 1989 has continuously been adapted since then to meet environmental standards (i.e. the EU-standard of 50 mg NO3− l−1) without reducing milk production intensity (11900 kg ha−1). After an initial decline in nitrate concentration from 193 mg l−1 to 63 upon implementation, it subsequently ‘stabilized’ at a level higher than the environmental standard: 55 mg l−1. The goal of this paper is to examine causes of excessive nitrate leaching. This was done by relating measured nitrate concentrations with management characteristics such as N balances, cropping patterns and grazing intensities. Special attention was paid to aspects that were supposed to be conducive for leaching: crop rotation of grass and maize and grazing. No evidence was found for enhanced nitrate leaching due to the rotation of grass with maize compared to permanent cultivation. This could be ascribed to the reduction in fertilization levels in first and second year maize with 90 and 45 kg N ha−1, respectively to account for the expected N release from the ploughed-in grass sod. Triticale was found to lead to higher leaching than grass or maize which is attributed to its poor growth in the period that it should function as catch crop in maize. Grazing contributed to a nitrate increase of about 30 mg NO3− l−1 on grassland. As grazing management and intensity is already strictly optimized in order to restrict nitrate leaching, this result underpins the need to develop sustainable grazing methods on soil that is susceptible to nitrate leaching.

Can Nitrogen Sequestration Explain the Unexpected Nitrate Decline in New Hampshire Streams?

Goodale and others (2003) pose an intriguingquestion: How can we explain the unexpected de-cline in nitrate concentration in New Hampshire’sstreams? Theory of forest nitrogen (N) biogeo-chemistry suggests that retention of atmospheri-cally deposited N can occur through vegetationuptake, accumulation in soil organic matter, abioticimmobilizaton, and accumulation in coarse woodydebris including roots and stumps. The lattermechanism occurs through microbial processesthat narrow the C:N ratios of this pool and may beunderappreciated, but is well documented (Har-mon and others 1986; Fahey and others 1988; Fa-hey and Arthur 1994). The relative strength ofthese sinks depends upon site conditions and, inparticular, on historical land use (Dise and others1998; Gundersen and others 1998a; Goodale andothers 2000). In general, the greater the previousdisturbance and resulting soil N depletion, thegreater the soil N sequestration potential. Forest Nsinks are limited, thus, N sequestration will notcontinue indefinitely (Stoddard 1994; Aber andothers 2003), but the capacity of this N sink and itsdynamics over time are uncertain. As an exampleof the uncertainty in the temporal dynamics of thisN sink, consider the lag between N additions andmeasurable effects on process. There can be a sig-nificant delay between the initiation of inorganic Nadditions and the induction of detectable increasesin net nitrification (Magill and others 2000).The unexpected decline in nitrate concentrationsin New Hampshire streams cannot be explained bytrends in forest maturation or atmospheric deposi-tion of N (Goodale and others 2003). Likewise,mechanisms for the decline, based on a re-equili-bration following recovery from insect defoliationand severe drought in the 1960s (Aber and others2002), and soil frost dynamics, were provisionallyruled out based on their failure to explain theregional nature of the stream water nitrate decline(Goodale and others 2003). Potential explanationbased on enhanced N uptake through growth in-creases caused by CO

Solute accumulation of chestnut oak and dogwood leaves in response to throughfall manipulation of an upland oak forest

To determine the biochemical basis of osmotic adjustment, seasonal and treatment differences in foliar water- soluble organic solutes and inorganic ions were investigated for two hardwood species that exhibited osmotic adjustment in a Throughfall Displacement Experiment at the Walker Branch Watershed near Oak Ridge, Tennessee. Leaf samples of overstory and understory chestnut oak (Quercus prinus L.) and understory dogwood (Cornus florida L.) were collected during the 1994 (wet) and 1995 (dry) growing seasons from each of three treatments: dry (-33% throughfall), ambient (control) and wet (+33% throughfall). Foliar soluble carbohydrates and organic acids were measured by gas chromatography-mass spectrometry. During May 1994, when the demand for sucrose was greatest, dogwood accumulated small amounts of glucose, quinic acid and Mg2+, offsetting a decline in nitrate concentration. As the mild drought continued and tree growth slowed, there was a significant accumulation of sucrose in dogwood in the dry treatment in June, and a trend toward increased K+. In overstory chestnut oak in the dry treatment over the same period, there were significant accumulations of fructose, glucose and K+, and a trend toward increased quinic acid accumulation. Sucrose did not become a key osmotically active compound in chestnut oak until August 1994. In 1995, with the exception of understory chestnut oak, there was no accumulation of K+ in either species. During the severe drought of 1995, monosaccharides, particularly glucose and fructose, accounted for most of the osmotic adjustment in both species. Among solutes, glucose constituted the largest accumulation in dogwood in the dry treatment in August 1995, followed by fructose and sucrose. There was only a moderate increase in solutes in chestnut oak trees in 1995, with fructose and glucose constituting over 50% of the predicted solute accumulation in July. Both species accumulated a wider array of solutes during the dry year than during the wet year, but treatment differences in solute accumulation in chestnut oak were partially limited by drought. The greater dehydration tolerance of chestnut oak than dogwood was evident in the higher baseline concentrations of organic solutes and the greater array of solutes accumulated in response to drought.

Pre- and Post-Flood Retention Efficiency of Nitrogen in a Sonoran Desert Stream

The objectives of this study were 1) to compare Sonoran Desert streams with other streams in terms of retention efficiency of nitrate; 2) to examine the effects of a flood on nitrate retention and to determine which factors control nitrate retention in the surface stream subsystem in Sycamore Creek, Arizona; and 3) to compare the short-term nutrient addition technique with computations based upon natural nutrient gradients. From June to September 1995, we did 8 short-term nitrate and chloride additions (4 additions before and 4 after a flood) in a 240-m reach to measure nitrate uptake length as an index of surface stream retention efficiency of nitrate. We also calculated nitrate uptake lengths based on a natural downstream decline in nitrate concentration, using data from the addition dates and from previous studies. Nitrate uptake lengths measured in Sycamore Creek were short (<120 m) compared to published values from other streams, indicating a high retention efficiency of nitrate in this nitrogen-limi...