Nitrate isotopes (δ15N, δ18O) in precipitation: best practices from an international coordinated research project.

Stable isotope ratios of nitrogen and oxygen (15N/14N and 18O/16O) of nitrate (NO3-) are excellent tracers for developing systematic understanding of sources, conversions, and deposition of reactive atmospheric nitrogen (Nr) in the environment. Despite recent analytical advances, standardized sampling of NO3-) isotopes in precipitation is still lacking. To advance atmospheric studies on Nr species, we propose best-practice guidelines for accurate and precise sampling and analysis of NO3- isotopes in precipitation based on the experience obtained from an international research project coordinated by the International Atomic Energy Agency (IAEA). The precipitation sampling and preservation strategies yielded a good agreement between the NO3- concentrations measured at the laboratories of 16 countries and at the IAEA. Compared to conventional methods (e.g., bacterial denitrification), we confirmed the accurate performance of the lower cost Ti(III) reduction method for isotope analyses (15N and 18O) of NO3- in precipitation samples. These isotopic data depict different origins and oxidation pathways of inorganic nitrogen. This work emphasized the capability of NO3- isotopes to assess the origin and atmospheric oxidation of Nr and outlined a pathway to improve laboratory capability and expertise at a global scale. The incorporation of other isotopes like 17O in Nr is recommended in future studies.

δ18O as a tracer of PO43- losses from agricultural landscapes.

Accurately tracing the sources and fate of excess PO43- in waterways is necessary for sustainable catchment management. The natural abundance isotopic composition of O in PO43- (δ18OP) is a promising tracer of point source pollution, but its ability to track diffuse agricultural pollution is unclear. We tested the hypothesis that δ18OP could distinguish between agricultural PO43- sources by measuring the integrated δ18OP composition and P speciation of contrasting inorganic fertilisers (compound vs rock) and soil textures (sand, loam, clay) in southwestern Australia. δ18OP composition differed between the three soil textures sampled across six livestock farms: sandy soils had lower overall δ18OP values (21 ± 1‰) than the loams (23 ± 1‰), which corresponded with a smaller, but more readily leachable, PO43- pool. Fertilisers had greater δ18OP variability (∼8‰), with fluctuations due to type and manufacturing year. Consequently, catchment 'agricultural soil leaching' δ18OP signatures could span from 18 to 25‰ depending on both fertiliser type and timing (lag between application and leaching). These findings emphasise the potential of δ18OP to untangle soil-fertiliser P dynamics under controlled conditions, but that its use to trace catchment-scale agricultural PO43- losses is limited by uncertainties in soil biological P cycling and its associated isotopic fractionation.

An integrated assessment of nitrogen source, transformation and fate within an intensive dairy system to inform management change.

From an environmental perspective optimised dairy systems, which follow current regulations, still have low nitrogen (N) use efficiency, high N surplus (kg N ha-1) and enable ad-hoc delivery of direct and indirect reactive N losses to water and the atmosphere. The objective of the present study was to divide an intensive dairy farm into N attenuation capacity areas based on this ad-hoc delivery. Historical and current spatial and temporal multi-level datasets (stable isotope and dissolved gas) were combined and interpreted. Results showed that the farm had four distinct attenuation areas: high N attenuation: characterised by ammonium-N (NH4+-N) below 0.23 mg NH4+-N l-1 and nitrate (NO3--N) below 5.65 mg NO3--N l-1 in surface, drainage and groundwater, located on imperfectly to moderately-well drained soils with high denitrification potential and low nitrous oxide (N2O) emissions (av. 0.0032 mg N2O-N l-1); moderate N attenuation: characterised by low NO3--N concentration in drainage water but high N2O production (0.0317 mg N2O-N l-1) and denitrification potential lower than group 1 (av. δ15N-NO3-: 16.4‰, av. δ18O-NO3-: 9.2‰), on well to moderately drained soils; low N attenuation-area 1: characterised by high NO3--N (av. 6.90 mg NO3--N l-1) in drainage water from well to moderately-well drained soils, with low denitrification potential (av. δ15N-NO3-: 9.5‰, av. δ18O-NO3-: 5.9‰) and high N2O emissions (0.0319 mg N2O l-1); and low N attenuation-area 2: characterised by high NH4+-N (av. 3.93 mg NH4+-N l-1 and high N2O emissions (av. 0.0521 mg N2O l-1) from well to imperfectly drained soil. N loads on site should be moved away from low attenuation areas and emissions to air and water should be assessed.

A rapid protocol for assessing sediment condition in eutrophic estuaries.

The enrichment of sediments with nutrients and organic matter (eutrophication) is a key anthropogenic stressor of estuaries worldwide, impacting their sediment condition, ecology and ecosystem service provision. A key challenge for estuary managers and scientists is how to effectively quantify and monitor these changes in ecological condition in a timely and cost-effective manner. We developed a Rapid Assessment Protocol (RAP) for characterizing sediment condition based on the qualitative characteristics of sediment colour, odour and texture. We evaluated its utility for assessing sediment condition, and particularly the degree and effects of sediment enrichment (as quantified by complementary measurements of total C, organic C and total N) across 97 sites throughout a eutrophic microtidal estuary. RAP results were strongly and significantly correlated with the degree of sediment enrichment, with RAP scores correctly identifying the assigned enrichment class (low, medium, high) of 83.5% of sites. More enriched sediments exhibited poorer condition, manifested as significantly lower RAP scores for sediment colour, texture and odour, particularly (but not only) where enrichment coincided with elevated mud content. The RAP was particularly successful (<12% misclassification) at identifying sites with low levels of enrichment, indicating its promise as a first-pass survey approach for identifying potential reference or control sites to support impact assessments. RAP approaches based on qualitative sediment characteristics can provide a useful proxy for the degree and impacts of inorganic and organic enrichment, with potentially broad applicability for supporting timely, cost-effective assessment and monitoring of sediment condition in estuaries worldwide.

Effects of denitrification and transport on the isotopic composition of nitrate (δ18O, δ15N) in freshwater systems.

Nitrate isotopes (δ15N-NO3- and δ18O-NO3-) are a potentially powerful tool for tracking the biological removal of reactive nitrogen (N) as it is transported from land to sea. However, uncertainties about, 1) the variability of the strength of biological isotopic fractionation during anaerobic benthic NO3- reduction (the kinetic enrichment factor: εdenit), and, 2) how accurately these εdenit values are expressed in overlying aerobic surface waters (the effective enrichment factor: εeff), currently limit their use in freshwater systems. Here we used a combination of incubation experiments and numerical modelling to construct a simple framework for defining freshwater εdenit based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the εdenit values produced in submerged soils (n = 3) and sediments (n = 4) with denitrification rates between 10 and 600 mg N m-2 d-1 ranged from -3‰ to -28‰. Critically, model results indicated that diffusive transport would homogenise this to an effective fractionation range of -6 ±â€¯4‰. Evidence for biological and hydrological variability of NO3- isotope fractionation means that values measured in aerobic surface water environments are most appropriately evaluated by a range of fractionation values, rather than commonly used single 'site specific' εdenit values.

Anoxic nitrogen cycling in a hydrocarbon and ammonium contaminated aquifer.

Nitrogen fate and transport through contaminated groundwater systems, where N is both ubiquitous and commonly limits pollutant attenuation, must be re-evaluated given evidence for new potential microbial N pathways. We addressed this by measuring the isotopic composition of dissolved inorganic N (DIN = NH4+, NO2-, and NO3-) and N functional gene abundances (amoA, nirK, nirS, hszA) from 20 to 38 wells across an NH4+, hydrocarbon, and SO42- contaminated aquifer. In-situ N attenuation was confirmed on three sampling dates (0, +6, +12 months) by the decreased [DIN] (4300 - 40 µM) and increased δ15N-DIN (5‰-33‰) over the flow path. However, the assumption of negligible N attenuation within the plume was complicated by the presence of alternative electron acceptors (SO42-, Fe3+), both oxidizing and reducing functional genes, and N oxides within this anoxic zone. Active plume N cycling was corroborated using an NO2- dual isotope based model, which found the fastest (∼10 day) NO2- turnover within the N and electron donor rich central plume. Findings suggest that N cycling is not always O2 limited within chemically complex contaminated aquifers, though this cycling may recycle the N species rather than attenuate N.

Multi-species measurements of nitrogen isotopic composition reveal the spatial constraints and biological drivers of ammonium attenuation across a highly contaminated groundwater system.

Groundwater under industrial sites is characterised by heterogeneous chemical mixtures, making it difficult to assess the fate and transport of individual contaminants. Quantifying the in-situ biological removal (attenuation) of nitrogen (N) is particularly difficult due to its reactivity and ubiquity. Here a multi-isotope approach is developed to distinguish N sources and sinks within groundwater affected by complex industrial pollution. Samples were collected from 70 wells across the two aquifers underlying a historic industrial area in Belgium. Below the industrial site the groundwater contained up to 1000 mg N l(-1) ammonium (NH4(+)) and 300 mg N l(-1) nitrate (NO3(-)), while downgradient concentrations decreased to ∼1 mg l(-1) DIN ([DIN] = [NH4(+)N] + [NO3(-)N] + [NO2(-)N]). Mean δ(15)N-DIN increased from ∼2‰ to +20‰ over this flow path, broadly confirming that biological N attenuation drove the measured concentration decrease. Multi-variate analysis of water chemistry identified two distinct NH4(+) sources (δ(15)NNH4(+) from -14‰ and +5‰) within the contaminated zone of both aquifers. Nitrate dual isotopes co-varied (δ(15)N: -3‰ - +60‰; δ(18)O: 0‰ - +50‰) within the range expected for coupled nitrification and denitrification of the identified sources. The fact that δ(15)NNO2(-) values were 50‰-20‰ less than δ(15)NNH4(+) values in the majority of wells confirmed that nitrification controlled N turnover across the site. However, the fact that δ(15)NNO2(-) was greater than δ(15)NNH4(+) in wells with the highest [NH4(+)] shows that an autotrophic NO2(-) reduction pathway (anaerobic NH4(+) oxidation or nitrifier-denitrification) drove N attenuation closest to the contaminant plume. This direct empirical evidence that both autotrophic and heterotrophic biogeochemical processes drive N attenuation in contaminated aquifers demonstrates the power of multiple N isotopes to untangle N cycling in highly complex systems.

Microbial nitrogen transformation in constructed wetlands treating contaminated groundwater.

Pathways of ammonium (NH4 (+)) removal were investigated using the stable isotope approach in constructed wetlands (CWs). We investigated and compared several types of CWs: planted horizontal subsurface flow (HSSF), unplanted HSSF, and floating plant root mat (FPRM), including spatial and seasonal variations. Plant presence was the key factor influencing efficiency of NH4 (+) removal in all CWs, what was illustrated by lower NH4 (+)-N removal by the unplanted HSSF CW in comparison with planted CWs. No statistically significant differences in NH4 (+) removal efficiencies between seasons were detected. Even though plant uptake accounted for 32-100 % of NH4 (+) removal during spring and summer in planted CWs, throughout the year, most of NH4 (+) was removed via simultaneous nitrification-denitrification, what was clearly shown by linear increase of δ(15)N-NH4 (+) with decrease of loads along the flow path and absence of nitrate (NO3 (-)) accumulation. Average yearly enrichment factor for nitrification was -7.9 ‰ for planted HSSF CW and -5.8 ‰ for FPRM. Lack of enrichment for δ(15)N-NO3 (-) implied that other processes, such as nitrification and mineralization were superimposed on denitrification and makes the stable isotope approach unsuitable for the estimation of denitrification in the systems obtaining NH4 (+) rich inflow water.

Biogeochemistry and community ecology in a spring-fed urban river following a major earthquake.

In February 2011 a MW 6.3 earthquake in Christchurch, New Zealand inundated urban waterways with sediment from liquefaction and triggered sewage spills. The impacts of, and recovery from, this natural disaster on the stream biogeochemistry and biology were assessed over six months along a longitudinal impact gradient in an urban river. The impact of liquefaction was masked by earthquake triggered sewage spills (~20,000 m(3) day(-1) entering the river for one month). Within 10 days of the earthquake dissolved oxygen in the lowest reaches was <1 mg l(-1), in-stream denitrification accelerated (attenuating 40-80% of sewage nitrogen), microbial biofilm communities changed, and several benthic invertebrate taxa disappeared. Following sewage system repairs, the river recovered in a reverse cascade, and within six months there were no differences in water chemistry, nutrient cycling, or benthic communities between severely and minimally impacted reaches. This study highlights the importance of assessing environmental impact following urban natural disasters.