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Streams play a critical role in attenuating the excess reactive nitrogen (N) generated from human activities. Consequently, streams can also emit significant amounts of the potent greenhouse gas N2O. Models and manipulative experiments now suggest that hydrology regulates the balance between N removal and N2O production, but validating this hypothesis under field conditions has been difficult. We aimed to redress this knowledge gap by measuring changes in the concentration and isotopic composition of NO3− (δ18O-NO3−, δ15N-NO3−) and N2O (δ18O-N2O, δ15N-N2O, 15N-N2O site preference) in the sediments and surface water of a 30 m stream reach as discharge dropped from 2.7 to 1.8 m3 s−1. Over the eight-day measurement period the changes in conductivity, δ18O-H2O, and 222Rn indicated that hyporheic mixing decreased and net groundwater inputs increased as discharge declined. This coincided with increases in surface water NO3− (1–3 mg N l−1) and N2O (700–1000% saturation) that were beyond what could be explained by increased groundwater N inputs. Instead, both N2O and NO3− isotopic composition indicated that concentration increases were caused by increasing within-stream production (nitrification), rather than decreased reduction (denitrification), as hyporheic exchange decreased. This highlights the importance of oxidising processes in regulating N cycling even under strongly heterotrophic conditions (productivity/respiration: 0.005–0.2). Together these findings provide a first empirical confirmation that relatively short term (daily-weekly) stream flow dynamics directly regulate biological cycling of both NO3− and N2O.

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.

Estuaries are productive ecosystems that provide important ecosystem functions such as the storage and cycling of dissolved organic matter (DOM) and nutrients. Intermittently closed/open lakes and lagoons (ICOLLs) can significantly impact biogeochemical processing and release of terrestrial nitrogen and carbon into the coastal environment due to longer residence times that can extend nutrient processing within the ICOLL. Pulses of nutrient release then occur when there is connectivity between the catchment and coastal waters. It remains unclear how modifications to estuaries and their catchments impact internal processes. To better understand the balance between autochthonous and allochthonous nutrients in a heavily modified ICOLL, multiple stable isotopes (δ13C, δ15N) of dissolved nutrients were used to evaluate seasonal and spatial changes to nitrogen sources and sinks in a southwest Australian ICOLL. The eutrophic status of water bodies has traditionally been based on concentrations of inorganic nitrogen (DIN) (particularly NH4+ and NOx) due to its presumed higher bioavailability and association with anthropogenic pollution. However, both NH4+ and NOx concentrations were low (0 – 12.5 µM) throughout the study area in both wet and dry seasons. Despite low surface water DIN concentrations, the system suffers from eutrophication issues such as algal blooms, low dissolved oxygen, and fish kills. The differences between the surface and porewater dissolved organic nitrogen (DON) and carbon (DOC) pools decreased in the wet season (high connectivity), suggesting that internal DOM turnover sustains eutrophication. This work demonstrates that including DON and its isotopic signature can be an effective way to study N in waterbodies with low DIN concentrations. It also highlights the need for DON, as a major constituent of total dissolved nitrogen, to be included in N studies and ultimately may have significant impact on the current understanding of the global N budget.

Coastal waters are known to emit globally significant quantities of CH4, a potent greenhouse gas, but the potential of the rapid and ongoing human alterations to coastal areas to alter these emissions remains undefined. Here we addressed this gap by quantifying water-to-air CH4 fluxes and δ13C-CH4 values in sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of human modification (agricultural land use, wastewater discharge), and sediment-to-water CH4 fluxes from the major benthic habitats in representative Low, Moderate, and High systems. An increase in water-to-air CH4 fluxes from 9.7 µmol m−2 d−1 (Low) to 28 µmol m−2 d−1 (Moderate) to 47 µmol m−2 d−1 (High) was accompanied by a shift from hydrogenotrophic to acetoclastic production pathways. Unexpectedly, benthic CH4 production, which ranged from −48 µmol m−2 d−1 to +180 µmol m−2 d−1 between habitats, estuaries, and seasons, was not the primary driver of this shift. Sediments produced more CH4 (∼600%) than emitted from the Low estuary, ∼90% of CH4 emitted from the Moderate estuary, but only 9% of CH4 emitted from the High estuary. Instead, a combination of wastewater, groundwater, and apparent water column production caused a ∼ 3-fold increase in estuary CH4 emissions. Our findings indicate that human alterations to the source, rate, and pathways of CH4 production are driving a net increase in emissions from estuaries, demonstrating a need to redefine how we quantify ‘anthropogenic’ CH4 emissions.

Combined pressures from inland agricultural intensification and coastal development are dramatically altering estuaries’ structure and function. Despite the established global significance of estuarine carbon (C) cycling, the impact of growing anthropogenic stress on coastal C inputs and exports is unclear. To address this gap, we evaluated the magnitude and drivers of estuary C fluxes in eight sub-tropical estuaries at Low (n = 3), Moderate (n = 2), and High (n = 3) levels of nutrient enrichment. We measured changes in the concentration and isotopic composition (δ13C) of the major C pools (organic and inorganic) and gaseous product of C turnover (CO2) over wet and dry seasons. Over both sampling periods estuaries classified Moderate and High emitted far more CO2 (37 ± 10 mmol m−2 d−1) than those classified Low (6.3 ± 4 mmol m−2 d−1). However, estuaries with both high nutrients and high turbidity produced less CO2, and thus exported more DIC, than expected from hydrodynamics (freshwater flushing time). Differences in estuary phytoplankton biomass (Chla concentrations) corresponded with differences in the biological CO2 production (respiration) rates estimated from δ13C-DIC variations, although respiration rates were higher than predicted based on hydrodynamics (surface area/discharge) in high nutrient, low turbidity systems. Together these findings demonstrate that land-use intensification can alter both the source and the production of estuary CO2, and suggest that the direction of this shift can depend on ancillary factors like turbidity as well as nutrient enrichment. Evidence that human alterations to coastal ecosystems can shift the balance between DIC downstream export and CO2 emissions outside of the range predicted by hydrodynamic factors like residence time, surface area, and discharge has implications for global C models.

Accurately tracing the sources and fate of excess PO

Benthic biofilms are often assumed to control terrestrially-derived dissolved organic carbon (tDOC) uptake in streams. We tested this by comparing 13C-enriched ryegrass leachate uptake in an agricultural and a forest stream, hypothesizing that a greater abundance of autotrophic biofilms in the agricultural stream would cause its whole-stream tDOC uptake to be comparatively low. We measured whole-stream and biofilm tDOC tracer uptake, metabolism, bacterial and algal diversity, and nutrient status of benthic epilithic biofilms, and assessed whole-stream hydromorphology. Whole-stream uptake of tDOC was six times lower in the agricultural (3.0 mg m−2 day−1) than in the forest (19.0 mg m−2 day−1) stream, and tDOC uptake velocity indicated lower tDOC demand in the agricultural (1.2 mm min−1) than in the forest (1.9 mm min−1) stream. The agricultural stream differed from the forest stream by slightly lower transient storage capacity and higher benthic biofilm bacterial abundance and production, lower biofilm biomass and lower biofilm molar C:N, C:P, and N:P ratios. Changes in epilithic biofilms contributed little to the differences in whole-stream tDOC tracer uptake between streams, as biofilm tDOC uptake only amounted to 4% and 13% of whole-stream uptake in the forest and agricultural stream, respectively. This comparison of a forest and an agricultural stream suggests that agricultural stressors have the potential to diminish both whole-stream tDOC uptake and uptake efficiency. Furthermore, the weak link between biofilm and whole-stream tDOC uptake implies that benthic biofilms characteristics are poor predictors for human impacts on tDOC uptake in agricultural streams and that hot spots of tDOC uptake are likely situated in the hyporheic zone or in the stream water column.

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: edenit), and, 2) how accurately these edenit values are expressed in overlying aerobic surface waters (the effective enrichment factor: eeff), 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 edenit based on interactions between benthic denitrification and diffusive transport to surface waters. Under non-limited, anaerobic conditions the edenit 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 ± 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’ edenit values.

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 (

Coastal waters are typically productive aquatic ecosystems and play an important role in the global greenhouse gas (GHG) budget. However, the uncertainty in the estimation of GHG emission from estuaries remains large due to significant variability in GHG concentrations in time and space. This study aimed to provide a more accurate estimation of GHG emissions from sub-tropical estuaries by validating and analyzing results from a 3D hydrodynamic-biogeochemical model used to capture the temporal and spatial dynamics of the major GHG (CO2 CH4, and N2O). The model was applied to the Brisbane, Maroochy, and Noosa Estuary in Queensland, Australia, representing systems under high, median, and low human impacts, and was validated with datasets from long-term monitoring stations and field campaigns along the freshwater-marine continuum. Distinct spatial heterogeneity of GHG distribution was found with the upstream acting as a hotspot for emission to the atmosphere, despite this area occupying a relatively small portion of the rivers. Seasonal variations of pCO2 at the surface were driven mostly by the changes in water temperature and DIC concentrations, while strong diurnal variation was also found, driven by the changes related to tidal forcing. All GHG showed distinct signatures in the three rivers, related to trophic statues and hydrology. The model allowed us to approximate the fraction of incoming carbon and nitrogen that was lost to the atmosphere as GHG emissions, which is a step towards improving regional and national GHG budgets. A link of the biogeochemical model to a parameter optimization software PEST is being used to assist in uncertainty analysis from the model outputs.

Anthropogenic activities, particularly fertilisation, have resulted in significant increases in reactive nitrogen (rN) in soils globally, leading to eutrophication, acidification, poor air quality, and emissions of the important greenhouse gas N2O. Understanding the partitioning of rN losses into different environmental compartments is critical to mitigate negative impacts, however, loss pathways are poorly quantified, and potential changes driven by climate warming and societal shifts are highly uncertain. We present a coupled soil-atmosphere isotope model (IsoTONE; ISOtopic Tracing Of Nitrogen in the Environment) to partition rN losses into leaching, harvest, NH3 volatilization, and production of NO, N2 and N2O based on a global dataset of soil δ15N, as well as numerous other geoclimatic and experimental datasets. The model was optimized in a Bayesian framework using a time series of N2O mixing ratios and isotopic compositions since the preindustrial era, as well as a global dataset of N2O emission factors (EF). The posterior model results showed that the total anthropogenic flux in 2020 (7.8 Tg N2O-N a-1) was dominated by indirect emissions resulting from N deposition, while the growth rate and trend in anthropogenic N2O was driven by both direct N fertilisation and deposition inputs. In contrast, inputs from fixation N drive natural N2O emissions, and were responsible for subdecadal interannual variability in total emissions.Total N gas (N2O + NO + N2) production and N2O losses were strongly dependent on geoclimate and thus spatially variable, therefore the spatial pattern of N inputs strongly impacted resulting EFs and total N2O emissions. The area-weighted global EF for N2O was 1% of anthropogenic N inputs in 2020, similar to the current IPCC default of 1.4%, however the N input-weighted global EF was 4.3%. Shifts in fertilisation inputs from the temperate Northern hemisphere towards warmer regions with higher EFs such as India and China have led to accelerating N2O emissions (1.02±0.7 Tg N2O-N a-1). In addition, N2O emissions have increased over the past decades due to climate warming (0.76±0.4 Tg N2O-N a-1). Predicted increases in fertilisation in India and Africa in the coming decades could further accelerate N2O-driven climate warming, unless mitigation measures are implemented to increase fertiliser N use efficiency and reduce N2O emission factors.

Refereed/Peer reviewed Estuaries play a key role in moderating the flow of nitrogen (N) to marine ecosystems. However, the magnitude of this N removal can vary dramatically both within and between estuaries due to the benthic habitats present. Here, we compare denitrification, coupled nitrification–denitrification, anammox, and dissimilatory nitrate reduction to ammonium (DNRA) across a mosaic of benthic habitats in the subtropical Noosa River Estuary, Australia. Using 15N tracer techniques and passive pore‐water samplers, we show that coupled nitrification–denitrification was the dominant pathway for N2 production across all habitats, with higher rates in vegetated habitats (10–70 μmol N m−2 h−1) compared to bare sediments (0.9–2 μmol N m−2 h−1). Unusual pore‐water profiles in the macroalgal sediments suggest the presence of sulfur‐driven anoxic nitrification of NH4+ to NO3− and N2. A benthic N budget showed that combined denitrification and coupled nitrification–denitrification accounted approximately 96% of the N2 production, while DNRA accounted for 9% of total NO3− reduction pathways in the Noosa River Estuary. The macroalgae habitat contributed 76% of total N removal via N2 production and 65% of N retention via DNRA, despite accounting for only 25% of the total surface area. We show a strong relationship between seagrass and macroalgae area and N2 production (r2 = 0.8; p

In North Atlantic Europe intensive dairy farms have a low nitrogen (N) use efficiency, with high N surpluses often negatively affecting water quality. Low feed input systems on heavy textured soils often need artificial drainage to utilise low cost grassland and remain profitable. Heavy textured soils have high but variable N attenuation potential, due to soil heterogeneity. Furthermore, drainage system design can influence the potential for N attenuation and subsequent N loadings in waters receiving drainage from such soils. The present study utilises end of pipe, open ditch and shallow groundwater sampling points across five sites in SW Ireland to compare and rank sites based on N surplus, water quality and “net denitrification”, and to develop a conceptual framework for the improved management of heavy textured dairy sites to inform water quality N sustainability. This includes both drainage design and “net denitrification” criterion, as developed within this study.N surplus ranged from 211 to 292 kg N/ha (mean of 252 kg N/sourha) with a common source of organic N across all locations. The predicted soil organic matter (SOM) N release potential from top-subsoil layers was high, ranging from 115 to >146 kg N/ha. Stable isotopes analyses showed spatial variation in the extent of specific N-biotransformation processes, according to drainage location and design. Across all sites, nitrate (NO3-N) was converted to ammonium (NH4+-N), which migrated offsite through open ditch and shallow groundwater pathways. Using the ensemble data the potential for soil N attenuation could be discriminated by 3 distinct groups reflecting the relative dominance of in situ N-biotransformation processes deduced from water composition: Group 1 (2 farms, ranked with high sustainability, NH4+ 5‰ and δ18O-NO3− > 10‰), low NH4+-N concentration coupled with a high denitrification potential; Group 2 (1 farm with moderate sustainability, NH4+ 0.23 mg N/l, 14‰ > δ15N-NO3− > 5‰ and 25‰ > δ18O-NO3− > −2‰), high NH4+-N concentration due to low denitrification. The installation of a shallow drainage system (e.g. mole or gravel moles at 0.4 m depth) reduced the “net denitrification” ranking of a site, leading to water quality issues. From this detailed work an N sustainability tool for any site, which presents the relationship between drainage class, drainage design (if present), completeness of denitrification, rate of denitrification and NH4-N attenuation was developed. This tool allows a comparison or ranking of sites in terms of their N sustainability. The tool can also be used pre-land drainage and presents the consequences of future artificial land drainage on water quality and gaseous emissions at a given site.

Estuarine N2O emissions contribute to the atmospheric N2O budget, but little is known about estuary N2O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N2O concentrations and water-air fluxes in three low-DIN (NO3−< 30µmol L−1) tropical estuaries in Queensland, Australia (Johnstone River, Fitzroy River, Constant Creek) during consecutive wet and dry seasons. Constant Creek had the lowest concentrations of dissolved inorganic nitrogen (DIN; 0.01 to 5.4µmol L−1of NO3−and 0.09 to 13.6µmol L−1of NH4+) and N2O (93–132% saturation), and associated lowest N2O emissions (– 1.4 to 8.4µmol m−2d−1) in both seasons. The other two estuaries exhibited higher DIN inputs and higher N2O emissions. The Johnstone River Estuary had the highest N2O concentrations (97–245% saturation) and emissions (– 0.03 to 25.7µmol m−2d−1), driven by groundwater inputs from upstream sources, with increased N2O input in the wet season. In the Fitzroy River Estuary, N2O concentrations (100–204% saturation) and emissions (0.03–19.5µmol m−2d−1) were associated with wastewater inputs, which had a larger effect during the dry season and were diluted during the wet season. Overall N2O emissions from the three tropical estuaries were low compared to previous studies, and at times water-air N2O fluxes were actually negative, indicating that N2O consumption occurred. Low water column NO3−concentration (i.e. < 5µmol L−1) appears to promote negative water-air N2O fluxes in estuary environments; considering the number of estuaries and mangrove creeks where DIN falls below this threshold, negative water-air N2O fluxes are likely common.

Viable indicators of nitrogen (N) attenuation at the catchment scale are needed in order to sustainably manage global agricultural intensification. We hypothesized that the dominance of a single land use (pasture production) and strong ground-to-surface water connectivity would combine to create a system in which surface water nitrate isotopes (δ15N and δ18O of NO3−) could be used to monitor variations in catchment-scale attenuation. Nitrate isotopes were measured monthly over a 2 year period in four reaches along a spring-fed, gaining stream (mean NO3−-N of 6 mg L−1) in Canterbury, New Zealand. The stream water NO3− pool indicated that the highest degree of denitrification occurred in the shallow upper reaches. Moving downstream through increasingly sandy soils, the isotopic signature of denitrification became progressively weaker. The lowest reaches fell into the expected range for NO3− produced from the nitrification of pasture N sources (urine and fertilizers), implying that the attenuation capacity of the groundwater and riparian systems was lower than the rate of N inputs. After excluding months affected by effluent spills or extreme weather (n = 4), variations in the degree of denitrification over stream distance were combined with the measured NO3− discharge to estimate N attenuation over time in the subcatchment. Attenuation was highly responsive to rainfall: 93% of calculated attenuation (20 kg NO3−-N ha−1 yr−1) occurred within 48 h of rainfall. These findings demonstrate the potential for detailed NO3− stable isotope data to provide integrative measures of catchment NO3− loss pathways.

Nitrous oxide (N2O) production and emissions are observed in estuary waters, yet little is known about estuary N2O fluxes under low dissolved inorganic nitrogen (DIN) conditions. We present high-resolution spatial surveys of N2O concentrations in three low-DIN (NO3−

Nitrate isotopes (δ

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.

Nitrate dual isotopes (δ 15 N NO 3 − and δ 18 O NO 3 − ) are increasingly used to assess the sources and sinks of nitrogen (N) pollution in freshwater systems. However, the application of this methodology to pasture agroecosystems is currently limited by the lack of information on how, or even if, the primary N inputs to the systems (livestock urine and urea fertiliser) are expressed in the isotopic signature of exported NO 3 − . To remedy this gap, direct measurements of fractionation during ammonia volatilisation were linked with changes in the concentration and isotopic composition of the residual soil inorganic N pool (NO 3 − , nitrite, and ammonium) following the addition of differing levels of bovine urine and urea fertiliser. Ammonia volatilisation, with a δ 15 N enrichment factor of +35 ± 5‰, removed from 5 to 40% of N inputs from the different treatments, which should have enriched the residual inorganic N pool to 25‰ and 3‰, respectively. However, this fractionation did not propagate into the soil NO 3 − pool due to a combination of urine-induced mineralisation (up to 120 μg N g soil −1 day −1 in the high urine treatment) and on-going nitrification. Consequently, NO 3 − measured within the treatments was not as enriched in 15 N as the values typically ascribed to excreta-N sources. Up-scaling these results, the whole-pasture NO 3 − isotopic composition primarily reflected time since fertilisation, regardless of urine inputs. These findings necessitate expanding the range of δ 15 N NO 3 − values ascribed to livestock sources to encompass values as low as −10‰, highlighting the need to account for post-deposition soil N cycling in order to accurately define NO 3 − isotopic source ranges.

Sustainable production of lowland rice (Oryza sativa L.) requires minimising undesirable soil nitrogen (N) losses via nitrate (NO3 −) leaching and denitrification. However, information is limited on the N transformations that occur between rice crops (fallow and land preparation), which control indigenous N availability for the subsequent crop. In order to redress this knowledge gap, changes in NO3 − isotopic composition (δ15N and δ18O) in soil and water were measured from harvest through fallow, land preparation, and crop establishment in a 7 year old field trial in the Philippines. During the period between rice crops, plots were maintained either, continuously flooded, dry, or alternately wet and dry from rainfall. Plots were split with addition or removal of residue from the previous rice crop. No N fertilizer was applied during the experimental period. Nitrogen accumulated during the fallow (20 kg NH4 +–N ha−1 in flooded treatments and 10 kg NO3 −–N ha−1 in treatments with drying), but did not influence N availability for the subsequent crop. Nitrate isotope fractionation patterns indicated that denitrification drove this homogenisation: during land preparation ~50 % of inorganic N in the soil (top 10 cm) was denitrified, and by 2 weeks after transplanting this increased to >80 % of inorganic N, regardless of fallow management. The 17 days between fallow and crop establishment controlled not only N attenuation (3–7 kg NO3 −–N ha−1 denitrified), but also N inputs (3–14 kg NO3 −–N ha−1 from nitrification), meaning denitrification was dependent on soil nitrification rates. While crop residue incorporation delayed the timing of N attenuation, it ultimately did not impact indigenous N supply. By measuring NO3 − isotopic composition over depth and time, this study provides unique in situ measurements of the pivotal role of land preparation in determining paddy soil indigenous N supply.

In February 2011 a M W 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 , 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.

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 (NH 4 + ) and 300 mg N l −1 nitrate (NO 3 − ), while downgradient concentrations decreased to ∼1 mg l −1 DIN ([DIN] = [NH 4 + N] + [NO 3 − N] + [NO 2 − 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 NH 4 + sources (δ 15 N NH 4 + 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 N NO 2 − values were 50‰–20‰ less than δ 15 N NH 4 + values in the majority of wells confirmed that nitrification controlled N turnover across the site. However, the fact that δ 15 N NO 2 − was greater than δ 15 N NH 4 + in wells with the highest [NH 4 + ] shows that an autotrophic NO 2 − reduction pathway (anaerobic NH 4 + 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.

Low-temperature pyrolysis of biomass produces a product known as biochar. The incorporation of this material into the soil has been advocated as a C sequestration method. Biochar also has the potential to influence the soil N cycle by altering nitrification rates and by adsorbing NH 4 + or NH 3 . Biochar can be incorporated into the soil during renovation of intensively managed pasture soils. These managed pastures are a significant source of N 2 O, a greenhouse gas, produced in ruminant urine patches. We hypothesized that biochar effects on the N cycle could reduce the soil inorganic-N pool available for N 2 O-producing mechanisms. A laboratory study was performed to examine the effect of biochar incorporation into soil (20 Mgha ― 1 ) on N 2 O-N and NH 3 -N fluxes, and inorganic-N transformations, following the application of bovine urine (760 kg N ha ―1 ). Treatments included controls (soil only and soil plus biochar), and two urine treatments (soil plus urine and soil plus biochar plus urine). Fluxes of N 2 O from the biochar plus urine treatment were generally higher than from urine alone during the first 30 d, but after 50 d there was no significant difference (P = 0.11) in terms of cumulative N 2 O-N emitted as a percentage of the urine N applied during the 53-d period; however, NH 3 -N fluxes were enhanced by approximately 3% of the N applied in the biochar plus urine treatment compared with the urine-only treatment after 17 d. Soil inorganic-N pools differed between treatments, with higher NH 4 + concentrations in the presence of biochar, indicative of lower rates of nitrification. The inorganic-N pool available for N 2 O-producing mechanisms was not reduced, however, by adding biochar.

Atmospheric concentrations of the greenhouse gas nitrous oxide (N 2 0) have continued to rise since the advent of the industrial era, largely because of the increase in agricultural land use. The urine deposited by grazing ruminant animals is a major global source of agricultural N 2 O. With the first commitment period for reducing greenhouse gas emissions under the Kyoto Protocol now underway, mitigation options for ruminant urine N 2 0 emissions are urgently needed. Recent studies showed that increasing the urinary concentration of the minor urine constituent hippuric acid resulted in reduced emissions of N 2 0 from a sandy soil treated with synthetic bovine urine, due to a reduction in denitrification. A similar effect was seen when benzoic acid, a product of hippuric acid hydrolysis, was used. This current laboratory experiment aimed to investigate these effects using real cow urine for the first time. Increased concentrations of hippuric acid or benzoic acid in the urine led to reduction of N 2 O emissions by 65% (from 17% to < 6% N applied), with no difference between the two acid treatments. Ammonia volatilization did not increase significantly with increased hippuric acid or benzoic acid concentrations in the urine applied. Therefore, there was a net reduction in gaseous N loss from the soil with higher urinary concentrations of both hippuric acid and benzoic acid. The results show that elevating hippuric acid in the urine had a marked negative effect on both nitrification and denitrification rates and on subsequent N 2 0 fluxes. This study indicates the potential for developing a novel mitigation strategy based on manipulation of urine composition through ruminant diet.

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 δ15N-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 δ15N-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.

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 data- sets (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. The authors thank S. Leach, C. Somers, D. Brennan, M.M.R. Jahangir and D. Peyton for assistance during the project. J. Patton provided data on the N balance and R. Fox and A. Lawless facilitated access to the research farm and helped with the management scenario section. peer-reviewed