Global Research Alliance N2 O chamber methodology guidelines: Recommendations for air sample collection, storage, and analysis.

Certain aspects in the collection, handling, storage, and subsequent analysis of discrete air samples from non-steady-state flux chambers are critical to generating accurate and unbiased estimates of nitrous oxide (N2 O) fluxes. The focus of this paper is on air sample collection and storage in small vials (<12 ml) primarily for gas chromatography (GC) analysis. Sample integrity is assured through following simple procedures including storage under pressure and analysis within a few months of collection. Concurrent storage of standards in an identical manner to samples is recommended and allows the storage period to be reliably extended. In the laboratory, an autosampler is typically used in batch analysis of ∼200 sequentially analyzed samples by GC with an electron capture detector (ECD). Some comparisons are given between GC and alternatives including optical N2 O detectors that are increasingly being used for high-precision N2 O measurement. The importance of calibration and traceability of gas standards is discussed, where high-quality standards ensure the most accurate assessment of N2 O concentration and comparability between laboratories. The calibration allows a consistent and best estimate of flux to be derived.

A hydrochemically guided landscape classification system for modelling spatial variation in multiple water quality indices: Process-attribute mapping.

Spatial variation in landscape attributes can account for much of the variability in water quality relative to land use on its own. Such variation results from the coupling between the dominant processes governing water quality, namely hydrological, redox, and weathering and gradients in key landscape attributes, such as topography, geology, and soil drainage. Despite the importance of 'process-attribute' gradients (PAG), few water quality models explicitly account for their influence. Here a processes-based water quality modelling framework is presented that more completely accounts for the role of landscape variability over water quality - Process-Attribute Mapping (PoAM). Critically, hydrochemical measures form the basis for the identification and mapping of effective landscape attributes, producing PAG maps that attempt to replicate the natural landscape gradients governing each dominant process. Application to the province of Southland (31,824 km2), New Zealand, utilised 12 existing geospatial datasets and a total of 28,626 surface water, groundwater, spring, soil water, and precipitation analyses to guide the identification and mapping of 11 individual PAG. The ability of PAGs to replicate regional hydrological, redox, and weathering gradients was assessed on the accuracy with which the hydrochemical indicators of each dominant process (e.g. hydrological tracers, redox indicators) were estimated across 93 long-term surface water monitoring sites (cross-validated R2 values of 0.75-0.95). Given hydrochemical evidence that PAGs replicate actual landscape gradients governing the dominant processes, they were combined with a land use intensity layer and used to estimate steady-state surface water quality. Cross-validated R2 values ranged between 0.81 and 0.92 for median total nitrogen, total oxidised nitrogen, total phosphorus and dissolved reactive phosphorus. Models of particulate species E. coli and total suspended sediment, although reasonable (R2 0.72-0.73), were less accurate, suggesting finer-grained land use, landscape attribute, and/or flow normalised measures are required to improve estimation.

Vertical stratification of redox conditions, denitrification and recharge in shallow groundwater on a volcanic hillslope containing relict organic matter.

Natural denitrification in groundwater systems has been recognised as an ecosystem service that reduces the impact of agriculturally-derived nitrate inputs to surface waters. Identification of this ecosystem service within the landscape would permit spatially differentiated land management and legislation. However, spatial variation in groundwater redox conditions poses a significant challenge to such a concept. To gain understanding of the small-scale mosaic of biogeochemical and hydrological controls on denitrification, we established a well field consisting of 11 multilevel well (MLW) clusters on a hillslope containing relict organic matter buried by volcanic deposits 1.8 ka before present. Based on site-specific redox classification thresholds, vertical redox gradients and denitrification potentials were detected at 7 of the 11 sites. Palaeosols or woody debris, which had previously been identified in laboratory experiments as resident electron donors fuelling denitrification, were visually recognisable at 4 of the 7 MLW sites with vertical redox gradients. Moderately enhanced groundwater dissolved organic carbon (DOC) concentrations occurred where resident electron donors were evident. DOC concentrations were lower where anoxic and nitrate-depleted groundwater was found but with an absence of resident electron donors. In these instances, it was assumed that nitrate reduction had occurred somewhere upgradient of the sampled well screen along the lateral groundwater flow path, with the proximate electron donor (DOC) largely consumed in the process, since no evidence was found for denitrification being fuelled by inorganic electron donors. Due to high variability in the isotopic signature of nitrate in oxidised groundwater, the nitrate dual isotope method did not yield firm evidence for denitrification. However, realistic vertical patterns were obtained using the excess N2 method. Tritium-based age dating revealed that oxic conditions were restricted to young groundwater (mean residence time ≤ 3 y), while anoxic conditions were observed across a wider age range (3-25 y).

Using stable isotopes to follow excreta N dynamics and N2O emissions in animal production systems.

Nitrous oxide (N2O) is a potent greenhouse gas and the dominant anthropogenic stratospheric ozone-depleting emission. The tropospheric concentration of N2O continues to increase, with animal production systems constituting the largest anthropogenic source. Stable isotopes of nitrogen (N) provide tools for constraining emission sources and, following the temporal dynamics of N2O, providing additional insight and unequivocal proof of N2O source, production pathways and consumption. The potential for using stable isotopes of N is underutilised. The intent of this article is to provide an overview of what these tools are and demonstrate where and how these tools could be applied to advance the mitigation of N2O emissions from animal production systems. Nitrogen inputs and outputs are dominated by fertiliser and excreta, respectively, both of which are substrates for N2O production. These substrates can be labelled with 15N to enable the substrate-N to be traced and linked to N2O emissions. Thus, the effects of changes to animal production systems to reduce feed-N wastage by animals and fertiliser wastage, aimed at N2O mitigation and/or improved animal or economic performance, can be traced. Further 15N-tracer studies are required to fully understand the dynamics and N2O fluxes associated with excreta, and the biological contribution to these fluxes. These data are also essential for the new generation of 15N models. Recent technique developments in isotopomer science along with stable isotope probing using multiple isotopes also offer exciting capability for addressing the N2O mitigation quest.

The impact of relict organic materials on the denitrification capacity in the unsaturated-saturated zone continuum of three volcanic profiles.

The denitrification capacity of wetlands, riparian zones, and aquifers in glacial outwash areas is well documented, but little or no information exists for volcanic profiles, particularly those containing relict organic matter contained in or on top of paleosols (old soils buried by volcanic deposits) below the groundwater table. Relict carbon contained in these layers could provide the necessary electrons to fuel heterotrophic denitrification. To the best of our knowledge, this is the first study investigating the denitrification capacity in both the unsaturated and saturated zone of volcanic profiles. Samples from three profile types with differing organic matter distribution were amended with N-enriched nitrate (NO-) and incubated in the laboratory under anaerobic conditions. Dinitrogen (N) dominated the N gas fluxes; averaged across all samples, it accounted for 96% of the total N (nitrous oxide [NO] and N) gas fluxes. Dinitrogen fluxes were generally highest in the A horizon samples (4.1-6.2 nmol N g h), but substantial fluxes were also observed in some paleosol layers (up to 0.72 nmol N g h). A significant correlation ( < 0.001) was found between the concentration of extractable dissolved organic carbon and the total N gas flux produced in samples from below the A horizon, suggesting that heterotrophic denitrification was the dominant NO attenuation process in this study. Extrapolation of lab-derived denitrification capacities to field conditions suggests that the denitrification capacity of profiles containing relict soil organic matter in the saturated zone exceeds the estimated N leaching from the root zone.

Nitrous oxide dynamics in a braided river system, New Zealand.

Recently the Intergovernmental Panel on Climate Change (IPCC) emission factor EF5-r was revised downward to a value of 0.0025 kg N2O-N per kg NO3-N leached. It was not reduced further due to the continued uncertainty surrounding the dynamics of N2O in river systems. There have been few studies where river system N2O yields and fluxes have been measured. In this study, we examined the relationship between NO3-N and N2O-N fluxes at 10 sites along a braided river system (84 km) over a 397-d period. Isotopic analysis of NO3-N river water samples and the potential agricultural nitrogen (N) sources demonstrated that the NO3-N came from agricultural or sewage sources. Percent saturation of N2O varied with site and date (average, 114%) and correlated with river N2O-N concentrations. Modeled N2O fluxes (16-30 µg m(-2) h(-1)) from five sites were strongly related to river NO3-N concentrations ( r² = 0.86). The modeled N2O-N fluxes ranged from 39 to 81% of the IPCC-derived emissions based on the NO3-N load in the river over 397 d and do not support further lowering of the EF5-r. Further in situ river studies are required to verify the N2O-N fluxes and the calculated gas transfer velocity values for these braided river systems.

Methods to adjust for the interference of N2O on delta13C and delta18O measurements of CO2 from soil mineralization.

In this paper we present an overview of the present knowledge relating to methods that avoid interference of N2O on delta13C and delta18O measurements of CO2. The main focus of research to date has been on atmospheric samples. However, N2O is predominantly generated by soil processes. Isotope analyses related to soil trace gas emissions are often performed with continuous flow isotope ratio mass spectrometers, which do not necessarily have the high precision needed for atmospheric research. However, it was shown by using laboratory and field samples that a correction to obtain reliable delta13C and delta18O values is also required for a commercial continuous flow isotope ratio mass spectrometer. The capillary gas chromatography column of the original equipment was changed to a packed Porapak Q column. This adaptation resulted in an improved accuracy and precision of delta13C (standard deviation(Ghent): from 0.2 to 0.08 per thousand; standard deviation(Lincoln): from 0.2 to 0.13 per thousand) of CO2 for N2O/CO2 ratios up to 0.1. For delta18O there was an improvement for the standard deviation measured at Ghent University (0.13 to 0.08 per thousand) but not for the measurements at Lincoln University (0.08 to 0.23 per thousand). The benefits of using the packed Porapak Q column compared with the theoretical correction method meant that samples were not limited to small N(2)O concentrations, they did not require an extra N2O concentration measurement, and measurements were independent of the variable isotopic composition of N2O from soil.

Real-time, high-resolution quantitative measurement of multiple soil gas emissions: selected ion flow tube mass spectrometry.

A new technique is presented for the rapid, high-resolution identification and quantification of multiple trace gases above soils, at concentrations down to 0.01 microL L(-1) (10 ppb). The technique, selected ion flow tube mass spectrometry (SIFT-MS), utilizes chemical ionization reagent ions that react with trace gases but not with the major air components (N2, O2, Ar, CO2). This allows the real-time measurement of multiple trace gases without the need for preconcentration, trapping, or chromatographic separation. The technique is demonstrated by monitoring the emission of ammonia and nitric oxide, and the search for volatile organics, above containerized soil samples treated with synthetic cattle urine. In this model system, NH3 emissions peaked after 24 h at 2000 nmol m(-2) s(-1) and integrated to approximately 7% of the urea N applied, while NO emissions peaked about 25 d after urine addition at approximately 140 nmol m(-2) s(-1) and integrated to approximately 10% of the applied urea N. The monitoring of organics along with NH3 and NO was demonstrated in soils treated with synthetic urine, pyridine, and dimethylamine. No emission of volatile nitrogen organics from the urine treatments was observed at levels >0.01% of the applied nitrogen. The SIFT method allows the simultaneous in situ measurement of multiple gas components with a high spatial resolution of < 10 cm and time resolution <20 s. These capabilities allow, for example, identification of emission hotspots, and measurement of localized and rapid variations above agricultural and contaminated soils, as well as integrated emissions over longer periods.