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Ruminant urine patches on grazed grassland are a significant source of agricultural nitrous oxide (N 2 O) emissions. Of the many biotic and abiotic N 2 O production mechanisms initiated following urine-urea deposition, codenitrification resulting in the formation of hybrid N 2 O, is one of the least understood. Codenitrification forms hybrid N 2 O via biotic N-nitrosation, co-metabolising organic and inorganic N compounds (N substrates) to produce N 2 O. The objective of this study was to assess the relative significance of different N substrates on codenitrification and to determine the contributions of fungi and bacteria to codenitrification. 15 N-labelled ammonium, hydroxylamine (NH 2 OH) and two amino acids (phenylalanine or glycine) were applied, separately, to sieved soil mesocosms eight days after a simulated urine event, in the absence or presence of bacterial and fungal inhibitors. Soil chemical variables and N 2 O fluxes were monitored and the codenitrified N 2 O fluxes determined. Fungal inhibition decreased N 2 O fluxes by ca. 40% for both amino acid treatments, while bacterial inhibition only decreased the N 2 O flux of the glycine treatment, by 14%. Hydroxylamine (NH 2 OH) generated the highest N 2 O fluxes which declined with either fungal or bacterial inhibition alone, while combined inhibition resulted in a 60% decrease in the N 2 O flux. All the N substrates examined participated to some extent in codenitrification. Trends for codenitrification under the NH 2 OH substrate treatment followed those of total N 2 O fluxes (85.7% of total N 2 O flux). Codenitrification fluxes under non-NH 2 OH substrate treatments (0.7-1.2% of total N 2 O flux) were two orders of magnitude lower, and significant decreases in these treatments only occurred with fungal inhibition in the amino acid substrate treatments. These results demonstrate that in situ studies are required to better understand the dynamics of codenitrification substrates in grazed pasture soils and the associated role that fungi have with respect to codenitrification.

Denitrification in pasture soils is mediated by microbial and physicochemical processes leading to nitrogen loss through the emission of N2O and N2. It is known that N2O reduction to N2 is impaired by low soil pH yet controversy remains as inconsistent use of soil pH measurement methods by researchers, and differences in analytical methods between studies, undermine direct comparison of results. In addition, the link between denitrification and N2O emissions in response to carbon (C) mineralization and pH in different pasture soils is still not well described. We hypothesized that potential denitrification rate and aerobic respiration rate would be positively associated with soils. This relationship was predicted to be more robust when a high resolution analysis is performed as opposed to a single time point comparison. We tested this by characterizing 13 different temperate pasture soils from northern and southern hemispheres sites (Ireland and New Zealand) using a fully automated-high-resolution GC detection system that allowed us to detect a wide range of gas emissions simultaneously. We also compared the impact of using different extractants for determining pH on our conclusions. In all pH measurements, soil pH was strongly and negatively associated with both N2O production index (IN2O) and N2O/(N2O+N2) product ratio. Furthermore, emission kinetics across all soils revealed that the denitrification rates under anoxic conditions (NO+N2O+N2 μmol N/h/vial) were significantly associated with C mineralization (CO2 μmol/h/vial) measured both under oxic (r2 = 0.62, p = 0.0015) and anoxic (r2 = 0.89, p<0.0001) conditions.

Pasture-based livestock systems are often associated with losses of reactive forms of nitrogen (N) to the environment. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N. Di-nitrogen (N2) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to financial and methodological constraints. Relatively few studies have focused on quantifying N2 losses in vivo and fewer still have examined the relative contribution of the different N2 emission processes, particularly in grazed pastures. We used a combination of a high (15)N isotopic enrichment of applied N with a high precision of determination of (15)N isotopic enrichment by isotope-ratio mass spectrometry to measure N2 emissions in the field. We report that 55.8 g N m(-2) (95%, CI 38 to 77 g m(-2)) was emitted as N2 by the process of co-denitrification in pastoral soils over 123 days following urine deposition (100 g N m(-2)), compared to only 1.1 g N m(-2) (0.4 to 2.8 g m(-2)) from denitrification. This study provides strong evidence for co-denitrification as a major N2 production pathway, which has significant implications for understanding the N budgets of pastoral ecosystems.

The nitrogen (N) cycle represents one of the most well-studied systems, yet the taxonomic diversity of the organisms that contribute to it is mostly unknown, or linked to poorly characterized microbial groups. While new information has allowed functional groups to be refined, they still rely on a priori knowledge of enzymes involved and the assumption of functional conservation, with little connection to the role the transformations, plays for specific organisms. Here, we use soil microcosms to test the impact of N deposition on prokaryotic communities. By combining chemical, genomic and transcriptomic analysis, we are able to identify and link changes in community structure to specific organisms catalysing given chemical reactions. Urea deposition led to a decrease in prokaryotic richness, and a shift in community composition. This was driven by replacement of stable native populations, which utilize energy from N-linked redox reactions for physiological maintenance, with fast responding populations that use this energy for growth. This model can be used to predict response to N disturbances and allows us to identify putative life strategies of different functional and taxonomic groups, thus providing insights into how they persist in ecosystems by niche differentiation. [ABSTRACT FROM AUTHOR]

Denitrifying bioreactors convert nitrate-nitrogen (NO3-N) to dinitrogen and protect water quality. Herein, the performance of a pilot-scale bioreactor (10 m long, 5 m wide, 2 m deep) containing seven alternating cells filled with either sandy loam soil or lodgepole pine woodchip and with a novel "zig-zag" flow pattern was investigated. The influent water had an average NO3-N concentration of 25 mg L-1. The performance of the bioreactor was evaluated in two scenarios. In Scenario 1, only NO3-N removal was evaluated; in Scenario 2, NO3-N removal, ammonium-N (NH4-N), and dissolved reactive phosphorus (DRP) generation was considered. These data were used to generate a sustainability index (SI), which evaluated the overall performance taking these parameters into account. In Scenario 1, the bioreactor was a net reducer of contaminants, but it transformed into a net producer of contaminants in Scenario 2. Inquisition of the data using these scenarios meant that an optimum bioreactor design could be identified. This would involve reduction to two cells: a single sandy loam soil cell followed by a woodchip cell, which would remove NO3-N and reduce greenhouse gas (GHG) emissions and DRP losses. An additional post-bed chamber containing media to eliminate NH4-N and surface capping to reduce GHG emissions further is advised. Scenario modeling, such as that proposed in this paper, should ideally include GHG in the SI, but because different countries have different emission targets, future work should concentrate on the development of geographically appropriate weightings to facilitate the incorporation of GHG into a SI. [ABSTRACT FROM AUTHOR]

Abstract: Laboratory denitrifying bioreactors, which use an organic carbon (C) rich media to enhance microbial reduction of nitrate (NO3 −) to nitrogen (N) gases, are used worldwide to protect surface and groundwater. To highlight potential adverse effects of denitrifying bioreactors, NO3 − removal rates (gNO3-Nm−3 d−1 removed), NO3 − removal efficiencies (% removed minus production of other N species) and release of greenhouse gases and solutes (ammonium (NH4 +), phosphorus (P) and organic carbon (C)) were compared in this study using different media: lodgepole pine woodchips (LPW), cardboard, lodgepole pine needles (LPN), barley straw (BBS) and a soil control. Results showed that NO3 − removals were consistently >99% for all media for initial leaching and steady-state periods. When pollution swapping was considered, this ranged from 67% for LPW to 95% for cardboard. Sustained P releases over the threshold for the occurrence of eutrophication were measured in all media. Greenhouse gas emissions were dominated by carbon dioxide (CO2) and methane (CH4) fluxes with little nitrous oxide (N2O) release due to the anaerobic conditions prevalent within the bioreactors. Comparisons of different media, under steady-state conditions, showed that C fluxes were highest for cardboard and BBS bioreactors. Carbon fluxes from cardboard bioreactors ranged from 11.6gCm−2 d−1 to 13.9gCm−2 d−1, whilst BBS emissions ranged from 3.9gCm−2 d−1 to 4.4gCm−2 d−1. These C emissions were correlated with the total surface area exposed within the media. Such investigations highlight the need to consider pollution swapping during the initial leaching period and should improve design criteria for field-scale bioreactors used to mitigate shallow groundwater NO3 −. [Copyright &y& Elsevier]

Soil N 2 O emission potential is commonly investigated under idealized denitrifying conditions (e.g. nitrate-N supplied and anaerobic soil), with pH commonly identified as a major determinant of N 2 O emission potential. However, under urine patch conditions in grazed pastures soils a more complex series of abiotic and biotic factors may influence emissions due to the complex N transformations that occur following urea hydrolysis. These transformations may decouple native and/or expected controls of N 2 O emissions encountered under classic denitrifying conditions. Here, we tracked O 2 , CO 2 , NO, N 2 O and N 2 emissions from urine amended soils (i.e. simulating a urine patch) to determine putative controls of N 2 O emissions within 13 different pasture soils from northern (Ireland) and southern hemispheres (New Zealand). Incubations were performed under aerobic conditions±artificial urine (13.3 mg N vial−1) equivalent to field ruminant urine deposition rates of 1000 kg N ha−1. Results revealed that pH was not an important regulator of the emission ratio (N 2 O/(NO + N 2 O + N 2)) in urine amended soils. Within urine affected soils, a new set of variables emerged as regulators of N 2 O emissions, likely due to the unique environment created within this system. We show that urine results in decoupling of the initial soil pH control of the emission ratio allowing other regulators such as nitrite to dominate. In addition, we observed that the emission ratio of N 2 O increased linearly with the rate of N- gas loss (NO + N 2 O + N 2 μmol N h−1), O 2 consumption was positively associated with ammonia oxidising bacteria (AOB) and that the production of NO and N 2 O were also enhanced under urine conditions. • Kinetic profiles reveal urine amended soils as significant sources of NO and N 2 O. • No relationship between soil pH and emission ratio of N 2 O following urine addition. • Ammonia oxidising bacterial (AOB) abundance positively correlated with oxic respiration. • NO 2 − was correlated with N 2 O emission ratio. [ABSTRACT FROM AUTHOR]