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A laboratory and a field study were conducted on a permanent grassland soil in Northern Ireland to investigate the effects of urea in combination with N process inhibitors such as the urease inhibitor N-(butyl) thiophosphoric triamide (NBPT) and/or the nitrification inhibitor dicyandiamide (DCD) on soil N dynamics. Urea enriched with n-butyl 15 N to 60 atom % was applied to soil at a rate of 100 μg N g −1 dry soil in the laboratory and 100 kg N ha −1 in the field. A numerical 15 N tracing model was used to quantify several simultaneously occurring gross N transformation rates in both studies. The changes in soil nitrate (NO 3 − ) and ammonium (NH 4 + ) concentrations and 15 N enrichment over a 25-day period as well as the concentration and 15 N enrichment of plant N at harvest were used to model soil gross N transformations. The results showed that the effect of N process inhibitors varied firstly between laboratory and field studies and secondly whether the inhibitors were applied individually or in combination. Overall DCD had a greater effect on the major soil N transformations than NBPT; reducing oxidation of NH 4 + , total nitrification, net NO 3 − produced, total mineralisation and the net adsorption of NH 4 + at both laboratory and field scale. The effect of DCD was similar for these transformations whether applied alone or co-applied with NBPT. In contrast NBPT had no significant effect on oxidation of NH 4 + , total nitrification, total mineralisation or total immobilisation compared to urea in the field, while the effect on these transformations in the laboratory was significant. The contrasting effects of inhibitors on gross N transformations between laboratory and field may relate to the differences in experimental conditions, e.g. soil preparation, environmental conditions and the contribution of plant biomass. To obtain a more realistic assessment of gross soil N transformations in situ , it is essential that laboratory experiments are supplemented with field studies. [ABSTRACT FROM AUTHOR]

• N 2 O emissions were measured by eddy covariance (EC) and static chambers (CH). • Mean daily CH fluxes were calculated by arithmetic and Bayesian statistics. • CH and EC were most alike when measurements were made over the same area and time. • CH and EC were least alike when CH sample size was low and calculated using Bayesian statistics. • Total N 2 O emissions by EC were most alike to CH by Bayesian statistics. Where nitrogen input from fertilizer application exceeds plant demands, hotspots of microbially produced nitrous oxide (N 2 O) can exhibit disproportionately high rates of emissions relative to longer periods of time, known as hot moments. Hotspots and hot moments of N 2 O are sensitive to changes in agricultural management and weather, making it difficult to accurately quantify N 2 O emissions. This study investigates the spatial and temporal variability of N 2 O emissions using both static chambers (CH) and eddy covariance (EC) techniques, measured at a grassland site subject to four fertilizer applications of calcium ammonium nitrate (CAN) in 2019. Daily mean CH emissions were calculated using the arithmetic method and Bayesian statistics to explicitly account for the log-normal distribution of the dataset. N 2 O fluxes measured by CH and EC were most comparable when flux measurements were > 115 N 2 O-N µg m −2 hr −1, and EC and CH measurements showed spatial and temporal alignment when CH n ≥ 15. Where n ≤ 5, the Bayesian method produced large uncertainties due to the difficulty of fitting an arithmetic mean from a log-normally distributed data set with few flux measurements. Annual EC fluxes, gap-filled using a multi-variate linear model, showed a strong correlation with measured flux values (R 2 = 0.92). Annual cumulative fluxes by EC were higher (3.35 [± 0.5] kg N ha−1) than CH using the arithmetic (2.98 [± 0.17] kg N ha−1) and Bayesian method (3.13 [± 0.24] kg N ha−1), which quantified emission factors of 1.46%, 1.30% and 1.36%, respectively. This study implies that a large sample size and frequent CH flux measurements are necessary for comparison with EC fluxes and that Bayesian statistics are an appropriate method for estimating realistic means and ranges of uncertainty for CH flux data sets. [ABSTRACT FROM AUTHOR]

Large-scale information regarding nitrous oxide (N 2 O) emissions is needed as an evidence base to underpin land use policy and mitigation approaches. However, the highly variable rates of denitrification make the prediction of N 2 O emission demanding. Here, we evaluated the role of abiotic and biotic factors on the potential denitrification of Irish soils, in order to identify the key factors regulating potential N 2 O emissions at a large scale. To do so, we collected 136 soil samples from 32 sites across Ireland, and characterised the soil physico-chemical properties, the prokaryotic and fungal community composition, the abundance of N-cycling genes and evaluated the soil potential nitrification, denitrification and end product N 2 O/(N 2 O + N 2). We found large differences in soil potential denitrification between sites (up to 41.5 mg N 2 O–N kg−1 soil day−1) with most of the emissions released in the form of N 2 O rather than N 2. Soils with highest potential nitrification rates also exhibited the highest potential denitrification rates, and similar parameters were linked to both processes. The factors most predictive of soil potential denitrification were soil physico-chemical properties and the prokaryotic community composition. Soil phosphorus content was as important for predicting potential denitrification as was pH and total nitrogen. Soil microbial community structure, rather than denitrifier abundance, was an important predictor of the potential denitrification and the end-product N 2 O/(N 2 O + N 2). The prokaryotic community composition was more strongly associated with denitrification rates and the resulting end-products than fungal communities. Increased relative abundance of the prokaryotic phyla Actinobacteriota and Crenarchaeota, were positively correlated to complete denitrification. Altogether, these results lay the foundation for a better understanding of the key factors regulating the potential denitrification in soils and identify important properties that enhance prediction of the potential denitrification at larger scales. • Soils with highest nitrification also had the highest denitrification potential (PD). • Soil phosphorus was as important predicting PD as was pH and total nitrogen. • The prokaryotic community was more predictive of soil PD than the fungal community. • Microbial community structure, rather than gene abundance, was an important predictor. • Actinobacteriota and Crenarchaeota were correlated to complete denitrification. [ABSTRACT FROM AUTHOR]

A laboratory incubation study was conducted on a temperate grassland soil to quantify the main mineral nitrogen (N) transformation rates and pathways via a15N tracing approach. Soil samples were taken from a long-term phosphorus (P) trial to investigate the effects on gross N-transformations under high and low phosphorus amendment. The soils were incubated over a 2-week period and treated with ammonium-nitrate (NH 4 NO 3) which was applied to the soil both with and without a glucose amendment and labelled with 15N either on the ammonium (NH 4 +) or nitrate (NO 3 −) moiety at 50% atom enrichment. The results showed immobilisation to greatly outweigh mineralisation and that NO 3 − was predominantly produced via heterotrophic nitrification. Individual pathways for NO 3 − production were quantified including oxidation of NH 4 +, recalcitrant and labile organic N. Oxidation of labile organic N to NO 3 −, a newly considered pathway, accounted for between 63 and 83% of total NO 3 − production across the various treatments and P levels. This process was significantly higher in the low-P rather than the high-P soils (p < 0.05), highlighting the effect of soil P on the microbial community. • The relative abundance of C, N and P significantly influences N-transformations. • A novel nitrification pathway of oxidation of labile organic N to NO 3 − was found. • This pathway of accounted for between 63 and 83% of total NO 3 − production. • This novel pathway was significantly higher in the low-P soils when not C-limited. [ABSTRACT FROM AUTHOR]

• Fertiliser and urine deposits can interact and increase N 2 O emissions. • Urine emission factors were higher in autumn and lower than the IPCC default value. • Rainfall and temperature are key drivers of seasonal variations in emissions. • Interactions between emission sources must be considered to improve IPCC assessments. Cattle excreta deposited on grazed pastures are responsible for one fifth of the global anthropogenic nitrous oxide (N 2 O) emissions. One of the key nitrogen (N) sources is urine deposited from grazing animals, which contributes to very large N loadings within small areas. The main objective of this plot study was to establish whether the application of N fertiliser and urine deposit from dairy cows synergistically interacts and thereby increases N 2 O emissions, and how such interaction is influenced by the timing of application. The combined application of fertiliser (calcium ammonium nitrate) and urine significantly increased the cumulative N 2 O emissions as well as the N 2 O emission factor (EF) from 0.35 to 0.74 % in spring and from 0.26 to 0.52 % in summer. By contrast, EFs were lower when only fertiliser (0.31 % in spring, 0.07 % in summer) or urine was applied (0.33 % in spring, 0.28 % in summer). In autumn, N 2 O emissions were larger than in other seasons and the emissions from the combined application were not statistically different to those from either the separately applied urine or N fertiliser (EF ranging from 0.72 to 0.83, p-value < 0.05). The absence of significant synergistic effect could be explained by weather conditions, particularly rainfall during the three days prior to and after application in autumn. This study implies that the interactive effects of N fertilisation and urine deposit, as well as the timing of the application on N 2 O emission need to be taken into account in greenhouse gas emission inventories. [ABSTRACT FROM AUTHOR]

This research investigated the effect of long-term phosphorus (P) addition relative to carbon (C) availability on nitrous oxide (N 2 O) emissions from an ungrazed grassland soil via two incubation experiments. No significant effect of soil P on N 2 O was found under C-limited conditions, while under added-C, cumulative N 2 O was significantly higher from low-P (p < 0.05) rather than high-P soils. CO 2 was not significantly different between P-levels. This highlights the influence of soil P on N 2 O emissions under non C-limiting conditions. This is one of the first studies demonstrating available-C moderating the effect of P on N 2 O emissions from temperate grassland soils. • Phosphorus effects on N 2 O emissions from soils are poorly understood. • Carbon limitation masks effects of phosphorus on nitrous oxide emissions. • When soil not C-limited, N 2 O reduced at higher P concentrations. • Manipulation of C:N:P ratio can reduce N 2 O emissions from grassland. [ABSTRACT FROM AUTHOR]