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Most nitrogen (N) lost to the environment from grazed grassland is produced as a result of N excreted by livestock, released in the form of nitrous oxide (N2O) emissions, nitrate leaching and ammonia volatilisation. In addition to the N fertiliser applied, excreta deposited by grazing livestock constitute a heterogeneous excess of N, creating spatial hotspots of N losses. This study presents a yearlong N2O emissions map from a typical intensively managed temperate grassland, grazed periodically by a dairy herd. The excreta deposition mapping was undertaken using high-resolution RGB images captured with a remotely piloted aircraft system combined with N2O emissions measurements using closed statics chambers. The annual N2O emissions were estimated to be 3.36 ± 0.30 kg N2O–N ha−1 after a total N applied from fertiliser and excreta of 608 ± 40 kg N ha−1 yr−1. Emissions of N2O were 1.9, 3.6 and 4.4 times lower than that estimated using the default IPCC 2019, 2006 or country-specific emission factors, respectively. The spatial distribution and size of excreta deposits was non-uniform, and in each grazing period, an average of 15.1% of the field was covered by urine patches and 1.0% by dung deposits. Some areas of the field repeatedly received urine deposits, accounting for an estimated total of 2410 kg N ha−1. The method reported in this study can provide better estimates of how management practices can mitigate N2O emissions, to develop more efficient selective approaches to fertiliser application, targeted nitrification inhibitor application and improvements in the current N2O inventory estimation. [ABSTRACT FROM AUTHOR]

The effect of long‐term soil phosphorus (P) on in situ nitrous oxide (N2O) emissions from temperate grassland soil ecosystems is not well understood. Grasslands typically receive large nitrogen (N) inputs both from animal deposition and fertiliser application, with a large proportion of this N being lost to the environment. Understanding optimum nutrient stoichiometry by applying N fertilisers in a relative balance with P will help to reduce N losses by enabling maximum N‐uptake by plants and microbes. This study investigates the N2O response from soils of long‐term high and low P management receiving three forms of applied N at two different rates: a nitrate‐based fertiliser (KNO3) and an ammonium‐based fertiliser ([NH4]2SO4) (both at 40 Kg N ha−1), and a synthetic urine (750 Kg N ha−1). Low soil P significantly increased N2O emissions from KNO3 and (NH4)2SO4 fertilisers by over 50% and numerically increased N2O from urine by over 20%, which is suggested to be representative of the lack of significant effect of N fertilisation on N‐uptake observed in the low P soils. There was a significant positive effect of soil P on grass N‐uptake observed in the synthetic urine and KNO3 treatments, but not in the (NH4)2SO4 treatment. Low P soils had a significantly lower pH than high P soilss and responded differently to applied synthetic urine. There was also a significant effect of P level on potential nitrification which was nearly three times that of low P, but no significant difference between potential denitrification and P level. The results from this study highlight the importance of synergy between relative nutrient applications as a deficiency of one nutrient, such as P in this case, could be detrimental to the system as a whole. Optimising soil P can result in greater N uptake (over 12, 23 and 66% in (NH4)2SO4, KNO3 and synthetic urine treatments, respectively) and in reduced emissions by up to 50% representing a win‐win scenario for farmers. Highlights: P deficiency in grassland soils causes greater N2O emissions.Insufficent soil P inhibits N‐uptake, regardless of rate or form of N.Optimising soil P levels can reduce N2O emissions and improve overall NUE. [ABSTRACT FROM AUTHOR]

Multispecies pastures containing grasses, N-fixing legumes and forage herbs can maintain high herbage dry matter (DM) yield, while reducing the need for fertiliser N inputs thus potentially reducing harmful N losses such as nitrous oxide (N 2 O) emission and nitrate (NO 3 −) leaching to the wider environment. However, our understanding of these processes in grassland swards of contrasting composition is still very limited. A lysimeter experiment was carried out over 12 months to test the hypotheses that (1) cumulative and (2) seasonal N 2 O emission and N leaching would be highest from the Perennial Ryegrass only (PRG, 250 kg N ha-1 yr-1) treatment compared to three other treatments; Perennial Ryegrass and Low White Clover (PRG+LWC, 90 kg N ha-1 yr-1), Perennial Ryegrass and High White Clover (PRG+HWC, 0 kg N ha-1 yr-1) and Perennial Ryegrass, White Clover and Ribwort Plantain (PRG+WC+P, 45 kg N ha-1 yr-1) and (3) that the soil N cycling pathways (nitrification/denitrification) linked to these N losses would be different between treatments. Cumulative N 2 O emissions (2.74–3.25 kg N 2 O-N ha-1), leached N (10.91–13.70 kg N ha-1), DM yields (5083–5493 kg DM ha-1 yr-1) and N uptake (122–169 kg N ha-1 yr-1) were not significantly different between treatments. Nitrogen yield scaled N 2 O emissions were significantly lower from all other treatments compared to the high fertiliser N input PRG monoculture. A significant interaction between treatment and season showed that in Spring, N 2 O emission was significantly higher from PRG (1.39 kg N 2 O-N ha-1) and PRG+LWC (1.19 kg N 2 O-N ha-1) than PRG+HWC (0.81 kg N 2 O-N ha-1) and PRG+WC+P (0.85 kg N 2 O-N ha-1). This result was linked to a numerically lower fraction of nitrification (F N) for PRG+WC+P potentially indicating that biological nitrification inhibition associated with ribwort plantain could lessen N 2 O emissions. This study demonstrates some of the environmental benefits of grassland management systems that require less intensive chemical fertiliser N input. • N 2 O and leached N were 1.1–1.3 % and 4.4–5.5 % of N applied, respectively. • Nitrogen yield-scaled N 2 O emission were highest for perennial ryegrass monoculture. • Treatment × season interactions show opportune time and management to cut N loss. • Lower F N indicates nitrification inhibition associated with ribwort plantain. [ABSTRACT FROM AUTHOR]

Nitrogen fertiliser application represents the largest anthropogenic source of nitrous oxide (N2O) emissions, and the magnitude of these emissions is dependent on the type of fertilisers applied in the agroecosystems. Despite N-P-K compound fertilisers being commonly used in agricultural soils, a lack of information exists regarding their effects on N2O emissions. This study aims at examining the effects of different commonly used N-P-K compound fertiliser formulations with contrasting nitrate to ammonium ratios (0.05 to 0.88) on N2O emissions, yield, and nitrogen use efficiency (NUE) in temperate grassland and to compare these variables with common straight N fertilisers. Compound fertilisers with varying NPK inclusion rates (18-6-12, 10-10-20, 24-2.2-4.5, and 27-2.5-5), and calcium ammonium nitrate (CAN) and urea + N-(n-butyl) thiophosphoric triamide (NBPT) were applied at 80 kg N ha−1 to experimental plots in managed grassland on two occasions in a growing season. Fluxes of N2O during the experiment period, yield and NUE following two harvests were measured. The cumulative N2O emission from urea + NBPT, 18-6-12, 10-10-20, and 24-2.2-4.5 treatments were significantly reduced by 44%, 43%, 37%, and 31% compared with CAN treatment under conducive soil moisture condition. Under the same soil condition, 18-6-12 and 10-10-20 treatments showed higher yield, N uptake, and NUE although did not significantly differ from the other fertiliser treatments. Our results suggest that ammonium-based compound fertilisers have a potential to reduce N2O emissions while maintaining yields. Further long-term study is needed to capture the full magnitude of variations in N2O emissions, including ammonia (NH3) volatilization from nitrate and ammonium-based compound fertiliser applications from multiple soil types and under different climatic conditions. [ABSTRACT FROM AUTHOR]

• Urea hydrolysis induced rapid changes in organic labile and recalcitrant N pools. • Gross mineralisation and immobilisation rates declined as soil moisture increased. • Recovery of 15N was higher in the recalcitrant N pool at lower soil moisture. • Labile N pool size and turnover was higher under increasing soil moisture. Ruminant urine patches in grazed grasslands significantly change the chemical and biological properties of the affected soils due to the predominance of urea within ruminant urine and the high rates deposited onto pastures. The net result is the loss of reactive N (N r) but little is known about the gross N transformation rates leading to N r losses or the long-term fate of urine-N in pasture soils. Using data from a previous incubation study, that simulated ruminant urine application by applying 15N-urea, we investigated the effects of differing soil moisture regimes on gross soil N transformation rates, including urea hydrolysis and ammonia (NH 3) formation. Gross transformation rates were quantified using a 15N tracing tool ' Ntrace Basi c ' that was extended with a urea submodel. The new model (Ntrace Urea) matched the measured data well (NH 4 +, NO 3 − concentrations and their respective 15N enrichments over time). Soil moisture affected urea hydrolysis dynamics and was postulated to regulate the magnitude of the NH 3 dynamics due to constraints on gas diffusion under wetter (−1 kPa) soil conditions. Under drier soil conditions (−10 kPa) sorption and release of NH 4 +, the movement of NH 4 + into and out of the soil labile N pool, the movement of NO 3 − into and out of the soil recalcitrant N pool, and the mineralisation of the recalcitrant N pool were all enhanced relative to −1 kPa, as were the gross N transformation rates in general due to the high urea N rate applied. This study shows that the time required for soils to re-establish equilibrium following urea deposition is substantial, and provides an explanation for the long-term 15N recoveries observed under ruminant urine patches in soils. The results highlight future research directions including the need to understand the potential role of NH 3 in contributing to the recalcitrant N pool. [ABSTRACT FROM AUTHOR]

Quantifying nitrous oxide (N 2 O) emissions from grazed pastures can be problematic due to the presence of hotspots and hot moments of N 2 O from animal excreta and synthetic fertilisers. In this study, we quantified field scale N 2 O emissions from a temperate grassland under a rotational grazing management using eddy covariance (EC) and static chamber techniques. Measurements of N 2 O by static chambers were made for four out of nine grazing events for a control, calcium ammonium nitrate (CAN), synthetic urine (SU) + CAN and dung + CAN treatments. Static chamber N 2 O flux measurements were upscaled to the field scale (F CH FIELD) using site specific emission factors (EF) for CAN, SU+CAN and dung + CAN. Mean N 2 O EFs were greatest from the CAN treatment while dung + CAN and SU + CAN emitted similar N 2 O-N emissions. Cumulative N 2 O-N emissions over the study period measured by F CH FIELD measurements were lower than gap-filled EC measurements. Emission factors of N 2 O from grazing calculated by F CH FIELD and gap-filled were 0.72% and 0.96%, respectively. N 2 O-N emissions were derived mainly from animal excreta (dung and urine) contributing 50% while N 2 O-N losses from CAN and background accounted for 36% and 14%, respectively. The study highlights the advantage of using both the EC and static chamber techniques in tandem to better quantify both total N 2 O-N losses from grazed pastures while also constraining the contribution of individual N sources. The EC technique was most accurate in quantifying N 2 O emissions, showing a range of uncertainty that was seven times lower relative to that attributed to static chamber measurements, due to the small chamber sample size per treatment and highly variable N 2 O flux measurements over space and time. • N 2 O emissions were measured using EC and static chambers (SC) from a grazed pasture. • The magnitude of mean N 2 O-N EFs measured by SC were CAN > dung + CAN > SU + CAN. • EFs of CAN, dung + CAN and SU + CAN were used to upscale SC N 2 O fluxes (F CH FIELD). • N 2 O-N EFs by EC and F CH FIELD were 0.72% and 0.96%, > IPCC default value 0.6%. • EC showed lower uncertainties in cumulative N 2 O emissions compared to F CH FIELD. [ABSTRACT FROM AUTHOR]

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]

• N 2 O emissions were measured at six site-years in Irish grassland. • 90 N 2 O-EF were observed from three N fertilisers and five rates. • Average direct N 2 O-EF for CAN (1.62 %)>urea + NBPT (0.6 %)>urea (0.46 %). • The urea EF was most sensitive to increasing N rate. • The EF increased significantly with increasing rate in 3 (CAN), 5 (urea) and 2 (urea + NBPT) site-years. The present study evaluated the impact of three nitrogen (N) fertiliser formulations, applied at five N rates, on nitrous oxide (N 2 O) fluxes and annual direct N 2 O-N emission factors (EF) in temperate grassland. Closed static chambers were used to measure direct N 2 O fluxes at three geographically dispersed locations in Ireland over a two-year period, generating a total of 90 EFs across the six site-years and treatments. The three fertiliser formulations tested were calcium ammonium nitrate (CAN), urea, and urea amended with the urease inhibitor N -(n -butyl) thiophosphoric triamide (NBPT) at 100, 200, 300, 400 and 500 kg N ha−1 yr−1. All treatments were applied in five equal split applications ranging from 20 to 100 kg N ha−1 split-1 over the growing season. The N 2 O-N EFs for CAN ranged from 0.39 − 4.68 with a mean of 1.62 (cv. 81 %), for urea from 0.04 – 1.7 with a mean of 0.46 (cv. 77 %) and for urea + NBPT from 0.18 – 1.7 with a mean of 0.60 (cv. 59 %). A significant positive relationship was found between the N rate and the annual N 2 O-N EFs in three (CAN), five (urea) and two (urea + NBPT) of six the site-years. For the remainder of the site-years EF was unaffected by N rate. These results indicate that fertiliser N choice and rate can be management factors that enable farmers to alter N 2 O losses in temperate grassland. Notably, the response of EF to increasing N rate was not consistent across the fertilisers, with the EF from urea being the most sensitive to the increasing N rate, urea + NBPT the least sensitive and CAN being intermediate. The accuracy of national greenhouse gas accounting could be improved by including N fertiliser formulation and its rate of application. Further research is also needed to understand the inconsistency in EF response to N rate across sites. [ABSTRACT FROM AUTHOR]