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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]

[Display omitted] • Assessing downstream impact of dairy diets on whole-farm emissions is crucial. • Process-based models may capture herd and interannual variation on emissions. • Pasture-based and confinement systems have different carbon footprints. • Process-based models capture dietary effects on-farm carbon and nitrogen cycling. Feed management decisions are crucial in mitigating greenhouse gas (GHG) and nitrogen (N) emissions from ruminant farming systems. However, assessing the downstream impact of diet on emissions in dairy production systems is complex, due to the multifunctional relationships between a variety of distinct but interconnected sources such as animals, housing, manure storage, and soil. Therefore, there is a need for an integral assessment of the direct and indirect GHG and N emissions that considers the underlying processes of carbon (C), N and their drivers within the system. Here we show the relevance of using a cascade of process-based (PB) models, such as Dutch Tier 3 and (Manure)-DNDC (Denitrification-Decomposition) models, for capturing the downstream influence of diet on whole-farm emissions in two contrasting case study dairy farms: a confinement system in Germany and a pasture-based system in New Zealand. Considerable variation was found in emissions on a per hectare and per head basis, and across different farm components and categories of animals. Moreover, the confinement system had a farm C emission of 1.01 kg CO 2 -eq kg−1 fat and protein corrected milk (FPCM), and a farm N emission of 0.0300 kg N kg−1 FPCM. In contrast, the pasture-based system had a lower farm C and N emission averaging 0.82 kg CO 2 -eq kg−1 FPCM and 0.006 kg N kg−1 FPCM, respectively over the 4-year period. The results demonstrate how inputs and outputs could be made compatible and exchangeable across the PB models for quantifying dietary effects on whole-farm GHG and N emissions. [ABSTRACT FROM AUTHOR]

Increased emissions of N 2 O, a potent greenhouse gas (GHG), from agricultural soils is a major concern for the sustainability of grassland agriculture. Emissions of N 2 O are closely associated with the rates and forms of N fertilisers applied as well as prevailing weather and soil conditions. Evidence suggests that multispecies swards require less fertiliser N input, and may cycle N differently, thus reducing N loss to the environment. This study used a restricted simplex-centroid experimental design to investigate N 2 O emissions and soil N cycling following application of urea-N (40 kg N ha−1) to eight experimental swards (7.8 m2) with differing proportions of three plant functional groups (grass, legume, herb) represented by perennial ryegrass (PRG, Lolium perenne), white clover (WC, Trifolium repens) and ribwort plantain (PLAN, Plantago lanceolata), respectively. Swards were maintained under two contrasting soil moisture conditions to examine the balance between nitrification and denitrification. Two N 2 O peaks coincided with fertiliser application and heavy rainfall events; 13.4 and 17.7 g N 2 O-N ha−1 day−1 (ambient soil moisture) and 39.8 and 86.9 g N 2 O-N ha−1 day−1 (wet soil moisture). Overall, cumulative N 2 O emissions post-fertiliser application were higher under wet soil conditions. Increasing legume (WC) proportions from 0% to 60% in multispecies swards resulted in model predicted N 2 O emissions increasing from 22.3 to 96.2 g N 2 O-N ha−1 (ambient soil conditions) and from 59.0 to 219.3 g N 2 O-N ha−1 (wet soil conditions), after a uniform N application rate. Soil N dynamics support denitrification as the dominant source of N 2 O especially under wet soil conditions. Significant interactions of PRG or WC with PLAN on soil mineral N concentrations indicated that multispecies swards containing PLAN potentially inhibit nitrification and could be a useful mitigation strategy for N loss to the environment from grassland agriculture. Unlabelled Image • Measurement of N 2 O emissions and N cycling from varying sward compositions. • Post N application (40 kg N ha−1) N 2 O loss increased with white clover proportion. • N 2 O emissions from PRG were 2.5 fold higher in wet soil (WFPS > 60%) compared to ambient. • Soil N dynamics suggest denitrification as dominant N 2 O source when WFPS > 60%. • Plantago lanceolata (forage herb) potentially regulates N cycling pathways. [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]

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]