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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]

Rationale: N2O isotopomers are a useful tool to study soil N cycling processes. The reliability of such measurements requires a consistent set of international N2O isotope reference materials to improve inter-laboratory and inter-instrument comparability and avoid reporting inaccurate results. All these are the more important given the role of N2O in anthropogenic climate change and the pressing need to develop our understanding of soil N cycling and N2O emission to mitigate such emissions. Cavity ring-down spectroscopy (CRDS) could potentially overcome resource requirements and technical challenges, making N2O isotopomer measurements more feasible and less expensive than previous approaches (e.g., gas chromatography [GC] and isotope ratio mass spectrometry [IRMS]). Methods: A combined laser spectrometer and small sample isotope module (CRDS & SSIM) method enabled N2O concentration, δ15Nbulk, δ15Nα, δ15Nβ and site preference (SP) measurements of sample volumes <20 mL, such as static chamber samples. Sample dilution and isotopic mixing as well as N2O concentration dependence were corrected numerically. A two-point calibration procedure normalised d values to the international isotope-ratio scales. The CRDS & SSIM repeatability was determined using a reference gas (Ref Gas). CRDS & SSIM concentration measurements were compared with those obtained by GC, and the isotope ratio measurements from two different mass spectrometers were compared. Results: The repeatability (mean ± 1s; n = 10) of the CRDS & SSIM measurements of the Ref Gas was 710.64 ppb (± 8.64), 2.82? (± 0.91), 5.41? (± 2.00), 0.23? (± 0.22) and 5.18? (± 2.18) for N2O concentration, δ15Nbulk, δ15Nα, δ15Nβ and SP, respectively. The CRDS & SSIM concentration measurements were strongly correlated with GC (r = 0.99), and they were more precise than those obtained using GC except when the N2O concentrations exceeded the specified operating range. Normalising CRDS & SSIM d values to the international isotope-ratio scales using isotopic N2O standards (AK1 and Mix1) produced accurate results when the samples were bracketed within the range of the d values of the standards. The CRDS & SSIM δ15Nbulk and SP precision was approximately one order of magnitude less than the typical IRMS precision. Conclusions: CRDS & SSIM is a promising approach that enables N2O concentrations and isotope ratios to be measured by CRDS for samples <20 mL. The CRDS & SSIM repeatability makes this approach suitable for N2O "isotopomer mapping" to distinguish dominant source pathways, such as nitrification and denitrification, and requires less extensive lab resources than the traditionally used GC/IRMS. Current study limitations highlighted potential improvements for future users of this approach to consider, such as automation and physical removal of interfering trace gases before sample analysis. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The effects of slurry application method and weather conditions after application on ammonia volatilisation are well documented, however, the effect on slurry N recovery in herbage is less evident due to large variability of results. The objective of this field experiment was to determine the recovery of cattle slurry NH4-N in herbage and soil in the year of application as affected by application method (trailing shoe versus broadcast) and season of application (spring versus summer), using 15N as a tracer. In 2007 and 2008, 15N enriched slurry was applied on grassland plots. N recovery in herbage and soil during the year of application was determined. Both spring and trailing shoe application resulted in significantly higher herbage DM yields, N uptake and an increased recovery of 15NH4-N in herbage. Additionally, the recovery of slurry 15NH4-N in the soil at the end of the growing season was increased. Spring and trailing shoe application reduced the losses of slurry 15NH4-N by on average 14 and 18 percentage points, respectively, which corresponded closely to ammonia volatilisation as predicted by the ALFAM model. It was concluded that slurry N recovery in temperate pasture systems can be increased by adjusting the slurry application method or timing. [ABSTRACT FROM AUTHOR]

Perennial species with the C4 pathway hold promise for biomass-based energy sources. We have explored the extent that CO2 uptake of such species may be limited by light in a temperate climate. One energetic cost of the C4 pathway is the leakiness (Φ) of bundle sheath tissues, whereby a variable proportion of the CO2. concentrated in bundle sheath cells, retrodiffuses back to the mesophyll. In this study, we scale Φ from leaf to canopy level of a Miscanthus crop (Miscanthus X giganteus hybrid) under field conditions and model the likely limitations to CO2 fixation. At the leaf level, measurements of photosynthesis coupled to online carbon isotope discrimination showed that leaves within a 3.3-m canopy (leaf area index = 8.3) show a progressive increase in both carbon isotope discrimination and Φ as light decreases. A similar increase was observed at the ecosystem scale when we used eddy covariance net ecosystem CO2 fluxes, together with isotopic profiles, to partition photosynthetic and respiratory isotopic flux densities (isofluxes) and derive canopy carbon isotope discrimination as an integrated proxy for Φ at the canopy level. Modeled values of canopy CO2 fixation using leaf-level measurements of Φ suggest that around 32% of potential photosynthetic carbon gain is lost due to light limitation, whereas using 4 determined independently from isofluxes at the canopy level the reduction in canopy CO2 uptake is estimated at 14%. Based on these results, we identify 4' as an important limitation to CO2 uptake of crops with the C4 pathway. [ABSTRACT FROM AUTHOR]