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- PublicationSward composition and soil moisture conditions affect nitrous oxide emissions and soil nitrogen dynamics following urea-nitrogen applicationIncreased emissions of N2O, a potent greenhouse gas (GHG), from agricultural soils is a major concern for the sustainability of grassland agriculture. Emissions of N2O 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 N2O 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 N2O peaks coincided with fertiliser application and heavy rainfall events; 13.4 and 17.7 g N2O-N ha−1 day−1 (ambient soil moisture) and 39.8 and 86.9 g N2O-N ha−1 day−1 (wet soil moisture). Overall, cumulative N2O 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 N2O emissions increasing from 22.3 to 96.2 g N2O-N ha−1 (ambient soil conditions) and from 59.0 to 219.3 g N2O-N ha−1 (wet soil conditions), after a uniform N application rate. Soil N dynamics support denitrification as the dominant source of N2O 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.
82Scopus© Citations 14
- PublicationAnalysis of N2O emissions and isotopomers to understand nitrogen cycling associated with multispecies grassland swards at a lysimeter scaleNitrous oxide (N2O) is a potent greenhouse gas associated with nitrogen fertiliser inputs to agricultural production systems. Minimising N2O emissions is important to improving the efficiency and sustainability of grassland agriculture. Multispecies grassland swards composed of plants from different functional groups (grasses, legumes, herbs) have been considered as a management strategy to achieve this goal.
- PublicationIncreasing Legume and Soil Moisture Content Increases N2O Emissions from Soil Following Fertiliser N ApplicationNitrous oxide is a potent greenhouse gas linked to nitrogen inputs, soil moisture and land management practices.
- PublicationInvestigating the impact of grassland sward composition and soil moisture conditions on nitrogen cycling and nitrous oxide emissions using Cavity Ring Down Spectroscopy(University College Dublin. School of Agriculture and Food Science, 2020)
;0000-0003-0533-3595Mitigating N losses such as N2O emissions is needed to improve the environmental sustainability of grassland agriculture. This thesis firstly aimed to develop a new Cavity Ring Down Spectroscopy (CRDS & SSIM) technique to measure N2O concentration and isotopomers in small volume (<20 ml) gas samples emitted from soil. The method was tested and compared to GC-IRMS. With a precision (1¿) of N2O conc. 8.64 ppb, ¿15Nbulk 0.91‰, ¿15N¿ 2.00‰, ¿15N¿ 0.22‰ and SP 2.18‰, CRDS & SSIM can be used to distinguish important N2O sources. Secondly, this thesis aimed to determine the effect of grassland sward composition and soil moisture conditions on N2O emissions. A simplex-centroid field experiment was used to quantify cumulative N2O losses and changes in soil mineral N concentrations from eight experimental swards, post a 40 kg N ha-1 urea fertiliser application. The swards contained various proportions of perennial ryegrass (PRG), white clover (WC) and ribwort plantain (PLAN). Plots were split into two distinct areas of contrasting soil moisture conditions. Cumulative N2O emissions were higher under wet soil conditions. Increases of WC from 0% to 60% resulted in predicted N2O emissions increasing from 22.3 to 96.2 and from 59.0 to 219.3 g N2O-N ha-1, under ambient and wet soil conditions, respectively. Soil mineral N dynamics suggested denitrification was likely the main source of the increased N2O emissions under wetter soil conditions. N2O isotopomers, determined by CRDS & SSIM, were used to source partition N2O fluxes into fractions of nitrification F(N) or denitrification F(D). Overall, 34.2% of N2O fluxes were attributed to nitrification, 29.0% to denitrification and 36.8% to a mixture of both. Simplex modelled SP and ¿15Nbulk indicated that increasing proportions of WC under wetter soil conditions could stimulate denitrification as a source of N2O and that PLAN could potentially lower N2O emissions under drier soil conditions through biological nitrification inhibition (BNI). The final experiment, a completely randomised block design, used intact soil cores from selected plots of the field experiment as lysimeters. Four sward compositions were compared at different annual N fertiliser rates; PRG (250 kg N ha-1 yr-1), PRG+LWC (90 kg N ha-1 yr-1), PRG+HWC (0 kg N ha-1 yr-1) and PRG+WC+P (45 kg N ha-1 yr-1). Annual N fertiliser rates were chosen based on WC content, to account for biologically fixed N and create a balanced N management approach. N2O emissions, leached N, herbage DM yields and N uptake were quantified. N2O isotopomers were measured around times of peak N2O fluxes to source partition N2O and quantify F(N) and F(D). Soil cores were X-ray CT scanned and soil physical properties were quantified and correlated with N losses. Cumulative N2O emissions, leached N, DM yields and N uptake were not significantly different between treatments. Cumulative N2O emissions were PRG (3.25 ¿ 0.5), PRG+LWC (2.98 ¿ 0.2), PRG+HWC (3.01 ¿ 0.7) and PRG+WC+P (2.74 ¿ 0.3) kg N2O-N ha-1. While not statistically significant, PRG+WC+P had lower cumulative N2O emissions and F(N). BNI associated with ribwort plantain may have contributed to lower N2O emissions and warrants further research. Significantly greater N2O emissions and organic N leached during Spring from the PRG and PRG+LWC than PRG+HWC and PRG+WC+P was associated with higher fertiliser N inputs and application timing. Less Winter activity and slower N uptake in Spring from PRG may have resulted in more soil mineral N vulnerable to loss during Spring. Further research is needed to understand the impact of multispecies swards, particularly those containing ribwort plantain, on soil N cycling and N losses at a field scale and across a range of soil types and textures. The findings of this thesis indicate that the adoption of different grassland sward compositions and appropriate N fertiliser management could be a useful N2O mitigation strategy. 45
- PublicationSource partitioning using N2O isotopomers and soil WFPS to establish dominant N2O production pathways from different pasture sward compositionsNitrous oxide (N2O) is a potent greenhouse gas (GHG) emitted from agricultural soils and is influenced by nitrogen (N) fertiliser management and weather and soil conditions. Source partitioning N2O emissions related to management practices and soil conditions could suggest effective mitigation strategies. Multispecies swards can maintain herbage yields at reduced N fertiliser rates compared to grass monocultures and may reduce N losses to the wider environment. A restricted-simplex centroid experiment was used to measure daily N2O fluxes and associated isotopomers from eight experimental plots (7.8 m2) post a urea-N fertiliser application (40 kg N ha−1). Experimental pastures consisted of differing proportions of grass, legume and forage herb represented by perennial ryegrass (Lolium perenne), white clover (Trifolium repens) and ribwort plantain (Plantago lanceolata), respectively. N2O isotopomers were measured using a cavity ring down spectroscopy (CRDS) instrument adapted with a small sample isotope module (SSIM) for the analysis of gas samples ≤20 mL. Site preference (SP = δ15Nα – δ15Nβ) and δ15Nbulk ((δ15Nα + δ15Nβ) / 2) values were used to attribute N2O production to nitrification, denitrification or a mixture of both nitrification and denitrification over a range of soil WFPS (%). Daily N2O fluxes ranged from 8.26 to 86.86 g N2O-N ha−1 d−1. Overall, 34.2% of daily N2O fluxes were attributed to nitrification, 29.0% to denitrification and 36.8% to a mixture of both. A significant diversity effect of white clover and ribwort plantain on predicted SP and δ15Nbulk indicated that the inclusion of ribwort plantain may decrease N2O emission through biological nitrification inhibition under drier soil conditions (31%–75% WFPS). Likewise, a sharp decline in predicted SP indicates that increased white clover content could increase N2O emissions associated with denitrification under elevated soil moisture conditions (43%–77% WFPS). Biological nitrification inhibition from ribwort plantain inclusion in grassland swards and management of N fertiliser source and application timing to match soil moisture conditions could be useful N2O mitigation strategies.
115Scopus© Citations 11