Investigating the impact of grassland sward composition and soil moisture conditions on nitrogen cycling and nitrous oxide emissions using Cavity Ring Down Spectroscopy

Mitigating 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.