Influence of soil bulk density and matric potential on relative gas diffusivity and urea/nitrate-derived N₂O and N₂ losses

Abstract

Soil compaction is a common problem, particularly when forages are grazed in winter. High animal stocking rates result in increased soil bulk density (ρb), increases in water‐filled pore space (WFPS) and reduced gas diffusion into and out of the soil. These changes in soil physical properties increase the potential for N₂O emissions via denitrification from soil due to enhanced anaerobic conditions, especially in the presence of suitable substrates. Nitrous oxide (N₂O) is a greenhouse gas whose production in soils is mediated by nitrifying and denitrifying organisms. It is also a significant precursor to compounds involved in the destruction of the stratospheric ozone. Agricultural practices, particularly ruminant urine deposited onto grazed pasture soils, are a significant source of agricultural N₂O emissions and under suitable conditions N₂O is completely denitrified to harmless dinitrogen (N₂) gas. This present study examined the fundamental effects of soil ρb on gas diffusivity and subsequent urea/nitrate derived N₂O and N₂ fluxes. The study also assessed which variables e.g. relative gas diffusivity (Dp/Do) or WFPS could be used as a suitable predictor of N₂O emissions. The first laboratory experiment (Chapter 4) examined the effect of increasing soil ρb on urea‐derived N₂O‐N and N₂‐N fluxes at a constant matric potential (ψ) of ‐10 kPa. Dp/Do decreased while WFPS increased with an increase in soil ρb. Mean cumulative N₂O‐N and N₂‐N fluxes after 35 days, as a percentage of applied N, ranged from 0.05 to 2.14% and 0.06 to 4.97%, respectively. There was a threshold Dp/Do value of 0.038 below which cumulative N₂‐N fluxes increased significantly. Cumulative N₂‐N: N₂O‐N ratios were higher at soil ρb values of 1.3 and 1.4 Mg m⁻³. Both Dp/Do and WFPS explained urea‐derived cumulative N₂O‐N and N₂‐N fluxes equally well. In Chapter 5, two laboratory experiments were performed using the same ρb levels as in chapter 4 but at different ψ levels, ‐6.0 kPa and ‐0.2 kPa, to provide wetter soil conditions. At both ψ levels (‐0.2 and ‐6.0 kPa), soil N transformations, soil pH and DOC concentrations showed comparable trends to those observed in chapter 4 demonstrating that denitrification occurred in the soil. At ‐0.2 kPa, cumulative N₂‐N fluxes were 20 to 25 times greater than the cumulative N₂O‐N fluxes at all soil ρb treatments. In Chapter 5, the maximum cumulative N₂O‐N flux measured as a % of applied N was 16%, at ‐6.0 kPa at 1.3 Mg m⁻³ treatment and these N₂O‐N fluxes decreased with further increases in soil ρb. Again both, Dp/Do and WFPS were equally good in explaining urea‐derived cumulative N₂O‐N and N₂‐N fluxes. It was also demonstrated in this chapter that soil ρb has the potential to change the soil’s pore size distribution. As soil ρb increased, macroporosity (% of total soil volume) decreased by 23% while soil mesoporosity and microporosity increased by 6 and 2%, respectively. In Chapter 6, data from the previous experiments (Chapters 4 and 5) was compiled to assess the overall impact of increasing soil ρb on urea‐derived cumulative N₂O‐N and N₂‐N fluxes at different levels of ψ. The maximum cumulative N₂O‐N flux occurred at an intermediate ψ level (‐6.0 kPa). Dp/Do showed better relationships than WFPS when these independent variables were plotted against cumulative N₂O‐N and N₂‐N fluxes and cumulative N₂‐N: N₂O‐N ratios. In Chapter 7, a controlled experiment was performed where NO₃ ‐ derived N₂O‐N fluxes were measured at varying ρb and ψ levels. This experiment showed that N₂O‐N fluxes peaked at a Dp/Do value that corresponded to the air entry value (ψa). This Dp/Do value, in the range of 0.006‐0.0067, was where maximum N₂O‐N fluxes occurred, regardless of changes in soil ρb. This critical value of Dp/Do was very similar to the Dp/Do value observed in chapter 6 where urea‐derived cumulative N₂O‐N fluxes peaked. At values of ψ > ψa, N₂O‐N was either entrapped in the soil or was reduced to N₂, while at values of ψ < ψa, N₂O‐N fluxes decreased, probably due to soil conditions becoming more aerobic. Soil entrapped N₂O concentrations measured in this experiment also support this finding since higher concentrations were measured at higher ψ levels. An equation was developed using data from this study to predict Dp/Do. It was better at predicting Dp/Do when compared to three other predictive Dp/Do models available in the literature. However, it requires further independent data sets for validation.