Influence of soil water potential, soil temperature and soil gas composition on the generation, absorption and transportation of soil gases

Abstract

Nitrous oxide, N₂O, and methane, CH₄, are long-lived, radiatively active trace greenhouse gases. Soils mainly act as net sources for N₂O and both sources and sinks for CH₄. Biological denitrification and nitrification are the main processes of N₂O production in soil, and methanogenic and methanotrophic organisms are responsible for CH₄ production and consumption, respectively. A number of environmental factors affect the production and consumption of these two gases, and ultimately their atmospheric concentrations. This study was initiated to identify the factors infiuencing the fluxes of these two gases in response to the application of organic and inorganic amendments and various soil management practices.

The study was carried out as three separate field experiments. The first involved different straw managements on bare arable soil (expt.1). The second experiment investigated concentrated animal slurry applied to a pasture soil (expt.2), while the third involved the split (summer and autumn/winter) application of mineral-N fertiliser or dairy shed effluent to large (80 cm diameter x 120 cm deep) intact soil cores in lysimeters (expt.3). Soil surface gas fluxes were determined in each experiment using a soil cover method in which changes in headspace gas concentrations were quantified using gas chromatography. Measurements were made, usually once daily, over the 19 months period spanning all three experiments. Less frequent measurements were made of soil and environmental variables. In expt. 3, soil atmospheric samplers were used to quantify gas concentrations at various depths below the soil surface.

In all three experiments, soil water content was the principal factor affecting N₂O fluxes, with values of > 60% water filled pore space (WFPS) promoting denitrification and high N₂O emissions. In contrast, soil moisture content had only a minor influence on CH₄ fluxes, with high soil moisture content tending to slightly reduce CH₄ uptake by soil. Both soil mineral-N and soil temperature played secondary roles. The soils in all three experiments acted predominantly as sources of N₂O and sinks for CH₄, but on rare occasions very small negative N₂O fluxes (uptake) and CH₄ emissions were observed. In expt. 1, the total N₂O emissions or CH₄ uptake for the three forms of straw management investigated: burning (SB), incorporation (SI) or removal (SR) were not significantly different. Total N₂O emissions, over 135 days, from SB, SI and SR were 400,476 and 870 g N₂O-N ha⁻¹, respectively, representing 0.29, 0.34 and 0.63 % of the total-N applied earlier as fertiliser. Daily fluxes ranged from 2.5 to 96 (SB), 2.1 to 75 (SI), and 2.9 to 842 µg N₂O-N m⁻²hr⁻¹ (SR). In contrast to N₂O fluxes there was little variation in CH₄ uptake in response to rainfall or straw management, with fluxes ≅10 µg CH₄-C m⁻² hr⁻¹. Total CH₄ uptake over 135 days from SB, SI and SR were 338,342 and 252 g CH₄-C ha⁻¹, respectively.

In expt.2, N₂O fluxes from the slurry-treated (ST) soil (20 to 1042 µg N₂O-N m⁻² hr⁻¹) always remained significantly higher (P≤0.05) than non-treated (NT) soil (-2 to 42 µg N₂O- N m⁻² hr⁻¹). The N₂0 fluxes showed dynamic fluctuations especially in response to changes in rainfall and hence soil moisture. Cumulative N₂O emissions from the ST soil (8.97 kg N₂O-N ha⁻¹) over the 90 day measurement period were 17 times higher than the adjacent NT plot (0.53 kg N₂O-N ha⁻¹). This represents 2.3 % of the total N applied as slurry (368 kg N ha⁻¹), or 3.3 % of the NH₄⁺-N applied. Highest CH₄ production (4.5 mg CH₄-C m⁻² hr⁻¹) was observed immediately following slurry application and decreased exponentially with time and ceased altogether after 12 days. These CH₄ emissions were due to the decomposition of the volatile fatty acids (VFAs) present in the slurry. Both CH₄ emissions and VFA disappearance followed similar exponential functions of the form: Y = Ae⁻kT, where: Y = CH₄ emission (µg CH₄-C m⁻² hr⁻¹) or VFA concentration (nmoles mL⁻¹ soil solution), k = first order decay constant (hour⁻¹) and was -0.3357 or -0.4473 for CH₄ and VFA, respectively and T = time (hour). This resulted in a very strong relationship between these two variables given by the equation: CH₄ emissions = 19.4 + 4.65 (VFA) (l= 0.95, P≤O.OO1). The total CH₄ emission after 12 days, was 1065 (SE ± 425) g CH₄-C ha⁻¹, while only 207 (SE ± 12.13) g CH₄-C ha⁻¹ was consumed by this soil over the rest of the 90 day measurement period. During this later period, the amounts of CH₄ consumed by the ST and NT treatments averaged ≅10 µg CH₄-C m⁻² hr⁻¹ and were not significantly different (P>0.05).

In expt. 3, total N₂0 emissions following a summer application of aqueous ammonium chloride (NH₄Cl) or dairy shed effluent (DSE) (both at 200 kg N ha⁻¹ ) and additional irrigation at 50 mm H₂0 month⁻¹ (i1), were: 1.35 and 0.6 kg N₂0-N ha⁻¹ , or 0.60 and 0.23 % of total-N applied, respectively. In contrast, plots receiving 100 mm H₂O month⁻¹ additional irrigation (i2) lost 2.9 and 0.4 kg N₂O-N ha⁻¹ or 1.3 and 0.11 % of total-N applied, respectively. In the 191 days following an early winter (autumn) application of the same N-treatments, total N₂O emissions from DSE-il, DSE-i2, NH₄Cl-il and NH₄Cl-i2 were: 0.88,0.56, 1.70 and 1.55 kg N₂O-N ha⁻¹ or 0.27,0.12,0.68 and 0.61 % of total N applied, respectively. The highest daily flux (3.1 mg N₂O-N m⁻² hr⁻¹) was measured 6 days following the summer application of NH₄Cl. Amounts of N₂O dissolved in leachate were less than 0.06 % of the total applied-N. Methane emissions amounting to 21.5 mg CH₄-C m⁻² and 21.3 mg CH₄-C m⁻² were observed for several days immediately following application of the DSE treatments in summer and early winter (autumn), respectively. Exponential decay functions, similar to those found in expt. 2, were observed with rate constants of -0.305 and -0.100 hour⁻¹ for the summer and winter applications, respectively. Over the rest of the measurement period, CH₄ uptake by soil was observed approximating 15 µg CH₄-C m⁻² hr⁻¹ (3.6 g CH₄-C ha⁻¹ d⁻¹ ) from all the treatments.

No significant negative relationship between surface N₂O and CH₄ gas fluxes was observed in any of the experiments. However, the use of lysimeters in experiment 3 provided the unique opportunity to quantify below-ground biogenic N₂O production (Y) and relate this to below-ground methanotrophic activity (X). Highly significant negative relationships were obtained between ln(Y) and (X) across all treatments (r = -0.52 to -0.82, P≤ 0.001).

In expt. 3, all treatments (including NT) showed an increase in below-ground N₂O gas-phase concentrations immediately following both the summer and winter N applications. Concentrations were generally higher during winter than in summer, mainly in response to high WFPS and low temperature, both of which reduce gas diffusion and increase N₂O solubility in water. Gas-phase N₂O concentrations at 15 cm depth were highly correlated with measured N₂O surface gas fluxes (r² = 0.83, P≤ 0.001). Multiple linear regression using additional soil parameters yielded the equation: F = -7.97 + 0.0085 (C) + 51.8 (θᵥ) - 0.024 (C x θᵥ) + 0.001 (C xθᵥ xT) Where: F = N₂O flux at the soil surface (µg N₂O-N m⁻² hr⁻¹), C = concentration of gas at 15 cm soil depth (µg m⁻³), T = temperature at 15 cm soil depth (K), θᵥ = volumetric water content at 15 cm soil depth (m³ m⁻³ soil). (r² = 0.94, P≤ 0.001, n=162).

On any particular sampling occasion, N₂O below-ground gas-phase concentrations typically increased with depth. These below-ground concentration gradients (0-15 cm depth) were used to calculate N₂O surface fluxes with a Fick's equation of the form: (equation), where: F = µgN₂O-N m⁻²hr⁻¹, [N₂O] g = gasphase N₂O concentration (µg N₂0-N m⁻³ ), Z = soil depth (m), Dair = diffusion coefficient of N₂O in air (m²hr⁻¹) , Ea= air filled porosity (m³ air m⁻³ soil ) and Et = total soil porosity(m³ voids m⁻³ soil), ‘a’= 0.05 and 'b'= 1.33. There was excellent agreement (r² = 0.87, P≤ 0.001) between the measured N₂O fluxes and those calculated using Fick's law. This work indicates that knowledge of the N₂O gas concentration at 15 cm depth, the soil moisture content, and the temperature should be sufficient to allow surface gas fluxes to be calculated. This may prove easier than taking direct surface flux measurements using enclosures and could form the basis of a new procedure for the integration of N₂O fluxes over large areas.