The biophysical regulation of methane oxidation by a Nothofagus forest soil
Authors
Date
2001
Type
Theses / Dissertations
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Fields of Research
Abstract
Methane (CH₄) is an important greenhouse gas (GHG) whose tropospheric concentration has risen from approximately [0.80µℓℓ⁻¹] prior to the industrial revolution, to the present concentration of
[1.72 µℓℓ⁻¹.] New Zealand has a unique anthropogenic GHG emission profile, with CH₄ the largest component (45%) on a CO₂ equivalent basis. Enteric fermentation from the sheep and cattle populations is the major source of this CH₄.
To refine New Zealand's net CH₄ emissions, an estimate of potential terrestrial sink activity is required. Methanotrophic bacteria are responsible for the uptake of CH₄ by soil, oxidising CH₄ to CO₂. Soils known to oxidise CH₄ rapidly are those found in undisturbed, low mineral- N ecosystems such as native forests. Southern beech (Nothofagus) forest represents a large proportion of the indigenous forest cover in New Zealand. This study investigated the CH₄ sink activity of such a forest soil (USDA classification, Andic Dystrochrept), located in the Craigieburn Range (914m altitude), W of Christchurch, New Zealand. The effects of various physical and chemical variables, including soil water content, temperature, pH, mineral-N and CH₄ concentration on CH₄ oxidation rates, were also considered.
Methane oxidation rates (FCH₄) were measured once each month for 17 months, using chambers placed on bases permanently positioned at 18 separate locations in a 60 x 60 m study area. Soil water content (0 - 300mm by Time Domain Reflectometry), and soil mineral-N were also measured monthly, while soil temperature was logged half hourly. Soil atmosphere samples were also taken at a range of depths monthly from 0 - 200 mm. In parallel with the field measurements, laboratory incubation studies were performed separately under controlled conditions to examine the influence of the various parameters described above. Soil from 50 - 100 rum depth was identified as the major zone of methanotrophic activity and was subsequently used in these incubation studies. All gas samples were analysed for CH₄ on a gas chromatograph (GC) using a flame ionisation detector (FID).
The main source of CH₄ for the methanotrophs at Craigieburn is the atmosphere and the beech forest soil was, accordingly, always a sink for CH₄. FCH₄ averaged 90 ± 7 µg CH₄-C m⁻² h⁻¹ (1 SEM), which is high when compared with most beech forests (Genus Fagus which are deciduous) in the Northern Hemisphere. FCH₄ was substrate limited as a consequence of being very strongly regulated by soil water content (0-300mm),(r² = 0.88). The greatest FCH₄ (144 µg CH₄-C m⁻² h⁻¹) was measured when the soil was dry and warm. This maximum, field- measured rate was much less than that under ideal conditions in the laboratory. Further, the laboratory maximum rate, increased with increasing atmospheric (headspace) CH₄ concentration. These results show how FCH₄ was substrate limited. Surface CH₄ fluxes calculated using Fick's law (ie using standard tortuosity coefficients of a = 0.9 and b = 2.3 for DCH₄soil plus CH₄ concentration gradients within the soil) elucidated the role of soil water content in regulating gas movement through soil and substrate supply to the methanotrophs. This analysis also showed that the chambers did not affect FCH₄ measurements. Altering the tortuosity coefficients showed how important the pore size distribution is to deriving fluxes which agree with surface fluxes. No net in situ CH₄ (or N₂O) emissions were measured over the course of the study.
Methanotrophic activity in soil stored at 4°C did not change significantly for periods of up to 6 months. The kinetics of CH₄ oxidation was shown to be fust order with respect to CH₄ with values for the apparent Km (substrate concentration at half the maximal rate) and Vmax (maximum MOL when saturated with substrate), of 17.35 µℓℓ⁻¹ and 0.122 µg CH₄-C g⁻¹ h⁻¹ respectively. The laboratory studies confumed that the methanotrophs were highly responsive to changes in soil water content over the range 0.16 - 0.35 m³m⁻³, commonly encountered in the field. Increases in temperature between 5 and 12°C also caused a corresponding increase in methanotrophic activity and yielded an Arrhenius activation energy of 90.2 kJ mol⁻¹. Both increasing and decreasing the natural soil pH (4.4) caused the CH₄ oxidation rate to decline. Reductions in methanotrophic activity were also induced by the addition of common atmospheric pollutants or agricultural fertilisers, although in this soil the effects were short lived. Specific anionic and cationic inhibitory effects were observed in the order NO₂⁻ > Cl⁻ ≈ NO₃⁻ > NH₄⁺ > Na⁺ > K⁺, > SO₄²⁻. Bicarbonate (HCO₃⁻) appeared to show no specific inhibitory effect.
The optimum water content and temperature for soil CH₄ oxidation were 0.15 m³m⁻³ and 19.3°C respectively. Knowledge of these optimum values and the CH₄ oxidation rates for the range of soil water contents and temperatures studied, allowed the development of a Multiplicative Methane Oxidation model (MMOx model) of the form: FCH₄ = A X CH₄max x {ƒθ x ƒT x ƒOLA X ƒD}. The 'ƒx' coefficients are dimensionless numbers taking values between 0 and 1, which operate on the maximum CH₄ oxidation rate measured in vitro (CH₄max). Measured and modelled FCH₄ gave a strong linear relationship (r² = 0.82) with a slope of 1.08, but a non- zero intercept, 'c' of -57 µg CH₄-C m⁻² h⁻¹. An empirical modification of this MMOx (I) model was the addition of a +c term. This term was defined as the absolute value of the offset in MMOx (I) and its incorporation into the model gave MMOx II: F CH₄= A X CH₄max x {ƒθ x ƒT x ƒOLA x ƒD} + c. A re-interpretation of the ƒθ and ƒT functions better linking the field and laboratory results and physical changes of the soil (ƒD) using the equation, FCH₄mod = F CH₄max (modelled) x {ƒθ x ƒT x ƒOLA (MMOx III), gave a potential explanation for the large negative intercept term seen in the original model. The alternative relation between FCH₄mod and F CH₄meas had a much smaller intercept and it takes the form FCH₄mod = 0.756FCH₄meas - 6.51, r² = 0.76. Despite this improvement more work is needed to understand how microbial activity changes when the soil is excavated.
In addition, a water balance model (WBM) was developed and used to generate soil moisture content values to retrospectively predict FCH₄ for the 1990's decade. Annual FCH₄ varied little (6.5± 0.7 kg CH₄-C ha⁻¹ y⁻¹) but these were much higher than FCH₄ in Northern Hemisphere forest ecosystems (1.2 kg CH₄-C ha⁻¹ y⁻¹). The MMOx model (especially versions II and III) were validated by incorporating modelled soil water content data for both the sampling day and on a monthly basis. The MMOx II model provided the closest estimate of FCH₄ in comparison to the measured FCH₄ data. The annual native forest soil CH₄ sink was estimated as 68 Gg. This offsets -4.5% of CH₄ emissions from New Zealand's 55 million ruminant animals.
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