Regulation of methane and nitrous oxide emissions from two contrasting natural wetland sites

Abstract

Natural wetlands are the major source of atmospheric methane (CH₄) and are less important producers of nitrous oxide (N₂O). However, the role of wetlands in the global CH₄ and N₂O cycles is not clearly understood due, in part, to an incomplete understanding of the processes controlling the emission, production and consumption of these gases. The present study investigated the environmental regulation of CH₄ and N₂O emission from a slightly brackish (low to medium sulphate) wetland site and a saline wetland (high sulphate concentrations) site on the margins of Te Waihora (Lake Ellesmere), Canterbury, New Zealand.

A static chamber method was used to determine field rates of CH₄ and N₂O emission. Emission rates of CH₄ were found to be weakly related to water table positions, and less directly, to temperature. Rates of N₂O emission at high water table positions ranged from 0 to 152 mg CH₄ m⁻² d⁻¹ (average 22.4 mg CH4 m⁻² d⁻¹) were comparable to an estimate for New Zealand wetlands based on overseas studies. However no CH₄ emissions were detected during a protracted period of decreased soil moisture, high soil redox potential and low water table positions. N₂O emissions were sporadic (observed on only 5 out of 13 sampling occasions) but when detected, fluxes of N₂O ranged from 117 -1132 µg N₂O-N m⁻² d⁻¹ (1.2 -11.3 g N₂O-N ha⁻¹ d⁻¹). These rates were lower values measured overseas from organic, intensively farmed agricultural soil and wetlands prior to waterlogging. Incidences of positive N₂O flux were associated with low or fluctuating water table positions.

The vertical distribution of, and seasonal changes in, the rates of the principal microbial processes which produce CH₄ and N₂O were measured in laboratory studies on six occasions in the latter part of the study. Anaerobic slurries were used to determine rates of methanogenesis and denitrification (acetylene was added to the headspace in the case of the latter to inhibit reduction of N₂O to dinitrogen). Rates of nitrification were measured by the monitoring of nitrate levels in an aerobic soil slurry. Methanogenic activity was greatest in the most organic layer of the soil (0 – 6 cm depth) and declined markedly with depth. High rates of activity were present in the soil well after field emissions of CH₄ had ceased. High sulphate concentrations (225 - 14801 mg L⁻¹) at the saline site failed to completely suppress CH₄ emissions and production although a negative relationship between sulphate and methanogenesis was apparent (Spearman's ρ = -0.406, p<0.01). Addition of an important methanogenic substrate (acetate) enhanced activity by a factor of 4.4, a more dramatic response than measured elsewhere. A positive correlation between DOC and methanogenic activity (Spearman's ρ = 0.413, P<0.01) further supported the finding that activity was substrate limited.

In contrast, the vertical distribution of denitrifying activity was not consistent. However despite high water soluble organic carbon (DOC) and KCl-extractable nitrate concentrations in the soil, nitrate-amended denitrifying activity was nevertheless weakly correlated with both these variables (together accounting for 20 % of the variability in nitrate-amended denitrifying activity). Furthermore, a dramatic response to added nitrate indicated the importance of electron donor and electron acceptor availability at the cellular level.

Similarly, nitrification rates appeared to be limited by substrate despite high levels of soil KCl-extractable ammonium. The positive correlation between NH₄⁺-amended nitrifying activity and soil nitrate concentrations (Spearman's ρ = 0.382, p<0.05) indicated the importance of this process in the supply of nitrate.

Surprisingly, the in situ CH₄ oxidation activity (determined by the rate of conversion of added ¹⁴CH₄ to ¹⁴CO₂ in the headspace of a flux chamber) was higher on a flooded occasion (0.39 µMol CH₄ m⁻² h⁻¹) than on a non-flooded occasion (0.011 µMol CH₄ m⁻² h⁻¹). It is proposed that CH₄ oxidation was suppressed either through an absence of CH₄ emissions or through inhibition by high ammonium concentrations.

The study indicated that the potential for high rates of N₂O and CH₄ production that were present in the soil did not necessarily lead to high emission rates in the field. N₂O emission appeared to be regulated by diffusional constraints which affect the supply of substrate to nitrifiers and denitrifiers and the extent to which N₂O can escape consumption in the soil. CH₄ emission is regulated both by the interacting factors of substrate availability and water table positions. However, the influence of sulphate concentration on methanogenic activity was smaller than expected.

The complex effects of water table position and differences in soil chemistry on CH₄ and N₂O emissions were the major factors regulating CH₄ and N₂O emissions. Factors affecting the production of CH₄ appeared to be more important than the factors which influence CH₄ emission after production (eg CH₄ oxidation).

At times of high water table positions, CH₄ emissions were highest, but N₂O was not emitted regardless of the rates of nitrification and denitrification. The limited observations of N₂O emission suggested that the converse was also true (despite high methanogenic activity). This potentially inverse relationship of CH₄ and N₂O emissions has major ramifications for (i) the estimation of greenhouse gas emissions from natural wetlands and (ii) possible mitigation strategies to reduce CH₄ emissions, particularly in light of the higher global warming potential of N₂O. A more highly resolved investigation of the relationship between CH₄ and N₂O emissions, soil chemical parameters (particularly sulphate) and water table positions would further develop the understanding of CH₄ and N₂O emission.