Freshwater wetlands are an important global carbon regulation system, with an estimated total organic carbon storage of about 450 gigatons in vegetation and soil, and a storage time of up to a thousand years.

Although wetlands release greenhouse gases such as carbon dioxide and methane through microbial activity and ecosystem respiration, higher water levels limit oxygen from entering the soil, thereby inhibiting aerobic bacteria from decomposing organic matter. This mechanism helps balance carbon emissions and makes freshwater wetlands an important global carbon sink in their natural state.

External disturbances such as grazing can have an impact on the carbon balance of various ecosystems. Moderate grazing helps promote vegetation growth, enhance carbon sequestration capacity, and increase soil carbon storage, while reducing carbon emissions. However, overgrazing may increase carbon emissions and weaken the carbon sequestration function of freshwater wetlands.

Research Design:

The research team conducted a one-year study in the Weimera region of southeastern Australia, focusing on comparing the effects of various grazing mechanisms on carbon storage (plant biomass and soil carbon) and seasonal carbon flux in two freshwater wetlands with abundant rainfall and seasonal flooding (Kealy Farm and Smith Farm).

Three grazing mechanisms were set up in the experiment:

Continuous grazing: sheep are present all year round

Short term concentrated grazing: grazing for only two weeks in spring

Isolation zone for grazing prohibition

Two experimental fields are located on overgrazed sheep farms, divided into different grazing areas by fences, known as "short-term concentrated grazing areas" and "continuous grazing areas". The white square represents the isolated experimental area where sheep are prohibited from entering, while the black square represents the open experimental area where sheep are allowed to enter freely.

Research methods:

The study used a portable PVC circular dark box, partially buried in soil (1-2 cm), and combined with an ultra portable greenhouse gas analyzer (UGGA) to measure the flux of carbon dioxide and methane on site. Each observation point is covered with wetland soil covered with vegetation to reflect the overall carbon exchange situation of the wetland.

OA-ICOS GLA132-GGA Ultra Portable Greenhouse Gas Analyzer "UGGA"

The air inside the box is maintained in circulation by a small fan driven by a 12 volt battery. The top of the box is covered with a PVC lid and connected to UGGA in a closed loop through a transparent tube. The analyzer records the concentration of CH ₄ and CO ₂ inside the box every 5-10 minutes. After each sampling, remove the box cover to restore natural light and gas exchange inside. All flux data are the average of multiple measurements taken day and night.

The study also conducted two consecutive days of measurements at each wetland observation point in different seasons (periods with significant differences in temperature, precipitation, and vegetation growth) to capture seasonal variations in carbon emissions.

Research has found that:

1. CO ₂ flux variation

Seasonal factors (temperature, precipitation, and water level fluctuations) are the main reasons for the differences in carbon dioxide flux between the two types of wetlands and various regions. In addition to grazing methods and seasonal influences, there is also a significant difference between day and night: CO ₂ emissions at night are 31% higher than during the day.

Seasonal variation of CO ₂ flux in isolated and open experimental areas in two wetland ecosystems studied (Kealy). The gray bar chart represents the isolation test area, and the white bar chart represents the grazing test area.

Observations show that once grazing is prohibited in wetlands, carbon dioxide emissions can be reduced by about 35%, indicating that grazing bans help maintain wetland carbon storage and enhance carbon sequestration potential.

2. CH ₄ emission characteristics

During the entire monitoring period, methane emissions were generally low and closely related to changes in water levels. For example, in the summer of 2017, methane emissions reached their peak due to high water levels; During the subsequent period of water level decline, almost no CH ₄ release was detected.

Changes in CH ₄ flux and water depth in various wetland ecosystems and different seasons. During the isolated storm event at Kealy Wetland in early summer 2017, methane emissions peaked under high water level conditions.

3. Biomass and soil carbon

Research has shown that regardless of whether it is continuous grazing or short-term concentrated grazing, the aboveground biomass in the isolation zone is 6.4% higher than that in the grazing zone. At the same time, the soil carbon concentration in the quarantine area is 30% higher than that in the grazing area. This result consistently indicates that grazing activities reduce wetland aboveground biomass and soil carbon storage, and increase carbon emissions.

Management insights and applications:

Research has found that although short-term centralized grazing can help improve carbon storage, reduce carbon dioxide emissions, and have good investment return potential to some extent, prohibiting grazing in rain fed freshwater wetlands is still the best management strategy to increase soil and vegetation carbon storage and reduce carbon emissions.

Clarifying suitable wetland grazing methods can not only enhance carbon storage but also control carbon emissions, providing important basis for the scientific management of wetland ecosystems.

Case source:

Limpert, K.E., Carnell, P.E.&Macreadie, P.I. Reasonably managing agricultural grazing to enhance the carbon sequestration capacity of freshwater wetlands, wetland ecological management [J], 2021, 29:231-244

The article is reproduced from ABB Measurement and Analysis. Interested friends are welcome to leave a message in the background for communication!