Landcare Research Soil Methane Study

Methane is a potent greenhouse gas that has 33 times the global warming potential of CO2. Dr Kevin Tate (research associate with a specialist interest in methanotrophic bacteria) discovered some bacteria in NZ forest soils (methanotrophs) that are extremely efficient in converting atmospheric methane to biomass and CO2, thus dramatically reducing its potential environmental effects. Even more efficient populations of methanotrophs have been found in soil covering a landfill site near Taupo. In the lab he and his colleagues Drs Adrian Walcroft and Chris Pratt have built a methanotroph farm to determine the most effective populations of bacteria from these soils and the conditions under which they operate best.

Dr Chris Pratt has put together a small-scale system for capturing methane from a dairy farm effluent pond, directing it through a container filled with methanotrophs and measuring the methane concentration before and after the container. After six weeks it was working well, but methane emissions from the pond were at least ten times higher than expected from New Zealands national inventory, so modifications were made so that virtually no methane was detected at the exit.

The system is showing excellent promise as a tool for dramatically reducing methane emissions from farm and other effluent ponds, and possibly from landfill sites.

In 1995 New Zealand had no information on the capacity of soils to remove methane from the atmosphere. Dr Kevin Tate took samples of three New Zealand soils to Rothamsted in England for analysis and found that two had incredibly high capacities to oxidise methane to CO2. Subsequent research showed that some soils under NZ forests showed greater oxidation rates than pastoral soils, and part of the reason was that they had a quite different population of methanotrophs (bacteria that use methane as a sole carbon source and oxidise it to CO2). These bacteria are very difficult to culture, so help is being provided by molecular biologists in Aberdeen, UK and British Columbia and Canada.

Kevin and his colleagues set out to select for a population that would be superior at oxidising methane and could be used in some form of technology. They thought about putting methanotroph-filled filters in farm dairy sheds but preliminary calculations indicated that even if every dairy farm in New Zealand had one it would not make much difference to the total methane budget.

A colleague found that Canadian scientists had been doing something similar but had stopped the work. Their interest was in offsetting methane emissions from cattle housed in wintering barns, and they offered Landcare Research their equipment in return for some collaboration.

Kevins colleague Adrian Walcroft set about automating the equipment and constructing a methanotroph farm in the laboratory to explore the capability of different types of populations for consuming methane and optimise the process. The farm consists of a series of pipes, and chambers filled with brown material of different hues. Methane of known concentration is fed in continuously at the bottom and bacteria consume it as it moves up through the column. Measurements are done on the exit gases to determine how effective the methanotrophs have been in reducing methane concentration.

Dairy effluent ponds are a potent source of methane emissions. Anaerobic digestion of organic matter in the sludge at the bottom of these ponds releases biogas that bubbles to the surface and enters the atmosphere. Kevin and colleagues reasoned that if this gas were trapped it could be treated effectively using methanotrophs.

A lot of work took the team to the point where in October 2009 they commissioned the first prototype biofilter on one of the Massey dairy farm effluent ponds. It consists of 4 m cover that floats on the pond surface and is weighted at the edges so that any methane bubbling up under it is trapped. It is piped to the land where it passes through a 70L container of a soil-based material containing methanotrophs the biofilter. The gas coming off the pond is measured in a flow device developed by the team (because nothing suitable was available), and the methane concentration of the gas coming out the top is also monitored. They are currently optimising the system, and it is proving very effective with mainly CO2 in the exit gas.

Methane capture for energy use in electricity generators is believed to be economic for large herd sizes above 800. For smaller herds and for herd homes and wintering barns the biofilter technology shows promise as an effective means of reducing methane emissions.

However, it is quite a leap in thinking to consider how a full-scale on-farm system might operate. Covering a whole pond would be expensive because a special type of material is needed to contain the small methane molecules, and there is currently no financial benefit to farmers to invest in such technology. However, future requirements under the Emissions Trading Scheme, and possible future border carbon taxes in our most lucrative overseas markets, could change that.

One surprising finding from the study is that the emissions coming off the Massey dairy farm (500 cows) pond are ten times the amount expected from New Zealands national greenhouse gas inventory. The biogas is 85% methane and the current flow rate is such that, if it continues long term, using the methane for energy may prove economic. Time will tell.

Another finding is that some of the most active organisms from New Zealand soils are relatively new to science or only distantly related to the ones that have been found in the Northern Hemisphere. The actual populations in our soils and the physical conditions in some soils are unique and both contribute to the organisms activity. Many of them grow only very slowly, and so molecular biology rather than standard microbiology techniques is used to identify them. A microbiologist in Vancouver at University of Victoria is currently trying to culture the very active organisms being used in the biofilter.

Says Kevin: The concentration of methane in air is about 1.8ppm. In the lab we put 35,000ppm in the filter chamber containing the most active populations and virtually none comes out the top, so they are incredibly efficient in converting methane to carbon dioxide. We want to know what they are because they are behaving differently to the ones that I discovered in our pasture and forest soils.

Amongst other things, this project shows the value of long-term fundamental research and the contact and collaboration with scientists overseas. The ultimate driver, says Kevin, is to develop cost-effective tools that could help farmers reduce methane emissions.