Methane-eating bacteria give farmers a filter for the future
The world is looking for innovative scientific discoveries in the wake of the global agreement to lower greenhouse gas (GHG) emissions, struck in Paris earlier this year.
Methane (CH4) is the main agricultural GHG emitted from grazing animals in New Zealand. These emissions increased by 8.2 per cent between 1990 and 2012.
Effluent deposited on milking-shed floors, feed pads and stand-off pads and washed into storage ponds contributes up to 10 per cent of total dairy farm CH4 emissions. Therefore, finding a solution to this problem would make a major dent in New Zealand’s GHG emissions.
A filter made of soil or other cheap and readily available materials (compost, biochar, weathered pine bark) could hold the key. Landcare Research, in association with Massey University, has trialled a filter made of volcanic soil. Biogas (mainly CH4) produced from one of the university’s dairy effluent ponds was captured and passed through the biofilter, placed beside or floating on the pond.
The key to the success of this biofilter is the presence of a very active methanotroph population – bacteria that eat CH4. These methane-eating (oxidising) bacteria are present naturally in many New Zealand soils (pasture, forest and landfill). But the methanotrophs in the volcanic soil-based biofilters are capable of consuming the very large amounts of CH4 produced from stored effluent.
Two versions of the biofilter have been tested on a Massey University dairy farm. In the first trial, lasting over five years, the volcanic ash-based biofilter was positioned beside the effluent pond and performed efficiently, consuming most of the CH4 delivered to it with virtually no maintenance required. Over this time small additions of hydrogen sulphide in the biogas caused the pH of the biofilter to fall, but despite becoming quite acidic (pH 4.2) the biofilter continued to consume CH4.
The second version of the biofilter was tested for one year and avoided the need to cover the effluent pond to capture the biogas because it floated on the pond, allowing biogas to pass up through the biofilter, where 70–80 percent of the CH4 was oxidised.
As yet no full-scale testing of the biofilter technology has been undertaken. However, testing of cheap, readily available alternatives to the volcanic soil, such as local farm soil or biochar, have shown that these materials can also support very active methanotroph populations and perform as well, but at lower cost.
Our current CH4 capture and mitigation research, co-funded by the New Zealand Agricultural Greenhouse Gas Research Centre and conducted at Massey University in collaboration with OPUS International, is quantifying the capacity of pasture soil to mitigate the low concentrations of CH4 produced in dairy housing. This is being achieved by injecting the CH4-rich air into the soil for oxidation by methanotrophs, and then measuring the potential mitigation by these bacteria and the influence of changes in soil moisture and aeration conditions. To ensure the ‘dairy shed air’ is representative of the air in a dairy house, a suitable level of ammonia is also included in the enriched air.
Mitigation of CH4 using biofilter technology could offer the most economical and environmentally friendly option for treating emissions for an average-sized dairy farm. Methane mitigation using biofilters could also be used to treat CH4 emissions from sources where energy capture (e.g. from places like landfills that use flaring) is not possible.
Landcare Research scientist Professor Surinder Saggar said, "This technology could be used on ponds and overseas where barn farming is more prevalent. The world is searching for answers in the post-Paris climate agreement environment and the biofilter, in my opinion, has great potential."
Saggar is also heavily involved in research on improving New Zealand agriculture’s GHG Inventory. It is mandatory for New Zealand to report its annual GHG emissions to the United Nations Framework Convention on Climate Change (UNFCCC) under the UNFCCC and the Kyoto Protocol. New Zealand currently uses country-specific (EF3) values for urine and dung to estimate direct nitrous oxide (N2O) emissions from excreta. These values have largely been developed from trials on flat pastoral land. The use of the same EF3 for hill pasture with medium and steep slopes was recognised from a number of national trials (jointly conducted by AgResearch and Landcare Research) as a possible source of overestimation of N2O emissions in New Zealand.
A national series of trials funded by the Ministry for Primary Industries has measured N2O emission and estimated EF3 from beef and sheep urine and dung deposited on low, medium and steep slopes. The results of the research suggest that EF3 values for medium and steep slopes are about one-fifth and one-twentieth, respectively, of those measured on low slopes. This phenomenon is consistent with the hypothesis that under limited fertility and moisture conditions, lower excreta-nitrogen (N) inputs across rhizosphere processes respond to a relative shortage of resources and nutrients. Thus the interacting effects of soil and climatic conditions on excreta-N transformations result in a tight N cycle on medium and high slopes compared with low slope areas, and lower N2O production on the medium and steep slopes compared with low slopes.
The findings are significant. The proposed new methodology using New Zealand-specific EFs calculated from a national series of hill country experiments resulted in about 58 per cent lower N2O estimates and reduced New Zealand’s total national agricultural N2O Greenhouse Inventory estimates by around 18 per cent. These reductions correspond to between $25.9 million and $35.4 million in reduced liability for our beef, sheep and deer sector based on the current New Zealand Unit price of $15, and also improve the N2O emissions intensity for hill country grazing livestock.
NOTES ON FUNDING:Between 2014 and 2016 Landcare Research has allocated $982,321 to the biofilter research programme [FY 14: $250,333, FY 15: $250,333, FY16: $481,655]. A total of $1,157,269 was allocated to the hillslope emissions research between 2013 and 2015 [FY 13 $376,423, FY 14 $390,423, FY 15 $390,423].