Fears that farm dams could be another significant contributor to agriculture's greenhouse gas emission problem appear to have been overblown according to fresh local research.
In fact, contrary to previous assumptions, irrigation storage dams on farms may be acting as sinks for carbon and nitrous oxide, and not emitting the levels of gases previously included in Australia's greenhouse gas balance sheet.
Farm dams have been assumed to make up a significant proportion of Australian agriculture's emissions based on previous global and Australian research relied on by the United Nations' Intergovernmental Panel on Climate Change (IPCC).
However, the first detailed assessment of the carbon footprint of irrigation dams tested in southern NSW showed existing emissions models had not accounted for their capacity to act as atmospheric sinks for nitrous oxide (N2O).
The earlier data had also dramatically overestimated methane (CH4) emissions from these dams.
In addition to their findings, researchers have also suggested emissions from any farm dams may be reduced by making them deeper, and therefore cooler, and by limiting the amount of nitrogen entering the water bodies.
Deakin University aquatic biogeochemist, Dr Jackie Webb, led the research team monitoring emissions from dams in the Murrumbidgee Valley.
The team wanted to test if Australian conditions delivered comparable emissions to global studies used by the IPCC which suggested small artificial water bodies were widespread emitters of carbon dioxide and methane.
"The IPCC does great work, but it's all based on data availability and there is no data from semi-arid irrigation waterbodies fed into those models," Dr Webb said.
She said data previously used in Australia's national emissions inventory was based on livestock dams in Victoria and NSW and artificial ponds in Queensland.
"Irrigation farm dams from semi-arid regions were assumed to be similar in their emissions," said Dr Webb from Deakin's Centre for Regional and Rural Futures.
The team's findings were quite different to earlier data.
It's good news for irrigation farmers that farm dams aren't emitting anywhere near the emissions of what is assumed based on the current research
- Dr Jackie Webb, Deakin University.
A quarter of the dams studied were found to be net greenhouse gas sinks for carbon dioxide equivalent emissions.
Just over half were sequestering carbon and 70pc were sinks of nitrous oxide.
"It's good news for irrigation farmers that farm dams aren't emitting anywhere near the emissions of what is assumed based on the current research," she said.
"Irrigation dams may also be worthy of their own emissions factor in IPCC guidelines, but we need to test more regions."
The study involved surveying 38 on-farm irrigation dams across horticulture and broadacre cropping properties in the Murrumbidgee Valley.
The work established a baseline carbon footprint and provided an initial insight into areas that could be targeted for emission management.
Drone and on-ground biomass surveys and continuous carbon dioxide monitoring with underwater sensors were also carried out in a subset of the dams.
Dr Webb's research team's work, funded by AgriFutures, was the first examination of irrigation dams and their emissions and storage of methane, carbon dioxide and nitrous oxide.
Most irrigation farm dams were sources of diffusive methane, which was the highest contributor to overall emissions.
However, compared with global averages, the irrigation dams tested had considerably lower methane emissions (34 kilograms a hectare per year) than the IPCC's annual emission factor (183kg/ha/) for constructed ponds.
The NSW trials also found a lower indirect nitrous oxide emission factor (0.06 per cent) compared with the IPCC default emission factor for agricultural surface waters (0.26pc).
Huge differences
"It's a huge difference - four times less emissions of nitrous oxide and five times for methane," Dr Webb said.
"The reason why the IPCC models are so out is because 70pc of the dams we sampled were atmospheric sinks of nitrous oxide, but the IPCC emission factor models do not account for agricultural water bodies being nitrous oxide sinks at all."
She said widespread nitrous oxide sinks had also been observed once before in northern hemisphere farm dams.
"To reproduce this finding in Australia is really exciting as it challenges our fundamental understanding of indirect aquatic N2O emissions from agriculture," Dr Webb said.
Given the study demonstrated the carbon footprint of these irrigation dams was smaller than expected, she said more research was needed.
Revisions of national emissions accounting should also be considered.
"When we're talking about climate change and net zero emissions, it's very important we get it right," she said.
"There's huge potential to harness these irrigation dams to cut their emissions even more and make them carbon sinks."
Dr Webb said emissions could be reduced by making dams deeper.
Excessive nutrients draining into water waterways could also lead to low oxygen conditions in dams which activated anaerobic microbes that produced methane.
Andrew Marshall
Andrew Marshall is the group agribusiness writer for ACM's state agricultural weeklies and websites. He is a former editor at The Land and has worked in various Rural Press group roles in Canberra, North Richmond (NSW) and Toowoomba (Qld).
Andrew Marshall is the group agribusiness writer for ACM's state agricultural weeklies and websites. He is a former editor at The Land and has worked in various Rural Press group roles in Canberra, North Richmond (NSW) and Toowoomba (Qld).
