7 June 2016 Wee puzzle for soil researcher

Unexpected and conflicting results from research around irrigation practices and their influence on greenhouse gas emissions have Lincoln University PhD student Jen Owens contemplating a more complex scenario and a few possible explanations.

Ms Owens carried out a field experiment on a commercial dairy farm measuring nitrous oxide emissions and soil oxygen concentrations under urine patches subjected to varying rates of irrigation. Her studies are integrated with a three year research programme by Landcare Research into greenhouse gas emissions from intensive dairying.

Nitrous oxide is a potent greenhouse gas linked to climate change. Most of New Zealand’s nitrous oxide emissions come from grazed pastures where soil is affected by nitrogen inputs such as ruminant urine. Nitrous oxide emissions from urine patches are a problem for New Zealand given the intensification of dairying.

“While many dairy farms are using irrigation to improve pasture production to meet the nutritional needs of their cows, little is known about what happens to the nitrous oxide emissions from urine patches when they are irrigated,” says Ms Owens.

Ms Owen’s field experiment explored the possibility that irrigation may modify soil oxygen concentrations, potentially reducing nitrous oxide emissions from urine patches. She found that soil oxygen concentrations under a more frequently irrigated urine patch were lower when compared to a less frequent irrigation regime.

These lower soil oxygen concentrations were expected to lead to lower nitrous oxide emissions. However, this didn’t happen, suggesting a more complex scenario.

“Nitrous oxide emissions were not affected by irrigation frequency but we were surprised to find the more frequent irrigation rate lead to potentially greater nitrous oxide uptake by microbes. These two results appear to conflict with each other.

A possible explanation for this is that the potential for the production of nitrous oxide has increased in response to lower soil oxygen, as well as the potential for soil microbes to consume nitrous oxide. So in effect we are measuring the net effect of these two processes.”

Based on the results from the study, funded by Landcare Research, Ms Owens has performed a number of smaller experiments under controlled conditions to better understand how soil biology is driven by the soil’s physical status. “These experiments have helped us interpret how irrigation influences nitrous oxide emissions from urine patches.

“We’ve also started looking at urine patch induced nitrous oxide emissions on other soil types and during different times of the year, winter instead of summer, to validate and further explore the impact of soil oxygen concentrations.”