Jan 13, 2010
Keeping tabs on nitrogen
The presence of nitrogen is essential for land to act as a carbon sink. But the element can be lost from soil by leaching and by the actions of denitrifying bacteria. To date it's been unclear how much of a role these bacteria play. Now researchers from the University of California Davis, US, have found that soil denitrification is responsible for around one-third of the nitrogen lost from unmanaged land.
"This is the first study to provide a defensible approach to quantifying the role of bacteria in stripping soils of available nitrogen," researcher Ben Houlton told environmentalresearchweb. "Nitrogen is necessary for all life; it's a major component of amino acids, proteins and the DNA molecule. Understanding how much nitrogen is available to support life – and ecosystem services such as carbon dioxide sequestration – is therefore fundamental to understanding the living earth system as a whole."
Houlton says his findings confirm the prediction that bacteria consume a significant amount of the nitrogen that supports the fertility of ecosystems. The team estimates that roughly 28 Tg of nitrogen is lost each year by nitrogen emission from soil.
"A key implication of our work is that the capacity of ecosystems to pull out and store additional carbon dioxide from the atmosphere will be limited by nitrogen availability and the loss of this key nutrient from the soil," he said. "In this way, bacterial gaseous nitrogen production is likely to affect the pace and magnitude of climate change."
At present vegetation on land absorbs roughly one-third of human carbon dioxide emissions.
To carry out the investigation Houlton and colleague Edith Bai devised a new technique involving bacterial fingerprinting, in which "bacterial preferences for the light nitrogen isotopes (atomic mass 14) vs. the heavy one (atomic mass 15) are taken as evidence for their effect on gaseous nitrogen production". Without this method it's hard to measure how much nitrogen bacteria produce against the high background concentration in the atmosphere – 78% of which is nitrogen.
"Using a simple model and taking advantage of this bacterial bio-signature (i.e. the nitrogen isotope abundance of land), we estimate that one-third of all of the nitrogen that enters the natural land environment is released back to the atmosphere every year," said Houlton.
Now the team is analysing different global ecosystems to investigate how gaseous nitrogen production by bacteria varies from habitat to habitat. "We are also working to understand precisely how these bacteria impart their signature on nitrogen isotopes, so that we can develop even better estimates of this process in the future" said Houlton.
The researchers reported their work in PNAS.
About the author
Liz Kalaugher is editor of environmentalresearchweb.