Jan 6, 2009
Faster nitrogen cycling may alter vegetation
One of the lesser known consequences of global warming is an increased rate of nitrogen cycling. New research shows that this imbalance may cause a dramatic change in the vegetation of sub-Arctic environments, with deciduous trees such as birch replacing evergreen trees.
In some areas this change is already visible. "We have seen this in Alaska over the last 30 years, where there are lots more deciduous trees and fewer evergreen," Rien Aerts, from the Free University of Amsterdam, told environmentalresearchweb. "I'm convinced that this is due to warming."
And it isn't just Alaska that is likely to be affected. Rising temperatures could radically change the face of all sub-Arctic landscapes.
Currently, low temperatures mean that soil nutrients are mineralized by bacteria and fungi at a slow rate. However, as temperatures rise bacteria and fungi are likely to become more active and mineralization rates will increase. This will lead to more ammonia and nitrate being released from soil organic matter and becoming available for plants.
To test the implications of this extra nitrogen availability in sub-Arctic ecosystems, Aerts has been deliberately increasing nitrogen levels in small plots of soil in Stordalen mire, northern Sweden.
In 1998 Aerts marked out 16 plots, each measuring 1.5 metres by 1.5 metres. Every plot contained a mix of evergreen shrubs, deciduous bushy trees and grasses. Eight of the plots acted as controls and to the other eight he added large amounts of nitrogen fertilizer every year.
This increase in soil nitrogen levels mimics the effect of future climate change, whereby plants start to absorb more nitrogen from the soil and return it at increasing rates via decomposing leaf litter. However, in his experiment Aerts speeded things up, adding one decade's worth of nitrogen every year.
To measure the impact he took samples from the plots every year, measuring nitrogen levels in both the leaves of the plants, as well as in the soil. In addition he kept a careful eye on the diversity of plant species in each plot, and the ratio of evergreens to grasses and deciduous trees.
Aerts found that the increase in nitrogen in the soil made the vegetation grow faster; but it wasn't a uniform speed-up.
"Deciduous trees such as birches responded fastest, and then the grasses, while the evergreens responded slowly," he said. Over time this meant that the evergreens began to be out-competed by the deciduous trees.
Ironically the change in vegetation could help to reverse the effects of global warming. The faster rate of vegetation growth and increase in deciduous trees pulls down more carbon dioxide from the atmosphere, helping to reduce the greenhouse effect.
But in a separate study Aerts has shown that any benefit will be wiped out by faster decomposition of peat bogs – also caused by rising temperatures and greater bacterial and fungal activity. "Some of the peat bogs are very deep and will release large amounts of carbon dioxide," he said.
The results are reported in Polar Biology.
About the author
Kate Ravilious is a contributing editor to environmentalresearchweb.