What was done
In a study of four Swedish forests where humus has been accumulating in the soil for up to 3000 years, the authors note that since nitrogen is nearly always the limiting nutrient for vegetation growth in these ecosystems, and if their rates of denitrification are low, as they typically are, "then very little nitrogen should be removed from these forests through leaching or runoff, and we should be able to 'find' in a careful inventory nearly all of the nitrogen that the forests had accumulated since the last major fire."  Consequently, they describe how they "used measured and calculated litter fall data, calculated limit values for litter decomposition, and the nitrogen concentration at the limit value" to attempt such a feat, which attempt, if successful, would allow them to "say with some confidence that nitrogen accumulation drives carbon accumulation in these boreal forests, and that both N and C may be retained within the system over a long time period."

What was learned
Comparing the nitrogen accumulated in the forests at maturity to that calculated from a nitrogen budget based on estimated rates of nitrogen input, N2 fixation, denitrification and leaching to the mineral soil, Berg and Dise obtained "a good agreement of about 0.30-0.35 g N m^-2 year^-1 stored in humus and vegetation at forest maturity."  Hence, they were able to conclude that these forests are indeed "highly efficient at accumulating and storing nitrogen for thousands of years," which finding likewise suggests they are equally highly efficient at accumulating and storing carbon for thousands of years.

What it means
The results of Berg and Dise's analysis strongly suggest that earth's boreal forests can continue to accumulate carbon in their soils for literally thousands of years, well beyond the anticipated end of what we could call the Age of Fossil Fuels.  Furthermore, and in light of what we know about the ability of atmospheric CO2 enrichment to enhance the productivity of trees indefinitely (see our Editorials of 5 Mar 2003 and 9 Jun 2004), there is reason to believe that earth's boreal forests, as well as its other forests, will serve as enhanced carbon sinks well beyond the time at which we cease to emit the copious quantities of CO2 that we do currently and that we will continue to do for some time to come.  Hence, we are fortunate to have a very tenacious natural partner (earth's forests, both new and old) with a long-term commitment to helping us maintain the air's CO2 content within bounds that are not injurious to life.