MADISON, WI, July 19th, 2010 – Soil scientists studying bacterial communities in hardwood forests have found evidence that extra human-derived nitrogen deposited from the atmosphere can change the composition of the soil microbial community, with implications for carbon cycling and sequestration.
Don Zak and Sarah Eisenlord from the University of Michigan conducted a study on the response the soil bacterial community to levels of nitrogen accumulation expected by mid-century. They used molecular techniques to quantify the abundance of actinobacteria, a microbe involved in plant litter decomposition, and compared the differences in bacteria species found in experimental versus normal conditions. The results are reported in the July-August 2010 edition of the Soil Science Society of America Journal, published by the Soil Science Society of America.
Contrary to the author’s expectations, simulated atmospheric nitrogen deposition did not affect the abundance of actinobacteria in the forest floor, but did decrease total extractable DNA and gene abundance in the surface soil. This indicates that nitrogen deposition from human activities has a negative effect on soil microbial communities.
Moreover, this study identified significant and consistent changes in the type and abundance of microbes across the study’s four sites. Experimental sites contained unique groups of bacteria compared to communities under a normal nitrogen environment.
Specifically, there were decreases in a family whose members are known to degrade lignin, a plant component, and increases in a poorly understood sub-order. Another unexpected result was that a species commonly thought to be a dominant soil bacteria made up less than 4% of the experimental communities.
These changes in community compositions coincided with the slowing of litter decay and the enhanced production of dissolved organic carbon, a by-product of plant matter decomposition.
“Our observations are consistent with the idea that compositional shifts in soil microbial communities can elicit functional responses that influence the rates of soil carbon cycling,” says Sarah Eisenlord, regardless of the current limited understanding of actinobacterial ecology and physiology.
Understanding the mechanisms which alter the decay of leaf litter debris is crucial in understanding the dynamics of soil carbon storage in a changing climate. Fungi and Actinobacteria are the primary mediators in plant litter decay in the forest floor.
According to Eisenlord, the analysis of these communities has given rise to more questions about the diversity, identity, and function of microbes in forest soil ecosystems. This study uncovered a surprising diversity and distribution of un-cultured and un-characterized species, and is a call to further understand how these organisms interact with their environment to complement the advances in molecular techniques in the field.
The study sites have continuously received experimental nitrogen deposition beginning in 1994. With support from the National Science Foundation, Don Zak from the University of Michigan, Kurt Pregitzer from the University of Idaho and Andy Burton from Michigan Technological University have examined the effects of simulated atmospheric nitrogen deposition on forest carbon dynamics in northern hardwood forests dominated by sugar maple, a dominant forest type in eastern North America.