It has long been known when soil nitrogen is in short supply, mycorrhizal fungi (those living symbiotically on the roots of plants) transfer nutrients to their host plants in exchange for plant sugars derived from photosynthesis, but the rates of transfer have never been quantified in the field. John and Erik Hobbie's study, published in the April 2006 issue of the journal Ecology, quantifies the role of mycorrhizal fungi in nitrogen cycling for the first time through measurements of the natural abundance of nitrogen isotopes in soils, mushrooms and plants. The researchers tested their technique using data from the Arctic LTER (Long Term Ecological Research) site near Toolik Lake, Alaska, in the northern foothills of the Brooks Range.
Previous research has found that when mycorrhizal fungi in the soil take up nitrogen from the soil and transfer it to small trees and shrubs, the heavy nitrogen isotope, nitrogen-15, is reduced in abundance in the plants and enriched in the fungi. Using a mass balance approach, an accounting of material entering and leaving a system, the researchers quantified the transfer of nitrogen and found that 61-86% of the nitrogen in plants at the site entered through fungal symbionts,
"Previous studies at this Arctic site have found a large range of nitrogen isotope content in plants and attributed the range to plants tapping into several different sources of nitrogen in the soil," says John Hobbie. "Our study indicates that the differences can be attributed mainly to the presence or absence of symbiotic mycorrhizal fungi."
The researcher's new technique is shedding light not only on the nitrogen cycle in arctic tundra ecosystems, but can be applied to other nitrogen-poor ecosystems. "In the future, studies of plant nitrogen supply in all nitrogen-poor ecosystems must include these important transfers between plants and fungi," says Hobbie.