Last summer a paper was published that examined mechanisms for nitrogen transport to plants by Mycorrhizal fungi.

Many had assumed that the fungus would play a modest role. The team found, however, that the fungus acts more like a four-lane highway than a two-track country road in shuttling the nitrogen into plant roots. More than a third of the total nitrogen taken up by the plants came by way of the fungus.

A new paper ups the amount, at least in some environments.

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.

Not all plants establish links with fungi, and there are different types of Mycorrhizal fungi. This one study can't be taken as an iron rule. Still, the importance of VAM in the environment is becoming ever clearer.