A COMPETITIVE advantage of biological and organic farming methods, their inherent ability to harness the capabilities of arbuscular mycorrhiza (AM) fungi, may be even more important than has been thought.
AM have long been known to be oustanding phosphorus scavengers. The form most relevant to croppers, vesicular-arbuscular mycorrhizae (VAM), are known to be able to transport phosphorus to cereal crop plants from over a much wider area than the crop’s roots alone can reach.
Recent work out of the United Kingdom now suggests AM are also remarkable scavengers of nitrogen from soil organic matter, to the point that researchers at the University of York surmise that globally, there is as much nitrogen contained in these fungi as there is in plant roots.
Various biological farming methods, and a general principle of “minimise harm”, inherent in the biological and organic systems, have been devised to support VAM.
According to work by Angela Hodge (pictured) and Alastair Fitter at the University of York, that might be even more important than is thought.
The researchers found the AM they studied had the remarkable ability to colonise patches of organic matter beyond the plant root zone, strip the organic matter of nitrogen (N), and transfer it back to the plant.
“Up to one-third of the N in such organic ‘patches’ was captured by the AM and transferred to the plant, so about 20 per cent of the plant N likely came from the patch,” Dr Hodge said.
“This is a high figure and about as well as plants do when they have access to such patches via their roots. So the AM fungi can have a significant impact on plant nutrition. Obviously we now need to understand the controls on this transfer.”
It’s not all selflessness on the fungi’s part. Dr Hodge’s team also found that if nitrogen was limited in the soil, the AM could appropriate most of the nutrient for itself.
“The fungus itself has a large N need that has previously been overlooked, so there may well be competition between the plant and the fungus when N is limiting, but transfer of N to the plant when the fungus’ own need is satisfied,” she said.
Although more work remains, Dr Hodge speculates the plant-fungi relationship will work best in soils relatively high in organic matter.
“AM fungi need and acquire substantial amounts of N,” she said.
“That implies they will do best in N-rich soils. However, adding inorganic N to soil has been shown to reduce colonisation of roots.
“It may well be that under these circumstances, the plant is competitive for N and gets more than the fungus, reducing both fungal N capture and the fungus’ ability to trade N for C (carbon).”
When nitrogen is present largely in organic form, however, the fungus appears to be very effective at capturing that N from decomposing organic matter and then cycling it rapidly into the soil.
“This might imply that AM fungi will be especially common in soils with large amounts of organic N and C. Whether they promote organic N and C build-up in soils is too hard to say yet.”
Dr Hodge also warned there were many forms of AM fungi.
“Our data also indicates there are large differences between the AM fungi in the amount of N they need, so it may also depend on which fungi the plant is associated with,” she said.
“However, we also know that AM hyphal turnover can be rapid, so as the hyphae die and turnover, they will release N back into the soil for plant-fungal capture.”
The Hodge and Fitter paper, “Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling”, was published by the Proceedings of the National Academy of Sciences (PNAS).
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