July 18, 2010 The nitrogen cycle is the natural process that makes nitrogen available to all organisms on earth. Scientists at the University of York have discovered that one of the world's most common and ecologically important groups of fungi plays an unsuspected role in this key natural cycle.
Almost all plants form symbiosis with fungi in their roots, know as mycorrhizas. The commonest type of mycorrhiza is called the arbuscular mycorrhiza (AM) and involves two-thirds of plant species. Unlike most fungi, the AM fungi get their supply of sugars for energy and growth from their plant partner and not from the decomposition of organic matter. . Surprisingly, the researchers found that AM fungi thrive on decomposing organic matter and obtain large amounts of nitrogen from it. The fungus itself is much richer in N than plant roots, and calculations suggest that there is as much nitrogen in AM fungi globally as in roots. Since fungal hyphae (the threads of which the fungus is composed) are much shorter-lived than roots, this finding has implications for the speed with which nitrogen cycles in ecosystems,
The research, by Dr Angela Hodge and Professor Alastair Fitter in the Department of Biology at York was funded by the Biotechnology and Biological Sciences Research Council, is published in the latest issue of Proceedings of the National Academy of Sciences (PNAS).
Because these fungi cannot be grown in pure culture, the researchers created microcosms to isolate the fungi from plant roots and to allow them access to a patch of organic matter, and used stable isotopes to track the movement of nitrogen and carbon. Fungi that exploited decomposing organic matter were also better able to colonize a new plant. In addition, reducing the carbon supply to the fungus by shading the host plant did not diminish he fungal growth in the organic matter. Dr Hodge said: "We have known for a long time that these fungi play a central role in the phosphorus cycle; now it seems that they are equally important in the nitrogen cycle, opening the possibility of exploiting them in the development of more sustainable forms of agriculture. "
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- Hodge et al. Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling. Proceedings of the National Academy of Sciences, 2010; DOI: 10.1073/pnas.1005874107
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