Root fungi turn rock into soil
3 July 2009
Trees help to break down barren rocks into soil, but how does that work exactly? It turns out that tiny fungi living on the trees' roots do most of the heavy work.
The fungi first bend the structure of certain minerals, weaken their crystals and then remove any useful chemical elements to pass on to their host tree. During the process, the rocks change their chemistry, lose their strength and in the long-run become soil.
These hard-working fungi are called mycorrhiza and cover the roots of trees like gloves. They are extremely small and thin, but they are everywhere: 'it is estimated that every kilogram of soil contains at least 200 km of fungi strands,' says Dr Steeve Bonneville, from the University of Leeds.
This is a significant advance on previous simplistic ideas of mineral breakdown.
Prof. Liane G. Benning,
University of Leeds
Bonneville explains: 'Mycorrhiza have a perfect business relationship with plants and especially trees.' They help the plant to get nutrients from the soil and in return they receive part of the carbon produced during photosynthesis.
About 90 per cent of tree roots in boreal forests have this symbiotic association with mycorrhiza.
Mycorrhiza play a major role in soil formation, but how do they do it? 'We created the first experiment that closely copies a natural system to find out how mycorrhiza help to break down minerals,' says Professor Liane G. Benning, the Leeds principal investigator of the project.
Scots Pine seedling growing along a biotite (Bt) flake.
Together with colleagues at Sheffield the team planted a Scots pine seedling with the fungi Paxillus involutus, a mycorrhiza species. 'This is a very common tree-fungi association that occurs naturally in boreal forests,' says Bonneville. The tree and fungi were allowed to grow together for about 10 weeks and were then placed in a transparent pot with flakes of biotite, a common rock-forming mineral rich in potassium, iron and magnesium.
The seedling's roots became covered with fungi, which soon attached to the biotite. After three months, the scientists removed the biotite from the experiment and sampled the crystal along a single strand of fungi-covered root from the tip, middle and close to the root.
Bend first, steal later
'The first change we observed in the biotite, at the tip of the mycorrhiza, was mechanical stress,' says Benning. The fungi can apply a pressure onto the minerals that can be as high as the pressure in an average car tyre. This pressure value is 'very high', for a tiny organism, but unsurprising to Bonneville: 'these fungi evolved to penetrate minerals and rocks and some species are capable of even higher pressures.'
As a consequence of the pressure at the tip, the biotite starts to bend and to lose its strength. 'Once the crystal structure is weakened, the chemical changes start,' explains Benning. The mycorrhiza then proceeds to remove the potassium and other useful nutrients from the biotite, passing them on to the roots and ultimately the tree. Without potassium, the biotite breaks down into vermiculite and ferrihydrate, two minerals common in soils.
The mechanism - bend the structure first, steal nutrients later - is an efficient way for the fungi to breakdown minerals and at the same time gather essential nutrients for its host tree, write the authors on the report, published in July's edition of the journal Geology.
'This is a significant advance on previous simplistic ideas of mineral breakdown,' says Benning.
S. Bonneville, M.M. Smits, A. Brown, J. Harrington, J.R. Leake, R. Brydson and L.G. Benning. Plant-driven fungal weathering: Early stages of mineral alteration at the nanometer scale Geology July 2009, v. 37, p. 615-618, doi:10.1130/G25699A.1
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