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The flow of carbon through soil is ten times greater than the amount of carbon moved around by the burning of fossil fuel but until now how this happens was at best poorly understood. Soil was almost literally a black box to scientists interested in carbon. Now researchers at the University of Warwick have been able to shed light in that black box by getting a particular class of insects to expose the key underground carbon traffic system - by eating it.
The University of Warwick team worked with researchers from Aberdeen, Lancaster and Sheffield, to try and establish if plant associated fungi - arbulscar mycorrhizal (AM) fungi - found on the roots of 80% of all land plants had any role in the movement of atmospheric carbon to soil (fixed by plants in the form of CO2). AM fungi produce filaments that spread widely throughout the soil (sometimes referred to as the mycorrhizosphere) and they are known to be important for effective uptake by plants of water and phosphates but they were not known to play any role in the movement of carbon through the soil.
The researchers developed novel soil cores that were engineered with openings covered by nylon mesh with tiny pores just big enough to allow AM mycelia to grow into them but too small for any insects or other micro-fauna (including Collembola, soil mites) to get into the cores. The cores were then filled with soil which was frozen -80oC to kill any other insects/microfauna and inserted into experimental grassland to enable colonization by AM fungi from the surrounding plants. Twenty mites from the order Collembola, which would view the AM mycelia as food stuff, were introduced to half of the cores. After another four weeks the grassland was exposed to a particular form of carbon dioxide (a stable isotope of carbon, 13C) for 7 hours, a technique called pulse labelling. Concentration of 13C in cores was then analysed. The soil cores which were exposed to the mites were found to have 32% less 13C than the control cores. This showed that Collembola's consumption of the arbulscar mycorrhizal mycelia had disrupted a key pathway transporting carbon from plants to soil.
As a final check the researchers examined both the cores with and without Collembola for a particular phospholipid fatty acid (PLFA) that is characteristic for AM mycelia. They found that this particular PLFA contained significant amounts of 13C in cores not exposed to Collembola. However those soil cores that were exposed to collembola which fed on the mycorrhizal mycelia did not have 13C enriched PFLAs..
This research establishes that arbuscular mycorrhizal mycelia provide a major highway in terms of transporting carbon from plants to soil. This new understanding of how both mycorrhizal mycelia and the insect population of soil impact on the transport of carbon will assist researchers trying to understand what preserves a healthy soil and provides recycled carbon for supporting below ground biodiversity. It will also open up a new understanding of the food-webs and nutrient flow in soil which is fundamental to sustainable agriculture.
Note for editors: The research paper Soil Invertebrates Disrupt Carbon Flow Through Fungal Neworks is in Science vol 309, 2005, p. 1047
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