Three ingredients for a stable and fertile soil

press release of January 26, 2009

Plant roots, mycorrhizae (fungi having a symbiotic association with roots) and earthworms are all indispensable components of a healthy soil that also ensure its stability and optimal structure. This has been shown in experiments with leek plants carried out within the framework of the National Centre of Competence in Research (NCCR) Plant Survival and published in the scientific journal Plant and Soil.

Roxane Milleret, a PhD student in the Soil and Vegetation lab at the University of Neuchâtel, is interested in the role that roots, mycorrhizal fungi and earthworms play in the belowground zone of leeks. Under the supervision of Jean-Michel Gobat and Claire Le Bayon, the young biologist assessed the separate and combined effects of these three components on soil structure and plant growth. She will present her results at the beginning of February during the Swiss Society of Pedology's annual assembly.

Concerning the first aspect, the biggest surprise comes from the effect of mycorrhizae. The close association that these fungi (Arbuscular Mycorrhiza Fungi or AMF) have with roots is well documented. On the one hand, plants transfer carbon to the fungus and on the other hand, the mycorrhizae, considered as an extension of the root network, facilitate the uptake of nutrients from the soil to the plant. Researchers have shown that important interactions between AMF and leek roots are beneficial to the soil by significantly increasing the proportion of macroaggregates (having a size > 0.25mm), which results in a more stable and fertile soil. However, contrary to what previous studies suggest, a soil enriched with only AMF has no consequences on the macroaggregates. "We think that it's mainly the combination of root exudates, the protein glomalin that is secreted by the AMF, and the ability of roots and mycelia to enmesh soil particles into larger aggregates that greatly improves soil stability", explains Roxane Milleret. As for the earthworms, they slightly decreased root colonisation by the AMF and affected the soil, mainly by destructuring the macroaggregates.   

Concerning plant growth, the earthworms and AMF had divergent effects on root biomass. The largest root mass was achieved with the presence of AMF only, followed by the combination of AMF and earthworms. A further decrease in biomass occurred when only earthworms were present and the worst case was measured in pots without any AMF and earthworms. These observations illustrate the positive effect of an association between living soil organisms, regardless of how different they are!

Finally, Roxane Milleret and her colleagues specify that the combined actions of the AMF and earthworms on leek biomass also depend on the presence of sufficient soil nutrients, principally nitrogen and phosphorus.


Roxane Milleret

Prof. Jean-Michel Gobat

Dr. Claire Le Bayon

(c) Roxane Milleret
Earthworm and roots