Do fungi work efficiently in croplands?
Arbuscular mycorrhizal fungi (group Mucoromycota, subgroup Glomeromycotina) are obligate root symbionts of most terrestrial plant species, including many important crops. But, unlike the fungal groups forming conspicuous fruiting bodies many of which are edible, arbuscular mycorrhizal fungi spend all their life cycle belowground. The name “arbuscular” stem from the distinctive branching structures they form known as arbuscules. This means that the fungus has the ability to penetrate the root cells of the plant by tiny hyphae as an efficient way of trading soil nutrients with the plant. The fungi improve nutrient acquisition and stress tolerance but not for free, the plant in return must provide photosynthetically fixed carbon to the fungi. These fungi also contribute with soil ecosystem functions as improved soil aggregation and stability. For all this, they are considered an important component of agroecosystems. However, arbuscular mycorrhizal communities associated with agricultural soils can be negatively affected by excessive fertilisation regimes and tillage, which alters their functioning, i.e., the extent of benefits that the fungi can provide to the plant and the soils.
The researchers conducted the study in a greenhouse where they induced the development of native communities of arbuscular mycorrhizal fungi from 28 conventionally managed arable soils. After adding the native fungal communities, or a reference isolate of a ubiquitous mycorrhizal species (Rhizophagus irregularis) for comparisons, they measured the root colonization and the potential to promote plant growth and nutrient uptake (phosphorus and nitrogen) in pots where leek (Allium porrum) plants were growing.
The research group found out that roots were colonized by arbuscular mycorrhizal fungi and the higher the percentage of colonization, the higher the benefit for the plant. As reported in previous works, fungi proliferated better and were more beneficial for plants in phosphorus deficient soils than in medium-high levels of phosphorus in soil. Positive plant growth responses to mycorrhiza appeared only in a small subset of the arable soils, but positive effects on plant phosphorus uptake were more frequent and pronounced. However, in comparison with the reference isolate, native fungi from arable soils were less infective and provided fewer benefits to the plants. Although a wide range of plant responses to colonisation were observed, the authors suggest that fungal communities might improve plant growth but only in soils with phosphorus deficiency. Because in nature arbuscular mycorrhizas not only benefit the plant with improved mineral nutrition, the authors also evaluated the effect of the native fungi on the stability of soil aggregates (as a proxy of an important ecosystem function) but they did not find a direct effect.
The findings corroborate once more the main role of soil fertility in determining the output of the symbiosis by arbuscular mycorrhizal fungi, since the trade balance fungus-plant is mainly driven by the availability of phosphorus, nitrogen in the soil, and carbon supply from the plant. For example, overfertilization in croplands reduces the presence and optimal functioning of arbuscular mycorrhizal fungi because more nutrients are directly available to the plants, mycorrhizas become useless, and the host plant reduces the carbon supply that the fungus needs to survive and develop. Tillage, on the other hand, limits the presence of fungi in the soil by damages on the mycelial networks which also affects the role of these underground organisms in maintaining the structure and the quality of the soils. The authors at the end, suggest different lines of research to fill the current gaps of knowledge on the functioning of these intriguing organisms in arable fields.