Nearly all plants form close partnerships with mycorrhizal fungi, a symbiosis that is crucial for taking up essential nutrients. In a new study, a team led by ecologist Christina Kaiser at the Centre for Microbiology and Environmental Systems Science (CeMESS) at the University of Vienna shows that this relationship is highly sensitive to nutrient imbalances — particularly involving nitrogen, phosphorus, and potassium — in the soil. Drawing on data from a 70-year long-term experiment, the researchers provide rare, high-value insights that can inform more sustainable agricultural practices. The findings were recently published in New Phytologist.

Mycorrhizal fungi colonize plant roots and extend a web of fine hyphae into the surrounding soil. Because these threads are much thinner than roots, they can penetrate tiny soil pores and access nutrients — especially phosphorus and nitrogen — more efficiently. The fungi transfer a substantial share of these nutrients to their host plants in exchange for carbohydrates produced through photosynthesis. When this partnership functions well, plants can thrive even in nutrient-poor soils. Many major crops, including wheat, maize, and potatoes, also depend on mycorrhizae; in agricultural systems, these fungi can additionally help protect plants from pests and improve tolerance to drought.

Evidence From a 70-year Field Experiment

Using measurements collected by Andreas Richter (CeMESS, University of Vienna) from an experiment running since 1946 at the Raumberg-Gumpenstein Agricultural Research Station in Admont, Styria, the team investigated how fertilization regimes affect mycorrhizal symbiosis in grasslands. In this long-term trial, plots have been regularly mown and harvested, and the nutrients removed have been replenished with different combinations and doses of nitrogen, phosphate, and potassium fertilizers. The researchers found that potassium deficiency — especially when combined with nitrogen fertilization — was particularly damaging: under these conditions, plants lost about half of their fungal partners, likely weakening a key natural defense against pests and dehydration.

Evidence of More Sustainable Use of Fungal Families 

“We also discovered that within the largely unexplored spectrum of mycorrhizal fungi, there are families that specialise in specific nutrient situations,” says study leader Christina Kaiser from the University of Vienna. While certain fungal families responded positively to specific nutrient deficiencies, other families were severely decimated. Potassium deficiency, for example, reduced the proportion of fungi from the well-known Glomeraceae family, but at the same time increased the proportion of fungi from other, less well-known families, according to a press release.

“In agriculture, mycorrhizal fungi mixtures from this very family of Glomeraceae are currently mainly used. Our results suggest that fungi from other families should also be researched, as they may be better adapted to certain nutrient deficiency situations,” says Kian Jenab, doctoral student at the CeMESS at the University of Vienna and first author of the study.

Unbalanced fertilisation has long-term effects on plant and soil health

The combination of low potassium and high nitrogen in the soil, which led to the greatest loss of mycorrhizal fungi in this study, is quite common globally – because potassium fertiliser is expensive and difficult to obtain in some parts of the world. As a result, potassium is often not fertilised sufficiently in relation to nitrogen. ‘This unbalanced fertilisation does not have an immediate effect on plant yield. However, it weakens the symbiosis between plants and fungi, which has long-term effects on soil and plant health,’ Kaiser summarises. ‘The current study once again confirms the value of long-term experiments, which act as “silent observers” of nature, documenting processes in our soils over decades and thus providing us with such deep insights,’ adds Erich Poetsch, project partner at the Raumberg-Gumpenstein Research Station.

Summary:

Almost all plants live in close symbiosis with so-called mycorrhizal fungi – an important symbiosis for absorbing essential nutrients.

In their new study, a team led by ecologist Christina Kaiser from the University of Vienna has now discovered that this mycorrhizal symbiosis is very sensitive to imbalances of certain nutrients (nitrogen, phosphorus and potassium) in the soil.

The combination of low potassium and high nitrogen in the soil, which led to the greatest loss of mycorrhizal fungi in this study, is quite common globally – because potassium fertiliser is expensive and difficult to obtain in some parts of the world.

The data comes from a long-term experiment in Styria that has been running for over 70 years.