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 Centre for Microbiology and Environmental Systems Science (CeMESS) at 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 data comes from a 70-year long-term study and therefore provides particularly valuable insights, enabling scientists to make a significant contribution to sustainable agriculture. The results have recently been published in the renowned journal The New Phytologist.

Mycorrhizal fungi colonise plant roots and grow a network of fungal hyphae into the soil from there. These fungal hyphae are much thinner than plant roots and can grow into even the finest soil pores. This enables them to absorb essential nutrients such as phosphorus and nitrogen from the soil much more efficiently. This is what makes them so valuable to their plant partners: the fungi pass on a large proportion of the nutrients they absorb to the plants in exchange for carbohydrates produced by the plants through photosynthesis. With a well-functioning mycorrhizal symbiosis, plants are able to grow relatively well even in nutrient-poor soils. Many of the plants used in agriculture today, such as wheat, maize and potatoes, also live in such a symbiosis. As in the wild, these fungi protect plants from pests and help them cope with water shortages in agriculture.

Data from a 70-year long-term experiment

In the current study led by the University of Vienna, scientists have now been able to show that mycorrhizal symbiosis is very sensitive to imbalances of certain nutrients (nitrogen, phosphorus and potassium) in the soil. The data comes from a measurement campaign conducted by Andreas Richter, also from the CeMESS at University of Vienna, in a long-term experiment that has been running for over 70 years at the Raumberg-Gumpenstein Agricultural Research Station in Admont (Styria). In this long-term experiment, grassland areas have been regularly mowed and harvested since 1946, with the nutrients removed by harvesting being replaced by various combinations and amounts of nitrogen, phosphate and potassium fertilisers. The study authors found that potassium deficiency in combination with nitrogen fertilisation had an extremely negative effect on the symbiosis: under these conditions, the plants lost about half of their valuable fungal partners and thus presumably part of their natural shield 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. '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.

About the University of Vienna:

At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.

About New Phytologist

New Phytologist is a leading international journal focusing on high quality, original research across the broad spectrum of plant sciences, from intracellular processes through to global environmental change. The journal is owned by the New Phytologist Foundation, a not-for-profit organisation dedicated to the promotion of plant science.