Plants and soil microbes are different in many ways, in particular in their biomass composition. Plant biomass is rich in nitrogen, but has less phosphorus, compared to microbial biomass.
This means that these organisms require different diets to thrive – richer in nitrogen for plants and richer in phosphorus for microbes. In soils, where both plants and microorganisms harvest the nutrients they need, nitrogen and phosphorus can be found in varying amounts. This creates a range of possible conditions, from poor in nitrogen but relatively rich in phosphorous, to poor in phosphorus but relatively rich in nitrogen. Fertilization further changes the relative availability of one nutrient or the other, potentially creating dangerous nutrient imbalances that may inhibit plant growth. In most cases fertilization is beneficial, as it provides the most needed nutrients for both plants and microorganisms, but in some cases it does not.
“Because soil microorganisms need relatively more phosphorus than plants, adding phosphorus to soil allows them to grow faster and use the soil nitrogen that would otherwise be available for plants. As a result, plants will run out of nitrogen and grow less”, says Stefano Manzoni, senior lecturer at the Department of Physical Geography, Stockholm University, and co-author of this study. Thus, the diet of soil microorganisms is key to understand fertilization effects, but that is not enough to explain experimental findings from the more than 50 studies considered by Čapek and co-authors.
Plants and soil microorganisms are not always competing for soil nitrogen and phosphorus – they often form symbiotic associations that allow plants to benefit from the presence of soil microorganisms. “When symbiotic associations are present, the negative effects created by nutrient imbalances disappear, because plants and soil microorganisms cooperate and share resources, instead of competing” says Stefano Manzoni. This study thus shows that maintaining a healthy relation between plants and microbes based on natural symbiotic associations is the key to sustain ecosystem services such as productivity of fields and forests.
The article “Nutrient effects on plant primary production dependent on plant-microbe relationship” is published in Nature Ecology and Evolution.