Nutrients change the effect of precipitation on plant growth

A new study published in the journal PNAS examines how the relationship between mean annual precipitation and grassland biomass changes with the addition of one or more nutrients. Researchers from the German Center for Integrative Biodiversity Research (iDiv), the Helmholtz Centre for Environmental Research (UFZ), Martin Luther University Halle-Wittenberg (MLU) and Leipzig University were also involved in the global analysis, which was led by the US Department of Agriculture. They were able to show that precipitation and nutrient availability are the most important factors influencing plant biomass, while the effects of plant diversity on biomass are minor.

Precipitation regimes are changing all over the world: while average annual precipitation is increasing in some regions, it is becoming drier in others. At the same time, the input of nutrients such as nitrogen, phosphorus and potassium is increasing in many ecosystems - whether intentionally or unintentionally. Fertilizers from agriculture contribute to this, as does urban wastewater that enters the ecosystem from nearby watercourses. Both factors - water and nutrients - directly alter plant growth.

The researchers’ investigations confirmed a strong correlation between mean annual precipitation and plant biomass: on average, biomass increased with increasing annual precipitation. However, other factors also play a role - such as whether important nutrients are available. It was unclear how the relationship between plant biomass and annual precipitation changes with the addition of one or more nutrients.

To answer this question, the scientists carried out measurements of above-ground plant biomass and species diversity on 71 grassland areas on six continents. These included both natural and cultivated areas, which differed in terms of soil composition, nutrient content and management. All of the areas studied are part of the "Nutrient Network", which also includes experimental areas in Bad Lauchstädt and Jena. A standardized procedure is used within the network, which comprises over 130 sites worldwide, so that the data collected is comparable.

More nutrients, more biomass

In order to investigate the effects of nutrients in combination with the average annual precipitation, the test areas were fertilized with all possible combinations of nitrogen, phosphorus and potassium. The result: fertilization generally led to stronger plant growth and thus to an increase in biomass. The more nutrients (especially nitrogen and phosphorus) were added, the more increasing precipitation increased plant growth.

While this result was unsurprising, the new study also revealed an unexpected result: factors such as plant diversity had little influence on the relationship between annual precipitation and biomass within the plant community, except at sites with a good nutrient supply. Here, a clearer relationship between precipitation and biomass emerged - a pattern that went unnoticed in earlier analyses that did not take biodiversity into account. "With the addition of nutrients, plant diversity decreases, but the indirect effects of these changes in biodiversity are weaker than the direct effects of annual precipitation and nutrients," says author Prof. Stan Harpole, head of the Physiological Diversity research group at iDiv, the UFZ and MLU. even if the effects on biodiversity are not so strongly transferred to biomass, it is still important to take plant diversity into account. This is the only way to understand exactly how precipitation affects biomass in ecosystems where the availability of nutrients such as nitrogen and phosphorus is not a limiting factor."

The results showed that factors such as precipitation and nutrient availability have a greater impact on biomass than plant diversity, according to the researchers.

Nutrient interactions shape the relationship between annual precipitation and biomass

The results of the new study confirm the assumption that ecosystems are often limited by more than one nutrient at a time. Accordingly, fertilizing with several nutrients can promote the relationship between biomass and precipitation. The interactions of nitrogen and phosphorus are particularly important here. To understand how grassland ecosystems respond to global changes in precipitation and nutrient enrichment, both nutrient interactions and changes in plant communities should be considered. A better understanding of how nutrient limitations affect biomass production can make an important contribution to management and conservation strategies of grassland ecosystems worldwide.

Publication in PNAS:
"Interactions among nutrients govern the global grassland biomass-precipitation relationship" , DOI: 10.1073/pnas.2410748122

Stan Harpole

Head of the Physiological Diversity working group

German Center for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig
Helmholtz Center for Environmental Research (UFZ)
Martin Luther University Halle-Wittenberg (MLU)

E-mail: stan.harpole(at)idiv.de