A new study published in PNAS, led by the United States Department of Agriculture and involving several researchers from the German Centre for Integrative Biodiversity Research (iDiv), the Helmholtz Centre for Environmental Research (UFZ), Martin Luther University Halle-Wittenberg (MLU) and Leipzig University, investigated how the relationship between mean annual precipitation (MAP) and grassland biomass changes when one or more nutrients are added. The authors show that precipitation and nutrient availability are the key drivers of plant biomass, while the effects of plant diversity are minimal.
Global changes in precipitation are putting ecosystems around the world to the test: while MAP is increasing in some regions, it is decreasing in others. At the same time, many ecosystems are being exposed – both intentionally and unintentionally – to increasing amounts of nutrients, such as nitrogen, phosphorus and potassium. This may result from agricultural practices like land use and fertilisation, or from urban runoff, which includes wastewater, entering nearby waterways. Both factors – precipitation and nutrients – directly affect plant biomass production.
The study found that plant biomass is closely related to mean annual precipitation in grasslands. On average, plant biomass increases with higher MAP, but other factors come into play as well, such as nutrient availability.
Until now, little has been known about how the relationship between MAP and plant biomass changes when one or more nutrients are added.
To answer this question, the researchers measured aboveground plant biomass and species diversity in 71 grasslands on six continents, including native and planted grasslands with varying soil textures and nutrient contents, as well as management regimes. All sites were part of the Nutrient Network , including the NutNet sites at the Jena Experiment and at the UFZ Research Station in Bad Lauchstädt, Germany. This experimental network follows a standardised methodology applied across all 130 sites globally, ensuring comparability of data.
Multiple nutrients, more biomass
To look at the effect of nutrients in combination with MAP, the researchers fertilised all sites with nitrogen, phosphorus and potassium – in all possible combinations. Fertilisation generally boosted plant growth and biomass. Notably, when multiple nutrients – particularly nitrogen and phosphorus – were added, plants became more responsive to rainfall, which led to a stronger relationship between biomass and precipitation.
While this outcome did not come as a surprise, the study also revealed something unexpected: factors like species diversity had little influence on the biomass-MAP relationship within the plant community. However, in sites that were not limited by nitrogen or phosphorus, the link between precipitation and biomass became clearer – a pattern overlooked in earlier analyses that did not account for biodiversity. "Adding nutrients decreases plant diversity, but the indirect effect of diversity change on biomass is weaker than the direct effects of MAP and nutrients," says co-author Stan Harpole, head of Physiological Diversity at UFZ, iDiv and MLU. "Even though the effects on diversity don't carry over strongly to biomass, considering plant diversity is still important for accurately understanding how rainfall influences the biomass in ecosystems where nutrient availability (like nitrogen and phosphorus) is not a limiting factor."
The results highlight that factors like precipitation and nutrient availability have a stronger influence than plant diversity.
Nutrient interactions shape the MAP-biomass relationship
The new findings support the idea that ecosystems are often co-limited by multiple nutrients. Accordingly, fertilisation with more than one nutrient can promote the relationship between biomass and precipitation – with interactions between nitrogen and phosphorus playing a particularly important role. To fully understand how grassland ecosystems respond to global changes in precipitation and nutrient enrichment, both nutrient interactions and changes in plant communities should be considered. Understanding nutrient limitations and their impact on biomass production can help inform land management and conservation strategies for grassland ecosystems worldwide.
