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A recent study featured in PNAS, led by the United States Department of Agriculture and involving collaboration with researchers from the German Centre for Integrative Biodiversity Research (iDiv), Helmholtz Centre for Environmental Research (UFZ), Martin Luther University Halle-Wittenberg (MLU), and Leipzig University, delves into the dynamics between mean annual precipitation (MAP) and grassland biomass when additional nutrients are introduced. The study underscores that precipitation and nutrient availability are pivotal factors influencing plant biomass, while the impact of plant diversity appears to be limited.
As global precipitation patterns undergo changes, ecosystems face varying challenges; some regions are experiencing increased MAP, while others see declines. Concurrently, many environments are subject to rising nutrient levels, including nitrogen, phosphorus, and potassium. This nutrient influx can be traced back to agricultural practices, such as fertilization and land use changes, as well as urban runoff that introduces wastewater into nearby ecosystems. These elements directly influence the productivity of plant biomass.
The research found a clear connection between plant biomass and mean annual precipitation in grassland environments. Generally, plant biomass tends to rise with an increase in MAP; however, nutrient availability is also a significant factor that affects this relationship.
Despite prior studies, little has been understood about how the relationship between MAP and plant biomass evolves when various nutrients are added.
To investigate, researchers assessed aboveground plant biomass and species diversity across 71 grassland sites on six continents, including both native and cultivated grasslands with differing soil types and nutrient contents, as well as varying management practices. All sites are part of the Nutrient Network, which includes established locations like the Jena Experiment and the UFZ Research Station in Bad Lauchstädt, Germany. This experimental framework employs a standardized methodology across all 130 global sites, ensuring a high level of data comparability.
Multiple nutrients yield greater biomass
In assessing the interaction of nutrients with MAP, the research team applied fertilizers containing nitrogen, phosphorus, and potassium in various combinations across the different sites. The results generally indicated an enhancement in plant growth and biomass due to fertilization. Notably, when multiple nutrients—especially nitrogen and phosphorus—were incorporated, the plants exhibited heightened responsiveness to rainfall, thereby strengthening the correlation between biomass and precipitation.
While this finding aligned with expectations, an unforeseen outcome emerged: the species diversity within the plant communities had minimal impact on the relationship between biomass and MAP. In sites where nitrogen and phosphorus were not limiting, the relationship between precipitation and biomass became more pronounced—a detail often overlooked in previous studies that did not consider biodiversity. “Although nutrient addition reduces plant diversity, the indirect effects of diversity on biomass are less significant compared to the direct influences of MAP and nutrient levels,” stated co-author Stan Harpole, who leads the Physiological Diversity division at UFZ, iDiv, and MLU. “Despite the limited carryover of diversity’s effects on biomass, recognizing plant diversity remains essential for a comprehensive understanding of rainfall’s impact on biomass in ecosystems where nutrient availability, like nitrogen and phosphorus, are sufficient.”
The findings emphasize that precipitation and nutrient availability exert a more substantial influence on plant biomass than species diversity.
Nutrient interactions inform the MAP-biomass connection
The insights from this study lend support to the notion that ecosystems are commonly co-limited by multiple nutrients. Consequently, the application of fertilizers containing more than one nutrient can enhance the relationship between biomass and precipitation, with particular interactions between nitrogen and phosphorus being especially influential. To comprehensively understand how grassland ecosystems react to global shifts in precipitation and nutrient enrichment, it is vital to consider both nutrient interactions and changes in plant communities. Grasping the limitations posed by nutrients and their influence on biomass production can guide effective land management and conservation strategies for grassland ecosystems globally.
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