The Science

Interactions between plant nutrient demand and nutrient supply and dictate key terrestrial ecosystem feedbacks to global climate change. Experimental warming of a tidal marsh ecosystem showed that plants and microbes respond to warming at different threshold temperatures. Modest warming (<2°C) caused plant demand for nitrogen to outpace soil nitrogen supply, while more extreme warming (3-5°C) caused the nitrogen supply to increase to meet plant nitrogen demand. This response changed further when the plants were exposed to both warming and elevated CO₂.

The Impact

Globally, terrestrial plant nitrogen demand has exceeded soil nitrogen supply in unfertilized ecosystems over the past century, which could be due to either warmer climates or elevated atmospheric CO₂ increasing plant growth. Results from this experiment indicate that this imbalance is driven primarily by elevated CO₂, implying that nitrogen supply will likely increase to meet plant demand as warming exceeds 2°C.

Summary

Terrestrial ecosystem responses to climate change are mediated by complex plant-soil feedbacks that are poorly understood, but often driven by the balance of nutrient supply and demand. SMARTX is an in situ whole-ecosystem active warming experiment crossed with elevated CO₂ that was established in a Chesapeake Bay tidal marsh in 2016 to understand the combined ecosystem-scale effects of warming and elevated CO₂. Heating treatments run year-round along a gradient from ambient to +5.1°C above ambient and warming spans from above the plant canopy to 1.5 m soil depth. Data from two years show that plants and soils respond to whole-ecosystem warming at different threshold temperatures, creating non-linear responses in biomass allocation to roots vs shoots. Peak belowground allocation occurred at 1.7°C, but declined back to ambient levels with further warming up to +5.1°C. Crossing elevated CO₂ with +5.1°C of warming reversed this pattern and dramatically increased the root-to-shoot ratio. The research team proposes that this non-linearity is explained by asynchronous patterns of increasing plant nitrogen demand vs soil nitrogen supply; even though plants increase growth and thus nitrogen demand at low temperatures, microbial nitrogen mineralization, which drives the soil nitrogen supply, does not respond until higher levels of warming.

Principal Investigator

Pat Megonigal
Smithsonian Environmental Research Center
megonigalp@si.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was supported by the Department of Energy, Office of Science, Office of Biological and Environmental Research Program (DE-SC0014413 and DE-SC0019110) and the Smithsonian Institution.

References