Elevated CO₂ increases belowground respiration in California grasslands

Area of Science:

• Ecology
• Plant Physiology
• Biogeochemistry

Background:

• Rising atmospheric carbon dioxide (CO2) levels influence plant productivity and ecosystem carbon cycling.
• Understanding the response of belowground respiration to elevated CO2 is crucial for predicting future carbon budgets.
• Belowground respiration encompasses root and heterotrophic respiration, key components of soil carbon dynamics.

Purpose of the Study:

• To investigate the effects of elevated CO2 on belowground respiration in both field and microcosm ecosystems.
• To assess the impact of altered belowground respiration on the annual carbon budget.
• To determine the relationship between plant biomass, CO2 levels, and respiration rates.

Main Methods:

• Respiration measurements were conducted monthly in sandstone grassland and microcosms from November 1993 to June 1994.
• Peak biomass respiration was measured in 1993 and 1994 in serpentine and sandstone grasslands, and in microcosms with monocultures.
• Supplementary monthly measurements of belowground respiration were made in adjacent grasslands to understand ecosystem carbon cycling.

Main Results:

• Belowground respiration in sandstone grassland was 42% higher under elevated CO2 (2.12 μmol m-2 s-1) compared to ambient CO2 (1.49 μmol m-2 s-1).
• Microcosm studies showed respiration increased by an average of 70% with elevated CO2, correlating positively with plant biomass.
• Estimated annual soil carbon efflux increased from 323 g C m-2 year-1 in ambient CO2 to 440 g C m-2 year-1 in elevated CO2.

Conclusions:

• Elevated CO2 significantly stimulates belowground respiration in grassland ecosystems.
• The increase in soil carbon efflux due to elevated CO2 is substantial, potentially offsetting increased photosynthesis.
• Belowground respiration plays a critical role in regulating the net carbon balance of ecosystems under elevated CO2 conditions.