PARENT SESSION
Symposium #26: Gasping for CO₂: Ecological Effects of Past Variations in Atmospheric CO2.

Organized by: JW Williams and S Cowling
Thursday, August 8. 9:00 AM to 11:30 AM. Turquoise Ballroom, TCC.

Grassland responses to past and future atmospheric CO2.

Jackson, Robert^*,1, Gill, Richard², Polley, H³, Johnson, Hyrum³, Maherali, Hafiz¹, Anderson, Laurel⁴, ¹ Duke University, Durham, NC² Washington State University, Pullman, WA³ USDA-ARS Grassland, Soil and Water Research Laboratory, Temple, TX⁴ Ohio Wesleyan University, Delaware, OH

ABSTRACT- Carbon sequestration in soil organic matter (SOM) may moderate increases in atmospheric CO₂ concentrations (C_a) as C_a increases to >500 mol mol^-1 this century from interglacial levels of <200 mol mol^-1. However, such storage depends on feedbacks between plant responses to C_a and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past C_a than to those forecast for the coming century. Along a continuous gradient of 200 to 550 mol mol^-1 C_a, increased C_a promoted higher photosynthetic rates and altered plant tissue chemistry. Soil C was lost at subambient C_a, but was unchanged at elevated C_a where losses of old soil C offset increases in new C. Along the experimental gradient in C_a there was a non-linear, three-fold decrease in N availability. The differences in sensitivity of C storage to historical and future C_a and increased nutrient limitation suggest that the passive sequestration of C in soils may have been important historically, but the ability of soils to continue as sinks is limited.

KEY WORDS: carbon sequestration, subambient CO2, grassland ecosystems, elevated CO2