Root dynamics in native grassland exposed to elevated CO2 and warming
Abstract
Responses of belowground processes to global change play a major role in terrestrial ecosystem carbon (C) storage and feedbacks to climate, but remain understudied in comparison to aboveground processes. In grasslands, roots comprise about 75 percent of the biomass, and are responsible for increased inputs of C to soil pools under elevated CO2. Root exudation may also be responsible for increased rates of soil organic matter decomposition, or priming, potentially offsetting inputs of new C. Understanding the fate of belowground C allocation requires a better understanding of root processes including growth, rhizodeposition, turnover and decomposition. We studied root dynamics in mixed C3/C4 grassland at the Prairie Heating and CO2 Enrichment experiment near Cheyenne, WY, where Free-Air CO2 Enrichment is applied at 600 ppm during daytime in the growing season, and temperature is elevated by 1.5/3 deg C day/night all year. We applied several belowground techniques, including direct biomass measurements coupled with C isotope labeling, root litter decomposition measured in litter bags and in plots with herbicide applied, and image analysis of intact and harvested root systems . Direct measurements indicated that elevated CO2 increased root biomass, a trend that became increasingly significant over the first four years of treatments. Warming by itself tended to decrease root biomass in the first two years, and this effect declined in the next two years of the experiment, suggesting a transient negative response of root growth to warming. Continuous 13C labeling in elevated CO2 plots allowed detection of a greater proportion of new C in warmed than ambient temperature plots, demonstrating greater allocation of C to roots exposed to both elevated CO2 and warming. A root litter bag decomposition experiment showed that C3 grass roots decomposed more rapidly with elevated CO2 alone, but more slowly when elevated CO2 was combined with warming, possibly due to soil drying. C4 grass roots decomposed more slowly than C3 roots, probably due to higher C/N values, and were less responsive to global change treatments. Recent research has shown that C4 grasses become more competitive with elevated CO2 and warming, which together with their lower decomposition rates and higher root growth under these conditions, suggests that roots are likely to play an increasingly important role in belowground C storage in grasslands in the coming century. Changes in root system structure detected in minirhizotron and harvested root images, including lengths, diameters, and demography, will contribute further understanding of how belowground ecosystems will respond to global change.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B13H..06P
- Keywords:
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- 0439 BIOGEOSCIENCES / Ecosystems;
- structure and dynamics;
- 0476 BIOGEOSCIENCES / Plant ecology;
- 0486 BIOGEOSCIENCES / Soils/pedology;
- 1615 GLOBAL CHANGE / Biogeochemical cycles;
- processes;
- and modeling