Root System Modifications in White Oak (Quercus alba L.) Saplings Grown Under CO2 Enrichment in the Field
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The responses of white oak (Quercus alba L.) sapling root systems to C02 enrichment of the atmosphere were investigated after four seasons of growth under field conditions. Seedlings were planted in the soil within open-top chambers and exposed to a variable environment through four growing seasons with atmospheric C02 concentrations of ambient + 0 (- 350 J.1mol mol-I), ambient + 150 (- 500 µmol mol-l ) and ambient + 300 (- 650 µmol mol-I). Fine root density, assessed by soil coring, was 29% and 138% higher in elevated C02 for saplings grown in +150 and +300, respectively, compared to ambient C02 controls. Soil respiration rates responded similarly, with a significant and consistent increase in CO2 efflux from soils of C02-enriched chambers. On the other hand, fine root specific respiration rate and nitrogen concentration tended to decrease with increasing C02 concentration. These results suggest that an increase in below-ground biomass will result in the face of a notable decrease in specific metabolic activity under conditions of C02 enrichment. Thus, saplings grown under elevated C02 may be presumed more efficient; able to maintain a larger biomass at less cost energetically. This change in physiology may also be supported by the observed increases in fine root density, which can lead to a greater opportunity for soil exploration, and, therefore, nutrient acquisition and symbiotic relations. With these modifications under elevated CO2 a sapling may be able to reach an optimal adult biomass in a shorter amount of time and also be in better condition for surviving times of environmental stress. Implications that these physiological modifications may have for the global carbon cycle depends on where the balance lies between carbon sequestered through photosynthesis and carbon lost via cellular respiration.