Long-term effects of elevated atmospheric CO2 on below-ground biomass and transformations to soil organic matter in grassland

We determined the effects of elevated [CO2] on the quantity and quality of below-ground biomass and several soil organic matter pools at the conclusio n of an eight-year CO2 enrichment experiment on native tallgrass prairie. P lots in open-top chambers were exposed continuously to ambient and twice-am bient [CO2] from early April through late October of each year. Soil was sa mpled to a depth of 30 cm beneath and next to the crowns of C4 grasses in t hese plots and in unchambered plots. Elevated [CO2] increased the standing crops of rhizomes (87%), coarse roots (46%), and fibrous roots (40%) but ha d no effect on root litter (mostly fine root fragments and sloughed cortex material > 500 mu m). Soil C and N stocks also increased under elevated [CO 2], with accumulations in the silt/clay fraction over twice that of particu late organic matter (POM; > 53 mu m). The mostly root-like, light POM (dens ity less than or equal to 1.8 Mg m(-3)) appeared to turn over more rapidly, while the more amorphous and rendered heavy POM (density > 1.8 Mg m(-3)) a ccumulated under elevated [CO2]. Overall, rhizome and root C:N ratios were not greatly affected by CO2 enrichment. However, elevated [CO2] increased t he C:N ratios of root litter and POM in the surface 5 cm and induced a smal l but significant increase in the C:N ratio of the silt/clay fraction to a depth of 15 cm. Our data suggest that 8 years of CO2 enrichment may have af fected elements of the N cycle (including mineralization, immobilization, a nd asymbiotic fixation) but that any changes in N dynamics were insufficien t to prevent significant plant growth responses.