Root production and turnover and carbon budgets of two contrasting grasslands under ambient and elevated atmospheric carbon dioxide concentrations

Abstract

Monoliths of two contrasting vegetation types, a species-rich grassland on a brown earth soil over limestone and a species-poor community on a peaty gley, were transferred to solardomes and grown under ambient (350 μl l⁻¹) and elevated (600 μl l⁻¹) CO₂ for 2 yr. Shoot biomass was unaltered but root biomass increased by 40–50% under elevated CO₂. Root production was increased by elevated CO₂ in the peat soil, measured both as instantaneous and cumulative rates, but only the latter measure was increased in the limestone soil. Root growth was stimulated more at 6 cm depth than at 10 cm in the limestone soil. Turnover was faster under elevated CO₂ in the peat soil, but there was only a small effect on turnover in the limestone soil. Elevated CO₂ reduced nitrogen concentration in roots and might have increased mycorrhizal colonization. Respiration rate was correlated with N concentration, and was therefore lower in roots grown at elevated CO₂. Estimates of the C budget of the two communities, based upon root production and on net C uptake, suggest that C sequestration in the peat soil increases by c. 0·2 kg C m⁻² yr⁻¹ (=2 t ha yr⁻¹) under elevated CO₂.