Elevated atmospheric CO 2 effects and soil water feedbacks on soil respiration components in a Colorado grassland

Abstract

The shortgrass steppe is a semi-arid grassland, where elevated CO2reduces stomatal conductance and promotes soil moisture storage. Enhanced biomass growth from elevated CO2has been attributed in part to soil moisture effects. However, the influence of this soil moisture feedback on C cycling has received little attention. We used open-top chambers to increase atmospheric CO2concentrations to twice-ambient for four growing seasons. Soil respiration rates and stable C isotopes of soil CO2were measured during the third and fourth seasons. Elevated CO2increased soil respiration rates by ∼25% in a moist growing season and by ∼85% in a dry season. Stable C isotope partitioning of soil respiration into its components of decomposition and rhizosphere respiration was facilitated on all treatments by a13C disequilibrium between currently growing plants and soil organic matter. Decomposition rates were more than doubled by elevated CO2, whereas rhizosphere respiration rates were not changed. In general, decomposition rates were most significantly correlated with soil temperature, and rhizosphere respiration rates were best predicted by soil moisture content. Model simulations suggested that soil moisture feedbacks, rather than differences in substrate availability, were primarily responsible for higher total respiration rates under elevated CO2. By contrast, modeling demonstrated that substrate availability was at least as important as soil moisture in driving CO2treatment differences in soil organic matter decomposition rates.