Three years of soil respiration in a mature eucalypt woodland exposed to atmospheric CO₂ enrichment

Abstract

The rate of CO₂ diffusion from soils to the atmosphere (soil CO₂ efflux, soil respiration; R_soil) reflects the integrated activity of roots and microbes and is among the largest fluxes of the terrestrial global C cycle. Most experiments have demonstrated that R_soil increases by 20–35% following the exposure of an ecosystem to an atmosphere enriched in CO₂ (i.e., eCO₂), but such experiments have largely been performed in young and N-limited ecosystems. Here, we exposed a mature and phosphorus-limited eucalypt woodland to eCO₂ and measured R_soil across three full years with a combination of manual surveys and automated measurements. We also implemented an empirical model describing the dependence of R_soil on volumetric soil water content (θ) and soil temperature (T_soil) to produce annual R_soil flux estimates. R_soil varied strongly with T_soil, θ, and precipitation in complex and interacting ways. The realized long-term (weeks to years) temperature dependence (Q₁₀) of R_soil increased from ~ 1.6 at low θ up to ~ 3 at high θ. Additionally, R_soil responded strongly and rapidly to precipitation events in a manner that depended on the conditions of θ and T_soil at the beginning of the rain event; R_soil increased by up to 300% within 30 min when rain fell on dry soil that had not experience rain in the preceding week, but R_soil was rapidly reduced by up to 70% when rain fell on wet soil, leading to flooding. Repeated measures analysis of R_soil observations over 3 years indicated no significant change in response to CO₂ enrichment (P = 0.7), and elevated CO₂ did not alter the dependence of R_soil on T_soil or θ. However, eCO₂ increased R_soil observations by ~ 10% under some constrained and moderate environmental conditions. Annual R_soil flux sums estimated with an empirical model were ~ 7% higher in eCO₂ plots than in aCO₂ plots, but this difference was not statistically significant. The lack of a large eCO₂ effect on R_soil is consistent with recent evidence that aboveground net primary production was not stimulated by eCO₂ in this ecosystem. The C budget of this mature woodland may be less affected by eCO₂ than the young N-limited ecosystems that have been studied previously.