Increased belowground biomass and soil CO2 fluxes after a decade of carbon dioxide enrichment in a warm‐temperate forest

Abstract

Atmospheric CO 2 concentrations have risen 40% since the start of the industrial revolution. Beginning in 1996, the Duke Free‐Air CO 2 Enrichment experiment has exposed plots in a loblolly pine forest to an additional 200 μL/L CO 2 compared to trees growing in ambient CO 2. This paper presents new belowground data and a synthesis of results through 2008, including root biomass and nutrient concentrations, soil respiration rates, soil pore‐space CO 2 concentrations, and soil‐solution chemistry to 2 m depth. On average in elevated CO 2, fine‐root biomass in the top 15 cm of soil increased by 24%, or 59 g/m 2 (26 g/m 2 C). Coarse‐root biomass sampled in 2008 was twice as great in elevated CO 2 and suggests a storage of ~20 g C·m−2·yr−1. Root C and N concentrations were unchanged, suggesting greater belowground plant demand for N in high CO 2. Soil respiration was significantly higher by 23% on average as assessed by instantaneous infrared gas analysis and 24‐h integrated estimates. N fertilization decreased soil respiration and fine‐root biomass by ~10–20% in both ambient and elevated CO 2. In recent years, increases in root biomass and soil respiration grew stronger, averaging ~30% at high CO 2. Peak changes for root biomass, soil respiration, and other variables typically occurred in midsummer and diminished in winter. Soil CO 2 concentrations between 15 and 100 cm depths increased 36–60% in elevated CO 2. Differences from 30 cm depth and below were still increasing after 10 years' exposure to elevated CO 2, with soil CO 2 concentrations >10 000 μL/L higher at 70‐ and 100‐cm depths, potentially influencing soil acidity and rates of weathering. Soil solution Ca 2+ and total base cation concentrations were 140% and 176% greater, respectively, in elevated CO 2 at 200 cm depth. Similar increases were observed for soil‐solution conductivity and alkalinity at 200 cm in elevated CO 2. Overall, the effect of elevated CO 2 belowground shows no sign of diminishing after more than a decade of CO 2 enrichment.