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

Abstract

Atmospheric CO2 concentrations have risen 40% since the start of the industrial revolution. Beginning in 1996, the Duke Free-Air CO2 Enrichment experiment has exposed plots in a loblolly pine forest to an additional 200 microL/L CO2 compared to trees growing in ambient CO2. 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 CO2 concentrations, and soil-solution chemistry to 2 m depth. On average in elevated CO2, fine-root biomass in the top 15 cm of soil increased by 24%, or 59 g/m2 (26 g/m2 C). Coarse-root biomass sampled in 2008 was twice as great in elevated CO2 and suggests a storage of approximately 20 g C x m(-2) x yr(-1). Root C and N concentrations were unchanged, suggesting greater belowground plant demand for N in high CO2. 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 approximately 10-20% in both ambient and elevated CO2. In recent years, increases in root biomass and soil respiration grew stronger, averaging approximately 30% at high CO2. Peak changes for root biomass, soil respiration, and other variables typically occurred in midsummer and diminished in winter. Soil CO2 concentrations between 15 and 100 cm depths increased 36-60% in elevated CO2. Differences from 30 cm depth and below were still increasing after 10 years' exposure to elevated CO2, with soil CO2 concentrations >10000 microL/L higher at 70- and 100-cm depths, potentially influencing soil acidity and rates of weathering. Soil solution Ca2+ and total base cation concentrations were 140% and 176% greater, respectively, in elevated CO2 at 200 cm depth. Similar increases were observed for soil-solution conductivity and alkalinity at 200 cm in elevated CO2. Overall, the effect of elevated CO2 belowground shows no sign of diminishing after more than a decade of CO2 enrichment.