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

Authors: Jackson, RB, CW Cook, JS Pippen, SM Palmer

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 µl l-1 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 m-2 (26 g m-2 C; P<0.05). Coarse root biomass sampled in 2008 was twice as great in elevated CO2 and suggests a storage of ~20 g C m-2 y-1 (P<0.05). Root C and N concentrations were unchanged (P>0.30), suggesting greater belowground plant demand for N in high CO2. Soil respiration was significantly higher by 23% on average (P<0.01) 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 CO2 (P<0.05). In recent years, increases in root biomass and soil respiration grew stronger, averaging ~30% at high CO2. Peak changes for root biomass, soil respiration, and other variables typically occurred in mid-summer and diminished in winter. Soil CO2 concentrations between 15 and 100 cm depths increased 36 to 60% in elevated CO2 (P<0.05). Differences from 30 cm depth and below were still increasing after ten years' exposure to elevated CO2, with soil CO2 concentrations >10,000 µl l-1 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 (P<0.05 for each). 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.
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