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Vose, J.M,  K.J Elliott, D.W. Johnson, D.T. Tingey,  M. Johnson. 1997. Soil respiration response to two years of elevated CO2 and N fertilization in ponderosa pine (Pinus ponderosa Doug. ex Laws.). Plant and Soil 190:19-28

We measured growing season soil CO2 evolution under elevated atmospheric [CO2] and soil nitrogen (N) additions. Our objectives were to determine treatment effects, quantify seasonal variation, and compare two measurement techniques. Elevated [CO2] treatments were applied in open-top chambers containing ponderosa pine (Pinus ponderosa L.) seedlings. N applications were made annually in early spring. The experimental design was a replicated factorial combination of CO2 (ambient, +175, and +350 L L-1 CO2) and N (0,10, and 20 g m-2 N as ammonium sulphate). Soils were irrigated to maintain soil moisture at >25 percent. Soil CO2 evolution was measured over diurnal periods (20-22 hours) in October 1992, and April, June, and October 1993 and 1994 using a flow-through, infrared gas analyzer measurement system and corresponding pCO2 measurements were made with gas wells. Significantly higher soil CO2 evolution was observed in the elevated CO2 treatments; N effects were not significant. Averaged across all measurement periods, fluxes, were 4.8, 8.0, and 6.5 for ambient +175 CO2, and +350 CO2 respectively.

Treatment variation was linearly related to fungal occurrence as observed in minirhizotron tubes. Seasonal variation in soil CO2 evolution was non-linearly related to soil temperature; i.e., fluxes increased up to approximately soil temperature (10cm soil depth) and decreased dramatically at temperatures > 8C. These patterns indicate exceeding optimal temperatures for biological activity. The dynamic, flow-through measurement system was weakly correlated (r=0.57; p<0.0001; n=56) with the pCO2 measurement method.

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