Soil respiration response to three years of elevated CO 2 and N fertilization in ponderosa pine ( Pinus ponderosa Doug. ex Laws.)

Abstract

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 μL 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).