Soil respiration response to three years of elevated CO2 and N fertilization in ponderosa pine (Pinus ponderosa Doug. ex Laws.)
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).
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 > 18°C. These patterns indicate exceeding optimal temperatures for biological activity. The dynamic, how-through measurement system was weakly correlated (r = 0.57; p < 0.0001; n = 56) with the pCO2 measurement method.