EFFECTS OF ELEVATED CO2 AND N FERTILIZATION ON SOIL RESPIRATION FROM PONDEROSA PINE (PINUS-PONDEROSA) IN OPEN-TOP CHAMBERS

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 determine regulating mechanisms. Elevated CO2 treatments were applied in open-top chambers containing 3-year-old ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings. Nitrogen applications were made annually in early spring. The experimental design was a replicated factorial combination of CO2 (ambient, +175, and +350 mu L . L(-1) CO2) and N (0, 10, and 20 g . m(-2) N as ammonium sulfate). Soils were irrigated to maintain soil moisture at >25%. Soil CO2 evolution was measured over diurnal periods (20-22 h) in April, June, and October 1993 using a flow-through, infrared gas analyzer measurement system. To examine regulating mechanisms, we linked our results with other studies measuring root biomass with destructive sampling and root studies using minirhizotron techniques. Significantly higher soil CO2 evolution was observed in the elevated CO2 treatments in April and October; N effects were not significant. In October, integrated daily values for CO2 evolution ranged from 3.73 to 15.68 g CO2 . m(-2)day(-1) for the ambient CO2 + 0 N and 525 mu L . L(-1) CO2 + 20 g . m(-2) N, respectively. Soil CO2 flux among treatments was correlated with coarse root biomass (r(2) = 0.40; p >F = 0.0380), indicating that at least some of the variation observed among treatments was related to variation in root respiration. Across ail sample periods and treatments, there was a significant correlation (r(2) = 0.63; p >F = 0.0001) between soil CO2 evolution acid percent fungal hyphae observed in minirhizotron tubes. Hence, some of the seasonal and treatment variation was also related to differences in heterotrophic activity.