SOIL RESPIRATION RESPONSE TO 3 YEARS OF ELEVATED CO2 AND N FERTILIZATION IN PONDEROSA PINE (PINUS-PONDEROSA DONG EX LAWS)

We measured growing season soil CO2 evolution under elevated atmospher ic [CO2] and soil nitrogen (N) additions. Our objectives were to deter mine treatment effects, quantify seasonal variation, and compare two m easurement 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 d esign was a replicated factorial combination of CO2 (ambient, +175, an d +350 mu L L-1 CO2) and N (0, 10, and 20 g m(-2) N as ammonium sulpha te). Soils were irrigated to maintain soil moisture at > 25 percent. S oil CO2 evolution was measured over diurnal periods (20-22 hours) in O ctober 1992, and April, June, and October 1993 and 1994 using a flow-t hrough, infrared gas analyzer measurement system and corresponding pCO (2) measurements were made with gas wells. Significantly higher soil C O2 evolution was observed in the elevated CO2 treatments; N effects we re not significant. Averaged across all measurement periods, fluxes, w ere 4.8, 8.0, and 6.5 for ambient + 175 CO2, and +350 CO2 respectively ). Treatment variation was linearly related to fungal occurrence as ob served in minirhizotron tubes. Seasonal variation in soil CO2 evolutio n 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 degrees C. These patterns indicate e xceeding optimal temperatures for biological activity. The dynamic, fl ow-through measurement system was weakly correlated (r = 0.57; p < 0.0 001; n = 56) with the pCO(2) measurement method.