Factors Affecting Carbon Dioxide Release from Forest and Rangeland Soils in Northern Utah

Laboratory and field CO2 efflux measurements were used to investigate the influence of soil organic C (SOC) decomposability and sod microclimate on summer SOC dynamics in seasonally dry montane forest and rangeland soils at the T.W. Daniel Experimental Forest in northern Utah. Soil respiration, soil temperature, and soil moisture content (SMC) were measured between July and October 2004 and 2005 in 12 control and 12 irrigated plots laid out in a randomized block design in adjacent forest (aspen or conifer) and rangeland (sagebrush [Artemisia tridentata Nutt.) or grass-forb) sites. Irrigated plots received a single water addition of 2.5 cm in July 2004 and two additions in July 2005, The SOC decomposability in mineral soil samples (0-10, 10-20, and 20-30 cm)was derived from 10-mo tab incubations. The amount of SOC accumulated in the A horizon (16 Mg ha(-1)) and the top 1 m (74 Mg ha(-1)) of the mineral soil did not differ significantly among vegetation type, but upper forest soils tended to contain more decomposable SOC than rangeland sods. The CO, efflux measured in the field varied significantly with vegetation cover (aspen > conifer = sagebrush > grass-forb), ranging from 12 kg CO2-C ha(-1) d(-1) in aspen to 5 kg CO2-C ha(-1) d(-1) in the grass-forb sites. It increased (similar to 35%) immediately following water additions, with treatment effects dissipating within 1 wk. Soil temperature and SMC, which were negatively correlated (r = -0.53), together explained similar to 60% of the variability in summer soil respiration. Our study suggests that vegetation cover influences summer CO2 efflux rates through its effect on SOC quality and the soil microclimate.