Efflux of CO₂ from the soil surface, soil respiration (R_S), is a predominate flux in the carbon cycle.  It accounts for 70% of total respiration from forest stands and is the single largest terrestrial source of atmospheric CO₂.  Comparisons among vegetation types in response to resource availability over meaningful time scales are rare.  In this study, soil respiration was measured monthly in cottonwood (Populus deltoids Bartr.) and loblolly pine (Pinus taeda L.) during the first seven years following stand establishment.  Fertilization and irrigation, delivered with drip tube irrigation, caused large stand productivity differences.  Much of the variation in R_S was explained by seasonal temperature fluctuations.  Temperature sensitivity of R_S was higher in poplar (Q₁₀ = 2.15) than pine (Q₁₀ = 1.90), and increased as the stands aged (0.15 Q₁₀ /year).  Consequently, annual R_S increased with stand age, indicating a larger autotrophic contribution.  Annual specific R_S (annual R_S divided by fine-root biomass) was higher in pine than in poplar, and lower with irrigation and/or fertilization.  These differences were attributed to changes in root biomass and imply that the coarser and less abundant pine roots have greater reliance on mycorrhizal symbionts for nutrient acquisition in limiting conditions.  Seasonally, pine R_S was higher during winter and lower during summer compared to poplar, especially in fertilized plots.  Drip tube heterogeneity increased root biomass in both species, but increased R_S only for pine.  Allocation of current assimilates for R_S appears to be lower under drip tubes especially in poplar.  Distinguishing processes responsible will require much greater measurement detail.