This study compares soil respiration and its heterotrophic and autotrophic components in four land use types: agriculture, 2 and 9 year old hybrid poplar plantations, grassland, and a native aspen stand in north central Alberta, Canada, over a period of two growing seasons (2006 and 2007). The differences were examined with respect to substrate quality and quantity, fine root biomass, and nutrient availability, in addition to soil temperature and soil water content. Cumulative soil C loss via soil respiration averaged over the two growing seasons was (in decreasing order) 781, 551, 523, 502, and 428 g C m(-2) for native aspen stand, 9 year old hybrid poplar plantation, grassland, agriculture and 2 year old hybrid poplar plantation, respectively. We found that similar to 75% of soil respiration in the native aspen stand originated from the top 7.5-10 cm litter-fibric-humus layer. Seasonal heterotrophic and autotrophic respiration among the land uses ranged from 97 to 272 and 333 to 560 g C m(-2), respectively, contributing up to 35% and 83% of total soil respiration, respectively. The variability in soil respiration across different land uses was explained mainly by site differences in soil temperature (88-94%). Soil respiration followed a pronounced seasonal trend: increasing during the growing season and converging to a minimum in the fall. Soil respiration under different land uses was influenced by (1) ecosystem C stock, (2) temperature sensitivity (Q(10)) of organic matter present, and (3) organic matter decomposability as indicated by the natural abundance of delta(13)C. Heterotrophic respiration was influenced by soil temperature, while autotrophic respiration was influenced by fine root biomass and nutrient (NO(3)(-) and P) availability. These results are useful in estimating potential responses of soil respiration and its components to future land management and climate change.