The effect of boreal forest composition on soil respiration is mediated through variations in soil temperature and C quality

Getting a better understanding of CO₂ efflux from forest soils is critical for increasing our comprehension of the global C cycle. We examined the influence of two common boreal tree species, either in pure stands (BS = black spruce; TA = trembling aspen) or in mixtures (MW = BS + TA mixedwood), on total (RS), heterotrophic (RH) and autotrophic soil respiration (RA) and their relationship with soil temperature and moisture, distance to the nearest tree, labile and total soil organic C (SOC), and root content. Stand-specific soil respiration–temperature models were developed to estimate annual soil CO₂ efflux. Soil temperature was the main factor explaining RS and its components, followed by labile and total SOC. These three variables were significantly affected by forest composition, while no difference in soil moisture, distance to the nearest tree and root content was observed between stand types. A reciprocal forest floor transplant experiment showed that the influence of stand types on mineral soil temperature was due to a difference in light penetration rather than forest floor characteristics. Annual RS and RH were significantly greater in MW and TA than in BS, whereas annual RA was greater in BS and MW than in TA. Temperature sensitivity (Q₁₀) of both RS and RH was significantly higher in BS than in MW and TA, suggesting that CO₂ efflux from BS soils could be increased more under climate warming than that from the other stand types. Our results show evidence that boreal forest composition affects soil CO₂ efflux and that litter quality is not the only factor explaining the differences between stand types. The influence of forest composition on soil CO₂ efflux would be mediated through effects on soil temperature as well as on factors affecting the accumulation and the quality of SOC.