Effect of temperature on soil organic matter decomposition in three forest biomes of eastern Canada

The sensitivity of soil organic matter decomposition to temperature change is critical tothe global carbon balance and to whether soils will respond with positive feedback to climate change. Forest cover determines litter composition, which controls to a large extent soil organic matter quality and its sensitivity to temperature. The effect of temperature on soil organic matter decomposition was studied along a latitudinal gradient encompassing sugar maple, balsam fir and black spruce forest types. Long-term laboratory soil incubations conducted at four different temperatures were used to discriminate the effect of temperature from that of organic matter quality on decomposition rates. The specific C mineralization rate of the humus layer was highest for balsam fir sites, intermediate for one sugar maple site and lowest for black spruce sites and the other sugar maple site. However, considering the total C pools of the FH layer and of the top 20 cm of mineral soil, it was estimated that coniferous sites exhibit a higher C efflux than sugar maple soils at any given temperature. Estimated C mineralization rates in the field using the temperature records for each individual site showed the same trends despite cooler temperature regimes for the coniferous sites. The Q10 respiration rates of the humus layer of all sites increased as the temperature got warmer. A significant effect of temperature on the pool size of labile C in the mineral soil was detected for some sites suggesting a potential long-term loss of C upon warming. The low estimated C evolution rates of sugar maple soils were perhaps due to the greater decomposition activity within the L layer, before the litter C enters underlying soil pools. These observations suggest that coniferous soils are not more resistant than deciduous forests to increasing their specific rates of soil heterotrophic respiration upon warming. Key words: Soil organic carbon, forest type, forest composition, warming, long-term incubation, labile carbon