Variation in stocks and distribution of organic C in soils across 21 eastern Canadian temperate and boreal forests

Quantifying soil organic carbon (SOC) stocks in forests and identifying the factors that control their size is crucial to predict how they will be affected by climate and land-use changes. Here, we assessed variations in SOC stocks in the forest floor (FF) and the mineral soil of 21 temperate and boreal forest sites in Quebec, and analyzed their relationships with 13 biophysical variables. Across the studied area soil C stocks ranged from 9.2 kg m(-2) to 27.8 kg m(-2) with on average 78% of this C located in the mineral horizons. Carbon stocks in the FF increased with Mean Annual Precipitation (MAP) and the Percentage of Conifers (Pc), and decreased with Mean Annual Air Temperature (MAAT). Total and mineral soil carbon stocks increased with MAP but were poorly correlated with the other variables mainly because of the high variability in C concentration in B-horizons and mineral soil thickness. When the latter was restricted to 50 cm, the explanatory power of the statistical model for total soil C stock shifted from 34% to 49%, illustrating the impact of soil sampling thickness on C stocks estimates and on the ability to predict them. Regression analyses showed that SOC stocks were mainly controlled by MAP. The depth at which 50% of the mineral soil C stock is reached (1350) was used as an index for the distribution of SOC across the soil profile. The 1350 values ranged from similar to 2 cm to similar to 40 cm and increased with altitude and Pc, and conversely decreased with the percentage of hardwoods and MAAT. In light of the recent findings on the origin of C stabilization in soils, these results suggest that these impacts on SOC distribution might also affect the stability of the SOC pool. They also emphasize the importance of sampling depth choice according to site characteristics and the importance of the vegetation not only on FF C stocks but also on C distribution across the soil profile. (C) 2015 Elsevier B.V. All rights reserved.