Simulating the effects of past changes in climate, atmospheric composition, and fire disturbance on soil carbon in Canada's forests and wetlands

[1] Using the Integrated Terrestrial Ecosystem Carbon Cycle model ( InTEC), six simulations with different input scenarios of climate, CO2, and nitrogen ( N) deposition are conducted to study the changes of soil carbon ( C) content in Canada's forests and wetlands during 1901 - 2000. Simulated total C stored in Canada's forest and wetland soils is 164.5 Pg C and accounts for about 7% of the global total of 2400 Pg C to the depth of 2 m, implying the significance of Canada's forest and wetland soils in the global terrestrial C cycle. Soils of Canadian forests and wetlands sequestrated 3.9 Pg C ( 2.6 Pg C in forests and 1.3 Pg C in wetlands) during 1901 - 2000 because of the integrated effects of climate, CO2 fertilization, N deposition, and forest age factors. The changes of soil C content during 1901 - 2000 ranged spatially from-2 kg Cm-2 to 4 kg Cm-2, depending on fire disturbance history, climate change pattern, and N deposition rates. Soil C increased by 2 to 4 kg C m(-2) in Eastern Hudson Plains, Eastern Middle Boreal Shield, Southern Boreal Shield, and Atlantic Maritime and decreased by more than 1 kg Cm-2 in Southern Boreal Plains. Simulations shows that climate influences growing conditions, growing season length, net N mineralization, and N fixation and therefore was the biggest driver of the increase in total soil C content during 1901 - 2000, followed by CO2 fertilization and N deposition. The climate- induced increase of soil C occurred mainly in the cool and wet period from the middle 1940s to the middle 1970s. Overall, an increase of 1 C in mean annual temperature induced heterotrophic respiration to increase by 62 Tg C a(-1). In contrast to the century- scale trend from 1901 to 2000, during the last two decades ( 1981 - 2000), CO2 fertilization was the biggest driver of the increase in soil C, while climate change alone caused soil C to decrease.