Carbon storage in forest soil of Finland .1. Effect of thermoclimate

A total of 30 coniferous forest sites representing two productivity classes, forest types, were investigated on a temperature gradient (effective temperature sum using +5 degrees C threshold 800-1300 degree-days and annual mean temperature -0.6-+3.9 degrees C) in Finland for studying the effect of thermoclimate on the soil C storage. Other soil forming factors were standardized within the forest types so that the variation in the soil C density could be related to temperature. According to the applied regression model, the C density of the 0-1 m mineral soil layer increased 0.266 kg m(-2) for every 100 degree-day increase in the temperature sum, and the layer contained 57% and 28% more C under the warmest conditions of the gradient compared to the coolest in the less and more productive forest type, respectively. Accordingly, this soil layer was estimated to contain 23% more C in a new equilibrium with a 4 degrees C higher annual mean temperature in Finland. The C density of the organic layer was not associated with temperature. Both soil layers contained more C at the sites of the more productive forest type, and the forest type explained 36% and 70% of the variation in the C density of the organic and 0-1 m layers, respectively. Within the forest types, the temperature sum accounted for 33-41% of the variation in the 0-1 m layer. These results suggest that site productivity is a cause for the large variation in the soil C density within the boreal zone, and relating the soil C density to site productivity and temperature would help to estimate the soil C reserves more accurately in the boreal zone.