Density of organic carbon in soil at coniferous forest sites in southern Finland

More detailed knowledge of the density of organic carbon in soils of boreal forests is needed for accurate estimates of the size of this C stock. We investigated the effect of vegetation type and associated site fertility on the C density at 30 mature coniferous forest sites in southern Finland and evaluated the importance of deep layers to the total C store in the soil by extending the sampling at eight of the sites to the depth of ground water level (2.4–4.6 m). The C density in the organic horizon plus 1 m thick mineral soil layer ranged from 4.0 kg/m² to 11.9 kg/m², and, on the average, increased towards the more productive vegetation types. Between the depth of 1 m and the ground water level the C density averaged 1.3–2.4 kg/m² at the studied vegetation types and these layers represented 18–28% of the total stock of C in the soil. The results emphasize the importance of also considering these deep layers to correctly estimate the total amount of C in these soils. At the least fertile sites the soil contained about 30% more C than phytomass, whereas at the more fertile sites the amount of C in soil was about 10% less than the amount bound in vegetation.