Persistent carbon loss from the humus layer of tilled boreal forest soil

Abstract

SummaryEffects of forest floor tillage on the soil organic matter (SOM) content of a podzol were studied across the boreal forest zone in Finland. At each of the 93 study sites, where an old natural forest stand is bordered by a tilled or non‐tilled clear‐cut‐regenerated stand, 20 topsoil cores were collected from both the old and regenerated stand for SOM determination. An analysis of covariance (ancova) revealed a significant decrease in the SOM pool related to tilled soil on the clear‐cut stands compared with the adjacent old‐forest stands. A relative increase in SOM on several of the most recent ‘clear‐cut’ sites appears to be a transient feature attributable to logging residue. The average decrease in SOM for all the > 10‐year tilled sites was 1260 g OM m−2, corresponding to a decline of about 15% from the old‐forest average. For clear‐cut sites without tillage, there was an average loss of only 300 g OM m−2, or 3.9%. Based on the current rates of forest floor tillage in Finland (1200 km2 annually), CO2 emissions were estimated to be in the order of 2.8 Tg year−1, which suggests a total of about 130 Tg CO2 has been emitted since the 1960s (about 58 000 km2 of forests have been tilled so far). The persistent reduction in carbon stocks observed in tilled forest soil contradicts the carbon balance neutrality claimed for the intensive forestry currently practised in Finland.Highlights Effects of clear‐cut forestry and soil tillage on SOM stores in Finland. Soil at clear‐cut borders of old‐forest stands reveals effect of forestry on soil C balance. Tilling of soil on clear‐cuts typically reduces SOM by 10–15% on average. The results suggest that present Finnish UNFCCC reporting for boreal forest soil is erroneous.