Forest continuity, soil depth and tree species are important parameters for SOC stocks in an old forest (Templiner Buchheide, northeast Germany)

Abstract

Forest mineral soils have the potential to accumulate large amounts of carbon (C). Numerous factors, which have often been insufficiently studied, affect soil organic C (SOC) stocks. Detailed knowledge of variation in SOC storage is important to assess the C accumulation potential of forest soils. To examine the impacts of forest continuity, soil depth and tree species on SOC stocks, 15 ancient (>230years of forest continuity) and 15 old (>100 but <200years of forest continuity) forest soils, topsoil and subsoil in the Templiner Buchheide (Brandenburg, NE Germany) were compared. The old forest sites were afforested on former grassland or wasteland. On all sites grew one of three dominant tree species: European beech (Fagus sylvatica), Scots pine (Pinus sylvestris) or oak (Quercus spec.). Pine forest sites had been underplanted with beech and were mixed-species stands. Soil samples were taken down to a mean depth of 55cm. Total contents of SOC, nitrogen (N), phosphorus (P), sulphur (S), calcium (Ca), potassium (K) and magnesium (Mg); soil pH; and bulk densities were determined.The soils of ancient forest sites stored significantly more total SOC, N, P, S, K and Mg than did the old ones. Mean total SOC stocks in ancient forests of all three tree species were 12–17% larger compared with those in old forests. Significant differences in SOC stocks between the two forest continuity groups appeared only in subsoil and not in topsoil. Pine forest stored larger SOC stocks than did beech and oak forests. Significant differences were found between ancient pine and oak forests and between ancient beech and oak forests. Soils in ancient beech and pine forests at depths of between 29 and 55cm contained, on average, even 50% larger SOC stocks than did soils at the same depths in ancient oak forests and in all old forests.Forest continuity significantly affected SOC stocks. These results support previous studies that old forests are still able to enrich SOC. Although soil samples were carried out to a mean depth of only 55cm, the results indicate that differences in SOC stocks between ancient and old forest could also be found in deeper soil layers. It was suggested that beech and mixed-species stands of beech and pine and total soil P stocks had a positive effect on SOC stocks in subsoil. To understand SOC accumulation in forests, especially in subsoil, with a forest continuity of >100years, the role of different tree species and of total P cycling in forests, deeper sampling depths and repeated sampling would be required.