Change in soil organic carbon following afforestation of former arable land

Abstract

The resolution of the Kyoto Protocol to include effects of land use and land-use change in global carbon budgets has put focus on C sequestration following afforestation of former arable land. Carbon is sequestered in the aggrading biomass of the new forests, but the question remains, to what extent the former arable soils will contribute as sinks for CO 2. The present study explored changes in soil C stores following afforestation of former arable land with oak ( Quercus robur L.) and Norway spruce ( Picea abies (L.) Karst.). Seven stands of each tree species on nutrient-rich soils made up a chronosequence ranging from 1 to 29 years. An adjacent ∼200-year-old mixed deciduous plantation was included to give information on the possible long term changes in soil C. Soil sampling included organic layers and three layers of the mineral soil to a depth of 25 cm. Development of forest floors sequestered most C in spruce stands. While oak stands sequestered around 2 Mg C ha −1, spruce stands sequestered approximately 9 Mg C ha −1 in forest floors over 29 years. There were no differences between the two tree species in C concentration and storage of the three mineral soil layers. Carbon concentration and storage increased in the upper 5 cm of the mineral soil but decreased in the 5–15 and 15–25 cm soil layers with increasing stand age. The soil C store appeared to be undergoing redistribution following afforestation, and mineral soil C stores in 0–25 cm tended to decrease over the 29-year period. Together with the C sequestration of forest floors, this led to fairly similar total soil C stores of around 65 Mg ha −1 along the 29-year chronosequence. Within this short time span, C sequestration mainly occurred in the biomass of trees while soil C stores were clearly higher in the ∼200-year-old plantation (81 Mg C ha −1). The ongoing redistribution of mineral soil C in the young stands and the higher soil C contents in the ∼200-year-old afforested stand suggest that nutrient-rich afforestation soils may become greater sinks for C in long term.