A New Approach Using Modeling to Interpret Measured Changes in Soil Organic Carbon in Forests: The Case of a 200 Year Pine Chronosequence on a Podzolic Soil in Scotland

Scotland is continuing to afforest land in order to combat climate change, but thelong-term capacity for carbon sequestration in forest soils is still uncertain. Here wepresent measurements that provide comparative estimates of soil organic carbonin grassland and forestry sites at steady state. We develop a new approach tointerpret these values based on simulation of organic carbon turnover in soils that areaccumulating carbon and use this to determine losses due to management operationsassociated with afforestation of grassland and deforestation/reforestation of foreststands. Soil organic carbon stock changes were studied in a >120 year-old Scotspine chronosequence and adjacent grassland sites on podzolic soils. Significant carbonaccumulation was measured in the top organic soil horizons with forest age, whileno changes were noted in the deeper mineral soil horizons. The simulations with theRothC-26.3 model revealed that pine forests on sandy soils could lose a significantamount of soil organic carbon through management operations. The lowest modeledstocks of soil organic carbon were not in the young sites (0–25 years old), but at 43years since reforestation. Using measured data from our study site, the simulationsof grassland afforestation suggested that accumulation of organic carbon under forestoccurs mainly in the organic horizons, while the deeper sandy mineral soil horizons arelikely to become depleted in organic carbon compared to grasslands. Our simulationssuggest that afforestation of grasslands would increase overall soil carbon stocks butmay deplete the more stable carbon pools in the deeper mineral horizons of the podzols