The impact of former land-use type to above- and below-ground C and N pools in short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations in hemiboreal conditions

Short-rotation forestry with fast-growing hybrid aspen (Populus tremula L. × P. tremuloides Michx.) on former arable lands is a novel land-use system in northern Europe that has substantially increased in the region during the last few decades. The objective of this study was to assess the potential of hybrid aspen plantations to sequestrate C and N to above- and below-ground pools from the age of 5 (young) to 15 years (midterm) depending on former land-use type. Data were collected from permanent experimental plots on former croplands (n = 28) and grasslands (n = 23). Based on repeated soil monitoring and destructive model tree sampling, the following C and N pools were estimated: above-ground biomass of trees, soil uppermost layer (A-horizon), coarse roots and subsoil (below A-horizon up to a depth of 75 cm). On average, A-horizon SOC and Ntot pools had decreased significantly on former grasslands, while no change was observed on croplands. Unexpectedly, considerable changes had occurred in subsoil, where SOC and Ntot pools increased significantly on both former land-use types. Therefore, grasslands' A-horizon C loss was compensated for by increased coarse-root fraction and subsoil gains. About one-third of below-ground C pool was stored in subsoil. In above-ground leafless part of 15-year-old hybrid aspen model trees the weighted average C concentration was 45.8% and the N concentration was 0.309%. From the age of 5-15, hybrid aspen plantations acted as C sinks because total ecosystem C pool increased significantly by 3.17 Mg C ha−1 yr−1 on former croplands and by 2.56 Mg C ha−1 yr−1 on former grasslands. The main C sequestration had taken place in the above-ground pool (for croplands 73.8% and for grasslands 94.9%). To conclude, hybrid aspen plantations already showed during the first 15 years a great potential to sequestrate C and N at the ecosystem level, whereas former SOC-exhausted croplands have a higher ability to sequestrate new C to the below-ground pool than already SOC-rich grasslands. Deeper subsoil should definitely be taken into account in SOC estimations.