Soil organic matter changes in a spruce chronosequence on Swedish former agricultural soil I.: Carbon and lignin dynamics

Afforestation of former agricultural soils is an important tool to reduce atmospheric CO2 levels because of the high capacity of both biomass and soil to store carbon (C). The long-term effect of afforestation on the role of soil as C sink was investigated in a chronosequence of 18-, 29-, 64-, 73-, and 91-year-old Picea abies stands, established on former agricultural soils in southwestern Sweden. The forest floor was sampled both as a whole and in the three horizons (unaltered, semialtered, and altered organic horizons), whereas the mineral soil was sampled at 0 to 5, 5 to 15, and 15 to 30 cm depth. The physicochemical characterization of soil, C and nitrogen content, and lignin characterization by the cupric oxide oxidation method was conducted. Within the first decades of afforestation, the soil properties reflected the former soil management, and C accumulation involved mainly the organic layer (3.41-5.92 Mg C ha(-1)), where the new litter was accumulating, and coniferous lignin showed a low degree of degradation. Meanwhile, the mineral soil behaved as a C source (95.5-80.2 Mg C ha(-1)), loosening the C inherited from the previous land use probably because of the nutrient needs of the growing trees that accelerated organic matter mineralization. Vanillyl moieties were almost absent, and the syringyl units, derived from angiosperms, were considerably oxidized, indicating a low C input from the conifer litter and high degradation of the inherited organic material. With age, the C stock started to increase both in the organic and mineral layers, reaching 191.5 Mg ha(-1) in 91 years. The accumulation in organic soil was accompanied by a litter nitrogen impoverishment and decrease of soil pH, which probably limited the microbial community to fungi, able to degrade lignin. In the oldest stands, the lignin material seemed to be intensively altered, mainly in the deeper organic horizons, where aromatic, mobile, acidic compounds were produced and/or translocated down in the profile. This could contribute to organic matter incorporation into mineral layers and to mineral weathering, driving the soil toward the restarting of the podzolization process. From these results, it seemed that even if the previous land use caused an initial acceleration of organic matter mineralization, soil started to accumulate C when the influence of the new vegetation dominated. The soil became a major C sink with a long forest growth of about a century. The accumulation was strongly affected by litter composition, which changed with spruce age.