Changes in humic substances along an age sequence of Norway spruce stands planted on former agricultural land

Afforestation is one of the most invoked practices to compensate for CO2 emission to the atmosphere because of the high capacity of both the biomass and the soil to store C. This is accompanied by changes in the degradation and transformation of soil organic matter (OM). We investigated the changes in humic substances upon afforestation along an age sequence of 18-, 29-, 64-, 73- and 91-year-old Picea abies (L.) Karst. stands, planted on former agricultural land in Sweden. The forest floor was sampled by separating the Oi, Oe and Oa horizons; the mineral soil was sampled at 0–5 and 5–15 cm depth. Humic acids (HA) and fulvic acids (FA) were extracted and quantified and the changes in HA composition over time were evaluated via characterization of lignin-derived phenols using CuO oxidation, in combination with chemical and spectroscopic analysis. The HA and FA contents in the organic horizons increased progressively with stand age up to 73 years and then remained constant or even decreased at the oldest site. FA progressively dominated over HA with depth, suggesting translocation of the more mobile fraction from the organic horizons to the mineral soil. The HA composition changed with stand age. Elemental composition and 13C nuclear magnetic resonance (NMR) spectroscopy suggest an increase in the HA alkyl component with time, because of selective preservation of highly recalcitrant compounds from resins, waxes and other lipids, and resynthesis of aliphatic microbial products. The aromatic component increased up to 64 years, and then decreased in both the organic horizons and the mineral layers, while lignin-derived phenols in the HA increased up to 29 years and then decreased. In the youngest stands, the lignin composition of the HA in the organic horizons was determined from the input of tree-derived OM, while in the mineral soil the HA showed a greater content of syringyl and cinnamyl units, still bearing some characteristics of OM inherited from the former agricultural land use. With stand age, the acid/aldehyde ratio of vanillyl units [(Ac/Al)V] increased, indicating progressively greater lignin alteration. This resulted in incorporation of lignin-derived phenols into the humic material and leaching of highly oxidized products from the organic horizons into the mineral soil horizons. The latter likely was responsible for the loss of aromatic C from the HA after 64 years and thus the selective preservation of alkyl structures. The translocation of altered lignin products to the mineral horizons and subsequent stabilization therein by association with mineral phases may account for the increase in C observed in the mineral layers of the oldest stands.