Self-restoration of post-agrogenic sandy soils in the southern Taiga of Russia: Soil development, nutrient status, and carbon dynamics

The focus of this chronosequential study was on the vegetation succession, profile morphology, nutrient dynamics, and carbon stocks of post-agrogenic sandy soils under self-restoration of the southern Taiga zone in the European part of Russia. The sites investigated were comparable in climate, texture, and land-use history, but differed in the duration of agricultural abandonment, covering 3, 20, 55, 100, and 170 years of self-restoration. During self-restoration, the vegetation developed towards natural spruce forests and the soils towards natural Podzols. After 55 years of self-restoration, an initial albic horizon under a 2–3 cm thick raw humus layer had developed and after 100 years, all typical Podzol horizons (O–E–Bsh), though relicts of former land use (Ap features), were still present after 170 years. Increasing podzolisation was indicated by a pH decrease from 6.7 to 3.6 (CaCl2), decrease of exchangeable Ca and Mg and decrease of base saturation from 54.0% to 4.3%, C/N ratios increasing from 15.6 to 31.2 and by the loss of pyrophosphate-soluble Fe in the top soil from 340 to 214 μg g− 1 and accumulation in the subsoil from 162 to 896 μg g− 1 in 170 years of self-restoration. During self-restoration, the contents of P and K in total and plant-available forms as well as total N decreased in the top mineral soil, causing a considerable nutrient depletion after 55 years and partly shifting the source of plant nutrients from the mineral part of the soil upwards to the forest floor. Despite this over-all release, the P contents stayed high (817 mg kg− 1) within the relictic ploughed horizons, which was also true for C. But mainly because of increasing SOC stores of the organic surface layer, carbon stocks increased from 4.5 kg C m− 2 to 6.3 kg C m− 2 during self-restoration, indicating a carbon sink. Because of the continued existence of C in parts of the former ploughed horizons, the carbon sink functioning is even larger in self-restorated Podzols than in natural ones.