Legacy of plaggen agriculture: High soil organic carbon stocks as result from high carbon input and volume increase

Plaggic Anthrosols are anthropogenic soils formed by former plaggen agriculture in north-western continental Europe. Here, we present an inventory of organic carbon (OC) concentrations and stocks of Plaggic Anthrosols and of reference soils in the vicinity without plaggen fertilization history from eight sites in northwest Germany (Lower Saxony, North Rhine-Westphalia) and the eastern parts of The Netherlands (North Brabant, Overijssel). To evaluate the enrichment of Plaggic Anthrosols with OC and the implications for OC stocks, soils were analysed for basic soil properties (bulk density, pH, OC − and nitrogen (N) concentrations), texture, and acid oxalate-(Fe_OX) and dithionite-citrate-bicarbonate-extractable iron (Fe_DCB) concentrations. Organic C stocks and plant-available water capacity (PAWC) were calculated. The plaggic topsoils are characterized by high OC concentrations (6.5–36.9 mg g⁻¹) and a thickness of 53–124 cm, in contrast to the modern cultivated ∼30 cm thick topsoils of the reference soils with 10.6–24.0 mg g⁻¹ OC concentration. The plaggen agriculture formed a soil with improved properties and conditions for plant growth. The addition of plaggen material to the topsoils induced a volume increase as well as higher OC concentrations in the Ap horizons, resulting in significantly higher OC stocks in the Plaggic Anthrosols (6.3–14.5 kg m⁻²) compared to the reference soils (4.9–10.5 kg m⁻²). The additional soil volume amounted to 30–56% to the total soil profile depth (1.0 to 1.6 m) and comprised 25–62% of the total OC stocks. Compared to the reference soils, the volume increase of the plaggic topsoil increased the PAWC in the effective rooting zone by 1.6 to 3.4 times. Plaggic Anthrosols represent a relic of historical farming with preserved high OC concentrations and improved storage capacity for plant-available water, causing higher nutrient availability and an increase in yields, which in turn result in higher in-situ OM input, additionally contributing to high OC stocks. The results of this study demonstrate the potential of soil melioration practices similar to the former plaggen management to induce high OC concentrations and increase OC stocks in sandy agricultural soils.