Sustainable increase of subsoil SOC in loamy soils by fractional deep tillage (FDT)

The melioration technique “fractional deep tillage” (FDT) was developed in the late 1950s to increase crop yields and crop yield security in East Germany (former GDR) by remediating subsoil compaction and increasing topsoil depth. It is characterized by the shaft-wise exchange of topsoil and subsoil material. Today, FDT is of particular interest due to its carbon accrual and CO₂ sink effect. On the one hand C-rich topsoil material is shifted into 50 cm deep shafts, which were created by special plough shares in 35 to 80 cm intervals. At the same time C-poor subsoil material is lifted into the topsoil. This reduces the SOC content of the topsoil and reinforces carbon sequestration due to the induced imbalance in the C cycle of the soil-plant-system. The C accrual until topsoil equilibrium is a quite fast process (Burger et al., Geoderma 2023) – due to the admixture of highly reactive, unsaturated subsoil mineral phases. The CO₂ sink potential of a soil strongly depends on the long-term fate of the SOC buried in the subsoil.

Here we present results from a unique 40 years old historical field trial in NE Germany (“Wolfshagen”). A complete erosion-deposition sequence, typical for soil landscapes of hummocky ground moraines, was studied: “Calcaric Regosol - Nudiargic Luvisol - Stagnic Luvisol – Luvic Stagnosol - Gleyic Colluvic Regosol”. As climate and farming practice along the sequence were identical soil-related factors influencing the long-term fate of buried topsoil SOC in subsoils can be identified. Soil samples were taken from the shafts and the area next to the shafts and the contents of carbon (SOC, carbonates), nitrogen, pedogenic oxides (Fe, Al, Mn) as well as soil texture were analyzed.

We found that the SOC content in the shafts was significantly increased compared to the subsoil next to the shafts. The average increase of subsoil SOC in the shafts was 224±125%. The highest increase in subsoil SOC was found in the Stagnic Luvisol with 383% and Calcaric Regosol with 345%, whereas the smallest increase was found in Gleyic Colluvic Regosol with 45%. To calculate the potential SOC increase in the subsoil (25-50 cm depth) by fractional deep tillage, the dimensions of the CarbonFarming plough currently under development and the SOC data in the shaft and subsoil after 40 years were used. The results show that SOC stocks in the subsoil could be increased by 5.7 t ha^-1 on average (range: 3.2 t ha^-1 in the Gleyic Colluvic Regosol, 7.7 t ha^-1 in the Luvic Stagnosol).