Sustainable increase of SOC stocks and nutrients in sandy subsoils by ameliorative fractional deep tillage (aFDT)

The sandy soils of Eastern Germany show a high gradient in soil organic carbon (SOC) between topsoil and subsoil. This is reflected in the low nutrient supply, water storage capacity and increased compaction susceptibility of the subsoil. In view of these productivity limitations, ameliorative fractional deep tillage (aFDT) was developed in the late 1950s.

The aim of this subsoil melioration measure is to create shafts enriched with organic-rich topsoil material in the subsoil to remediate root restricting layers and promote accessibility of subsoil resources. At the same time, organic C sequestration is induced by mixing of subsoil material low in organic matter into the topsoil. The 50 cm deep and 7-15 cm wide shafts are created at 35-80 cm intervals by a special plough or modified loosener. Thereby, solid zones remain between the shafts to ensure stability and reduce the risk of re-compaction of the strip wise deep tillage.

We studied three 37- to 43-year-old historical field trials representing a soil quality gradient near Müncheberg to investigate the long-term effects of aFDT on SOC accumulation as well as nutrient stocks in the subsoil and organic carbon sequestration in the topsoil. Besides the shaft, the topsoil, the area next to the shaft, the subsoil material below the shaft and reference topsoil without aFDT outside the trial plot were sampled. In total, 43 shafts were sampled.

We found that the 50-75% of the original SOC was still preserved in the shafts. The shafts had significantly higher SOC contents (+286%) and nutrient contents (P+75%, K +33%, Mg +50%) compared to surrounding subsoils (E/Bw horizons). These results indicate that aFDT is an effective melioration method to increase the SOC and nutrient stocks.