Long-term improvement of subsoil pore structure in sandy soils by meliorative fractional deep tillage

• Pore structure effects of meliorative Fractional Deep Tillage (mFDT) persist for more than 40 years. • mFDT sustainably improves subsoil pore structure by increasing biopores, particularly in sandy soils. • Long-term decrease of bulk density in subsoils after mFDT. • Long-term increase in plant-available P and SOC after mFDT. Soil compaction remains a major constraint on agricultural productivity, exacerbated by increasing machinery weights since the mid-20th century. While deep tillage is traditionally used to alleviate compaction, its long-term effectiveness is often limited by re-compaction due to field traffic. This study investigates the long-term effects of meliorative fractional deep tillage (mFDT) on subsoil properties more than 40 years after its application in four field trials in Eastern Germany. Soil samples were collected from historical mFDT shafts and surrounding subsoil, with analyses conducted using X-ray computed tomography (X-ray CT) to assess pore structure, connectivity, and biopore formation. Soil chemical analyses were performed to evaluate nutrient availability and soil organic carbon (SOC) content. Results indicate that mFDT significantly improved subsoil macroporosity, particularly in sandy soils, with increased pore connectivity facilitated by root-derived biopores. In contrast, in more loamy soils, where stable biopores already existed in the subsoil, significant structural changes were not observed. However, in all soils, particulate organic matter (POM) accumulation within the shafts suggests that these zones continue to function for preferential root growth into the subsoil creating an active SOC reservoir. These findings underscore the long-term agronomic and environmental benefits of mFDT, particularly in sandy soils, by mitigating compaction, enhancing root penetration, and potentially supporting carbon stability in subsoils.