Developing topsoil structure through conservation management to protect subsoil from compaction

The problem of subsoil compaction is increasing because agricultural machinery is becoming heavier and heavier and exerts pressure to the subsoil. Conservation management promotes edaphic soil structure and increases the bearing capacity of the soil. This may even prevent subsoil compaction. To assess the ability of conservation management to mitigate subsoil compaction, we evaluated hydraulic, biological and structural parameters across the soil profile of two agricultural conservation management systems, no-till and shallow minimum tillage in transition to no-till, in comparison to conventional tillage at two sites in Austria. We found that conservation management resulted in higher aggregate and percolation stability compared to conventional management in the tillage horizon down to 30 cm. In undisturbed soils, this structural stability was correlated with organic carbon content. However, mechanical soil disturbance reduced the structural stability, even if the organic carbon in the soil was still elevated. We found no difference in correlation with soil stability parameters between microbial biomass carbon, dissolved and total organic carbon, despite the different processes by which they contribute to the formation of soil aggregates. Below the tillage horizon, subsoil structural stability was not affected by management. However, the different bearing capacity and thus protective effect of the upper soil layers resulted in lower penetration resistance in depths below the tillage depth for both conservation management systems. As the positive effects of shallow tillage are less concentrated on the soil surface than with no-tillage, we recommend shallow minimum tillage if the subsoil is at risk of compaction.