Protecting the subsoil from compaction through conservation management.

The intensification of agriculture has increased production but has also had negative effects on the soil. One negative effect of intensified management is soil compaction caused by heavy agricultural machinery. The depth to which problematic compaction occurs depends on the load, the bearing capacity of the soil and the prevailing soil conditions. The most persistent problem is subsoil compaction, as natural or mechanical loosening in great soil depths is ineffective or difficult. A key strategy to prevent subsoil compaction is to increase the carrying capacity of the soil through appropriate management. We investigated whether two common conservation agriculture systems, reduced shallow tillage and no-tillage, can protect the subsoil from compaction compared to conventional tillage management with similar loads applied through agricultural machinery. We assessed the effects on soil structure in the soil profile down to 50 cm depth at two sites in Austria (temperate continental climate) in terms of compaction and structural, hydraulic and biological parameters. Both conservation management systems led to better structural stability at the soil cultivation horizon than in conventionally managed soils. Based on measurements taken at three depths down to 30 cm soil depth, we measured higher aggregate stability of 142 % (reduced shallow tillage) and 135 % (no-tillage) for conservation tillage than for conventional tillage. In undisturbed soil layers, the dependence of structural stability on humus content was observed. However, this stability decreased when the soil was mechanically disturbed, regardless of the remaining humus content. Under the soil cultivation horizon, differences in structural and biological parameters were negligible. For hydraulic parameters, there was a slight trend towards higher water storage capacity (+ 1-2 %) and hydraulic conductivity. This is attributed to the lower subsoil compaction achieved by both conservation systems.  At a soil depth of 45-50 cm, we measured a 6% (reduced shallow tillage) or 9% (no-tillage) lower bulk density with conservation tillage than with conventional tillage. As the positive effects of no-tillage on soil structure are more limited to the soil surface, and the reduced shallow tillage has a more uniform effect on deeper soil layers, we recommend reduced shallow tillage in similar pedoclimatic regions where there is a risk of subsoil compaction.