How does subsoil compaction affect topsoil structure? Insights from the ROCSUB project

Subsoil compaction is a major environmental threat adversely affecting soil functions with potentially negative effects on topsoil too. In the present study, that is part of a long-term experiment called ROCSUB (restoration of compacted subsoil), we aim at better understanding how subsoil compaction and loosening can affect topsoil quality and plant development. The study takes place in a loamy field in western Switzerland and was setup in 2020 after a severe compaction event. Only the subsoil was severely compacted by a heavy pile of excavation material, while the topsoil was removed and stored gently aside during the construction process. Visible signs of compaction were detected up to 70 cm depth. The study includes 2 mechanical treatments (subsoil compaction Vs mechanical loosening) and 2 culture treatments (Salix Vs crop rotation) with 4 plots per treatment, totaling 16 plots. The Salix trees were planted in 2021 as a potential biological soil loosening technique. The crop rotation treatment included temporary grassland in 2021 and 2022, winter wheat in 2023 and maize in 2024.

Soil sampling took place in 2023 in the Salix plots and in 2024 in the cropped plots (during maize). We sampled at 5-10 cm depth and at 30-35 cm depth for bulk density, water content and air content at -60 hPa. Soil structure quality was visually evaluated with the CoreVESS method. Plant productivity was measured for the Salix trees, wheat and maize. Root biomass allocation in the root system was recorded for Salix and maize plants. Earthworm activity was recorded in the Salix treatment.

Our study found that in compacted subsoil, the topsoil structure quality was superior to that in loosened subsoil. This conclusion was drawn by the results from the Salix treatment. In the Salix treatment, subsoil compaction negatively affected aboveground biomass productivity only during the first year after planting but not in the following years. This was attributed to the ability of the Salix roots to adapt to physical constraints through changes in biomass allocation. We hypothesized that this phenomenon occurred because biological activity was concentrated in the topsoil, intensifying its effects and compensating for the inaccessibility of the compacted subsoil. This surprising finding highlights the resilience of soil systems when operating in a "natural" state. In contrast, in agricultural settings such as maize cultivation, the topsoil structure quality did not compensate for the compacted subsoil and root biomass, aboveground biomass, and grain yield were negatively impacted. Taken together, these findings suggest that the impact of subsoil compaction on topsoil structure quality is highly dependent on soil management practices: natural systems demonstrate resilience and compensatory mechanisms, while high-disturbance systems do not.