Model-based evaluation of the impact and longevity of a novel sustainable subsoil melioration method (Soil³ method) on root growth

The subsoil, commonly defined as the soil beneath the tilled or formerly tilled soil horizon, contains large amounts of nutrients and water. Large fractions of these subsoil resources are not readily available to agricultural crops due to compacted layers of high bulk density. Although there are conventional methods for loosening compacted subsoils (e.g., mechanical subsoiling and deep ploughing), their effects are often quickly reversed or can even be harmful to the soil structure. Eventually, the brief enhancement in subsoil access for crops is often insufficient to justify the considerable expenses associated with the methods. To facilitate a more efficient use of subsoil resources, the Soil³ project for sustainable subsoil melioration derived a novel  approach, which is carried out in a single crossing of the field. First, the top soil of a 30 cm wide strip is excavated and deposited on the soil surface beside the strip, creating a furrow. The subsoil in this furrow (30-60 cm depth) is then loosened and intermixed with organic material (e.g., compost). After mixing, the excavated topsoil is lead back into the furrow, thus closing it again. The method therefore preserves the natural soil structure by not mixing the top and subsoil substrates, while the loosened subsoil structure is stabilized by incorporating organic material. Additionally, the operating costs are kept reasonable by only loosening the soil in a strip-wise manner.

Here, we use and extend the 3D functional-structural plant model CPlantBox to investigate the impact of the Soil³ method on root growth. On the soil side, we employ pedo-transfer functions to model the evolution of soil bulk density (soil setting) and the resulting changes in soil hydraulic properties in time. The pedo-transfer functions are parameterized based on data of the Soil³ field trials and are solved for different soil depths, as well as for the soil layers on and beside the melioration strips. In our model, we account for the time-dependent changes in soil hydraulic properties of all soil layers by implementing the usage of variable Van-Genuchten parameter sets within a single simulation run. Based on the parameterized soil domain, we simulate root growth and root water uptake from the different soil layers. Experimental data is used to parameterize general root growth parameters (e.g., root length density, planting density, transpiration). The explicit 3D root system architecture, however, is a result of the model, and its growth is modeled as a function of bulk density, water content and penetration resistance. By performing virtual replications of the field trials over multiple consecutive years, we can evaluate the impact and longevity of the subsoil melioration on root growth and its underlying processes.