Development and implementation of a soil compaction risk assessment system

Soil compaction due to traffic with modern agricultural machinery is one of the major threats to soil quality. Ever increasing weights of farm machinery particularly result in compaction of the subsoil, a particularly serious issue because it is effectively persistent. Compaction adversely affects soil ecosystem services, in particular regulating and production services, resulting in significant ecological and economic damage to farmers and society. Despite its importance, soil structure remains difficult to define at scales relevant to field applications. Typically, it characterization relies on laboratory measurements on core samples or on episodic small scale field measurements, offering limited perspectives on its spatial and temporal variations. The motivation of this thesis rests on the need for characterization techniques that provide soil compaction information at relevant spatial scales and can guide strategies to maintain favorable soil structure. Therefore, the major aim is to integrate the use of different geophysical techniques to tackle the challenging non-invasive characterization of soil compaction directly or indirectly by measurement of surrogate variables. First, we identified and tested the preponderant geophysical methods employed in soil science based on the electrical properties of soil materials, which manifest main concomitant variations with hydraulic states. We explored the ability of Electrical Resistivity Tomography (ERT) and Frequency Domain Electromagnetic Method (FDEM) for monitoring the effects over time of different agricultural practices, for a better understanding of the complex soil-plant-water interactions that take place underground. Subsequently, we tested the ability of the same techniques (i.e. ERT and FDEM) to assess the effects of compaction on agricultural soil, highlighting the pros and cons of each. Both DC-resistivity and FDEM data revealed that soil compaction leads to a persistent decrease in soil electrical resistivity (>30%), although, however, the electromagnetic response of the soil is influenced by various factors (mineralogy, texture, porosity, fluids, organic matter). In any case, it is necessary to always consider sensitivity to the phenomenon and optimize target-specific acquisition schemes. Thereafter, we moved to seismic methods, little or not used at all in agronomy, given their ability to provide unmatched insights into the mechanical properties of materials. The combined approach of volume waves and surface waves made it possible to clearly characterize and isolate surface compaction, paving the way for the realization of a seismo-electric system capable of mapping large areas in a short time. Finally, we explored the opportunities of hydrogeophysics in characterizing the hydrological dynamics of compacted and uncompacted systems. Within an experimental site, equipped with borehole electrodes and soil sensors, we monitored water infiltration processes by means of multiple geophysical data. Most of these results were achieved at the L. Toniolo experimental farm, located nearby Padova’s university campus of Legnaro, Italy. The research reported here is a contribution to the use of geophysical methods to address the challenge of spatially characterizing soil structure and dynamics, to diagnose chronic soil degradation, and to assist agronomic activities by providing more detailed, complementary, and spatio-temporal soil information. The work, however, also demonstrates the limitations of the approach, discussing a range of perspectives for future improvements. While not sufficient on their own, geophysical methods remain a useful tool for the emerging field of agrogeophysics and can provide valuable insights for shaping future agricultural practices. Above all, the thesis provides a reference for the continued integration of geophysics and soil research through careful monitoring combined with modelling.