Contribution of Geophysics to the Study of Urban Soils Amended with Biochar

By 2050, climate changed-induced extreme heat waves and urban heat island phenomenon will lead to severe soil drought, which can affect trees health and sustainability of green spaces. By-product of the pyrolysis of biomass, biochar incorporation can favor soil water retention and can be seen as a potential tool to mitigate future soil drought. In order to test the ability of biochar incorporation into soil to improve water retention, experimentations are necessary to evaluate its efficiency. To do so, experimental tests involving monitoring would require destructive, costly and sparse spot measurements that are not representative of the heterogeneous nature of studied technosols. This work explores how geophysics can overcome such limitations, by enabling spatialized, non-invasive monitoring of soils at the scale of experimental plots. Control and biochar-amended plots (n > 3) were monitored over time with spectral induced polarization (SIP) and active seismic methods. Initial results suggest that SIP can distinguish the presence of biochar through marked contrasts in resistivity and phase, which evolve with time suggesting a potential monitoring of biochar aging (e.g. fragmentation, migration and oxidation). At the same time, seismic surface-wave velocity measurements show sensitivity to seasonal variations, as well as a quasi-systematic decrease in velocities in amended soils, which could reflect changes in porosity and/or water content. This approach serves as a proof of concept for highlighting the potential of geophysics as an in situ diagnostic tool, capable of monitoring the effect of biochar by providing reliable and integrative indicators of water content in heterogenous urban soils.