Wood-Biochar Influence on Rill Erosion Processes and Hydrological Connectivity in Amended Soils

Although there is increasing interest in biochar as a soil amendment, its antierosive effectiveness is still uncertain. This investigation aims at evaluating how wood-biochar affects rill erosion and hydrological connectivity in amended soils. In this paper, at first, plot experiments were performed entering a clear inflow into two rills, named rill₃ and rill₅, incised in a soil amended with an initial biochar concentration BC in weight of 3% and 5%, respectively. For each rill, terrestrial photogrammetry was used to obtain the Digital Elevation Models (DEM) before and after the experimental runs, and the consequent DEM of difference (DoD) was used to calculate the total volume of the eroded mixture (sediment and biochar), while three samples of rill outflow discharge were collected to determine the biochar and sediment rates. Then, small laboratory samples of the soil, biochar, and mixtures with different BC (1%, 3%, 5%, 10%, and 30%) were used to determine size and distribution of pores, and thus measure the structural and functional connectivity, by nuclear magnetic resonance (NMR) relaxometry with the fast field cycling (FFC) layout. The DoDs highlighted that the mixture volume for rill₅ was lower than that for rill₃. Moreover, the rill₅ condition yielded a higher biochar percentage in the mixture. The NMR measurements demonstrated that the biochar addition increases the size of micropores and mesopores, and the macro-pore component is never dominant. Biochar concentrations greater than 5% do not produce appreciable changes in the pore distribution inside the mixture. The biochar component improves the structural connectivity up to BC = 5%. In the BC range of 0%–3%, FCI decreased as BC increased. In conclusion, a target biochar concentration of 5% allows for the mitigation of the rill erosion phenomena, favours the improvement of soil structural connectivity, and does not appreciably modify the functional connectivity.