How Rilling and Biochar Addition Affect Hydraulic Properties of a Clay-Loam Soil

Rill erosion is a significant problem worldwide as it determines relevant amounts of soil loss on hillslopes. Although, in the last few years, many studies have focused on rill erosion and biochar as soil amendment, their influence on soil hydrological properties and relevance on soil conservation strategies is still uncertain. In this paper, the effects of rill formation and biochar addition on the physical and hydraulic properties of a clay-loam soil were assessed by laboratory measurements (water retention, hydraulic conductivity, minidisk infiltrometer data and ¹H Nuclear Magnetic Resonance (NMR) relaxometry with the fast field cycling (FFC) setup) and field tests (rill formation tests at the plot scale). The rilled and non-rilled soils did not show any difference in the volume of pores with a diameter (d) > 300 μm, but the former showed a smaller volume for the pores in the size range between 300 and 0.2 μm. As compared with an untreated rilled soil, the addition of 5% (w w⁻¹) biochar in the soil in which the rill is incised did not change the volume of pores with d > 300 μm, while there were more pores of both 30 ≤ d ≤ 300 μm and 0.2 ≤ d ≤ 30 μm. Moreover, there were less pores with d < 0.2 μm. Shaping the rill did not influence the hydraulic conductivity of the nearly saturated soil (pressure head, h = −1 cm), while it determined a significant decrease of the soil ability to transmit water in more unsaturated conditions (h ≤ −3 cm). The addition of biochar to the soil improved, in general, the soil aptitude to transmit water, regardless of the pore size. However, this improvement was statistically irrelevant in the case of a transport process governed by larger pores. The hydrological measurements also demonstrated that the addition of a large amount of biochar (5%) impedes soil characteristics alteration as the changes due to rilling are balanced by adding biochar in the soil. NMR was also used to measure the structural and functional connectivity of the original soil, the biochar and a mixture with three biochar concentrations (i.e., BC = 1%, 3% and 5% w w⁻¹) traditionally applied in agronomical activity. These measurements revealed that the mixture of soil and biochar was characterised by longitudinal relaxation time (T₁) values, which are related to pore sizes, longer than those measured for the soil. In addition, the soil empirical cumulative frequency distribution of T₁ was always skewed towards shorter T₁ values, thereby suggesting that the macro-pore component (i.e., the largest T₁ values) was never dominant while biochar addition increased the size of mesopores and micropores. Biochar concentrations larger than 3% (w w⁻¹) did not produce appreciable changes in the pore distribution inside the mixture. The biochar component improved the structural connectivity up to BC = 5%, while decreased the functional connectivity up to BC = 3%. A relationship between the water volume contained in soil pores and the NMR data were established for the micropores (d ≤ 0.2 μm). The biochar-amended soil was characterised by fewer small pores, but these micropores were greater than those in non-treated soil.