Characterization of microstructural stability of biochar-amended Planosol under conventional tillage for irrigated lowland rice ecosystem

Soil structural degradation through compression from machine and animal traffic affects soil functioning and threatens environmental sustainability. However, the structural stability of the soil against the external stresses can be enhanced through the addition of organic substances. The study aimed to investigate the efficacy of biochar addition on the microstructural stability of a Gleyic Luvic Planosol under yearly conventional tillage of rice-fallow-rice lowland ecosystem in southern Brazil. Organic amendment consisted of four levels of rice husk biochar rates namely 0 % (Bio0), 2 % (Bio2), 5 % (Bio5) and 10 % (Bio10) by dry mass, corresponding to 0, 20, 50, and 100 g biochar/kg soil. As organic matter is known to closely interact with density, we decided further to investigate the impact of varying density, namely 1.15, 1.33, 1.45, 1.60, and 1.75 g/cm3. Biochar-incubated soil samples were subjected to four levels of matric potentials (0, -3, -6, and -10 kPa). The microstructural stability parameters of strain (gamma), shear resistance (tau), storage and loss moduli (G', G"), and coefficient of structural rigidity (Iz) of the amended soil samples were evaluated using the amplitude sweep test in a rheometer. Increasing biochar rate significantly (p < 0.05) increased the soil organic matter and, consequently, the water retention capacity of the biochar-amended Planosol. The stabilizing effect of biochar addition was reflected in higher microstructural stability of the amended soil compared to the control (no biochar). Increasing density in the biochar-amended Planosol increased the shear resistance, reduced the strain at linear viscoelasticity and yield point, and decreased the stiffness degradation. Increased strength and quantity of the menisci are the driving forces for the improved microstructural stability of the Planosol and at the nucleus of environmentally sustainable application. This is important when considering potential carbon sequestration and storage effects of biochars.