Incorporation of Biochar Carbon into Stable Soil Aggregates: The Role of Clay Mineralogy and Other Soil Characteristics

Aggregation and structure play key roles in water-holding capacity and stability of soils. In this study, the incorporation of carbon (C) from switchgrass biochar into stable aggregate size fractions was assessed in an Aridisol (from Colorado, USA) dominated by 2:1 clays and an Alfisol (from Virginia, USA) containing weathered mixed 1:1 and 2:1 mineralogy, to evaluate the effect of biochar addition on soil characteristics. The biochar was applied at 4 levels, 0, 25, 50, and 100 g kg⁻¹, to the soils grown with wheat in a growth chamber experiment. The changes in soil strength and water-holding capacity using water release curves were measured. In the Colorado soil, the proportion of soil occurring in large aggregates decreased, with concomitant increases in small size fractions. No changes in aggregate size fractions occurred in the Virginia soil. In the Colorado soil, C content increased from 3.3 to 16.8 g kg⁻¹, whereas in the < 53 µm fraction C content increased from 5.7 to 22.6 g kg⁻¹ with 100 g kg⁻¹ biochar addition. In the Virginia soil, C content within aggregate size fractions increased for each size fraction, except the > 2 000 µm fraction. The greatest increase (from 6.2 to 22.0 g kg⁻¹) occurred in the 53–250 µm fraction. The results indicated that C was incorporated into larger aggregates in the Virginia soil, but remained largely unassociated to soil particles in the Colorado soil. Biochar addition had no significant effect on water-holding capacity or strength measurements. Adding biochar to more weathered soils with high native soil organic content may result in greater stabilization of incorporated C and result in less loss because of erosion and transport, compared with the soils dominated by 2:1 clays and low native soil organic content.