Soil aggregate size distribution and total organic carbon in intra-aggregate fractions as affected by addition of biochar and organic amendments

A two-year field trial on maize (Zea mays L.) production was established to determine the influence of biochar, maize straw, and poultry manure on soil aggregate stability, aggregate size distribution, total organic carbon (TOC), and soil microbial biomass carbon (MBC). Seven treatments with four replications, namely CK, control; S, 12.5 Mg ha-1 straw; B1, 12.5 Mg ha-1 biochar; B2, 25 Mg ha-1 biochar; SB1, straw + 12.5 Mg ha-1 biochar; SB2, straw + 25 Mg ha-1 biochar; and M, 25 Mg ha-1 manure were tested at four soil depths (0–10, 10–20, 20–30, and 30–40 cm). Aggregates were grouped into large macro-aggregates (5–2 mm), small macro-aggregates (2–0.25 mm), micro-aggregates (0.25–0.053 mm) and silt + clay (<0.053 mm). Biochar, straw, and manure applications all had significant effects (p < 0.05) on aggregate stability, with B2 at 20 cm soil depth showing the greatest increase (62.1%). SB1 of small macro-aggregate fraction showed the highest aggregate proportion (50.59% ± 10.48) at the 20–30 cm soil depth. The highest TOC was observed in SB2  (40.9 g kg-1) of large macro-aggregate at 10–20 cm soil depth. Treatment effects on soil MBC was high, with B1 showing the greatest value (600.0 µg g-1) at the 20–30 cm soil depth. Our results showed that application of biochar, straw, and manure to soil increased aggregate stability, TOC as well as MBC