Biochar–Clay–Compost Composite Materials for Sustainable Soil Management: Effects on Soil Biochemical Activity, Pore Architecture, and Water Retention

The development of sustainable soil amendment materials derived from natural and recycled resources represents an important strategy for improving soil functionality and mitigating the impacts of land degradation and climate change. This pot study evaluated the effects of biochar (BC), poultry litter (PL), smectite-silica clay (SSC), and two composted combinations, C(PL+BC) and C(PL+BC+SSC), on soil biochemical activity, pore structure, and water retention. Seven treatments included an unfertilized control, mineral fertilization (MF), and MF combined with each amendment. The highest cumulative oxygen demand was recorded in MF+SSC (1.805 mg O2 g−1 DM), whereas the lowest occurred in MF+C(PL+BC) (1.356 mg O2 g−1 DM). Mineral fertilization produced the highest dehydrogenase activity. Compared with CTR, MF+BC had a larger volume of storage pores (0.5–50 μm) and produced the highest field capacity (0.385 cm3 cm−3) and available water content (0.265 cm3 cm−3). MF+SSC had the highest bulk density (1.309 g cm−3) and permanent wilting point (0.120 cm3 cm−3), but its available water content did not differ significantly from CTR. The first two principal components explained 58.0% of the variance and represented gradients associated with water retention and with bulk density and fine porosity. Among the tested materials, BC and C(PL+BC+SSC) provided the most balanced improvements in soil functionality by simultaneously enhancing water retention and maintaining moderate biological activity. The results indicate that biochar-, clay-, and compost-based amendment materials have potential for improving soil quality and supporting sustainable land management.