Enhancing soil organic carbon sequestration at different depths: The role of chitin-rich organic amendment in salt-affected soils

Chitin-rich organic amendments are known for enhancing soil aggregation and mitigating salinity, yet their effects on subsoil organic carbon (SOC) remains underexplored. An experiment was conducted in a greenhouse vegetable production system, employing three treatments: CK (no amendment), SC (chitin-rich organic amendment), and ST (bio-organic amendment). Soil samples were collected from both the topsoil (0–20 cm) and subsoil (20–40 cm) layers. The results revealed significant increase in soil electrical conductivity (EC), salt ion content, labile SOC fractions under organic amendment treatments in both soil layers. In the topsoil, a notable reduction in pH and enhanced desalination rate were observed. The bio-organic amendment notably increased the proportion of large macroaggregates and aggregate stability in both layers. In contrast, the chitin-rich organic amendment specifically enhanced small macroaggregates in the topsoil and large macroaggregates in the subsoil (P < 0.05). Redundancy analysis identified soil pH as a crucial factor influencing topsoil aggregation, while EC, Mg²⁺, and Ca²⁺ were critical for subsoil aggregation. Organic amendments increased SOC, with higher Ca²⁺ and Mg²⁺ levels under lower pH in the topsoil improving aggregate stability. In the subsoil, elevated EC, primarily driven by increased Ca²⁺ and Mg²⁺ (excluding Na⁺), facilitated aggregate formation and accumulation of recalcitrant organic carbon, thereby contributing to SOC stabilization. These findings underscore the potential of chitin-rich and bio-organic amendments to improve soil structure and to enhance carbon sequestration in both topsoil and subsoil. This has important implications for sustainable soil management in secondary saline greenhouse vegetable systems.