Effects of cricket feces application on soil colloid surface charges and carbon fractions: A nutrient retention improving opportunity in tropical acidic sandy soil

Acidic sandy soils in tropical regions have inherently low nutrient retention due to their highly weathered nature and low organic matter content. Organic amendments, like cricket feces, offer potential for improving soil properties; however, the temporal dynamics of soil charge characteristics under different application methods and rates remain unclear, particularly in variable-charged soils. This study evaluated the effects of application methods (incorporation versus surface placement) and rates (0, 3.13, 6.25, and 12.50 Mg ha-1) of cricket feces on charge characteristics of an acidic sandy soil in Northeast Thailand. An incubation experiment monitored changes in cation exchange capacity (CEC), zero point of net charge (ZPNC), and their related properties. CEC increased with the application rates and was negatively related to ZPNC throughout the experimental period. In the early stage, during 1–5 days, the incorporation treatments produced significantly higher CEC (2.40–3.80 cmol kg-1) than the surface application treatments (2.00–3.50 cmol kg-1), associated with higher pH, organic C, dissolved organic C, and decomposition rate (k). In the later stage, by day 45, surface application treatments showed significantly higher CEC (up to 2.73 cmol kg-1) than the incorporation treatments (2.33 cmol kg-1), corresponding with greater dissolved organic C and lower k, indicating the effects of organic matter quality affected by the decomposition rate. These findings demonstrated that incorporating cricket feces led to an immediate enhancement of nutrient retention, whereas surface application brought about a more prolonged retention effect.