Impact of Sugarcane Management Practices and Time Periods on Soil Organic Carbon and <scp> δ ¹³ C </scp> Signature After Paddy Rice Conversion

ABSTRACT Land use change (LUC) from paddy rice to sugarcane cultivation strongly influences soil organic carbon (SOC) stocks, with the extent and direction of change depending on residue management and time since conversion. This study aimed to (i) evaluate SOC stock changes under different residue management practices and conversion periods following rice‐to‐sugarcane transition, and (ii) determine variations in the proportions of old rice‐derived and new sugarcane‐derived SOC. Eight sites were selected under two residue management practices—burned (B) and unburned (UB)—across four conversion periods: 1 year (SC1), 3 years (SC3), 5 years (SC5), and 10 years (SC10), with a paddy rice field as reference. Soil samples were collected from 0 to 20 and 20 to 40 cm depths. SOC stocks were measured, and δ 13 C analysis was used to track rice‐ and sugarcane‐derived carbon. The interaction between residue management and conversion period significantly affected SOC stocks ( p ≤ 0.05). Burned management resulted in significant SOC decreases in SC3 (4.90 Mg ha −1 topsoil; 3.18 Mg ha −1 subsoil) compared to the reference, whereas SOC under unburned management in SC5 did not differ significantly, indicating rapid recovery. δ 13 C analysis showed a sharp decline in rice‐derived carbon within the first 3 years, stabilizing thereafter under both managements. Unburned residue enhanced the incorporation and early stabilization of sugarcane‐derived carbon in SC3 and SC5.