Dynamics of 13C and 15N isotopes in fire-affected soils under rotational shifting cultivation in Northern Thailand

Understanding the biogeochemical consequences of fire and land-use history in tropical upland systems is essential for sustainable soil management. We investigated the vertical distribution of stable carbon (δ13C) and nitrogen (δ15N) isotopes in soils under rotational shifting cultivation (RSC) in Northern Thailand. Three fields with distinct land-use histories were analyzed: a continuously fallow site for seven years (CF-7Y), a six-year fallow RSC site (RSC-6Y), and a twelve-year fallow RSC site (RSC-12Y). The RSC-6Y and RSC-12Y fields were left fallow for 6 and 12 years, respectively, with both fields burned in 2022 and entered a new two-year fallow phase. By contrast, CF-7Y field was last burned in 2017 and has remained under continuous fallow since that time. In 2024, soil samples were collected from upper, middle, and lower slope positions to analyze total organic carbon (TOC), total nitrogen (TN), TOC:TN, and δ13C and δ15N signatures across the 0–100 cm soil profile. Results revealed that longer fallow periods (RSC-12Y) enhanced vertical movement and stabilization of TOC and TN, with more enriched δ13C and δ15N values at depth—indicative of legacy fire effects and microbial transformation. The CF-7Y field showed high surface TOC and more negative δ13C values, reflecting active C₃ vegetation inputs and minimal decomposition. The δ13C values were significantly affected by both site and slope position, suggesting independent influences of land-use history and topography on soil carbon dynamics. In contrast, δ15N was shaped by a significant interaction between site and slope, indicating that nitrogen cycling processes vary with the combined effects of land use and topographic gradient. The δ15N values consistently increased with depth across all fields, particularly in lower slope positions, suggesting deposition of 15N-enriched material and persistent alteration of nitrogen pools post-fire. Slope position significantly influenced nutrient distribution, with lower slopes acting as nutrient sinks and upper slopes experiencing erosion-driven losses. These findings underscore the decoupled recovery of soil carbon and nitrogen cycles after disturbance, highlighting the need for slope-sensitive and nutrient-aware restoration strategies.