Soil organic carbon (SOC), one of the largest terrestrial carbon reservoirs, is formed as photosynthetically fixed CO2 is incorporated into soils via microbial decomposition. The stable carbon isotope ratio (delta C-13) is an effective tracer of SOC transformation. This study investigates delta C-13 variations in plant materials, SOC and soil-respired CO2 (soil CO2) from agricultural and forest soils across semi-arid to sub-humid regions of Gujarat, western India. The analysis aims to quantify isotopic fractionation during litter and SOC decomposition at different stages of development and to assess how land-use practices influence soil carbon sequestration. Fractionation factors (epsilon) were calculated for the conversion of plant materials to SOC, and contributions of different organic matter pools to soil CO2 were quantified using an isotopic mixing model. The estimated epsilon ranged from -0.58 parts per thousand to -3.48 parts per thousand, with no clear dependence on vegetation, land use or climate. Transformation from plant matter to surface SOC caused 93-99% carbon loss, with C-13 enrichment of 10.30 +/- 3.44, 7.18 +/- 2.61, and 5.95 +/- 3.59 parts per thousand in agricultural, dry deciduous and wet deciduous forests, respectively. Soil CO2 delta C-13 indicated mixing, with similar to 76% from root respiration, similar to 19% from fresh SOC, and similar to 5% from older SOC. SOC delta C-13 reflects pre-agricultural C3 vegetation, despite current C4 cropping in an agricultural site, indicating minimal carbon sequestration in agricultural soils. Distinct delta C-13 signatures from different pools, arising during plant-to-SOC conversion and further decomposition enable quantitative estimation of contributions to the soil CO2 from various carbon pools. delta C-13-based partitioning reveals limited sequestration under current land use practices in semi-arid tropics.