[Spatial Characterization of Stable Isotope Composition of Organic Carbon from Farmland Soils in Chongqing].

Soil was sampled from 182 profiles in typical farmlands of Chongqing and analyzed for the stable carbon isotope composition of organic matter ( δ ¹³ C _SOC ). The results showed that the values of δ ¹³ C _SOC for each soil profile were gradually increasing with increasing soil depth, and the mean values were (-23.63±1.53)‰, (-22.43±1.59)‰, and (-21.42±1.90)‰ for surface, middle, and bottom layers, respectively. The δ ¹³ C _SOC values in the northeastern region of Chongqing tended to be more negative, whereas those in central Chongqing were less negative. Paddy fields showed the most negative values of δ ¹³ C _SOC , followed by rice-upland rotating fields and upland fields, with the average being (-25.32±0.93)‰, (-23.17±1.37)‰, and (-24.75±1.28)‰ for the surface layers, respectively. For different soil types, the δ ¹³ C values in the surface layers were in the order of paddy soil<alluvial soil<purplish soil<calcareous soil<yellow soil. According to the regression-tree analysis, the crop types predominantly influenced the variation in δ ¹³ C _SOC in surface soils, and soil types mainly affected that in the middle- and bottom-layer soils. Other factors, such as soil properties (TN, SOC, and pH) and meteorological conditions (precipitation and air temperature) played only minor roles in the variation of δ ¹³ C _SOC . In short, the stable isotope composition of organic carbon in the surface soils was primarily controlled by the input carbon source, whereas that in the deeper layers was closely linked with carbon cycling processes within the soils.