Characteristics of soil organic carbon and its isotopic compositions

Based on the data of soil organic carbon (SOC) content and its isotopic compositions (δ¹³C and Δ ¹⁴C) in three drilling cores at typical elevations at the Chongmingdongtan salt marsh of Yangtze River Estuary and in six profiles at different altitudes in the Dinghushan Biosphere Reserve, the characteristics of SOC content and its isotopic compositions during the turnover of soil organic matter (SOM) were investigated. The results indicated that SOC contents and δ¹³C values were negatively correlated in the soil sections above the depth with maximum δ¹³C (δ ¹³ C_max), and positively correlated below this depth. The negative correlation indicated that SOM reduction degree was low, and the rapid turnover compartments were in greater proportions of SOM which were fresh organic matters in the upper soil sections with SOM apparent ¹⁴C ages less than 200 years. The positive correlation was in adverse, and the SOM apparent ¹⁴C ages were usually over 300 years. The degradation rate of SOC was negatively correlated with pedogenesis ages, and remarkably lower below the soil depth with δ¹³ C_max than above the depth. The greater the pedogenesis age, the less the decreasing rate of SOC content with time, suggesting that the SOM was mainly of slow turnover compartments. The temporal changes of δ¹³C values in different soil profiles showed similar tendency, i. e., the δ ¹³C values above the soil depth with δ ¹³ C_max increased with time, while those below this depth were in adverse, which correlated well with the temporal changes of SOC contents, due to the distribution of different SOM compartments with soil depth.