Changes in the Organic Carbon Content of Agricultural Soils in the Middle and Lower Reaches of the Yangtze River in China: Based on Data from Long-term Localization Experiments

The literature of long−term fertilization experiment in the Yangtze River Economic Belt from 1993 to 2023 was collected, and the data of farmland soil organic carbon were extracted and integrated. Using the normalization treatment and the analysis method of relative annual variation, the overall change of soil organic carbon content in farmland in the Yangtze River Economic Belt under long-term different fertilization measures was studied, and the change differences of soil organic carbon content under three tillage modes were compared, so as to judge and analyze the influence of the duration of the experiment on soil organic carbon dynamics. The results showed that under long-term different fertilization measures, the organic carbon content of farmland soil in the Yangtze River Economic Belt in China showed an overall upward trend. NP, NPK, O, and NPKO treatments all increased the organic carbon content of agricultural soils, with the NPKO treatment being the largest. The sole application of inorganic nitrogen fertilizer reduced the organic carbon content of the soil. The rates of change in soil organic carbon content were 0.11g·(kg·a) −1 , 0.31 g·(kg·a) −1 , and 0.30 g·(kg·a) −1 for dryland, paddy and water−dry rotation farmland, respectively. There is some variation in the rate of change of soil organic carbon content between soil types. The average rate of change of organic carbon was 0.20 g·(kg·a) −1 for red soils, 0.13 g·(kg·a) −1 for tidal soils and 0.19 g·(kg·a) −1 for rice soils. The trend of NPKO>O>NPK>NPK>NP>N is basically maintained for the rate of change of soil organic carbon content. N treatment showed a reduction in organic carbon content in all soil types. Considering the carbon fixation of farmland soil, the combined application of organic and inorganic fertilizers is a more suitable fertilization method in this area.