Effects of long-term addition of exogenous organic materials on soil inorganic carbon pool

The specific effects of long-term addition of exogenous organic materials on soil inorganic carbon pool in farmland is still unclear. Our study was conducted to figure out the effects of different fertilization on soil carbon pool distribution and related physicochemical properties based on a 16-year long-term experiment in North China. Six treatments were set up as follows: control (CK), nitrogen fertilizer (N), chemical fertilizer (NPK), nitrogen fertilizer combined with organic fertilizer (MN), chemical fertilizer combined with organic fertilizer (MNPK), and chemical fertilizer combined with straw (SNPK). In this study, the effects of different fertilization treatments on soil organic carbon (SOC), soil inorganic carbon (SIC) content and other key soil physicochemical properties were analyzed in detail. Principal component analysis was used to investigate the mechanism of exogenous organic material addition on soil carbon pool change. The results showed that the soil organic carbon (SOC) content in 0-20 cm soil layer was increased by 45% on average by long-term addition of exogenous organic materials compared with CK. MNPK treatment increased the soil inorganic carbon (SIC) content by 21% in the 40-50 cm soil layer, while SNPK treatment reduced the SIC content by 28% in the 50-60 cm soil layer. Soil pH value and water condition are the key factors of SIC change, and SOC content, organic materials input, soil parent material and other factors affect the process. The decomposition of straw produces organic acids and CO₂, and promotes the mineralization of background SOC to produce CO₂, which leads to a significant decrease in soil pH. This is the main reason for the significant loss of SIC in deep layer of SNPK. In conclusion, fertilizer combined with organic fertilizer is conducive to the accumulation of soil organic carbon and inorganic carbon, while fertilizer combined with straw may lead to the loss of inorganic carbon in deep soil. This not only provides a theoretical basis for revealing the transformation mechanism of soil carbon pool after the addition of foreign organic materials, but also has important significance for evaluating the key role of SIC in soil carbon storage.