Objectives Soil organic carbon (SOC) and inorganic carbon (SIC) play an important role in the global carbon cycle and the mitigation of climate change. Better understanding of transformations between them has important implications for reliable estimation of soil carbon stocks. However, there are few studies on it, and the research results are discrepant.

Methods This study collected 41 literatures published from 1990 to 2018 in China, on the effect of different climatic zones, different land uses and different soil layers on the relationship of soil organic and inorganic carbon.

Results SOC/SIC values were higher in 0−20 cm soil layer than those in 20−100 cm soil layer in all climatic zones and land uses. Specifically, in the temperate continental climate zone the SOC/SIC value in the 0−20 cm soil layer was the lower (0.53) in the grassland, and higher in forestland (0.90) and the cropland (0.80). There was a significant positive correlation between SOC and SIC under all three types of land uses. However, the SOC/SIC value of the 0−20 cm layer in the temperate monsoon climate zone was in the order of grassland (0.82) ≈ cropland (1.05) > forest (0.29). SOC and SIC had a positive correlation in forest and cropland, but a negative correlation in grassland. In addition, the SOC/SIC value of the 20−100 cm soil layer was the highest in the forest, followed by grassland and cropland in the temperate continental climate zone. In contrast, there was no significant difference in the SOC/SIC value among the three land uses in the temperate monsoon climate zone. The relationship between SOC and SIC was positive in forest and cropland, but negative in grassland. The climate type and land use affected not only the content of SOC and SIC, but their relationship. In the temperate continental climate zone, the SOC/SIC value of the forest was higher than cropland and grassland. In the temperate monsoon climate zone, however, the SOC/SIC value was higher in grassland than those in forest and cropland. This may be due to 1) the different vegetation coverage, resulting in different amounts of crop carbon, and 2) the different vegetation cover also affects the various biochemical processes in the soil, leading to different transformation process of carbon, the content of SOC and SIC.

Conclusions There is a cyclic transformation between SOC and SIC, and climatic condition, land use and soil type have significant effects on SOC and SIC cycling and conversion. SOC and SIC show different correlations under different conditions. The results of this study can provide a theoretical basis for estimating soil organic and inorganic carbon pools accurately.