Key factors influencing the spatial distribution of soil organic carbon and its fractions in Mollisols

Soil organic carbon (SOC) is a key component of the global carbon pool, which is crucial to the fertility and health of soils and deeply influences global carbon cycling. "Mollisols belts" with high SOC storage are considered as the natural breadbasket of the world, and SOC transformation between the sources and sinks profoundly affects global climate change. In this review, based on the newest publications, the effects of both natural and anthropic factors on the decomposition, migration, and transformation of SOC and further influencing the spatial distribution of SOC and its fractions in Mollisols were summarized. Furthermore, we summarized the debates on the spatial changes of SOC and its fractions in Mollisols from publications and analyzed the causes of these debates. Generally, climate change, land use types, fertilization, tillage practices combined with topographic factors, hydrologic process, and soil erosion process significantly influence the spatial distribution of SOC. As well, in the Mollisols, (1) Global warming and elevated CO2 may increase the content of labile organic carbon fractions and affect the soil-atmosphere carbon cycle. (2) The conversion of abandoned cropland to forest or grassland, and the conversion of dry cropland to paddy field may improve the carbon sequestration capacity of terrestrial ecosystems, while rational fertilization and organic inputs reduce the SOC loss of natural ecosystems converting to cropland. (3) Conservation tillage in cropland improves the SOC storage by increasing the pool of labile organic carbon in soils. (4) A reasonable combination ratio of organic and chemical fertilization is crucial for achieving the stability of the SOC pool and the increase of the SOC storage in soils. At the end of this review, the key issues on the spatial distribution of SOC and the driving mechanisms were prospected in the future.