Retention of soil organic matter by occlusion within soil minerals

The stabilization of soil organic matter is crucial for global carbon cycling processes as soil stores large amounts of organic carbon. The occlusion of SOM within minerals sequesters these organic molecules, rendering them inaccessible to interference from biotic and abiotic factors. However, the microscopic mechanisms of occlusion are lacking. In the past few years, many researchers have focused on the elucidation of the occlusion process, and the results are summarized in this review. The occlusion of representative SOM such as natural extracted or commercial humic substances, sugars, amino acids within minerals including calcite, clay, metal oxides, were observed by various in situ and ex situ methods, such as atomic force microscopy, nano-scale secondary ion mass spectrometry and synchrotron-based infrared micro spectroscopy. These results have shown that minerals can occlude SOM either via organo-mineral aggregation or within growing hillocks, which are classical growth features on crystal surfaces, and the microscopic mechanisms have been illustrated in this review. The occlusion process is influenced by various factors, including the characteristics of minerals and the composition of SOM and soil solution conditions, which are mediated by the interactions of organo-mineral interfaces. Finally, some new perspectives for future research of occlusion are provided in order to give new possibilities for observing and comparing the detailed occlusion process in soils from different areas. In summary, SOM can be retained, protected and stabilized by soil minerals via occlusion either by aggregation or within growth hillocks, influenced by various factors. The results have implications for global carbon cycling in soil ecological systems. Graphical abstract Soil organic matter could be occluded within soil minerals via aggregation and hillocks.