The role of minerals in fractionated soil organic matter stabilization in the subsoil

Soil organic matter (SOM) content is an essential indicator of soil fertility, functionality, and health. Increasing SOM content also improves the hydrological conditions of the soil and, therefore, is a helpful tool against climate change-affected extreme precipitations and droughts. Moreover, the carbon amount additionally stored in the soil may support the reduction of greenhouse gas concentration in the atmosphere. SOM is primarily stabilized by minerals being resistant to decomposition. Many land use and agrotechnical-related initiations have succeeded in increasing SOM in the surface layer. However, half of the SOM is globally stored in the subsoil (>30cm). The primary way of SOM migration to the subsoil is through water-solved transport via infiltration and leaching. Nonetheless, we have only limited knowledge of potential SOM increases in the subsoil. The present study aimed to investigate the roles of soil mineral types on dissolved SOM stabilization. Quartz, illit, muscovite, goethite standards, and a mixture of them (model soil) were treated with the IHSS Suwannee River fulvic acid standard III. The adsorbed Carbon content was measured by mass spectrometry, and the SOM composition was measured by X-ray photoelectron spectroscopy (XPS). SOM changes in the liquid phase due to adsorption were investigated using high pressure liquid chromatography (HPLC). Results indicated that the adsorption order of the minerals was quartz<muscovite<<illite<<goethite. The adsorbed carbon amount was not linked directly to the minerals' surface area, indicating the highlighted role of mineralogy. The model subsoil adsorbed less SOM compared to the prediction based on the single mineral adsorption results, indicating mineral aggregation and active surface decrease. HPLC results of the remaining SOM compounds in the solution after adsorption showed inverse linkage to the solid surface related XPS results, proving the parallel applicability of the two approaches. HPLC indicated four SOM peaks, of which three had lower measured than predicted adsorption for the model soil. Quartz preferred to adsorb hydrophobic organic matter, whereas goethite showed a hydrophilic preference. Overall, the results proved the importance of mineral composition on organic matter compound preference and stabilization. This work was supported by the National Research, Development and Innovation Fund of Hungary [project no. K 142865].