Cultivation reduces quantities of mineral-organic associations in the form of amorphous coprecipitates

Abstract. Mineral-organic associations are crucial carbon and nutrient reservoirs in soils. However, soil cultivation disrupts these associations, leading to carbon loss and reduced soil fertility. Although, identifying the specific type(s) of mineral-organic associations susceptible to destruction or transformation upon cropping remains challenging, it is essential for devising strategies to preserve organic matter in croplands. Here we aimed to determine the predominant mineral-organic associations and to identify which types of associations are transformed upon cultivation. To achieve this, we sampled an andosol from both a forested and a cultivated area. We then analyzed cultivation-induced changes in soil physicochemical parameters and characterized mineral-organic associations using an array of spectro-microscopic techniques (TEM-EDX, TEM-EELS, and STXM), for comprehensive structural and compositional analysis. At the micro and nanoscale, we observed mineral-organic associations in the form of coprecipitates composed of amorphous oligomers containing Al, Si, and Fe (referred to as nanoCLICs for nanosized coprecipitates of inorganic oligomers with organics). Down to a few hundred nanometers, the nanoCLICs displayed elemental enrichments with C+Al+Si, C+Fe+Al+Si, or Al+Si dominance with less C. In contrast, organic matter exhibited various C speciation without compound-specific enrichments. These findings suggest that mineral-organic associations in andosols are nanoCLICs-type coprecipitates rather than organic matter associated solely with secondary minerals. NanoCLICs were present in both forest and crop andosols, and while cropping led to a 50 % decrease in nanoCLICs, it did not alter their nature. This novel conceptualization of mineral-organic associations as nanoCLICs shifts our understanding of their persistence in andosols and demonstrates their vulnerability to crop-induced changes.