Interplay of coprecipitation and adsorption processes: deciphering amorphous mineral–organic associations under both forest and cropland conditions

Abstract. Mineral–organic associations are crucial carbon and nutrient reservoirs in soils. However, conversion from forest to agricultural systems disrupts these associations, leading to carbon loss and reduced soil fertility in croplands. Identifying the types of mineral–organic associations within a single soil is already challenging, and detecting those susceptible to disruption during forest-to-crop conversion is even more complex. Yet, addressing this identification challenge is essential for devising strategies to preserve organic matter in croplands. Here, we aimed to identify the predominant mineral–organic associations within an Andosol (developed on Fe-poor parent material) under both forest and cropland conditions. To achieve this, we collected Andosol samples from both a forested and a cultivated area, located 300 m apart. We then analyzed differences between the two soil profiles in soil physicochemical parameters and characterized mineral–organic associations using an array of spectro-microscopic techniques for comprehensive structural and compositional analysis. At microscale and nanoscale spatial resolution, we observed mineral–organic associations in the form of amorphous coprecipitates, composed of a mix of C+Al+Si and C+Al+Fe+Si nanoCLICs (inorganic oligomers with organics), proto-imogolites and organic matter, some Fe nanophases associated with organic matter, and some metal–organic complexes. This challenges prior conceptions of mineral–organic associations in Andosols by demonstrating the presence of amorphous coprecipitates rather than solely organic matter associated with short-range-order minerals (i.e., imogolite and allophanes). Moreover, chemical mappings suggested that these amorphous coprecipitates may adhere to mineral surfaces (i.e., phyllosilicates and imogolites), revealing secondary interactions of mineral–organic associations in soils. While the presence of similar amorphous coprecipitates in both the forest and crop Andosols was confirmed, the crop soil had 75 % less C in mineral–organic associations (in the 0–30 cm depth). Although the sample size for comparing land use types is limited, these results suggest that the nature of mineral–organic associations remains identical despite quantitative differences. This study highlights the crucial role of amorphous coprecipitates in C stabilization in Andosols and also suggests their vulnerability to disruption after 30 years of a forest-to-crop conversion, thereby challenging our understanding of the persistence of mineral–organic associations in Andosols.