Amorphous mineral-organic associations in soils: Effective carbon stabilizers undermined by oxalic acid root exudates

Abstract

Mineral-organic associations are key to soil organic matter dynamics, serving as major carbon sinks and nutrient sources for plants and microbes. While most studies focus on associations with crystalline or short-range ordered minerals, recent spectromicroscopy reveals amorphous and heterogeneous associations in some soils, notably nanosized coprecipitates of inorganic oligomers with organics (nanoCLICs). Composed of C, Al, Fe, and Si, these nanostructures may be less stable and more reactive than associations with short-range ordered and crystalline minerals, raising question about their C stabilization potential and the extent of their disruption in the rhizosphere. This study investigates the reactivity of nanoCLICs, emphasizing their C stabilization and disruption potential following root exudate additions (oxalic acid). Laboratory-synthesized and a nanoCLICs-rich Andosol fraction were incubated in microcosms, and C mineralization was tracked over 42 days. Compared to free organic molecules, the nanoCLICs structure reduced C mineralization by 90 %. However, 1–6 % of the C in nanoCLICs was still mineralized, demonstrating a stabilization capacity comparable to that provided by associations with crystalline or short-range ordered minerals. Adding oxalic acid, a model root exudate, increased mineralized C, by a factor of 2 to 8, confirming nanoCLICs’ susceptibility to disruption by root exudates made of oxalic acid. These findings call for greater consideration of very disordered and heterogeneous mineral-organic associations, such as nanoCLICs, in soil C stabilization and nutrient cycling, offering a broader perspective beyond the traditionally emphasized role of crystalline and short-range ordered minerals.