Microbial stability of mineral-associated root exudates governed by mineral association capacity, exudate nitrogen availability and their pH

Low-molecular-weight organic compounds, like root exudates, can directly associate with clay minerals, reducing their microbial availability. However, it remains unclear whether the microbial stability of mineral-associated organic carbon (MAOC) is influenced by the properties and their interactions of the original minerals and organic compounds. This study quantified the maximal association capacity of four typical clay minerals (kaolinite, illite, vermiculite, and montmorillonite) and five common root exudate compounds (oxalic acid, citric acid, glucose, alanine, and N-acetylglucosamine) within sorption experiments, and assessed the microbial stability of MAOC through model soil incubation with the same microbial inoculum and texture for 120 days. Maximal MAOC content ranged from 3.42 to 93.27 mg C g^-1 clay, with 0.69 % to 33.98 % subsequently mineralized. MAOC content showed mineral association capacity increased with molecular size of substrates associated with kaolinite and illite. Montmorillonite preferentially associated with glucose, alanine, and N-acetylglucosamine through physical entrapment and hydrogen bonding for C - H, N - H, and O - H groups; vermiculite interacted mainly with oxalic and citric acids via COO^--cation bridging, and kaolinite bound with glucose primarily through surface pore entrapment and C - H hydrogen bonding. Illite primarily associated with organic matter through van der Waals forces and electrostatic interactions. Negative correlations between mineral association capacity and mineralization were observed for all compounds except for oxalic acid due to very low pH. For oxalic acid, vermiculite and montmorillonite exhibited stronger association capacity but lower microbial stability than illite, attributed to their higher cation exchange capacities and disordered porous structures that facilitated chemical adsorption and physical entrapment, as well as stronger dissolution reactions induced by ligand substitution. During early incubation (day 0-30), MAOC mineralization was faster for N-containing compounds, particularly for alanine compared with N-acetylglucosamine in kaolinite and illite, due to alanine's lower C:N ratio. These findings suggest that microbial stability of mineral-associated root exudates is governed by mineral association capacity, exudate nitrogen availability, and their pH.