Influence of organic carbon amendments on weathering of pulverized silicate minerals: interplay of biotic and abiotic processes

Microbial oxidation of organic compounds produces organic and inorganic acids in soil, and these ligands and proton sources drive the weathering of soil minerals. Understanding the linkages between these biological and chemical processes has stimulated research on technologies to accelerate the rate of a C stabilization associated with weathering minerals, especially in highly disturbed agricultural soils. One approach involves applying organic and inorganic amendments together (e.g., compost, biochar, pulverized rock) to induce a synergistic effect via microbial and chemical mechanisms. However, it is unclear if a co-application of organic amendment appreciably enhances the weathering of mineral amendments and C accumulation. Also, the surface of weathering minerals can provide reactive binding sites for secondary mineral formation or metal co-precipitation with dissolved organic C, thus coating the surface and inhibiting weathering processes. To address this gap, a series of bench-scale experiments (soil incubation and batch dissolution) were conducted to link olivine weathering rates to different microbially derived proton sources (e.g., respired CO2, organic ligands) from different stages of microbial metabolism. We hypothesized: 1) olivine weathering will be linearly related to the increase of microbial metabolic products; and 2) the change of weathering rate will depend on dominant sorption mechanisms of organic ligands associated with weathering olivine surface. We will measure olivine weathering products (Mg, Fe, Si, etc.) and temporal changes of inorganic C pools induced from respired CO2 by incubating olivine and soil with 13C labeled glucose. Similarly, 13C labeled rice residue will be used to identify microbially derived C pools (microbial biomass, necromass, dissolved organic C) associated with mineral surfaces. Finally, olivine dissolution will be examined to verify the dominant mechanisms (proton or ligand promoted dissolution versus metal-DOC co-precipitation) in the presence of compost extracts and soil colloids. Incubation results will elucidate the biogeochemical roles of organic amendments in accelerating biochemical weathering of minerals; this information will be critical for developing management strategies to promote soil C sequestration in working lands.