9:15 AM - 9:30 AM
[ACG46-02] Mechanisms of soil organic carbon storage through mineral association:
a trade-off between adsorption capacity and microbial decomposability
Keywords:Mineral-associated organic carbon, Soil carbon persistence
Soil organic carbon (SOC) is the largest terrestrial reservoir of carbon (C), more than half of which is stored by binding to minerals as mineral-associated organic C (MAOC). MAOC not only has the capacity to store a large amount of C, but also is responsible for a long-term persistence of C in soils. Although recent studies emphasize the role of minerals in soil C sequestration and thus climate change mitigation, underlying mechanisms for the protection of MAOC against microbial decomposition remain an open question. This study aims to evaluate: 1) the chemical binding patterns in MAOC between amorphous/crystalline minerals and organic materials by batch isothermal adsorption; and 2) the microbial resistance of amorphous-MAOC and crystalline-MAOC by incubation experiments.
We employed three proxies to represent SOC components: humic acid sodium salt (HA) for soil humic substances, sodium gluconate (GA) for cellulose, and vanillic acid (VA) for lignin, respectively. These were adsorbed by two types of minerals, kaolinite (crystalline) and allophane (amorphous), with the particle sizes of less than 5 microns. In addition, Fe-removed allophane was prepared by dithionite citrate bicarbonate method. Mineral surface affinity for organic matter was estimated based on isothermal adsorption using Dubinin-Radushkevich fitting. To evaluate the microbial resistance of MAOC, MAOC produced through the batch adsorption was incubated with a microbial inoculum solution under unsaturated moisture conditions. The microbial inoculum solution was extracted from a soil sample (0 to 20 cm depth) using an NPK solution (0.04 mg N, 0.03 mg P, and 0.08 mg K per mL). During incubation, CO2 emission rates from MAOC were monitored using gas chromatography.
Here, we show a chemically driven mechanism that accounts for the MAOC persistence through a trade-off between the mineral surface affinity and adsorption capacity. First, focusing on the organic matter, we found that all molecules (HA, GA, and VA) become more resistant against microbial degradation when adsorbed onto mineral surfaces than when remained dissolved in soil solution. In particular, GA and VA, when adsorbed onto amorphous minerals, exhibited microbial resistance consistent with the mineral affinity trend of GA > VA. Meanwhile, HA, which exhibited the strongest affinity for mineral surfaces, remained thermally recalcitrant even at elevated temperatures. Regarding the mineral affinity property, results revealed that although amorphous minerals, with a higher adsorption capacity, can store greater amounts of SOC, their relatively weak resistance against microbial degradation compromises the recalcitrance of the adsorbed SOC. In contrast, kaolinite showed a lower adsorption capacity than amorphous minerals, but SOC adsorbed on kaolinite was more recalcitrant than that on allophane and Fe-removed-allophane. Specifically, the persistence of mineral-associated GA corresponded to the trend of mineral surface affinity: kaolinite > allophane > Fe-removed-allophane, rather than the trend of adsorption capacity: Fe-removed-allophane > allophane > kaolinite. Our results indicate the abiotic contribution of minerals to microbial resistance and question the hypothesi of persistence of MAOC on millennial timescales.
We employed three proxies to represent SOC components: humic acid sodium salt (HA) for soil humic substances, sodium gluconate (GA) for cellulose, and vanillic acid (VA) for lignin, respectively. These were adsorbed by two types of minerals, kaolinite (crystalline) and allophane (amorphous), with the particle sizes of less than 5 microns. In addition, Fe-removed allophane was prepared by dithionite citrate bicarbonate method. Mineral surface affinity for organic matter was estimated based on isothermal adsorption using Dubinin-Radushkevich fitting. To evaluate the microbial resistance of MAOC, MAOC produced through the batch adsorption was incubated with a microbial inoculum solution under unsaturated moisture conditions. The microbial inoculum solution was extracted from a soil sample (0 to 20 cm depth) using an NPK solution (0.04 mg N, 0.03 mg P, and 0.08 mg K per mL). During incubation, CO2 emission rates from MAOC were monitored using gas chromatography.
Here, we show a chemically driven mechanism that accounts for the MAOC persistence through a trade-off between the mineral surface affinity and adsorption capacity. First, focusing on the organic matter, we found that all molecules (HA, GA, and VA) become more resistant against microbial degradation when adsorbed onto mineral surfaces than when remained dissolved in soil solution. In particular, GA and VA, when adsorbed onto amorphous minerals, exhibited microbial resistance consistent with the mineral affinity trend of GA > VA. Meanwhile, HA, which exhibited the strongest affinity for mineral surfaces, remained thermally recalcitrant even at elevated temperatures. Regarding the mineral affinity property, results revealed that although amorphous minerals, with a higher adsorption capacity, can store greater amounts of SOC, their relatively weak resistance against microbial degradation compromises the recalcitrance of the adsorbed SOC. In contrast, kaolinite showed a lower adsorption capacity than amorphous minerals, but SOC adsorbed on kaolinite was more recalcitrant than that on allophane and Fe-removed-allophane. Specifically, the persistence of mineral-associated GA corresponded to the trend of mineral surface affinity: kaolinite > allophane > Fe-removed-allophane, rather than the trend of adsorption capacity: Fe-removed-allophane > allophane > kaolinite. Our results indicate the abiotic contribution of minerals to microbial resistance and question the hypothesi of persistence of MAOC on millennial timescales.