Microbial mediation of nepheline-bearing rock weathering releases nutrients for crop growth

Enhanced rock weathering (ERW) has been considered a new technology to mitigate climate change. Organic acids exuded from plant roots and microbes, e.g., oxalate, gluconate, and citrate, can enhance the rates of rock weathering. Additionally, cations released from rock dust to the soil environment will require ionic neutrality, which is mainly balanced by bicarbonate in soil. This process increases carbon sequestration, and the carbon can be long-term stored either in the form of carbonate minerals or bicarbonate. The first stage of this study is to demonstrate whether microbes (Streptomyces sp.) can enhance rock weathering.

Streptomyces sp. are filamentous bacteria capable of decomposing organic matter and therefore very important in the soil environment. Three Streptomyces strains were isolated from the surfaces of weathered dolerite and screened for their ability to mobilize potassium (K) from rock dusts. All three strains, Mid SCVA3, Mid SCVA1 and MO AIA1, can grow on synthetic minimum medium agar with rock dust as K source. The rock dusts used in this study were nepheline-bearing rocks, Bo Phloi alkaline basalt (ABP), refined nepheline syenite (RNS) and nepheline syenite tailings (NST). Nepheline is a fast-weathering mineral under Earth’s surface conditions. There is 3 to 25% of K substitution in the position of the sodium ion, so nepheline-bearing rocks are a natural and sustainable alternative K source for crop nutrition. An amount of rock dust correlating to 250 ppm K was applied to a weathering experiment. Results show that growth rates of strains using rock dust as a sole source of nutrients were different, referred from glucose consumption. MO AIA1 was the best-growing strain, followed by Mid SCVA1 and Mid SCVA3 for all treatments. NST inoculum showed the highest glucose consumption, followed by RNS and ABP. In contrast, looking at elemental releases, Mid SCVA1 was the best strain mobilizing K from all rock treatments, while MO AIA1 and Mid SCVA3 presented similar rates of K releases. However, K release concentration in all inoculums was distinctively higher than abiotic control, supporting that microbes can increase weathering rates of rock dust. Significantly higher concentrations of K were released from ABP than NST and RNS, respectively. For agronomic purposes, ABP will be the best source of K nutrition, and bioaugmentation of Mid SCVA1 will assist in mobilizing more K from the rock. Next step, we will further investigate carbon removal related to ERW in plant growth experiment, using Streptomyces sp. to enhance rock weathering.