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Contributors: Paul Reginato1, Pritha Ghosh2, Petra Pjevac3, Derek Bell4

Lead contact: Paul Reginato ([email protected])

Problem Statement

Motivating Factor

Atmospheric carbon dioxide removal and point-source carbon capture technologies are well-accepted as being necessary for meeting climate goals [1]. Rock weathering in the environment naturally generates alkalinity that draws ~0.3 GtCO2/yr from the atmosphere and converts it to solid carbonates or (bi)carbonates which are transported to the ocean and stably stored [2][3]. Enhanced rock weathering (ERW) technology seeks to accelerate alkalinity generation via mineral dissolution for carbon storage by grinding rocks to increase reactive surface area and exposing them to weathering conditions [4][5]. A core challenge of ERW is cost-effectively increasing mineral dissolution kinetics to enable scaling [6]. A leading ERW approach applies finely-ground rock to agricultural soils, where mineral dissolution can capture atmospheric carbon dioxide and also benefit crop yields [7].

Specific Constraint

Microbes can accelerate mineral weathering [8][9][10][11]. Biologically-enhanced weathering (bio-ERW) in soil has been proposed, wherein microbial weathering would be leveraged, for example by managing or amending soil to encourage a favorable microbiome for weathering; by choosing ERW sites with favorable microbiomes; or via soil inoculants [12][13][10].

To develop bio-ERW, we must understand the soil microbial community functions that result in weathering and their requisite soil conditions. However, such understanding is lacking or limited [10][14]. Bioweathering studies have generally been performed in the lab under simplified conditions, where weathering rates can differ from the field by orders of magnitude [10]. Existing field studies have mostly been performed in forests on rock slabs, not in agricultural soils on rock dust. The one microbial function study performed in a soil ERW context led to proposal of a bio-ERW strategy [15]. Further microbial function studies are necessary. Of particular interest are studies incorporating arbuscular mycorrhizal fungi: ectomycorrhizal weathering has been demonstrated in forests, but arbuscular mycorrhizae, which associate with crop roots and are relevant to soil ERW, are less well-studied [12].

Actionable Goals

Work should be carried out to characterize 1) metabolic pathways that promote weathering in natural environments; 2) the occurrence and expression of those pathways by microbial communities in situ; and 3) the physical imprints microbes make on minerals and the soil structures they are associated with [10]. 

Metagenomic, metatranscriptomic, and metabolomic tools should be applied [10]. Long-read sequencing may help accurately identify key microbes. It is crucial to measure functions associated with mineral grains, versus bulk soil. Soil-derived grains would best represent ERW, but separating them while retaining sample quality would be challenging. Bags of rock powder buried in soil might be used to obtain samples enriched in weathering activity [15].

Mesocosm or soil column experiments can achieve controlled settings in complex soil [16][17]. Diverse soils should be sampled to reflect the diversity of deployment environments. Soils where the parent material includes minerals of interest for ERW may be useful for understanding bioweathering in soils that have been treated with rock dust over many years.

Additional information

Open Questions

• Any process involving microbial inoculation into soil must be evaluated for risks it may pose if the microbes enter natural soils.

Assumptions

• Microbial community function can be measured for its impacts on mineral weathering, in isolation from physical soil properties.

Related problem statements

• Platforms for high-throughput characterization of biologically-enhanced mineral weathering [18]

• Propose and assess microbial community functions for reactor-based bio-enhanced weathering [19]

• Systematically characterize microbially-enhanced mineral weathering [20]

Other information

• Soil structure (water retention capacity, bulk density, etc.) affects hydrological properties that influence weathering and the environment available to microbial communities. Microbial communities also affect soil structure in turn. Therefore, agricultural practices that modify soil properties may indirectly impact microbial weathering, and microbial inoculations may indirectly influence weathering through soil structure. 

• The one study of microbial function in ERW demonstrated microbial community enrichment of siderophore-producing genes on basalt grains from ERW field trials. This led to proposal of a bio-ERW method involving application of the synthetic chelator K-EDDHA to soils, which directly weathered basalt and stimulated microbial siderophore production that further weathered basalt in soils in the lab, resulting in ~2.5-fold enhancement of alkalinity generation [15].

• One other study demonstrated microbial enhancement of alkalinity generation in soil, using inoculation with Bacillus subtilis, which had previously been shown to weather minerals. Inoculation of soil columns, containing one corn plant each, with a corn-root-derived Bacillus subtilis strain resulted in ~7-fold increased alkalinity liberation from feldspars present in the soil. In field trials, the same study found a 0.57%  increase in soil inorganic carbon in inoculated soils compared to 0.15% in untreated soil [17].