Understanding the geochemical basis for soil organic matter storage at the global scale

Much of the carbon assimilated by terrestrial

terrestrial

The geographical and biological features that are associated with the Earth's solid, rocky surface, residing on or resembling the land or terrain found on our planet.

Source: NASA Source

ecosystems ends up shed by plants to become soil organic matter (SOM), the largest carbon reservoir

reservoir

A manmade lake that is created when a dam is built on a river. River water backs up behind the dam creating a reservoir.

Source: USGS Source

in the terrestrial biosphere. The capacity of soil to retain SOM and stabilize additional carbon is moderated by the activity of soil microbial decomposers—but also by the geochemical environment. Soil minerals transform, dissolve, and interact with organic molecules released and synthesized by microbial decomposers. Soil mineral reactivity hence defines the abiotic environment in which soil biological processes unfold, and is an overarching control on SOM stabilization. Despite its significance in the carbon cycle,

carbon cycle

The movement and exchange of carbon through living organisms, the ocean, the atmosphere, rocks and minerals, and other parts of the Earth.

Source: a student's guide to Global Climate Change Source

soil mineral reactivity is virtually uncharacterized at the scales relevant to Earth system modeling. We will address this knowledge gap by synthesizing a global database of soil geochemical measurements and modeling the global-scale processes that govern soil mineral reactivity and associated SOM storage. Specifically, we will use this database to test the hypothesis that warmer, wetter, more productive biomes tend to host less reactive soil minerals—but mineral reactivity can be amplified by geologic factors: lithology, topography,

topography

The land's physical attributes within a specific area or region, comprising both natural formations like mountains, valleys, and rivers, alongside human-made additions such as roads and buildings.

Source: Source

and depositional history. Consequently, soil mineral reactivity and accompanying soil organic matter storage vary substantially as a function of geologic factors within major biomes. Our analysis will involve harmonizing global geochemical databases and developing approaches to infer soil mineralogical composition from bulk chemical composition. We will then use the harmonized global database to develop gridded maps of soil mineralogy and mineral reactivity, which we will quantify using weathering indices (e.g., the chemical index of alteration). In the final stage of our analysis, we will quantify spatial correlations between soil mineral reactivity and SOM while controlling for confounding environmental variables in a series of regional case studies. The data synthesis

synthesis

The process of combining a number of things into a coherent whole to form a theory or system.

Source: National Library of Medicine Source

and modeling approaches described here will thus provide a scaffolding for the next generation of belowground biogeochemical models. This research will also test our fundamental understanding of how soils develop and function in different ecological and geological contexts.