Biogeochemistry of the Critical Zone: Origin and Fate of Organic Matter

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Changing temperature, precipitation, and land use intensification has resulted in global soil degradation. The accompanying loss of soil organic matter (SOM) decreases important soil health services. Soil organic matter is a major global pool of carbon; if SOM can be increased, soils can mitigate elevated atmospheric CO2. However, there are major knowledge gaps in SOM persistence. This project looks to understand the processes that create SOM and to discover how SOM could persist for millennia.

Scientists digging a soil pit

Digging a soil pit to sample soil organic matter and how it changes over time, Emily Kykker-Snowman (former USGS) and Marjorie Schulz (USGS) at the Mattole soil chronosequence near Petrolia, California.

(Credit: Corey Lawrence, USGS. Public domain.)

Statement of Purpose: The processes of the critical zone sustain all life on earth. The critical zone is the earth’s outer skin, extending from the top of the trees to the bottom of groundwater. Plants remove CO2 from the atmosphere through photosynthesis and move carbon to the soil either through litter falling to the soil surface or through root processes, which inject organic compounds directly into the soil in the rooting zone.

This research investigates SOM; how it reacts with minerals, moves with soil water, how microorganisms process SOM, and how SOM glues mineral phases together. These processes all affect what happens to carbon in soils. Microbes (bacteria, archaea, and fungi) change the quality of the SOM by metabolizing SOM and creating microbial biomass. This research investigates microbial processes and how microbes inhabit soil minerals, which in turn affects how SOM is retained across millennia.




Why this Research is Important: Characterization of SOM under changing land use (e.g. restoration), soils of different ages (e.g. chronosequences), and changing ecosystems (e.g. different precipitation) will guide us toward best practices for long term SOM retention. This research has the potential to point to new and enduring pathways for accumulating carbon in soils, which has ramifications for future climate change.





Scanning electron microscope image of root in soil matrix

To study carbon interaction with soil minerals we use a scanning electron microscope. Here a root remnant persists in soil matrix. Mattole soil chronosequence, Terrace 6, soil depth, 100cm.

(Credit: Marjorie Schulz, USGS. Public domain.)




Objective(s): This project seeks to resolve several important issues;

  1. Do soil minerals control the potential for SOM accumulation?
  2. How important are root processes in SOM and secondary mineral formation in the sub-surface?
  3. Under what conditions does microbial cycling of SOM lead to its persistence?
  4. How does drought and rewetting affect the vulnerability of SOM to loss?
  5. What processes drive microbial assemblages and carbon deposition on mineral surfaces?





Scanning electron microscope image of soil biofilm

A soil biofilm imaged with a scanning electron microscope, Mattole soil chronosequence 120 cm soil depth.

(Credit: Marjorie Schulz, USGS. Public domain.)

Methods: This project uses a variety of methods that interrogate SOM over many spatial and temporal scales: regionally across gradients of temperature and precipitation, at the landscape level (km) across gradients of soil age (12,000 to 225,000 years), at sampling sites (meters) in soil pits to examine soil development (cm) and characterize short-term (days to years) microbial processes, down to the sub-micron scale to characterize SOM-mineral connections with electron microscopy and characterization of microbial communities through DNA-based analyses.