Abstract:
Since the dawn of agriculture, cultivated soils have lost a vast amount of carbon to the atmosphere. To repay this debt, and help limit global warming impacts, we must quickly scale processes of atmospheric CO2 removal (CDR). Soils store 3000 Gt of carbon in organic matter and inorganic carbonate minerals—more carbon than the atmosphere and terrestrial biosphere combined. Despite the loss of nearly 500 Gt CO2 equivalents (CO2e) due to soil cultivation in the past century, there is clear potential for soil-based CDR. Recent estimates suggest global croplands could sequester 1–5 Gt CO2e per year. But there is significant debate regarding the best approach for soil CDR, and the mechanisms that lead to persistent soil carbon. Most of the organic matter in soil is microbial necromass, shaped by the traits of diverse organisms. To understand how soil microorganisms lead to stable soil carbon, it is critical to understand how microbial ecophysiological traits are linked to soil organic matter formation, and how complex cross-kingdom interactions—involving diverse bacteria, fungi, archaea, protists, microfauna and viruses—shape soil carbon availability and loss. Geographic patterns of biophysical constraints and mineral capacity also constrain the potential for promoting stabilized soil carbon. I will present results from studies where we have used quantitative stable isotope probing (SIP), metagenomics/transcriptomics, and biogeochemical modeling to understand soil carbon persistence and will share new ideas for soil-based opportunities for climate mitigation.
Bio:
Jennifer Pett-Ridge is a LLNL Distinguished Member of Technical Staff adjunct professor at UC Merced and investigator at UC Berkeley’s Innovative Genomics Institute. She studies microbiome interactions in soil ecosystems, using quantitative systems biology tools such as stable isotope probing, NanoSIMS imaging, and computational modeling. She leads several DOE team projects, focused on the mechanisms that lead to soil carbon persistence (the Microbes Persist Soil Microbiome Scientific Focus Area), the capacity and costs of carbon dioxide removal in the USA (Roads to Removal) and novel bio- and geoengineering approaches for soil-based carbon drawdown (Terraforming Soil Energy Earthshot Research Center). Pett-Ridge is a fellow of the Ecological Society of America and recipient of a DOE Early Career award, Geochemical Society Endowed Biogeochemistry Medal, and the DOE Office of Science Ernest Orlando Lawrence Award.
Summary:
Focus: modeling the biogeochemistry of soils and their interactions with plants, microbes, etc.
Reports
Microbes Persist: https://www.genomicscience.energy.gov/llnlsoil/
Roads to Removal: https://roads2removal.org/
Soils and agriculture
We farm half the world’s land surface
Agricultural soils have lost >= 487 GTons CO2
Can we put it back?
Many studies and startups looking for new ways to recapture carbon
But there are real limitations that need to be considered
Challenge: we don’t really understand the dynamics of carbon in soils
How does persistent soil carbon form?
Starts as mostly dead roots
Microbes colonize the roots, encase them in proteins, dead cell bodies, etc.
This “goo” forms aggregates of dirt clumps, carbon etc.
Persistence depends on the microbial environment (whether there are bacteria that will eat it)
Roots -> Microbes -> Minerals
Deep roots:
Perennial grasslands have more C residence than forests, croplands
E.g. switchgrass roots can go down meters (corn is cms)
Roots are always growing, refreshing the soil carbon stock
Perennial grass soils vs annual cropland soils:
Have more carbon and the carbon is more recent (continually deposited)
Higher concentration of the EPS (Exopolysaccharide) “goo”
Highly weathered soils in US South are most able persist carbon
Measurement challenge:
Soil measurements are physically sparse, creating noise in the measurements
This measurement noise means that initial SOC (Soil Organic Carbon) is not that strongly related to SOC 10 years later
Trees:
Roots are also deep and last longer but grow more slowly than perennial grasses
Also, most tree biomass is above ground
Upper bound on storage ~.2 Pg/year
One technique is to keep plants on the ground for more of the year (e.g. cover crops)
Photosynthesis may place an upper bound on the possible sequestration rates
Working on gene editing to increase efficiency of photosynthesis to increase carbon fixation rate
How to prove new carbon has been added?
Using CO2 with C13 isotope in greenhouse plants to track propagation of C into roots
Can measure the relationship between the plant’s environment and carbon storage in plant body, roots and soil
Used regression analysis to connect DNA-based and metabolic datasets on plant behavior
Showing an effect of soil state (Nitrogen, water availability)
Need multi-prong effort:
Atmospheric CO2 fixation
Enhanced carbon flow
Soil carbon retention
Measuring soil microbiome
Many microbial traits identified: life history, biophysical, cellular composition, resource acquisition, stress tolerance, antagonism/defense, emergent traits
Can identify these traits via DNA analysis
Challenge:
We can only culture 1-5% of microorganisms in the lab (others only grow in the soil with the other organisms)
Only 20-30% of the microbiome is actually active at any given point in time
Stable isotope probing helps to identify the active organisms
Showed that the state of the soil near a living root vs dead root is very different (much more life and microbial activity near living roots)
The range of bacteria phyla active in the soil is diverse but there are a few that are mostly active at any point in time
There are also many viruses that infect the bacteria
Role of mycorrhizal fungi in soils
Provide significant amount of plant N, P and water
Plants share resources with the fungi
Transport carbon outside roots
Fungal tendrils reach much further than plant roots into soils
In experiment fungi transported carbon from soil with roots to plant-less soil (increased C in plant-free soil by 2% in just 6 weeks)
Recent observation: potential for fungal carbon to persist in soils
Mineralogical C capacity
There’s a saturation point where no more C can be globbed into minerals
Rate of accrual depends on C saturation
Rates 3x higher in soils at 10% saturation than at 50% saturation
Have conducted a whole-US analysis to identify regions where soils have the most persistence capacity
Pacific Northwest, Midwest, New England
Enhanced Rock Weathering and Biochar can make this even more productive
“TerraFarming”
Soil solutions with impact:
Focus on producers and sustainable systems
Many things we can do now
Cover cropping
Changing soil management practices