How can we increase soil carbon in cropping systems?

Inputs

Depth

C pool

Losses

Kate ScowRussell RanchDept of LAWRkmscow@ucdavis.edu

And what does soil carbon have to do with soil life?

How is soil organic matter NOT made?

Previously we thought process was “humification”.Decomposition/modification (e.g. condensation) of plant and

other organic matter, leading to humus as final product.

These are laboratory artifacts

New conceptual model—a lot has changed.

New non-invasive tools to look at soil organic matter, isotopes to track carbon flow, technologies to characterize soil microbial communities.

Microbes are enzymatic drivers who convert plant residue C into their cells, die and

provide the “feedstock” for SOM formation

Soil organic matter formation: (STEP 1) Transformation of plant residues into microbial cells and by-products

plant material

50-80% of SOM is simply

dead microbial bodies. If

you want to increase SOM,

you must build microbial biomass.

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Earth & ClimateLandslidesGeochemistryAcid Rain

ReferenceHumusSoilSoil lifeSoil pH

Dec. 14, 2012 — Remains of dead bacteria have

far greater meaning for soils than previously

assumed. Around 40 per cent of the microbial

biomass is converted to organic soil components,

write researchers from the Helmholtz Centre for

Environmental Research (UFZ), the Technische

Universität Dresden (Technical University of

Dresden) , the University of Stockholm, the Max-­

Planck-­Institut für Entwicklungsbiologie (Max

Planck Institute for Developmental Biology) and the

Leibniz-­Universität Hannover (Leibniz University

Hannover) in the journal Biogeochemistry.

Until now, it was assumed that theorganic components of the soil werecomposed mostly of decomposedplant material which is directlyconverted to humic substances. In alaboratory experiment and in fieldtesting the researchers have nowrefuted this thesis. Evidently theeasily biologically degradable plantmaterial is initially converted tomicrobial biomass which thenprovides the source material to soilorganic matter.

Soil organic matter represent thelargest fraction of terrestrially boundcarbon in the biosphere. Thecompounds therefore play animportant role not only for soil fertilityand agricultural yields. They are alsoone of the key factors controlling theconcentration of carbon dioxide inthe atmosphere. Climatic changecan therefore be slowed down oraccelerated, according to themanagement of the soil resource.

In laboratory incubation experiment,the researchers initially labelledmodel bacteria with the stable

isotope 13C and introduced thebacteria to soil deriving from the long-­term cultivationexperiment "Ewiger Roggenbau" in Halle/Saale. Following theincubation time of 224 days the fate of the carbon of bacterialorigin was determined. "As a result we found fragments ofbacterial cell walls in sizes of up to 500 x 500 nanometresthroughout our soil samples. Such fragments have also beenobserved in other studies, but have never been identified orquantified," declares Professor Matthias Kästner of the UFZ.The accumulation of the bacterial cell wall fragments appearsto be supported by peptides and proteins from the liquid interiorof the cells, which remain to a greater extent in the soil thanother cell components. These materials enable the formationof a film of organic molecules on the mineral components ofthe soil, on which the carbon from the dead bacteria isaccumulated and stabilised.

When the fragments of the bacterial cell walls dry out, theymay lose their rubber-­like properties and can harden like glass.If the soil subsequently becomes moist again, however, undercertain circumstances they cannot be re-­wetted -­-­ an importantprerequisite for their degradation by other bacteria. This wouldprovide the simplest explanation for the stabilisation oftheoretically easily degradable carbon compounds in soil. "Thisnew approach explains many properties of organic soilcomponents which were previously viewed as contradictory,"says Matthias Kästner. In the late 1990s, Kästner and histeam arrived at this idea on the basis of earlier investigationson the degradation of environmental contaminants likeanthracene in polluted soils of former gas work sites. In these

The electron micrograph shows bacteria(Hyphomicrobium sp;;. Yellow) growing up partly onsolid surfaces, floors and sediment grains. Duringgrowth whatsoever cells die and deformed orfragmenting cell envelopes remain. Small-­scalefragments of these shells (red) then set themicroparticulate matrix in soils and sediments.(Credit: Burkhard Schmidt-­Brücken, Institute ofMaterial science/TU Dresden;; Colored by ChristianSchurig/ UFZ)

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And then? STEP 2

plant material

Soil organic matter formation: (STEP 2) stabilization of organic material in soil

Microbial products

Chemical

ProtectionPhysical

protection

(occlusion)

To stabilize SOM, the C

from microbial bodies

needs to bind with soil

minerals and/or be trapped in small pores

Schmidt et al. 2011 Nature 478, 49–56; Miltner et al. 2012

Variety of soil carbon-mineral associations

Protection in soil aggregates

Gross, C.D. and Harrison, R.B., 2019. The case

for digging deeper: soil organic carbon storage,

dynamics, and controls in our changing

world. Soil Systems, 3(2), p.28.

Putting it all together

C pool

Implications and questions about microbial feedstocks

• We must feed the soil--synthetic fertilizer is not

enough. And continuously. How much, how

often?

• Are more degradable substrates better than

less degradable substrates to build carbon?

• Are microbes getting all nutrients they need to

build cells: N, P, K, S? Enriched inputs?

• How does microbial community composition

(e.g. fungi vs bacteria) and diversity affect

SOM formation?

• Can SOM be formed under conditions harsh

for microbial activity (dry, hot, high salinity)?

What can we do to increase soil carbon?

Inputs

Depth

C pool

Losses

Add organic amendments

• aim for slow release of carbon over the course of the year

• steady food supply for microbes

• variety of organic compounds

Sources:• organic amendments: compost (from variety of sources), manure solids and

slurries, biodigestate, biosolids (?)

• Integrated animal systems: manure and urine

• Others?: biochar not very degradable, compost tea low in carbon concentration

Create carbon “in place”

• Crop residues (biomass varies)

• Soil cover for more of year• steady food supply

• Roots--variety of exudates/dead roots • promote metabolic diversity• deep roots

• Cover crops (e.g. legumes)• N inputs

Crop rotations

Cover crops

Intercropping

Depth

Increase “size” (or capacity) of soil reservoir

• Texture: Some (but not too much) clay is best. Is it possible to build SOM in sandy soils? Can added minerals help?

• Soil structure: good aggregation essential to preserve SOM

• Increase size of reservoir by increasing target depth. Is it important to distribute carbon inputs throughout soil (e.g. increase mineral/OM ratio, move C deeper where clay content is higher? What is role of water as vehicle?

Improve root systems

Annual crop varieties have been bred for systems that provide water and mineral fertilizers.• Maximized

• above-ground biomass (fruits, leaves etc)

• Reduced• root system extent• root colonization by beneficial

organisms• root exudates• organic mass when plants die

15

Kernza ® is a grain-producingperennial wheatgrass

• Cousin of annual wheat; dual-usecool season rhizomatous grass1

• 40 farmers in Midwest U.S.A.

growing Kernza (170 ha)2

• 40 international research groups

working on Kernza3

• Recent studies observed economic and

environmental benefits associated

with Kernza vs. annual wheat

cultivation 2,4

Rationale | Overall Approach | Experimental Design | Chapter 1 | Chapter 2 | Chapter 3 | Conclusions

• Kernza established in 2018 at Russell Ranch Sustainable Agriculture Facility

• Split-split plot design with 4 field replicates

• Cropping system type (main plot), irrigation (subplot), nitrogen fertilization (sub-sub plot)16

How does Kernza’s aboveground productivity and ecosystem services (including C sequestration) compare to tilled and no-till annual wheat along gradients of nitrogen and water in the Mediterranean climate of California?

17

• Non-irrigated Kernza yields were almost 2X higher than irrigated yields

• N fertilization slightly increased grain yield but dramatically increased aboveground biomass

• Kernza grain yields were significantly lower than annual wheat, but total aboveground biomass was greater

• 2019 biomass and grain yields were comparable to results from other US regions

Reduce disturbance

Benefits• Stalk incorporation

• Weed managment

Conventional tillageNo Till

Issues• Loss of soil C through

exposing C, adding oxygen

• Fungal symbiont network disturbance

• Compaction

Benefits• Increased soil C near

surface

• Improved soil structure

Issues• Soil C doesn’t have as

much access to minerals.

• Soil C doesn’t move deeper?

• Herbicide use

Take home messages

• Soil is a dynamic living system

• Need to consider deeper soil profiles when accounting for soil C

• To maximize soil C and health:

• Add organic amendments

• Increase plant cover

• Improve root systems

• Reduce disturbance