Soil Carbon Mineralization Limits.

A new study titled Is the rate of mineralization of soil organic carbon under microbiological control? suggests that:

  • the rate limiting step in SOC mineralization is abiotic (physical rather than biological).
  • that mineralization of SOC may be a two-stage process: firstly, non-bioavailable forms are converted abiologically to bioavailable forms, which, only then, undergo a second process, biological mineralization.

I’ll speculate and say that mineralization is perhaps limited by dissolved organic carbon in the form of plant exudates, and humates with complex and random chemical structures that are produced by weathering and detritivores that consume plant litter. Liquid compounds that tend to be lost when composting.

Compounds that are probably why wet climates and seagrasses sequester the most carbon.

Why drying and wetting of these compounds leads to CO2-Bursts.

Compounds we should be trying to keep in the soil profile doing good, and not in aquifers or waterways creating algal blooms, like we really need to with this recently discovered underground molten lake the size of Mexico!

That depending on the complexity of the carbon molecule as it gets passed down the soil carbon continuum food web it may be more than a two-stage process, with more than one in both the physical and biological realms based upon chemical energy and chemisorption of the carbon compounds.

*shrugs* Just my guess, I suck at chemistry.

Chemisorption:Screenshot from 2017-05-21 12-34-28.png

Add 1.5% Biochar to a 3.5% SOC Soil And Wait 9 Years.

Add 1.5% biochar to a 3.5% (0-100mm sampled) soil organic carbon soil, wait 9 years and it begins to positively prime. Clearly they didn’t add enough to reach a tipping point faster, or depending on your perspective and availability, adding biochar to low carbon soils is a long-term investment. I assume it was surface applied as the study is paywalled.

Screenshot from 2017-05-21 07-29-17

Paywalled: Biochar built soil carbon over a decade by stabilizing rhizodeposits : Nature Climate Change : Nature Research

Soil Carbon, Grazing & Cropping Grasslands of Alberta, Canada

114 site study.

Grazing:

  • Grazing increased plant diversity
  • Grazing increased introduced species in higher rainfall (>350mm May – Sept) areas
  • Grazing increased aboveground biomass productivity in high rainfall areas, decreased it in lower rainfall areas.
  • Grazing saw no change in SOC in the 6 study regions.
  • Grazing increased root growth in the top 30cm of soil in high rainfall areas.
  • Grazing increased decomposition of plant litter in high rainfall areas.
  • Grazing lowers CO2/N20 flux.
  • Rotational grazing (High intensity, low frequency) significantly lowered CH4 production.
  • Native grasslands store most carbon.

Conclusion: Rotational grazing native grasslands in high rainfall areas FTW.

Cropping:

root mass.png

  • Perennial grasslands produce higher below ground biomass than above
  • Cultivation leads to a rapid loss 30-60% of soil C
  • Continuous wheat cropping led to 19% loss of C
  • Cropping saw 30-40% C loss after 5 years.
  • Silvopasture (perennial pasture system) produced least CO2
  • Naturally re-vegetated areas failed to recover even after 50 years.

Conclusion: Silvopasture intercropping FTW.

Basically what Colin Seis does with Pasture Cropping native grasses.

skpcap

Residue Amendment and Soil Carbon Priming for Richer or Poorer

Feed the microbes carbon in C-poor soil and they’ll have a party.
Feed the microbes carbon in C-rich soil and they’ll put it in the C-bank.
This quote is of particular interest, emphasis mine:

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

Predator-prey carbon sequestration? Sounds similar to the Arthropod predator results. May the shift be with you.

The study:

A 150-day incubation experiment was conducted with 13C-labelled soybean residue (4%) amended into two Mollisols differing in SOC (SOC-poor and SOC-rich soils). …

The amounts of residue-C incorporated into the coarse particulate organic C (POC), fine POC and mineral-associated C (MOC) fractions were 4.5-, 4.3– and 2.4-fold higher in the SOC-rich soil than in the SOC-poor soil, respectively.

Residue amendment led to negative SOC priming before Day 50 but positive priming thereafter.

The primed CO2 per unit of native SOC was greater in the SOC-poor soil than in the SOC-rich soil. This indicates that the contributions of residue-C to the POC and MOC fractions were greater in the SOC-rich soil while residue amendment had stronger priming effect in the SOC-poor soil, stimulating the C exchange rate between fresh and native SOC.

The shift of bacterial community composition in response to residue amendment contributes to the sequestration of residue-C in SOC fractions.

The fate of soybean residue-carbon links to changes of bacterial community composition in Mollisols differing in soil organic carbon

Arthropod Predator Community & Soil Carbon Sequestration

The composition of the arthropod predator community and associated cascading effects on the plant community explained 41% of variation in soil C retention among 15 old-fields across a human land use gradient. We also evaluated the potential for several other candidate factors to explain variation in soil C retention among fields, independent of among-field variation in the predator community. These included live plant biomass, insect herbivore community composition, soil arthropod decomposer community composition, degree of land use development around the fields, field age, and soil texture. None of these candidate variables significantly explained soil C retention among the fields. The study offers a generalizable understanding of the pathways through which arthropod predator community composition can contribute to old-field ecosystem carbon storage.

Predator community composition is linked to soil carbon retention across a human land use gradient. – PubMed – NCBI

Cover Crops May Increase Soil Microbial Biomass 3x More Than Compost

Three to six times more microbial biomass carbon and nitrogen depending on soil type.

These results provide evidence that carbon (C) inputs from frequent cover cropping are the primary driver of changes in the soil food web and soil health in high-input, tillage-intensive organic vegetable production systems.

Fresh is best.

Cover cropping frequency is the main driver of soil microbial changes during six years of organic vegetable production

Microbial communities affected by type of carbon “food” sources

carbontypes.png

A new study has found that:

The type of carbon source affects not only the composition and activity of natural microbial communities, but also in turn the types of mineral products that form in their environment.

“We’ve illustrated that as microorganisms alter their environment, their environment then affects the type of microorganisms that are there and their activity.”

Researchers took anaerobic respiration microbial communities and presented them with one of three carbon sources: glucose, a six-carbon sugar; lactate, a four-carbon compound; or acetate, a simple two-carbon compound.

Their analysis showed that a distinct series of changes occurred consistently when microbes were exposed to lactate or acetate-rich environments. However, in glucose-rich environments, they observed varying patterns of changes.

“We think that, because glucose is a larger, more complex compound that can be broken down into many simpler compounds, this opens up more chemical pathways in the community through which it can be used, and that this diverse metabolic potential accounts for the different patterns we’re seeing,” said O’Loughlin.

Impact of Organic Carbon Electron Donors on Microbial Community Development under Iron- and Sulfate-Reducing Conditions