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

Why microbes sometimes fail to break down organic carbon in soils

A new study has found that:

In oxygen-starved places such as marshes and in floodplains, microorganisms do not equally break down all of the available organic matter. Instead, carbon compounds that do not provide enough energy to be worthwhile for microorganisms to degrade end up accumulating. This passed-over carbon, however, does not necessarily stay locked away below ground in the long run. Being water soluble, the carbon can seep into nearby oxygen-rich waterways, where microbes readily consume it.

Tests found that, in contrast to the layers where oxygen was available, leftover carbon compounds in the sediment samples where sulfur had been used for respiration instead of oxygen were mostly of the sort that requires more energy to degrade than would be liberated through the degradation itself. Making these carbon compounds of no use, then, to growing microbes, and had remained within the deeper sediment layers.

Shunned by microbes, organic carbon can resist breakdown in underground environments, Stanford scientists say | Stanford School of Earth, Energy & Environmental Sciences

Soil Carbon, Grazing & Cropping Grasslands of Alberta, Canada

114 site study.


  • 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.


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.


New Slow-release Nitrogen Calcium Phosphate Fertilizer


Researchers have used nanoparticles to create a a fertilizer that releases nutrients over a week, giving crops more time to absorb them (ACS Nano 2017, DOI: 10.1021/acsnano.6b07781).

They attached urea molecules to nanoparticles of hydroxyapatite, a naturally occurring form of calcium phosphate found in bone meal. Hydroxyapatite is nontoxic and a good source of phosphorous, which plants also need.

In water, the urea-hydroxyapatite combination released nitrogen for about a week, compared with a few minutes for urea by itself. In field trials on rice in Sri Lanka, crop yields increased by 10%, even though the nanofertilizer delivered only half the amount of urea compared with traditional fertilizer.

Slow-release nitrogen fertilizer could increase crop yields | Chemical & Engineering News

They should call it UreaCa! Geddit?

Alternately you could just use fresh plant litter or cover crop residues that leach nitrogen over two weeks and also feed soil microbes carbon. Or faba bean that will release it over three years[1] and build soil carbon so eventually you don’t need to add any.

[1] Carbon and Nitrogen Release from Legume Crop Residues for Three Subsequent Crops
Abstract | Digital Library

[2] Formation of soil organic matter via biochemical and physical pathways of litter mass loss : Nature Geoscience : Nature Research

Synthetic Fertilizer Disrupts the Carbon Cycle.

David writes: Does Synthetic Nitrogen Fertilizer Destroy Soil Carbon? – THE SURVIVAL GARDENER

“According to a recent article at Grist”

While I can’t tell if David is being sarcastic about the recent part of the Grist article since it’s from 2010, I totally agree with what he says and quotes.

Surface application of synthetic nitrogen alone has been shown to deplete the surface layer of organic carbon and increase the amount of dissolved organic carbon. This then leaches into the lower layer and initially increases C mineralization there[1]. As a result there is then less carbon in the surface layer for microbes to use and to bind nitrogen from air and so more fertilizer is then needed, leading to even more leaching over time and the creation of the more-on farmer.

Now there are studies showing that if synthetic nitrogen is added in moderation with organic matter that it can actually increase carbon mineralization, I just can’t find where I filed them. 🙂 What really matters is the form of organic matter added.

Recent studies[2,3] indicate a C:N of 100:1 or less is good for carbon mineralization depending on the clay in your soil, with between 11:1 and 50:1 being near ideal for priming in lab conditions. Split the difference and you get 30:1 which is often recommended for compost piles for microbes to break them down, not a coincidence!

Responses of priming of organic matter (OM) decomposition to OM C:N ratios (horizontal axis) and labile C:N ratios (vertical axis).
This contour figure was made based on all priming results of four OM forms from Fig. 2, using C:N ratios in OMs as x-axis, C:N ratios in the labile inputs as y-axis, and all priming data as z (color) axis. Priming effects vary strongly among substrates along the white dashed line, where labile carbon inputs are high and nitrogen is low. Priming effects do not vary strongly among substrates along the black dashed line where labile carbon is low and nitrogen is high. The dashed pink line indicates the substrate C:N threshold ratio where priming changes from negative to positive.

Ramial(small branch) chipped wood tends to be at the upper end of the region between 30:1 and 170:1 and can be a good choice long-term. However getting the C:N down should build soil faster. Chopping and dropping is one approach. Growing and harvesting cover crops before grazing and trampling them in with animals like Gabe Brown does would be another.

There are also mechanical versions like those used in biodynamics and regenerative practice. Hand operated versions of the latter are also possible.

Growing lawn grass and leaving any mower clippings can even build soil carbon[7]. However it may take 30 years to raise Total SOC levels by 1% doing it this way…

However in cases where you’re adding heartwood chips like many Back to Eden peeps, then nitrogen addition may help, I believe this is why many like Mr. Back to Eden himself Paul Gautschi has found that high nitrogen chicken manure helps.

Another example would be after forest fire or biochar creation where you have a lot of carbon but are nitrogen, phosphorus and sulfur limited as these tend to boil off at fire temperatures.


boiing points.png

In the comments on David’s post Bob also mentions the N impact on ectomycorrhizal fungi.

I found a carbon-13 labelled study that seems to suggest that by adding nitrate in the form of calcium nitrate, that trees significantly reduce below-ground C allocation, probably because the trees are getting their nitrates from the fertilizer and don’t need the microbes to fix the nitrates for them. As a knock on effect the fungi also reduce their C allocation to soil biota by 60%, likely because fungi need continuous carbon input to grow and fruit and can only afford to give up so much. This all suggests that nitrate addition short-circuits the carbon-nitrogen cycle when it doesn’t increase below-ground plant C allocation[4]. Those papers Bob quotes suggests the effect is not limited to nitrate but also ammonium further up the nitrogen cycle.


I’ve also read that the symbiosis between legumes and their rhizobia breaks down with use of nitrogen fertilizer[5]. And that there’s a consistent change in soil microbial communities with additions of N and P[6].

If N doesn’t do it, I’m wondering what will actually increase below-ground C allocation to build soils fast… any suggestions? [Other than legumes or perennials with long thin roots]

Off to Google Scholar I go…

[1] Carbon mineralization in response to nitrogen and litter addition in surface and subsoils in an agroecosystem

[2] Is the fate of glucose-derived carbon more strongly driven by nutrient availability, soil texture, or microbial biomass size?

[3] Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum, Figure 5 doi:  10.1038/srep19865

[4] Quantification of effects of season and nitrogen supply on tree below-ground carbon transfer to ectomycorrhizal fungi and other soil organisms in a boreal pine forest – Högberg – 2010 – New Phytologist – Wiley Online Library

[5] Long-term nitrogen fertilizer use disrupts plant-microbe mutualisms

[6] Consistent responses of soil microbial communities to elevated N & P nutrient inputs in grasslands across the globe PNAS | Mobile

[7] Turfgrass Selection and Grass Clippings Management Influence Soil Carbon and Nitrogen Dynamics

How much Soil Organic Carbon is best?

David Johnson’s excellent talk tackles this very question.

His finding correlates very well with my own readings on soil organic carbon (SOC) reaching a tipping point along with fungal diversity at about 3-4% SOC. It also correlates with biochar inputs and excess dissolved organic carbon (DOC) created in soils with higher than 8% SOC, which produce some of the highest crop yields in studies I’ve read. Higher than 8% SOC creating higher DOC would also mean the carbon can move deeper into the soil profile. So if you were top dressing with biochar then you probably want more than 8% biochar. However the higher DOC also has potential with phosphorus bound to the carbon to cause eutrophication when it reaches rivers and streams, as those elements are the major cause of algal blooms.

Building Soil.

I’m a firm believer that inputs drive the microbial community. For without regular water, food, shelter and play area, organisms don’t thrive. The best inputs do all of those and more.
Living plants do all these things and more, but sometimes they can use a helping hand to maximise the rate of soil production. What builds soil? Carbon life forms. Without the life forms, soil is called dirt.
The most productive soil to date appears in a protected bay growing sea grass, it has the highest natural soil organic carbon recorded so far. All because when the grass dies it gets buried in the anaerobic sediment and stays there a long time aided by the fact the bay is well protected and nutrients aren’t washed out to sea. The edge effect in action. Other ecosystems that are highly productive are low lying grasslands that receive nutrient run-off from higher elevations. Many of these are in high rainfall areas and actually store more soil carbon than forests. Grasslands tend to create a linear soil carbon profile down to about two foot, whereas forests tend to have a lot up top but less as you go deeper. The key with grasslands is the long, thin and deep rooted perennials that maximise photosynthetic area and can penetrate subsoil. The deeper you go, the less air, the less oxidation, the longer living the soil carbon.

Knowing this, I like to think about what different inputs put into the soil by way of the nutrients, structure, and biology, to change it. I also like to think about what houses and holds onto those nutrients in the soil and builds fertility. Components of soil like the sand, silt, clay, and the succession of these towards structured organic matter. The structure that is formed by carbon-based lifeforms creating the soil mucilages, exudates and porous structures as those lifeforms feed on nutrients and swim and burrow though soils.
How do different farming practises compare at creating these microbial community structures and soil carbon?
Distinct soil microbial diversity under long-term organic and conventional farming
FYM=Farm Yard Manure, CONMIN=Conventional Mineral, BIOORG=Bioorganic, BIODYN=Biodynamic
There is now Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls.
Basically that means microbes fed glucose can build soil structure without any added plant matter aka humus. Glucose is just carbon, oxygen and hydrogen, and these all exist in the air that plants and some microbes then photosynthesise into sugars. The organisms that can’t, like mycorrhizal fungi, rely on photosynthesising hosts.

When I see just how often carbon appears in the below image, it reminds me of its importance. Carbon beats out nitrogen by 4 atoms and it also forms the sugar phosphate backbone.

So along with carbon, nitrogen, and oxygen, if you just add nitrogen from the air and phosphorous from the soil then you’re ready to spin the spiral staircase and begin to churn the primordial soup.

As a result, carbon is responsible for so much diversity.
Soil organic carbon (SOC) appears to be correlated with microbial diversity and abundance.
Bacterial abundance and diversity appears to increase linearly, at least to 4% SOC.
Fungal abundance also increases linearly, however diversity appears to taper off up to 4% SOC, or it could just be that there isn’t enough data on high carbon soils.
I’ve read that “Higher soil pH values were consistently associated with higher bacterial diversity indices.”
I’ve also read the same of fungi and that Soil-available potassium was positively correlated with mycorrhizal colonization and species richness.
There’s also the Bacterial:fungal ratios are of limited utility for understanding soil processes that goes against some people beliefs.

One subject kept popping up in my soil carbon learnings, and that was just how important soil moisture is in keeping microbes active and not dormant or dead. Increasing aridity reduces soil microbial diversity.

Biocrusts are an amazing example at one end of the extreme. They can wake at a drop of rain landing on hydrophilic hairs on their surfaces, yet sleep an age during the dry times. Sleeping because without that rain the protein folding within cells doesn’t happen. The fine hairs on their surfaces even harvest atmospheric water, it’s that important.
A large Australian Soil Carbon study showed that soil moisture was correlated with 76% of SOC. That pH and soil bulk density were also important, but after that, not much else. The better the soil bulk density is, the better the soil/air exchange, while pH makes for happy microbes that breathe that air. It’s not a coincidence that soil carbon tends to buffer pH then.
Higher levels of soil carbon also increase soil moisture holding and reduces soil bulk density.
Another Brazilian forests study found 86% of soil nitrogen correlated with SOC.
I watched a Terra Preta documentary the other day. They were harvesting the high carbon black soils and trucking it out. The land owner claimed that if he left 20cm of the soil, that the deep black soil “grew back” over time. Fact or fiction? Are there microbes in the soil somehow creating long lasting soil carbon or are the soils simply at a tipping point with the 20cm and it’s enough to create or hold shorter-term carbon that may oxidise over time? Beats me. I do however know there are different forms of soil carbon and not all are equal.

Why I really love the notion of a Soil Carbon Continuum, it’s never ending. 🙂
Carbon and nitrogen additions induce distinct priming effects along an organic-matter decay continuum

As for adding biology to soils, there is a lot to consider. I wrote about Bokashi and Natural Farming in this thread recently. The results at the bottom of that post seem to indicate it works, but is that the ingredients or biology or a combination? Work With Nature on YouTube and his friend are performing a trial where they boil a compost tea and compare with a biologically active tea. I’m interested to see their results.