Nutrient Availability in Soil Amended with Wheat Straw and Legume Residue

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Study: Residue addition frequency influences respiration, microbial biomass and nutrient availability in soil amended with high and low C/N residue

In the image above I’ve basically highlighted the mature dried wheat straw in yellow with a C:N of 80:1 that was first applied to soil. After two weeks the same amount of the green young dry faba bean with a C:N of 20:1 was applied at differing amounts and frequency for two more weeks.

After application of the wheat straw you can see a decline in plant available nitrogen by 75% & phosphorus by 50% in the first two weeks.

After that two week period, adding the equivalent amount of faba bean residue then doubled the original available nitrogen and phosphorus availability, and it seems to me like it may have sustained much higher levels for longer had the study continued. Soil carbon priming in action.

The H1-L4 (High C:N wheat followed by 4 applications of Low C:N faba over two weeks) part of the study however is the most interesting for me. Instead of applying all the faba bean reside in one go, applying it in stages gradually increased (red line) the available N and P. This approach would probably be the most efficient nutrient wise as plant nutrient removal increases as the plant grows, so it makes sense to add the nutrients as it needs them. Plants typically remove nutrients in a sigmoid curve.

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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

Mulch! The Soil Carbon Continuum Way

Matt’s video prompted this reply. 🙂 Now you may know my views on compost, well…

In the video above they show a comparison of the effect of soil imprinting on run-off vs not imprinting on the left. Soil imprinting is a wonderful way to start a garden if you don’t have much compost or organic matter in topsoil, say in a dry climate. Throw what compost you have in those troughs, or not. Soil imprinting simply makes your soil look like one big egg carton helping to collect rain and wind debris. Soil moisture is the single biggest factor when it comes to building soil carbon and fertile productive soils, living plants and the microbes they host in the soil are the other. Harvesting that water where plants can use it to suck carbon from the air and feed it to soil microbes is critical, but so is the material that blows into the troughs. This material is very nitrogen rich, like the air that blows it in, and tends to have a high cation exchange capacity. It’s why weeds thrive in cracks and crevices.
This is mulching in place.

Live in a wet climate? Try planting in the peaks.

Cattle, bigfoot and astronaut grazing systems have a similar effect to soil imprinting, this can be a useful feature in intensive rotational grazing systems, so long as you have enough astronauts…

soil imprinting.jpegcattle-footprintbigfootmoon-imprint

Once you’ve imprinted, simply broadcast seeds and what compost you have into the troughs where water and debris blown by wind and rain collect and away you grow.

And while compost is great and helps get plants and a garden get started, what really gets the party going after that is when you chop and drop or roll and crimp just when plants flower and reach peak biomass. Studies have shown you get 3x more microbial biomass carbon and nitrogen than from compost if you chop a cover crop. More if you roll and crimp it, and even more on sandy soils.

It’s the leaching of soluble compounds from fresh litter that helps achieve this by feeding microbes that glue soil particles together and make houses for themselves, along with the moisture management properties of the bulk material once soluble nutrients are leached. Rolling and crimping helps by slowing the moisture loss.

I’ve previously written that when mulching ideally mulch should have a C:N of less than 50:1 to prime soil carbon, otherwise until the soil interface layer with mulch has biodegraded enough it will have a negative carbon priming effect that leads to problems like nitrogen deficiency. Planting legumes can help here if you’ve made that mistake, so can running chickens over the mulch, or simply moving it aside to expose the soil to more air.


Ultimately, we should aim for the Soil Carbon Continuum approach that consists of increasing C:N and chip sized mulch layers. Increasing in chip size is similar in many ways to the soil imprinting above.

It’s not just plants that need layers in the permaculture world.

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I like to start with soil imprinting, then compost, fresh plant litter, dry leaf/twig/bark mulch, branch mulch, sapwood mulch and then heartwood mulch layers. Each increasing in C:N ratio and halving the amount applied with each layer. All followed by a living root.


Because ultimately it’s the inputs that drive microbial diversity and power the soil food web, not the other way around. Fungi for example need a continuous supply of carbon to grow and fruit. In forests massive old growth trees do this in large quantities when they suck carbon from the atmosphere. It’s been shown that adding nitrogen fertilizer disrupts this feeding of soil microbes by trees, upsetting the highly tuned ecosystems.


So don’t forget the living root in those layers above. They host symbiotic microbes like mycorrhizal fungi that can shunt nutrients and water at 3mm per hour, that’s 72mm or 3″ a day!

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And roots pump the carbon in the air into the soil to feed soil microbes, but only up to a point. What’s that point I hear you ask? It’s soil moisture dependant. It’s been shown that plants exhale more carbon than they sequester when when moisture deprived during droughts. And increasing carbon in our atmospheres means plants thirst for more water leading to even worse droughts in some climates as plants suck that water up.

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That’s it. Follow the path of the living root.

In another post I’ll cover some different forms of carbon that make up the soil carbon continuum and play different roles and why we want diversity there too.

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


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

Mulch Decomposition Rule of Thumb

I made a chart and in the process learned that as a rule of thumb mulch C:N more or less follows an exponential time decomposition rate for carbon. Your decomposition time will vary depending on location temperature, latitude etc.


I based it on carbon decomposition rate data I could find, and Australian Native Forests and Plantations.

My conclusion is;

  1. build soil with with grass/legumes/compost/green leaves in the first year.
  2. add leaf litter in the second year.
  3. add twig & bark in the third year.
  4. add branch in the fourth year.
  5. add sapwood in the fifth.
  6. add heartwood in the sixth.

Basically; grow to chop and drop and by year 7 your soil will be awesome.