Adding Carbon to Soils increases Microbial Nitrogen Use Efficiency

… and that in the short-term carbon through microbial form immobilizes already available N, potentially making less available for plants or loss through leaching.

The belief that adding high-carbon mulch locks up nitrogen may yet have some merit, not necessarily in the mulch, but in the microbes!

From Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining:

Although C input promoted microbial growth and N demand, we did not find indicators of increased N mobilization from SOM polymers, given that none of the soils showed a significant increase in protein depolymerization, and only one soil showed a significant increase in N-targeting enzymes. Instead, our findings suggest that microorganisms immobilized the already available N more efficiently, as indicated by decreased ammonification and inorganic N concentrations. Likewise, although N input stimulated ammonification, we found no significant effect on protein depolymerization. Although our findings do not rule out in general that higher plant-soil C allocation can promote microbial N mining, they suggest that such an effect can be counteracted, at least in the short term, by increased microbial N immobilization, further aggravating plant N limitation.


Should you use plastic weed mat?

Comments and Questions 6 20 2017

Kevin answers questions and comments on his videos about a range of topics and emphasises the importance of soil, water and air.
Here I want to emphasise the importance of plants and organic mulch too.
One of the comments on his Q&A video was the following after seeing Kevin’s use of weed mat “to keep the soil covered”:

My only experience with weed mat, was pulling one up, and finding stinky dead earth beneath

Now it sounds like this person’s plastic weed mat caused the soil below it to turn anaerobic due to either a lack of air penetrating the mat or retaining too much moisture which then excluded the air from entering the soil. When this happens the soil begins to anaerobically digest and produce alcohols, phenols, and gasses such as methane, the scent of sewer gas.

Not ideal plant growing conditions!

However there can be benefits when using plastic weed mat in the right conditions that does let air and water through, as the mat can help keep the soil moist, especially in drier regions. Weed mats can also warm or cool the soil depending on how much sunlight they absorb or reflect. They can also help prevent erosion. But the main reason people use it, is because the mats also reduce weed competition for plants planted into or near the weed mat. Reduced competition for water, sunlight, and for nutrients.

However the biggest problem with plastic weed mat when growing and assuming the correct mat was chosen for the conditions, is that if left on for extended periods, the soil often tends to breed plant pathogens. This happens when soils are not amended with a high carbon source such as plant leaf litter, plant exudates, or an organic mulch.

In soils that are kept moist and aerated and warm the microbiology and fauna become more active and will chomp through organic matter like there is no tomorrow. In doing so there is an increase in soil carbon respiration in the form of carbon dioxide and methane along with other gasses. This increase in respiration can actually help increase plant growth by providing carbon dioxide concentrated around the plant leaves. The increase in microbiological activity also increases nutrient cycling and plant available soil nutrients. However if the plants aren’t putting the carbon back into the soil via their roots, exudates, or plant litter when they die – then over the long term the soil community suffers.

When there is a lack of high carbon input that organic mulch provides to soil organisms, competition for that soil carbon increases. And the less soil carbon, the less complex organisms will survive. This is particularly important for fungi that rely on the carbon because they are made up of more carbon than other microbiology. Worse, the fungi that do survive the hostile conditions are often those that are plant predators able to fight for the carbon needed in order to survive because they now lack competition. As a result those predators infect plants and reduce yields or even kill them, and so gardeners and farmers search for solutions to their fungal problems in the form of fungicides. As a result fungi get a bad name. The same happens to nematodes.

However for short season plants like seasonal crops in Kevin’s example, this may not be much of a problem as the plant may be ready for harvest before the predators have overcome a plants defences, and he’s adding organic matter every year.

To conclude, when organic resources are plentiful, everyone’s happy and works in symbiosis, and when they’re not happy it’s war. Not quite extremophile Star Wars, but certainly localised Planet Wars, and eventually those wars include us higher order carbon beings in Human Wars that result from desertification and a lack of resources.

Plastic mulch is also plastic. Did you know that most sea salt already has microplastics in it after a little over 100 years of plastics use?

Mulch, mulch, mulch, mulch, mulch; that’s the sound of happy soil biology.

The Natural Farmer

Great episode from Jagannath.

Comments: straw tends to be high in lignin which needs white rot fungi to break it down, which requires moisture and gives rotted wood it’s soft spongy characteristic. Straw is probably better as a winter rather than summer mulch which is why I have a rule of thumb to use soft mulch in summer and hard mulch in winter. Grow your mulch in spring for summer! Prune for winter!
Interestingly leaf litter can also almost contain as much lignin as straw. Sugar cane mulch on the other hand has about 1/3 the lignin of straw and corn stalks are half that again, sugar cane mulch has an ideal carbon to nitrogen ratio of 50:1 though. Corn can actually be quite good for building soil carbon so long as the mulch is left in place and a green added to it, like Jagannath’s planned legume companion.
Also, the allelopathic leaves mentioned can be used after a quick 3-day hot compost. Putting them in a black bag in the sun with some urine may work too.
I find legumes like beans are a great indicator of soil bulk density. They tend to thrive in loose soil where air gets the root zone. The other thing Jagannath may be missing for his soil and environment with such dry lifeless soil is the rhizobia inoculant that form the nitrogen-fixing symbiosis in the form of root nodules. Here in Australia there’s a trial going on to match the microbe with the legume so that farmers can inoculate at sowing time, apparently we don’t get the same level of root nodulation and yield compared to other countries because of our microbial species, so efforts are being made to breed and select for them.
Today I also learned that leaf litter suppresses herbivorous nematodes likely by increasing the fungi population that trap them, while on the other hand just leaving roots in the soil increases nematodes and bacteria over time.
Lastly, I like to measure my mulch depth in buried finger knuckle joints. One knuckle joint is good, two is better, three is best! And so is using three different mulches: green, brown and white. Green for bacteria (nitrifying), brown for brown rot fungi (cellulose), and white for white rot fungi (lignin).
As you can probably tell, I consider mulch the most underrated of topics! šŸ˜›

C:Nhoosing Your Mulch? Think of the Fungi.

Ideally we want a diverse range of mulch and C:N ratios such that we get a diverse range of biology. I’d wondered what that range might be so did some digging, or should that be mulching? Here’s one paper.

Effects of carbon concentration and carbon to nitrogen (C:N) ratio on six biocontrol fungal strains are reported in this paper. All fungal strains had extensive growth on the media supplemented with 6ā€“12Ā gĀ lāˆ’1 carbon and C:N ratios from 10:1 to 80:1, and differed in nutrient requirements for sporulation. Except for the two strains of Paecilomyces lilacinus, all selected fungi attained the highest spore yields at a C:N ratio of 160:1

Effects of carbon concentration and carbon to nitrogen ratio on the growth and sporulation of several biocontrol fungi

Seems to confirm that for fungi to reproduce and sporulate, that they need a constant supply of carbon. Note that the study only went to 160:1, and more carbon could be more desirable.

If you look at this chart I made, the curve is steepest at the peak between about 18:1 and 50:1, this range seems likely to be the sweet spot for fungi and for soil carbon priming, however to reproduce fungi, material with a higher C:N is also desirable.

fungi vs c:n.png


Supercharge mulch using this basic principle

eat your garden

This guy gets it.

  1. Add high nitrogen greens with the high carbon mulch,
  2. Wet/soak the lot,
  3. Mix a little to cover it all with microbes.
  4. Keep adding moisture.
  5. Soil profit!

You can also premix the mulch with greens and a handful of soil and soak in a container to ensure the mulch absorbs the water and adsorbs the microbes.

Soaking mulch in watered down 5:1 aged urine also adds nitrogen and other minerals, remember to add some soil for microbes.

Mulch! The Soil Carbon Continuum Way

Matt’s video prompted this reply. šŸ™‚ 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.

permaculture layers.gif

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!

mycorrhizal fungi.jpg

root soil carbon exudates.jpg

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.

living root.jpg

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.