… 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!
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.
Comments and Questions 6 20 2017
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?
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! 😛
Burning for weeks.
I can’t help but think this:
It’s in French but you don’t need to understand a word he says to understand what he does… at least until he plants his antenna. ;^)
The termite infested wood is a nice touch.
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 6–12 g l−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
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.
This guy gets it.
- Add high nitrogen greens with the high carbon mulch,
- Wet/soak the lot,
- Mix a little to cover it all with microbes.
- Keep adding moisture.
- 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.