Persistence of soil organic matter

Soil humic substances

The prevalence of humic substances in soil has been assumed for decades19. Previous generations of soil chemists relied on alkali and acid extraction methods20 and observations of the extracted (or residual) functional-group chemistry to describe the presence of operationally defined 'humic and fulvic acids' and 'humin'. Humic substances were thought to comprise large, complex macromolecules that were the largest and most stable SOM fraction. However, we now understand that these components represent only a small fraction of total organic matter13,21-23: direct, in situ observations, rather than verifying the existence of these large, complex molecules, in fact find smaller, simpler molecular structures, as visualized in Fig. 2 (refs 13, 22, 23). Some of what is extracted as humic acids may be fire-derived24,25, although these compounds are rare in soil without substantial fire-derived organic matter. In any case, there is not enough evidence to support the hypothesis that the de novo formation of humic polymers is quantitatively relevant for humus formation in soils.

Influence of roots

Root-derived carbon is retained in soils much more efficiently than are above-ground inputs of leaves and needles40-42. Isotopic analyses and comparisons of root and shoot biomarkers confirm the dominance of root-derived molecular structures in soil43 and of root-derived carbon in soil microorganisms44. Preferential retention of root-derived carbon has been observed in temperate forests45,46, for example, where below-ground inputs, including fungal mycelia, make up a bigger fraction of new carbon in SOMthan do leaf litter inputs44,47. In addition to many above-ground inputs being mineralized in the litter layer, root and mycorrhizal inputs have more opportunity for physico-chemical interactions with soil particles40. At the same time, fresh root inputs may 'prime' microbial activity, leading to faster decomposition of older organic matter48,49 as well as changing community composition50. Carbon allocation by plants thus plays an important part in soil carbon dynamics, but it is not known how future changes in plant allocation will affect soil carbon stocks51.

Taken together, these eight insights paint a broad picture of carbon cycling in soil that has implications for fundamental research, landmanagement, and climate change prediction and mitigation (Fig. 3). They suggest that themolecular structure of plant inputs and organicmatter has a secondary role in determining carbon residence times over decades to millennia, and that carbon stability instead mainly depends on its biotic and abiotic environment (it is an ecosystem property).Most soil carbon derives from below-ground inputs and is transformed, through oxidation by microorganisms, into the substances found in the soil.

Dylan Mulder wrote:Hello Permies,

I’m a long time lurker and a first time poster. For my first post, I’d like to share some interesting knowledge with y’all, since y’all have shared your knowledge with me for some time.

What I’d like to discuss comes from a scholarly article called, “Persistence of Soil Organic Matter as an Ecosystem Property”. This a good article, which discusses a number of topics related to soil organic matter - I’ll be posting only a few parts that I found especially thought provoking and relevant to agriculture.

In any case, there is not enough evidence to support the hypothesis that the de novo formation of humic polymers is quantitatively relevant for humus formation in soils.

What do you mean it’s not important?

In my area, when one has their soil tested the soil report contains a measure of humic substances. As growers, we desired to increase that percentage because the higher it is, the greater your cationic exchange capacity, and the less amendment you had to put down. One can even purchase humic substances as an amendment.

Years ago I put a soil extract through a mass spectrometer. The result was one big blur. So many different chemical compounds that we couldn't distinguish any of them. Humic and fulvic acids seem to be important, but that's little more than an educated guess.

A simple test: Weigh a sample of soil. Dry it and weigh it again. Burn it and weigh it a third time. This tells you how much water and how much organic matter (living and dead animals, plant, fungi and microbial material, humus etc) there is in the soil. OK this leaves a lot of unknowns, but soil and humus is basically a big black hole, very hard to study without destroying it. A good farmer will be able to estimate the organic matter content just by rubbing the soil between his fingers.

We know that organic matter is inherently a good thing. Peat contains almost no soluble minerals (unless they have been added artificially), but adding it to soil improves crop yields. It improves structure for one thing. It also seems to act as a reservoir for water and minerals, preventing them being lost from the soil and slowly releasing them to the plant roots, but at this point we are starting to peer into the black hole, because we don't know exactly which components of the soil organic matter/humus break down quickly and which slowly. We know that some of the soil organic matter will not release its nutrients for 100 years.

My attitude is that on a large scale over long time periods, the best way to increase soil organic matter is to grow stuff like grass, trees, clover etc. because their roots will ultimately be the best thing for the soil. However if you wish to create a small highly productive garden quickly, its better to bring in lots of organic matter from elsewhere, perhaps cut back vegetation or collect leaf litter from somewhere with lots of grass and trees (making sure you don't take too much over too big an area and trash the place), or, if it is available, rescue some "waste" organic matter which would otherwise go to landfill - spoiled fruit and vegetables, grass clippings

Bryant RedHawk wrote: ...Earthworms do not actually eat leaves, nor do they eat cardboard, rather they feast on the microbes that decompose the organic matter.

Peter Ingot wrote: ...OK this leaves a lot of unknowns, but soil and humus is basically a big black hole, very hard to study without destroying it.

Alder Burns wrote: ...All those interested in the issue of persistence of soil organic matter should look into biochar.