How organic matter breaks down in the soil to make humus:soil carbon pathways

When plants and animals die in the soil the soil organisms such as earthworms, woodlice, slugs, snails as well as the bacteria and fungi use the organic matter as food. The carbon in dead plants and animals is broken down in a number of different ways but two main stages are evident:

1.      the cellular structure and recognised organic substances and minerals in the decomposing organism disintegrate and become unrecognisable as big molecules are broken down to smaller ones.

2.    

           totally new combinations of these broken-down products develop. It is defined as humus when it becomes impossible to distinguish what the original material came from. So humus is an amorphous mix of black or brown gel-like substances of high molecular weight modified from the original tissues by various soil organisms.

There seem to be two main methods by which humus is made in soils

Process 1. The decomposition of organic materials such as cellulose and starch from crop residues and manure. Soil invertebrates eat detritus (dead plants and animals) and make the particles smaller for bacteria to work on, first by breaking them down, then by building them up slowly into humus. An alternative to this is the decomposition of woody organic material such as lignin in crop residues and compost by white rot fungi. Then the products are eventually built up by bacteria into humus.

Organic matter decomposition and formation.  Ankush J

Process 2. Carbon compounds which have been exuded by plant root are used by certain soil fungi which produce mycorrhiza. These form a symbiotic relationship with plant roots. The mycorrhiza use these sugar exudates to make glomalin, a major component of decomposing organic matter and this will eventually be built up into humus.

The process of humus formation through decomposition of organic matter is not very efficient at building humus in soils. Of 100 g of organic matter that is added to the soil maybe 60-80g will be converted back to carbon dioxide by the invertebrates and the bacteria, 3-8 g will be taken up by bacteria to help them to grow, 3-8 g used to make other plant organic compounds and possibly 10-30 g used to make humic compounds.

It is known that around 40% of the sugars made by photosynthesis in plants leaks out of the roots. This can be used by mycorrhizal fungi, some of which invade the plant roots. These fungi supply many nutrients including up to 90% of the plant's nitrogen and phosphorus requirements, plus calcium, potassium, magnesium and iron, as well as essential trace elements such as zinc, boron, copper, cobalt, molybdenum and manganese. They often supply water as well - all in exchange for liquid carbon! (Smith, 2008). So these mycorrhiza effectively increase the coverage of the plant’s root system in the soil and can make the plant much more efficient at absorbing plant nutrients (particularly phosphate) from the soil solution.

Mycorrhizal hyphae colonising the roots of a pine seedling. 

As the mycorrhiza grow they produce a protective surface coat of glomalin, a glycoprotein (protein containing a plant sugar). The glomalin drops off into the soil where it acts as a "super glue," helping sand, silt and clay particles stick to each other as pea sized lumps called aggregates. The small spaces between the aggregates help rainwater to move through the soil more easily. Moisture is absorbed in these aggregates which is protected from evaporation, but the mycorrhiza are able to access this water and supply it direct to the plant roots at times of water shortage. It is substantially due to the gel-like substance of glomalin that it is often stated that 1 kg of humus can hold 4-20 times its own weight of water. And it helps us to understand that soils rich in humus will have a good structure, improved water holding capacity, enhanced infiltration and drainage and enhanced nutrient exchange capacities

Glomalin also stores approximately 25-33% of the total soil carbon and can last in the soil for 7-40 years as part of the soil active fraction.  At some stage the glomalin is either respired to carbon dioxide if the soil is dug, or is converted to humus if the soil is not dug and there are plenty of mycorrhizal fungi in the soil.  

It seems that the process of making humus in soils through the activity of mycorrhizal fungi in process 2 is more efficient than soils which make humus by process 1 because soils that have not been dug generally have higher humus levels.

Another benefit of soils which have abundant mycorrhiza, glomalin and humus is that they have a markedly increased resistance to climate variability.

HCF's has a policy of feeding the soil by adding lots of animal manure and as much compost as is available. However as this article suggests this method is not very effective at converting soil organic matter to humus and may not actually increase soil carbon levels if other farming practices such as digging and leaving the soil bare destroy soil carbon.

Most of the cultivated plots at HCF are likely to be acutely short of mycorrhiza as digging or rotivating breaks up the hyphal strands and disrupts their relatively slow growth. Furthermore our practice of leaving the soil bare and plant free for several months of the year means that during this period the mycorrhiza have no living plants to exchange nutrients with and so decline. One of the challenges for us at HCF is to work out how we can help to develop mycorrhizal growth in the vegetable plots. Maybe we should think more clearly about planting winter cover crop?

References

Organic matter decomposition and formation.  Ankush J

Mycorrhizal hyphae colonising the roots of a pine seedling. Aberdeen mycorrhiza Research Group

What is soil organic matter?

Soil organic carbon is organic matter is made up of living organisms, undecomposed plant, animal and micro-organism residues and humus, formed over many years. You cannot have good soil structure without high levels of organic matter. It is the main food reserve for living soil organisms and many of them provide the simple substances that are taken up by the plant roots. The most reliable method of finding out the soil organic matter (SOM) concentration of a soil is by burning (dry combustion) at temperatures of over 900⁰C. The result is normally given as percentage organic matter in the soil. Farmers weekly provide a simple chart for farmers to score the quality of their soil.

Less than 1% very low

Less than 2% low

Less than 4% Medium

Less than 8% High

Over 8% Very high

Typical components of a soil

http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/PlantHormones/soil.gif

Organic matter is made up of three main components which have been called the living, the dead and the very dead!

Living organisms, plants, animals, soil invertebrates, bacterial and fungi are all considered to be part of soil organic matter, and they play a big role in contributing organic residues to the soil and in formation of more stable types of organic matter. Up to 15% of soil organic matter is living organisms and fresh organic material.

Figure 2 The structure of soil

Active soil organic matter is primarily made up of freshly dead plant and animal residues that break down in a very short time, from a few weeks to a few years. It is sometimes called detritus if it contains partially broken down cells and tissues that are only gradually decomposing. One third to one half of the SOM is active soil organic matter, the detritus of partially and slowly decomposing plant and animal material that may last decades.

Stabilised or passive soil organic matter, is known as humus. It is very dead(!) and not biologically active because it provides very little food for soil organisms. Humus may take hundreds or even thousands of years to fully decompose! Humus is very important as (a) it acts like a sponge and can absorb 4-10 times its weight in water,  (b) it is a way of taking carbon dioxide out of the atmosphere and burying it in the ground (sequestering it) and so helps to mitigate the effects of rising CO₂ levels.

Soil organic carbon from
http://soils.usda.gov/sqi/concepts/soil_biology/images/soil_f1_high_res.jpgaption

Soil organic matter may enter the soil in a number of ways, such as addition of manure, compost, mulch such as sweetcorn stalks or woodchip, leaf mould, coffee grounds, eggshells, chicken pellets or the growing plants in their little pots of compost. But it may also be produced in the soil by the crop and the surrounding weeds and soil animals grow and then die and remaining in or on the soil surface.

Soil organic matter may be removed from the soil when crops are harvested, often with some sticky soil around them, on our boots, when the wind blows it away, or heavy rain washes or leaches it away through the soil.

The global climate is becoming more unstable due to global warming. Last year it was "the Beast from the East" for us at HCF, followed by a warm and fairly dry summer. Who knows what it will be this year? One of our aims at HCF should be to create a soil that is as resilient as possible to climate change. To be this soil it needs high levels of Soil Organic Matter. This soil will be able to hold more moisture, which, among other things, will enable it to cope with longer periods of water shortage or heavy and excessive rains which would otherwise cause leeching or runoff. 

Refs

http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/PlantHormones/soil.gif
http://soils.usda.gov/sqi/concepts/soil_biology/images/soil_f1_high_res.jpg