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
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