Soil Organic Matter

Chapter 12

Soils and the greenhouse effectC cycle

Decomposition of organic matter C / N ratio

Humus

Factors affecting soil organic matter

Managing soil organic matter

Other related stuffHistosolsComposts

Soils and the Greenhouse Effect

Increasing concentration of certain gasesin atmosphere increases retention of solarenergy

Anaerobicorganisms

Wetlands bad?

Produced by soil microorganisms

Size of wedges indicaterelative effect, not absoluteconcentrations.

What if this rate is larger?

Carbon Cycle

The C cycle is complex, with many differentpools and rates of transformation from one toanother. However, in essence, it can becondensed into cycling between C in organiccombinations and C in inorganic forms, likeCO2, HCO3

-, etc.

Well, you get a build-up ofinorganic C and perhaps anthropogenic globalclimate change.

Deforestation, drainage and tillage increase decompositionof soil organic matter

According this accounting,annual net loss of C fromthe soil (62 – 60) as CO2

is > 1/3 of emissions dueto burning fossil fuels –a major contribution. Thus,we can slow on-set of climate change by reversing this situation, i.e., sequestrationof atmospheric C (CO2) into soil organic C.

Decomposition of Organic Matter

Releases CO2 + H2O

N, P and S released as inorganic ions

mineralization

This is microbially-mediated.

And at same time

N, P and S reincorporated into organics immobilization

Taken up by soil bugs and plants, and incorporated into their biomass.Thus, these nutrients are immobilized.

Decomposition rate depends on

Composition of residue

Aeration

C / N ratio

Composition should make sense. More complexmolecular structures like lignin are less susceptibleto decomposition than, say, starch. As for aeration,biological processes are faster under aerobicconditions. The last factor, C / N, should also makesense. If a substrate contains other essential elements,like N, P, S, etc., then the bugs decomposing it mayobtain these nutrients from it and the more of thesein the substrate the better for the bugs. N is often thelimiting nutrient, thus, the focus on C / N ratio.

C / N Ratio

Relatively constant in soils = 12

Variable in plants Increases as plants mature

Decomposition faster if C / N low

That is, if N is present in a relativelyhigh concentration.

Compare rye and vetch, N-fixing plant (viaassociation in the actual atmospheric N-fixers, certain microbes). The vetch has more N, therefore,is decomposed faster than the rye. The less mature

rye, killed 4 / 8,has a higher Nconcentrationthan the olderrye.

C / N of fresh organic matter also affectsavailability of N to plants

This is a matter of competition between the microbes decomposing theresidue and plants. If the organic matter has little N (high C / N), thenthe bugs use available soil N to satisfy the demand of their increasingpopulation (growth stimulated by added organic matter). The next coupleof slides shows the arithmetic of the matter.

What happens if C / N is > 24 / 1?

C / N in microorganisms = 8 / 1

Microbes incorporate about 1/3 of Cmetabolized into biomass

So OM must have C/N 24 / 1 to meetN needs of microbes

≤

(24 – 2/3 x 24) / 1 = 8 / 1, no?

What if C / N of residue = 36 / 1

2 / 3 of C respired and 1 / 3 makes biomass

So C / N = 12 / 1

To get to C / N = 12 / 1½ = 8 / 1

Microbes use available soil N and plant- available N decreases

The ½ temporarily depletes the available soil N.

Contrast the two data sets.

Top: Addition of high C / Norganic matter does stimulatemicrobial growth but to buildmicrobial biomass, N in add-ition to what is in the organicmatter is needed. Thus, thelevel of available N (statedhere as NO3

-, but NH4+ also)

is depleted. Eventually, theN in microbial biomass isreleased and the level of available N in the soil ishigher than initially, but thereis this temporary (could belong) depletion.

Bottom: High N / C in organicmatter results in increasingconcentration of available Nfrom the time the organicmaterial was added.

C / N Ratio Problem

Wheat straw is incorporated in soil during the spring and to avoidN-depression supplemental N is also added. Assume the wheat strawcontains 48 % C and 0.50 % N by weight (dry matter), the C / N ratio ofmicrobial biomass is 8 and soil microorganisms assimilate 1/3 of the carbon in the wheat straw. 1. What is the C / N ratio of the wheat straw? 2. How much C from the wheat straw is assimilated into microbial biomass (kg C per 100 kg straw)?

3. How much supplemental N (kg) must be added for every 100 kg of wheat straw in order to supply all the N required by microorganisms? Addition of N will prevent the temporary reduction in available inorganic N that otherwise would occur as the growing microbial population satisfies its demand for N.

The stable end-product of organicresidue decomposition in soil is called

A) humusB) humorousC) humongous

Humus

Soil OM includes

Biomass

Partially decomposed residue

Decomposed and colloidal residuehumus

So, soil organic matter is everythingand humus is just an important partof soil organic matter.

Humus includes

Humic substances

Slowly degraded polymers

Less resistant biomolecules

Non-humic substances

About 80% of humusis humic substances andthe balance is slowlydecomposed biomolecules.

But compared to the humicsubstances, these slowlydecomposed biomoleculesare rapidly decomposed.

Humic substances

Substance MW Decomposition

Fulvic acid lowest slow

Humic acid higher very slow

Humin highest extremely slow

The ½ life of fulvic acid (or acids, there is no set structure to these things, justcommonalities among what we call fulvic acid and humic acid) is maybe +1 year.That for humic acid, +10 years and for humin, +100 years. If curious about structure,look at the earlier figure for lignin –proposed structures are somewhat similar.

In lab you extracted fulvic and humicacid from the organic soil using base.When you added acid, the humic acidbegan to come out of solution and youreversed this by adding base again.

Does this help explain whythere is more organic matter inheavier texture soils?

Adsorption on clay increases resistance ofhumus to microbial decomposition

Yes, and this ought to make sense.

Properties of humus

High surface area / mass > silicate clays

High CEC (pH-dependent or no?)

High water holding capacityOf course it’s pH-dependent. The concept of permanentcharge due to isomorphic substitution in an organic structureis nonsense. Please recall the charge on organics is dueto ionization –COOH, Ar-OH, etc., giving – sites, or protonationof C-NH2, etc., giving + sites, mostly -, though.

Effects of OM on

Plants, Soil and Water Quality

Effects on plants

Greater water holdingSupplies nutrientsBetter aerationStimulates microbes

Growth + / - compounds

Effects mostly indirect. The betteraeration comes from the effectof organic matter on increasingsoil aggregation, thus, inter-aggregate porosity (large pores).

To a much lesser extent, certainorganics may directly affectplant growth, as with allelopathicsubstances.

Effects on soil

ColorAggregationWater holding capacity CEC

Effects on water quality

Greater infiltration

Reduced storm flow, soil erosion andsurface transport of contaminants

Greater chemical adsorption

Higher soil fertilitySo, less mobility of chemicalsin the soil or loading into runoff.

So, need less in fertilizers. You know thatsoluble N and P may degrade water qualityin certain circumstances.

Factors Affecting Soil OM

ClimateType vegetationTextureDrainage

CroppingTillageRotationsFertilizer

Natural conditions

Managed conditions

Climate

Temperature andMoisture

Aerated

Wet

If you examine these curves,you should see maximalaccumulation of organic matterwhere not only relative wet butalso cool.

Vegetation

Grasslands > forests

Texture and drainage

Clay > sandPoorly drained > well drained

Dry Wet NativeFarmed

Weird and Wet Forest

Forest Prairie

Typical distribution of organic matter (or organic C) with depth. More organic Cwhere wet, in grassland than forest, and less where a soil initially high in organicC has been farmed from a long time. So, what is the subsurface bulge in organic Cin the Spodosol called?

Repetitious but good –more where wetter and more in prairie.

Cropping and tillage

Virgin land > cropped land ?No-till > conventional-till ? Rotations and fertilizers ?

Take a good look at the next slide for answers. The data are from an oldfield plot study from the U. of Illinois. Famous in the agricultural domain.

What a drop in organic C with continued farming!The unfertilized rotation was not as bad as alwayscorn. The fertilizer with rotation treatment was bestbut results are probably biased because manurewas included. What do you think?

Established in 1876, the Morrow Plots (University ofIllinois) are the oldest agronomic experiment fields in the United States. They include the longest-term continuous corn plot in the world.Designated a National Historic Landmark in 1968.

Management Guidelines

Plant growth goodReduced tillage goodAdding OM good

Know limitations –a ceiling exists

Is N also required for OM accumulation?

These make sense, right?Regardless, the level of organicC (proportional to organic matter)is set by a balance between ratesof organic C addition to the soiland its decomposition. Bothdepend on how the soil is

managed.

If there is something of a equilibrium between N and C (C / N ratio ~ 12), can youexpect the addition of 100 tons of organic C in sawdust to result in a persistentlevel of organic C ≥ 10% (assuming an acre of soil weighs 1,000 tons)?

Histosols

Peat partially decomposed residue

Muck highly decomposed residue

General properties

Low bulk density 0.2 - 0.4 g cm-3

High water holding 2 - 3 x masscapacity

Importance

Role in C cycle 20 % of soil OM

Agricultural use oxidation andwind erosionproblems

Potting media

Composts and Composting

Create humus-like matter outside soil

Aerobic process in which

Nutrients conserved and concentrated

Of course, this is a misnomer because it ain’t. This makes it a wholelot cheaper to transporton a per mass ofnutrient basis. Good.

High internaltemperaturekills weed seeds and pathogens

MulchPotting mixSlow-release fertilizer

But you sure wouldn’t want to usethe stuff unless you were confidentit didn’t contain weed seeds andpathogens. Good thing the compostingprocess generates really high temperatures.

Possible Lignin Structure

Possible HS Structure (Part)

Biochemistry of Humic Substance Formation

Formation of HS not understood but thought to involve 4 stages

1Decomposition of biomolecules into simpler structures2Microbial metabolism of the simpler structures3Cycling of C, H, N, and O between soil OM and microbial biomass4Microbially-mediated polymerization of the cycled materials

Lignin (lignin-protein) theory

(Waxman, 1932)

Lignin incompletely used by microbes and residual part makes up HS

Lignin

Microorganisms Use Part Residual Unused Part

Demethylation, oxidation and condensation with N compounds

Humic acids

Fulvic acids

Polyphenol theory

These from either from lignin decomposition or derived bymicrobes from other sources such as cellulose

Oxidation of polyphenols to quinones leads to ready addition of amino compounds and development of structurally large condensation products

Sugar-amine condensation theory

Simple reactants derived from microbial decomposition undergo polymerization

All may occur but relative importance is site-specific