Soil Organic Matter Chapter 12

Size of wedges indicate relative effect, not absolute concentrations.

The C cycle is complex, with many different pools and rates of transformation from one to another. However, in essence, it can be condensed into cycling between C in organic combinations and C in inorganic forms, like CO 2, HCO 3 -, etc. Well, you get a build-up of inorganic C and perhaps anthropogenic global climate change.

According this accounting, annual net loss of C from the soil (62 – 60) as CO 2 is > 1/3 of emissions due to burning fossil fuels –a major contribution. Thus, we can slow on-set of climate change by reversing this situation, i.e., sequestration of atmospheric C (CO 2 ) into soil organic C.

This is microbially-mediated.

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

Composition should make sense. More complex molecular structures like lignin are less susceptible to decomposition than, say, starch. As for aeration, biological processes are faster under aerobic conditions. The last factor, C / N, should also make sense. If a substrate contains other essential elements, like N, P, S, etc., then the bugs decomposing it may obtain these nutrients from it and the more of these in the substrate the better for the bugs. N is often the limiting nutrient, thus, the focus on C / N ratio.

That is, if N is present in a relatively high concentration.

Compare rye and vetch, N-fixing plant (via association 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 N concentration than the older rye.

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

≤ (24 – 2/3 x 24) / 1 = 8 / 1, no?

The ½ temporarily depletes the available soil N.

Contrast the two data sets. Top: Addition of high C / N organic matter does stimulate microbial growth but to build microbial biomass, N in add- ition to what is in the organic matter is needed. Thus, the level of available N (stated here as NO 3 -, but NH 4 + also) is depleted. Eventually, the N in microbial biomass is released and the level of available N in the soil is higher than initially, but there is this temporary (could be long) depletion. Bottom: High N / C in organic matter results in increasing concentration of available N from the time the organic material was added.

C / N Ratio Problem Wheat straw is incorporated in soil during the spring and to avoid N-depression supplemental N is also added. Assume the wheat straw contains 48 % C and 0.50 % N by weight (dry matter), the C / N ratio of microbial 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.

So, soil organic matter is everything and humus is just an important part of soil organic matter.

About 80% of humus is humic substances and the balance is slowly decomposed biomolecules. But compared to the humic substances, these slowly decomposed biomolecules are rapidly decomposed.

The ½ life of fulvic acid (or acids, there is no set structure to these things, just commonalities 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 humic acid from the organic soil using base. When you added acid, the humic acid began to come out of solution and you reversed this by adding base again.

Yes, and this ought to make sense.

Of course it’s pH-dependent. The concept of permanent charge due to isomorphic substitution in an organic structure is nonsense. Please recall the charge on organics is due to ionization –COOH, Ar-OH, etc., giving – sites, or protonation of C-NH 2, etc., giving + sites, mostly -, though.

Effects mostly indirect. The better aeration comes from the effect of organic matter on increasing soil aggregation, thus, inter- aggregate porosity (large pores). To a much lesser extent, certain organics may directly affect plant growth, as with allelopathic substances.

So, less mobility of chemicals in the soil or loading into runoff. So, need less in fertilizers. You know that soluble N and P may degrade water quality in certain circumstances.

If you examine these curves, you should see maximal accumulation of organic matter where not only relative wet but also cool.

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

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

Take a good look at the next slide for answers. The data are from an old field 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 always corn. The fertilizer with rotation treatment was best but results are probably biased because manure was included. What do you think?

Established in 1876, the Morrow Plots (University of Illinois) 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.

These make sense, right? Regardless, the level of organic C (proportional to organic matter) is set by a balance between rates of organic C addition to the soil and its decomposition. Both depend on how the soil is managed. If there is something of a equilibrium between N and C (C / N ratio ~ 12), can you expect the addition of 100 tons of organic C in sawdust to result in a persistent level of organic C ≥ 10% (assuming an acre of soil weighs 1,000 tons)?

Of course, this is a misnomer because it ain’t.This makes it a whole lot cheaper to transport on a per mass of nutrient basis. Good.

But you sure wouldn’t want to use the stuff unless you were confident it didn’t contain weed seeds and pathogens. Good thing the composting process 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 structures 2Microbial metabolism of the simpler structures 3Cycling of C, H, N, and O between soil OM and microbial biomass 4Microbially-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 PartResidual Unused Part Demethylation, oxidation and condensation with N compounds Humic acids Fulvic acids

Polyphenol theory These from either from lignin decomposition or derived by microbes 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