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|
Location |
Type |
Carbon retained % |
Reference |
|
England |
Ryegrass tops and roots |
Approximately 33% first year irrespective of soil type or plant material |
Jenkinson |
|
West Germany |
Wheat straw and chaff |
31% after first year fro fallow and cropped soil |
IAEA |
|
Canada |
Wheat straw |
35-45% after first growing season |
Shields and Paul |
|
Colorado, U.S. |
Blue grama grass |
43-46% after 412 days |
Nyhan |
(Slide 7 of 31) Long Term C Loss from Organic Residues as Estimated from Studies with 14C-labeled Ryegrass
 |
Component |
Recovery of residual 14C |
|
Fulvic acid |
11.8-18.9% |
(Slide 8 of 31) Priming Effect
Influence of applied plant residues labeled with 14C on loss of native soil C as CO2-
(Slide 9 of 31) Degradation of Plant Components by Fermentative Organisms
- Initially short chain organic acids and alchols
- Further degradation by:
- Denitrifiers
- Sulfat-reducers
- Methanogenic organisms
(Slide 10 of 31) Reactions of Pyruvate in Anaerobic Systems
Reactions of pyruvate in anaerobic systems, showing mahor organic acid and CH4 formatioin. Carbon dioxide produced during fermentation serves as an electron (hydrogen) acceptor, with formation of CH4.
(Slide 11 of 31) Reactions of Pyruvate in Anaerobic Systems
Class |
Comments |
|
Fermentation products |
Incomplete oxidation leads to production of Ch4, organic acids, amines, mecaptans, aldehydes, and ketones |
|
Modifed or partially modified remains of plants |
In addition to slightly altered lignins, long-chain aliphatic hydrocarbons, carotenoids, sterols, and porphyrins of chlorophyll origin are preserved |
|
Synthetic organic chemicals |
Many man-made chemicals (e.g. DDT) degrade very slowly under anaerobic conditions and can thus persist for long periods |
|
Carcinogenic compounds |
Synthesis of methylmercury, dimethylarsine, dimethylselenide, and nitrosamines of various types |
(Slide 12 of 31) Pools of Organic Matter
Litter: Macroorganic matter (e.g. crop residues) that lies on the soil surface
Light fraction: Plant residues and their partial decomposition products that reside within the soil proper
Microbial biomass: Cells of living microorganisms, notably bacteria, actinomycetes, and fungi
Faunal biomas: Tissues of animals (primarily invertebrates)
Belowground plant constituents: Primarily roots with lesser amounts of dead roots and exudates
Water-soluble organics: Organic substances dissolved in the soil solution
Stable humus: Humified remains of plant and animal tissues that have become stabilized by microbial and chemical transformations and/or by association with inorganic soil components
(Slide 13 of 31) Composition of Soil Organic Matter
(Slide 14 of 31) Changing Forms of Soil Organic Matter
The root systems in the same area after a run of drought years.
(Slide 15 of 31) What Does Organic Matter Do?
(Slide 16 of 31) What is Organic Matter
Organic matter is the vast array of carbon compounds in soil. Originally created by plants, microbes, and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles.
For simplicity, organic matter can be divided into two major categories: stabilized organic matter which is highly decomposed and stable, and the active fraction which is being actively used and transformed by living plants, animals, and microbes. Two other categories of organic compound are living organisms and fresh organic residue. These may or may not be incuded in some definitions of soil organic matter.
(Slide 17 of 31) Stabilized Organic Matter
Many soil organisms decompose plant and animal tissues, and transform the organic matter into new compounds. After years or decades of these tranformations, what remains are large, complex compounds. After years or decades of these transformations, what remains are large, complex compounds that the microbes can degrade. Other compounds become bound inside soil aggregates where microbes cannot reach. These hard-to-decompose, or stabilized, substances make up a third to a half of soil organic matter. Scientists often divide stabilized organic matter into three chemical groups: humic acids, fulvic acids, and humins. Fungi and actinomycetes create many of the humic acids that combine in soil to become stable commpounds. Much of the stabilized matter in your soil originated from plants that grew one or more centuries ago. Some of thes old compounds are bound to clay, and are important in gluing together tiny aggregates of soil particles.
(Slide 18 of 31) Stabilized Organic Matter Acts Like a Sponge
Stabilized organic matter can absorb six times its weight in water. In sandy soils, water held by organic matter will make the difference between crop failure or success during a dry year.
Both organic and clay particles can hold on to nutrients electro-chemically - like a magnet holds on to iron filings. The amount of nutrients that the organic compounds and clay could carry and make available to plants is called the soil's cation exchange capacity (CEC). In Minnesota, a pound of stabilized organic matter can have five times as much CEC as a pound of clay. In other words, organic matter can hold five times as much nutrients for plants to use. Half or more of the CEC in Minnesota topsoil comes from organic matter. Although you cannot change the amount of clay in your soil, you can easily decrease or (with more difficultly) increase the amount of organic matter in your soil.
(Slide 19 of 31) OM and Pesticides
In addition to nutrients, stabilized organic matter holds on to pesticides. This prevents pesticides from moving into water supplies and improves the decomposition of the compounds, but it also makes pesticides less effective by preventing their contact with the target organism.
This pie chart represents organic matter in soil before cultivation. After land has been cultivated for one or two decades, much of the active fraction is lost and the stabilized organic matter makes up mmore than half of the soil organic matter.
(Slide 20 of 31) The Active Fraction
Up to 15% of soil organic matter is fresh organic material and living organisms. Another third to one half is partially and slowly decomposing material that may last decades. This decomposing material is the active fraction of soil organic matter.
The active organic matter, and the microbes that feed on it, are central to nutrient cycles. Many of the nutrients used by plants are held in organic matter until soil organisms decompose the material and release ammonium and other plant-avialable nutrients. Organic matter is especially important in providing nitrogen, phosphorus, sulfur, and iron. A soil with 3% organic matter contains about 3,000 pounds of nitrogen per acre. Depending on the rate of decomposition, 25-100 pounds may become available to plants in a year, but it is difficult to predict the decomposition rate. In Minnesota, decomposition rates are typically about 2%. This means that one acre of crop plants on a soil with 3% soil organic matter may get 60 pounds of their nitrogen and 6 pounds of their phosphorus from soil organic matter.
(Slide 21 of 31) The Active Fraction cont'd
Organic matter also affects nutrient cycles by chelating (chemically holding on to) nutrients, and preventing them from becoming insoluble and therefore unavailable to plants. For example, humic substances help make iron availalbe to plants, even in medium-to-high pH soils.
Regular additions of organic matter are important as food for microorganisms, insects, worms, and other organisms, and as habibtat for some larger organisms. Soil organisms degrade potential pollutants, help control disease, and bind soil particles into larger aggregates. Well-aggregated, crumbly soil allows good root penetration, improves water infiltration, makes tillage easier, and reduces erosion.
Very fresh organic matter can cause problems to crops in two ways -- nitrogen tie-up and allelopathy. A temporary nitrogen deficiency for crops occurs if the organic matter is low in nitrogen. "Allelopathic" chemicals are formed when some residues decay, and can inhibit plant growth.
(Slide 22 of 31) Organic Matter is More than Fertilizer
Organic matter is not just N,P,K, and carbon. Two sources of organic matter with the same nutrient content or total organic matter content might not have equal effects on your crops and soils.
In one research trial, fields treated with animal manure had different microorganisms and enzymes than fields where green manure or mineral fertilizers were used. The importance of these differences are not well studied, but they probably affect nutrient cycling and pests. In your system, manure may mean positive effects such as reducing some diseases, or negative effects such as increasing weed growth.
Plant residues also differ greatly as a source of organic matter. Above-ground growth has a different action in soil than roots, even when it is tilled into the soil. All roots do not act the same. For example, tap-rooted plants such as alfalfa create vertical pores in the soil, whereas the finely branched roots of grasses enhance soil aggregation.
(Slide 23 of 31) What Determines Soil Organic Matter Levels?
The amount of organic matter in soil is the result of two processes: the addition of organic matter (roots, surface residue, manure, etc.), and the loss of organic matter through decomposition. Five factors affect both additions and losses.
Management. Practices that increase plant growth on a field (cover crops, irrigation, etc.) will increase the amount of roots and residue added to the soil each year. On the other hand, intensive tillage increases the loss of younger organic matter, older, protected organic compounds can be exposed to decomposition if small aggregates are broken apart. In addition to hanging the amount of soil organic matter, tillage practices affect the depth of soil organic matter.
Soil texture. fine-textured soils ccan hold much more organic matter than sandy soils for two reasons. First, clay particles form electrochemical bonds that hold organic compounds. Second, decomposition occurs faster in well-aerated sandy soils. A sandy loam rarely holds more than 2% organic matter.
Climate. High temperatures speed up the degradation of organic matter. In areas of high precipitation (or irrigation) there is more plant growth and therefore mmore roots and residues entering the soil.
Landscape position. Low, poorly-drained areas have higher orgnaic matter levels, because less oxygen is avialable in the soil for decomposition. Low spots also accumulate organic matter that erodes off hii tops and steep slopes.
Vegetation. In prairies, much of the organic matter that dies and is added to the soil each year comes from grass roots that extend deep into the soil. In forests, the organic matter comes from leaves that are dropped on the surface of the soil. Thus, farmland that was once prairie will have higher amounts of organic matter deep in the soil than lands that were previously forest.
(Slide 24 of 31) What are Some Sources of Organic Matter?
Crop residue. For most farms, roots and above-ground plant growth are the most important sources of organic matter. When choosing varieties and crop rotations, consider how much residue will be generated, and how many months each year plants will be growing and creating organic matter.
Green manure. Finding appropriate cover crops is difficult in regions with a short growing season, but there are options and the research and development of appropriate crops is expanding.
Livestock manure. Spreading manure over your land is an excellent way to enhance organic matter, supply nutrients, and prevent manure from becoming a pollutant. If animals are not part of your operation, neighboring farmers may be a source.
Sewage sludge. Human wastes are also nutrient-rich. Spreading sludge on land is cheaper for communities than some other methods of treatment and disposal. If your local communicty will make it available to farmers, ask about the source and content of the sludge. Contaminants such as heavy metals are an important concern to investigate and test for.
Processing wastes. Vegetable processing plants, wood processing plants, breweries, and other industries generate organic wastes that can be useful soil amendments.
(Slide 25 of 31) Don't Forget the Roots
Surface residue is only part of what plants contribute to soil organic matter. Roots can add half again as much material. One quarter of the organic matter produced by corn or soybeans is produces by the roots. In prairies, half of plant production is underground
How does that translate into weight? Corn may produce 1/2 to 2 tons of root organic matter in each acre. Soybeans may produce 1/3 of a ton, and the prairie makes over 2 tons.
In no-till situations where, surface residue is not regularly tilled into soil, roots becomes especially critical as a course of soil organic matter.
(Slide 26 of 31) How do Organic Matter Levels Change?
To build organic matter levels in topsoil, more organic matter must be added than is lost to decomposition and erosion. Like a person trying to lose or gain weight, increasing organic matter is about changing the balance between how much energy goes in and how much is burned off.
Another way to think of soil is like a giant wood stove. You continually add organic matter (wood), and it burns to release energy and nutrients that will be used by palnts and microorganisms. Ideally, you want a slow steady burn that releases nutrients to plants as needed.
Intensive tillage aerates the soil and is like opening the flue or fanning the flames. Decomposition is desirable because it releases nutrients and feeds soil organisms. But if decomposition is faster than the rate at which organic matter is added, soil organic matter levels will decrease.
Reducing decomposition is only half the equation. It is just as important to increase the amount of organic matter added to the soil. Organic matter can be either grown in the field or brought to the field.
(Slide 27 of 31) How Long Does it Take?
Building organic matter is a slow process. First, the amount of residue and active organic matter will increase. Gradually, the species and diversity of organisms in the soil will change, and amounts of stabilized organic matter will rise. It may take a decade or more for total organic matter levels to significantly increase after a management change. Fortunately, the beneficial effects of the changes appear long before organic matter levels rise. These improvements, however, can be reversed in a year or two by returning to previous practices.
(Slide 28 of 31) Why Does it Take so Long for OM Levels to Increase?
An acre of soil six inches deep weighs about 1000 tons, so increasing the proportion of organic matter from two to three percent is actually a 10 ton change. However, you cannot simply add 10 tons of manure or residue and expect to measure a one percent increase in soil organic matter. Only ten to twenty percent of the original material becomes part of the soil organic matter. Much of the rest is converted over several years into carbon dioxide.
(Slide 29 of 31) An illustration
Most organic matter losses in soil occurred in the first decade or two after land was cultivated. Native levels of organic matter may not be possible under agriculture, but many farmers can increase the amount of active organic matter by reducing tillage and increasing inputs.
(Slide 30 of 31) Changes in Soil Organic Matter
(Slide 31of 31) Soil Organic Matter Levels under Three Management Practices
