THE MAJOR EFFECTS OF AGRICULTURAL OPERATIONS ON SOIL ORGANIC MATTER

In agricultural operations and practices, so many factor influences the effects of the soil organic matter which are acting on the soil, amongst these effects are;

• Carbon sequestration
• Tillage
• Cropping rotation
• Fertilization

In the soil, the loss of soil organic matter resulting from conversion of native vegetation to farm land has now been extensively examined and is one of the best-documented ecosystem consequences of our agricultural activities and practices. Agriculture has affected the qualitative as well as the quantitativeness of soil organic matter on many different stages. The biggest loss of soil organic carbon which is associated with agriculture occurs during the first few years of cultivation; with losses of 50% of farm produce is now being common.

Agricultural practices contribute to the depletion of soil organic matter through deforestation and biomass burning, drainage of wetlands, tillage, crop residue removal, summer fallow, cultivation, and over utilization of pesticides as well as other chemicals on the soil. Crop land soils generally store less soil organic matter than grazing land due to the fact that crop land has greater disturbance from cultivation, lack of manure being returned to the system, has less root biomass, and less biomass returned to the soil surface. Most of the factors which are related to agricultural management can affect the rate and amount of carbon which is lost from the soil system. The factors which are affecting soil carbon loss from agricultural soils include are climate and soil type, tillage intensity and depth, crop rotation decisions, amount of organic inputs, amount of plant residue on the soil surface, quality of plant residues returned to the soil, soil biological activity, length and time of fallow as well as erosion.

Carbon Sequestration

Soil organic matter plays an important and critical role in the global carbon cycle. The importance of soil in the carbon cycle is due to its role as both a major source and sink for carbon which is present in the biosphere. The total soil carbon pool is three times greater than the atmospheric carbon pool also 3.8 times greater than the biotic carbon pool. The soil carbon pool contains approximately 1.7 X 10¹² tons of organic carbon and about 8.3 X 10¹¹ tons of inorganic carbon to a depth of 3.3 feet.

However, the soil carbon cycle is complex; the concept of carbon sequestration for mitigating the release of greenhouse gases is relatively straight forward. Carbon which is stored in soils ties up carbon that would otherwise be released to the atmosphere as carbon-containing greenhouse gases, particularly carbon dioxide (CO₂) as well as methane (CH₄).

Tillage

In the practice of tillage, it results in the loss of soil organic matter primarily through three mechanisms which are mineralization of carbon due to breakdown of soil aggregates and changes in temperature and moisture regimes, leaching of organic carbon, and accelerated rates of erosion. Even in cropping systems that return almost none of the above ground residue back to the soil, such as silage corn production and some biofuel systems, reducing tillage operation intensity which can result in maintaining or increasing the soil organic fraction that is most readily decomposable. Reduced tillage has been shown to result in increased soil microbial biomass levels before measurable changes in total soil carbon occur.

Tillage is responsible for substantial loss of carbon from the soil. As carbon is released from the soil as a result of tillage, it leaves in the form of carbon dioxide (CO₂). The deeper and more aggressive the tillage, the more water is released to the atmosphere. The moldboard plow, the most aggressive system used lost 579 lbs /ac of water in a period of one day. Disk-rip was the intermediate tillage system and it also lost 271 lbs of water per acre which is 47% of moldboard plow. Strip tillage, a tillage system that tills less than 30% of the soil and leaves the rest undisturbed, lost 106 lbs of water per acre in a period of one day. Strip till lost only 18% of the carbon dioxide that the moldboard plow system lost from the soil.

Cropping Rotations

The rotations of crop enhance the productivity of all crops in the rotation, also benefit the soil. Some of the advantages of a well-managed crop rotation are improved yields, breaking plant pest cycles, maintaining soil fertility and reducing fertilizer inputs as well as the controlling of erosion. In many sustainable land management systems, cover crops and green manures are also part of the crop rotation. A cover crop is any crop that is grown to provide soil cover, regardless of whether it is later incorporated into the soil or not. Cover crops are grown primarily to prevent soil erosion to soil. When a cover crop is grown to reduce nutrient leaching or retrieve nutrients deep in the soil profile, it is referred to as a catch crop.

Crop rotations that include cover crops, perennial grasses and legumes, and reduced tillage are an important factor in soil organic matter management and can be adapted to any cropping system. Crop rotations also affect the biological diversity of an agro ecosystem. The biological diversity is important for maintaining a high-functioning, disease resistant, and stable ecological system. Crop rotations that maximize soil carbon inputs and maintain a high proportion of active carbons are important factors in establishing a sustainable cropping system.

Fertilization

In considering fertilization, the advent of affordable nutrient fertilizers in crop production was established. With ammonium and nitrate-based fertilizers, marginally fertile land could suddenly be cropped profitably and yields improved on land which is already fertile. The big question now is how does fertilization affect soil organic matter levels? There is no true picture to the answer of this question since it appears that many other factors including vegetation present, soil type, and climate are factors that must also be considered when considering the question. However, different soil organic matter fractions the active fraction, stable fraction, and so on are affected differently by fertilization.

Fertilization affects the soil microbial community both directly and indirectly by supplying mineral nutrients for microbial use and by allowing increased production of plant biomass to serve as a microbial food source. A larger microbial community can result in either a net carbon increase or decrease to the soil system, depending on how much carbon stays in the soil system as microbial biomass versus how much is lost as respired carbon gases, a greater microbial community results in a greater amount of soil respiration.