Due to human activities releasing carbon dioxide into the atmosphere (particularly fossil fuel consumption and land use practices), the carbon pool in the atmosphere has increased and the emmited carbon dioxide is considered one of the causes of global warming and climate change.
However, Soil Organic Carbon is a very crucial component of the global carbon cycle. It is the largest element of the terrestrial carbon pools, approximately twice the amount of carbon in the atmosphere and in vegetation.
The carbon cycle is an integral part of life on earth. ‘Soil organic carbon’ (SOC) – the amount of carbon stored in the soil is a component of soil organic matter – plant and animal materials in the soil that are in various stages of decay.
It makes up approximately 60% of the soil organic matter (SOM), with the remaining 40% of SOM containing other important elements such as calcium, hydrogen, oxygen, and nitrogen. It accounts for less than 5% on average of the mass of upper soil layers, and diminishes with depth.
According to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) – a federal government agency for scientific research in Australia, in rain-forests or good soils, soil organic carbon can be greater than 10%, while in poorer or heavily exploited soils, levels are likely to be less than 1% substances.
Soil carbon is one of the most neglected yet most important factors in soil fertility, disease control, water efficiency and farm productivity. It is the basis of soil fertility. It releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a defense against harmful substances in the soil.
It promotes soil structure by holding the soil particles together as stable aggregates improves soil physical properties such as water holding capacity, water infiltration, gaseous exchange, root growth and ease of cultivation.
As a food source for soil fauna and flora, soil organic Carbon plays an important role in the soil food web by controlling the number and types of soil inhabitants which serve important functions such as nutrient cycling and availability, assisting root growth and plant nutrient uptake, creating burrows and even suppressing crop diseases.
The amount of Soil Organic Carbon in the soil is determined by factors such as rainfall, temperature, vegetation and soil type. The main inputs of SOC to soil in rain-fed farming systems are from plant material, such as crop residues, plant roots, root exudates and animal manure Losses of SOC from soil are from decomposition by microorganisms, erosion of surface soil and off-take in plant and animal production.
Decomposition occurs when microorganisms use Soil Organic Carbon in soil to obtain the carbon, nutrients and energy they need to live. During decomposition, SOC is lost from soil because microorganisms convert about half of the SOC to carbon dioxide gas (CO2). Without continual inputs of SOC, the amount stored in soil will decrease over time because SOC is always being decomposed by microorganisms.
Historically, excessive cultivation using inappropriate implements led to soils being ‘over-worked’, and the consequent loss of Soil Organic Carbon has caused many land degradation problems such as erosion and soil structural decline.
Keeping the soil bare was also a common cropping practice. Fallowing was maintained by repeated cultivation for weed control. Soil Organic Carbon declines rapidly under fallowing because of the increased decomposition of organic matter due to the cultivation operations as well as the higher soil moisture conditions prevailing in the fallowed soils.
How then can farmers retain and increase soil organic carbon considering the important role it plays in agricultural productivity?