Growing concern about global climate change has focused increasing research attention on the carbon-regulating role played by soil. Collectively, the Earth’s soils contain more than twice the amount of carbon found in the atmosphere. Scientists report that global warming may significantly alter soil composition at the molecular level and that such changes could have a major impact on atmospheric levels of carbon dioxide (CO2).
Organic matter, the decaying remains of plants and animals, enables soil to support plant life, providing plants and soil microbes with the energy and raw materials they need for growth. Soil microbes facilitate the decomposition of organic matter from litter fall (the leaves, twigs, and other plant materials that fall to the ground), and CO2 is a natural by-product of this process. Rising atmospheric temperatures and/or CO2 levels are likely to increase photosynthesis and plant productivity, according to the results of several studies over the past two decades; consequently, litter fall is expected to increase. Global warming may change present-day decomposition patterns by altering the soil microbial communities and activities, thus changing the overall flow of carbon into and out of the soil and affecting soil fertility as well.
Increased microbial activity due to higher CO2 concentration and temperature produces greater amounts of polysaccharides and other soil stabilizers. Increases in litter or crop residues, root mass and organic matter content tend to stimulate the activity of soil macro fauna, including earthworms, with consequently improved infiltration rate and bypass flow by the greater number of stable bioprocess. The greater stability and the faster infiltration increase the resilience of the soil against water erosion and consequent loss of soil fertility. The increased proportion of bypass flow also decreases the nutrient loss by leaching during periods with excess rainfall. This refers to the available nutrients in the soil, including well-incorporated fertilizers or manure, but not to fertilizers broadcast on the soil surface. These are subject to loss by runoff or leaching.
These changes increase the resilience of the soil against physical degradation and nutrient loss by increased intensity, seasonality or variability of rainfall, as well as against some of the unfavorable changes in rate or direction of soil-forming processes discussed in the next sections.
If the partial pressure of CO2 in the soil air would rise, and that of O2 decrease to levels impairing root function, part of the benefits indicated would not materialize. The improved gas exchange with the atmosphere through increased numbers of stable bioprocess would tend to keep CO2 and O2 in the soil at ‘safe’ levels, at least in naturally or artificially well-drained soils. Wetland crops such as rice or jute have their own gas exchange mechanisms and would not be affected; neither would natural wetland vegetation.
The positive effect on weathering rate and plant nutrient availability would occur in soils with significant amounts of weather able minerals, not in very deeply and strongly weathered or otherwise very poor soils.