Environment Counts | Soil Degradation
It is estimated that within the next 20 years, due to the growing of world population, the global demand of food will increase by 50%, demand for water by 35-60%, and demand for energy by 45%. The worldâ€™s soils are consequently under increasing pressure. Soil carbon plays a vital role in regulating climate, water supplies and biodiversity, and therefore in providing the ecosystem services that are essential to human well-being. Managing soils to obtain multiple economic, societal and environmental benefits requires integrated policies , and decisive actions need to be taken to limit soil carbon loss due to erosion and emissions of carbon dioxide and other greenhouse gases to the atmospere. The top metre of the worldâ€™s soils stores approximately 2 200 Gt (billion tonnes) of carbon, two-thirds of it in the form of organic matter. This is more than three times the amount of carbon held in the atmosphere. However, soils are vulnerable to carbon losses through degradation. They also release greenhouse gases to the atmosphere as a result of accelerated decomposition due to land use change or unsustainable land management practices.
Soil carbon exists in both organic and inorganic forms. Soil inorganic carbon is derived from bedrock or formed when CO2 is trapped in mineral form (e.g. as calcium carbonate). Soil inorganic carbon is far less prone to loss than soil organic carbon (SOC). Although it can dissolve, particularly under acidic conditions, soil inorganic carbon is not susceptible to biodegradation. SOC is the main constituent of SOM. SOM is formed by the biological, chemical and physical decay of organic materials that enter the soil system from sources above ground (e.g. leaf fall, crop residues, animal wastes and remains) or below ground (e.g. roots, soil biota).
The elemental composition of SOM varies, with values in the order of 50 per cent carbon (Broadbent 1953), 40 per cent oxygen and 3 per cent nitrogen, as well as smaller amounts of phosphorus, potassium, calcium, magnesium and other elements as micronutrients. Soil biota (from microbes to earthworms) contribute living biomass to SOM mixing and breaking down the organic matter through physical and biochemical reactions. These biochemical reactions release carbon and nutrients back to the soil, and greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) to the atmosphere (Figure 2).
Soil management can affect the relative balance of these processes and their environmental impacts. As SOM is broken down, some carbon is mineralized rather rapidly to CO2 and is lost from the soil. SOM may also be lost through physical erosion. Organic nitrogen contained in biodegrading SOM is transformed to N2O and other nitrogen oxide (NOx) compounds. However, some fractions of SOM are not readily degraded. SOC content therefore tends to increase as soil develops undisturbed over time. In water-saturated soils, SOM may even accumulate as thick layers of peat (Beer and Blodau 2007). Organic matter binds to minerals, particularly clay particles, a process that further protects carbon (Von LÃ¼tzow et al. 2006). Organic matter also provides cohesive strength to soil and improves soil fertility, water movement, and resistance to erosion.
Scientists have characterized many thousands of different soils. Each has a distinctive composition of minerals, living organisms, organic matter, water and gases (WRB 2006, FAO et al. 2009). Soils are formed over thousands of years as rock is broken down and colonized by plants and soil biota, leading to the formation of soil organic matter (SOM). While SOM is primarily carbon, it also contains nutrients essential for plant growth such as nitrogen, phosphorus, sulphur and micronutrients (Box 1). Organisms in the soil food web decompose SOM and make these nutrients available (Brussaard et al. 2007). The rate of SOM decomposition and turnover mainly depends upon the interplay between soil biota, temperature, moisture and a soilâ€™s chemical and physical composition (Taylor et al. 2009).