M8 Soil Degradation and Biogeochemical Cycles

Topic

Soil degradation encompasses the processes that gradually reduce a soil's productive capacity and ecological functions—such as water and wind erosion, salinization, acidification, compaction, and the loss of organic matter. On a global scale, it already affects nearly one-third of the planet's agricultural land, with consequences impacting food security, the carbon cycle, and water quality.

Of all these forms of degradation, sheet erosion (a type of water erosion) is the most widespread: when a raindrop strikes the soil surface, the impact causes soil aggregates to break apart, freeing particles to be subsequently transported downslope by surface runoff.

The soil carbon cycle is one of the system's most critical biogeochemical processes: collectively, soils store approximately 1,500 petagrams of organic carbon—nearly three times the amount found in the atmosphere—primarily in the form of soil organic matter. This organic matter is decomposed by soil microorganisms, which release the resulting carbon dioxide back into the atmosphere at a rate determined by temperature, moisture, and the type of material undergoing decomposition. When erosion or tillage causes significant loss of organic matter, the soil can shift from acting as a carbon sink to becoming a net source of carbon emissions into the atmosphere.

Secondary salinization, caused by irrigating with saline water in arid regions, is another widespread form of soil degradation; it currently affects approximately 15 percent of all irrigated agricultural soils worldwide. Soil acidification occurs when the basic cations contained in the soil—calcium, magnesium, and potassium—leach out and are replaced within the soil's exchange complex by hydrogen and aluminum; it is precisely this replacement that causes soil acidity to rise and fertility to decline.

The cycles of essential nutrients—nitrogen, phosphorus, potassium, calcium, and magnesium—involve four successive processes within the soil: the weathering of minerals that release them, the biological fixation of atmospheric nitrogen, the mineralization of organic matter that returns them to the soil, and the eventual loss of these nutrients through leaching.

The expected behavior of this entire system includes a clear correlation between soil erosion rates and nutrient loss, as well as a progressive decline in soil fertility under intensive agricultural use that fails to replenish what the soil loses. Thus, from the reduction in productive capacity mentioned at the beginning of this document to the progressive fertility decline noted at the end—and encompassing sheet erosion, the carbon cycle, secondary salinization, and acidification—soil degradation and its biogeochemical cycles demonstrate that land fertility is never a fixed value; rather, it is the ever-changing result of the balance between the demands placed on the soil and what is returned to it.

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