Erosion accelerated by human actions
Soils are the main terrestrial reservoir of organic carbon, nitrogen and phosphorus. Thus, any type of disturbance, e.g. landuse changes, that alters soil integrity is a threat to planetaryscale biogeochemical cycles that sustain the life-support systems of the Earth. Soil erosion impacts the residence time of these elements (carbon, nitrogen, phosphorus) in soils, as well as their flux rates, storage and distribution.
Soil erosion is the removal, transport and deposition of soil particles by water or wind from their original place to another location. Factors that influence soil erosion are topography (slope steepness and length), soil erodibility (determined by properties such as texture, structure, moisture, organic carbon content, etc.), vegetation cover and management practices. When soil is left bare, the fine, carbon- and nutrient- rich fractions are removed first, altering physical, chemical and biological soil properties, such as soil albedo, temperature, evapotranspiration, water holding capacity and soil biodiversity at a variety of scales, which disrupt ecosystem functioning. Recent scientific insights, however, place soil degradation, including erosion, in a broader perspective. Soil erosion is not only a biophysical factor but also a feedback component in complex socio-environment systems that disrupts fundamental ecosystem services and the human economic systems that rely on them.
Anthropogenic soil erosion, as caused by improper agricultural practices and overgrazing, accelerates erosion and has repercussions on carbon and phosphorus emissions. Human induced erosion accounts for as much as one third of the carbon emissions that result from land-use change. Moreover, on much of the global farmland, soil is lost at greater rates through erosion than can be replenished through natural soil-forming processes. By contrast, deposition of eroded sediment can also sequester carbon. Understanding the global feedbacks of soil carbon to climate change is a major challenge. The precise role and impact of soil erosion in terrestrial carbon cycling is uncertain due to lack of harmonised global erosion estimates. Beyond carbon, phosphorus is another essential element in terrestrial ecosystems that is vital for agriculture. It, too, can be dislocated and transported in dust emissions caused by wind erosion. Recently, attention has been given to the deposition effects that may increase the fertility of weathered soils offsite. For example, dust from the Sahara may contribute phosphorus to America’s tropical forests. Also, dust emissions from cultivated areas may accelerate eutrophication of inland lakes where dust eventually is deposited. Erosion associated with human activities affects land productivity and has economic effects on-site and offsite. On-site costs that directly affect farm or pasture production are mostly absorbed by the land owner. Off-site costs are mostly borne by society at large through attempts to mitigate their impacts. On-site loss of topsoil is the most severe and affects both short-term and long-term land productivity through losses in fertility, water-holding capacity and changes in soil structure. In smallholder areas, the onsite costs of losing soil nutrients, nitrogen and phosphorous, due to sediment runoff that reduces yields and long-term land productivity can be substantial. Ultimately, off-site economic effects, such as adverse human and animal health effects, sedimentation of reservoirs, damages to inland infrastructure and ports by dust pollution and eutrophication of surface water, can often be higher than on-site consequences. Sediment retention in reservoirs not only threatens reservoir longevity, but also may accelerate coastal erosion by reducing sediment flows to the coast.
Vegetative cover protects soil against erosion, hence control measures focus first on keeping vegetation cover intact. Agronomic practices that help protect soil cover include not ill and reduced tillage, cover crops (in lieu of fallow), residue and management, vegetative filter strips, riparian buffers, agroforestry (i.e. introduction of trees into fields for production of food, fodder, fuel and construction materials) and soil synthetic soil conditioners. The implementation of such sustainable land management (SLM) practices can increase productivity particularly by improving water use efficiency, optimising nutrient cycles for crop production, enhancing vegetation cover and ultimately improving food security. Soil management strategies must seek to balance soil resource depletion (through erosion and carbon and nutrient exchange) and the effects of climate change, to ensure sustainable food supply for future generations. A recent global assessment of erosion confirms that land use changes are a prime cause of erosion and led to a 2.5 % increase in predicted erosion from 2001 and 2012. Over 50 % of erosion takes place on cropland, with hotspots in China, Brazil, Equatorial Africa, India, south-eastern United States and to a lesser extent centraleast Ethiopia, Mexico, Peru and Mediterranean Europe. Over the 2001-2012 period, increased erosion was found in south-eastern Asia, African and South America countries and a decrease in China and India. Adoption of conservation practices had a notable positive impact on erosion rates.
Wind and water erosion
Wind erosion is most commonly associated with arid areas. It is a function of soil erosivity, or the ability of wind to mobilise fine solid surface particles such as clay and silt to be transported as dust. Saharan dust has been observed far offshore. Independent of climate, wind erosion is a major driver of land degradation in areas where agricultural management leaves soils exposed to the wind. The best-known example wis the dust bowl in the Great Plains of the USA during the mid-1930s. This example illustrates that the loss of vegetation is both a cause and result of degradation. Water erosion is another serious cause of soil degradation. It is caused by raindrop impact and the drag force of running water. Once dislodged, soil material is transported by overland flow (sheet erosion) or concentrated flows (rill erosion). The exposure of the Earth’s surface to the energetic input of rainfall is a key factor in water erosion. Among many human actions, deforestation, biomass burning, forest fires, ploughing, extensive soil sealing, urbanisation and land abandonment are practices that favour water erosion.
Source: Nick Brooks. Flickr.com
Source: Hans Birger Nilsen. Flickr.com
Soil erosion has increased global sediment transport by rivers to about 2.3 (+/-0.6) billion metric tonnes per year. However, due to sediment retention in reservoirs, only about 1.4 (+/- 0.3) billion metric tonnes per year reach the oceans. This International Space Station image of the Rio de la Plata shows Buenos Aires on the right and Montevideo on the left. Sediments carried by the Paraná and Uruguay rivers colour the water brown.
Source: NASA, 17/03/2003; eol.jsc.nasa.gov