Soil is one of Earth's most vital natural resources. Erosion removes topsoil, reduces soil fertility, lowers the primary productivity of plants, and thus diminishes the land's capacity to hold CO2 and other GHGs. Climate change, especially in precipitation and temperature, can exacerbate soil erosion, which in turn may help drive climate change.
Soil is the material formed from weathering rocks and decaying plants and animals. It is considered a nonrenewable resource on a human timescale, because it takes thousands of years to form fertile soil (Hillel, 1991). The degradation of soil quality is thus considered permanent and irreversible. President Franklin D. Roosevelt once wrote, "A nation that destroys its soils, destroys itself." Soil is the foundation on which agriculture produces all goods that sustain human society. Thus, it is paramount to preserve this resource in order to maintain agricultural productivity and environmental quality. A changing climate, especially in terms of temperature and precipitation, affects soil quality through heightened erosion and accelerated runoff.
About 80 percent of the world's agricultural land is affected by soil erosion. Intensive agriculture practices have resulted in changes in soil structure and its microbial community and made the land more prone to erosion and surface runoff. Because topsoil supplies plant roots with much of the nutrients required for growth, the loss of topsoil necessitates the application of chemical fertilizers to replenish the lost nutrients. The short-term solution of using fertilizers and other chemicals, however, has led to several long-term detrimental effects. First, the resulting structural changes reduce the porosity and infiltration rate of soil and lower the land's capacity to hold water. These in turn increase surface runoff and soil erosion. Second, both topsoil loss and applied chemicals act to reduce microbial diversity and population levels in the soil. As a result, even a very limited amount of organic residue cannot be readily converted to useful soil ingredients.
The overuse of chemical fertilizers also creates other environmental problems. For example, there exists a progressively diminished return from fertilizer applications. As a result, higher doses of fertilizers are needed to maintain the same level of crop yields. The hiked application of agro-chemicals further damages the soil structure and microbial community, creating a vicious cycle of soil erosion. Historically, natural erosion rates varied from 38 to 161 grams per square meter per year. Erosion from present-day farmland reaches an average of 577 to 966 grams per square meter per year. In addition, excess fertilizers are washed off to streams and lakes, causing eutrophication. The most potent greenhouse gas (GHG), nitrous oxide, is also released from excess fertilizers. Other agricultural practices that are known to intensify soil erosions include row-cropping, tillage, and monocropping.
Several major climate factors contribute significantly to soil erosion, including temperature, precipitation, carbon dioxide (CO2) concentration, and wind. Increasing temperatures contribute to soil erosion indirectly in several ways. First, warmer temperatures can increase biomass production rates as a result of a faster accumulation of the required growing degree-days for crop maturity. Second, temperature affects microbial activity levels and subsequently the decomposition rates of plant residues in the soil. Higher temperatures mean faster decomposition, which may lead to more soil erosion due to a low level of ground cover by residues.
Third, temperature has major impacts on evapotranspiration rates, which affect soil moisture, which in turn influences water infiltration rates and runoff amounts. Finally, extended periods of high temperatures may lead to drought and make soil prone to wind erosion or water erosion when storms do fall.
Soil erosion and runoff patterns often follow those of annual precipitation. Predictions from various modeling systems all seem to suggest that, where precipitation increases significantly, erosion also increases significantly. A 7 percent increase in precipitation in Great Britain could lead to a 26 percent increase in erosion (Lindert, 2000). A 10 percent rainfall increase in South Africa could lead to a 20-40 percent increase in runoff. For the U.S. Corn Belt, a 20 percent precipitation hike was predicted to result in a 37 percent increase in erosion and a 40 percent increase in runoff. Wetter soil from precipitation entails lower infiltration rates and increased surface sealing, both of which may increase runoff rates. Increased runoff leads to heightened shear stress, which in turn increases the detachment force of the flow and thus increases erosion.
Atmospheric CO2 concentrations affect soil erosion directly through the amount of biomass accumulation and transpiration by plants and indirectly as a GHG. Higher CO2 levels may increase photosynthetic rates, which lead to higher biomass accumulation, which in turn affects ground residue cover. Increased plant residue results in reduced soil erosion and runoff. At the same time, increased CO2 may suppress transpiration through stomatal resistance, which leads to wetter soil, conducive to higher runoff-induced erosion. Increased air CO2 levels may warm the climate and exert their effects through higher temperatures as well. Increased wind erosion is often associated with rising air temperatures, excessively dry soil due to drought, lack of ground cover or wind breaker, and high wind speed.
Despite the known damaging effects of soil erosion, there is little reliable information regarding its global impacts, nor is there a comprehensive monitoring system to gather data. There is little doubt, however, that soil erosion has been accelerated during the past century. Studies using various modeling systems have all demonstrated that climate changes contribute to soil erosion. Up to 68 billion metric tons of topsoil are eroded annually, an equivalent of 90,000 square kilometers of productive land lost.
Erosion results directly in the loss of topsoil and reduction of soil fertility, both of which lead to lower crop yields and higher production costs. Indirectly, erosion causes sediment movement and deposits that create problems downstream, including the clogging of water ways, increased potential for floods, decreased reservoir capacity, and poorer water quality resulting from agro-chemical contamination. Minimizing erosion is important not only for the present but also for generations to come. Soil erosion should be monitored through a comprehensive network that is fully equipped and standardized across the globe. Sustainable agricultural practices will make significant differences in erosion control: Conservation tillage, crop rotations, intercropping, and cover crops will all help reduce soil erosion.
1. Hillel, D. J. Out of the Earth. New York: Free Press, 1991.
2. Lindert, Peter H. The Shifting Ground: The Changing Agricultural Soils of China and Indonesia. Cambridge, Mass.: MIT Press, 2000.
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