Human Dimensions That Drive Soil Degradation - Principles of Sustainable Soil Management in Agroecosystems - page 173

transferred to rivers via irrigation, and then deposited into other soils when down-
stream farmers irrigate (Stocking 2006; Tse-Yan Lee 2005). Unfortunately, inputs of
nitrogen increase pesticide use. Nitrogen encourages growth of weeds better adapted
to high levels of nitrogen. In addition, nitrogen can lead to greater damage from
animal pests, because plants become more nutritious for herbivores as nitrogen level
increases (Vogt et al. 2010).
7.3.3 c onversIon of L and c over
Conversion of land cover often degrades soil resources. Two especially problematic
types of land cover conversion are deforestation and urban sprawl (conversion of land
to impervious surfaces such as roads and buildings).
Deforestation has been well documented as a leading cause of land degradation,
especially in developing countries and tropical regions (Barbier 1997, 2000; Lambin
et al. 2001). Deforestation is estimated to account for 40% of soil erosion in Asia and
South America, 22% of soil erosion in Mexico and Central America, and 14% of soil
erosion in Africa (Barbier 1997, 892). The conversion of forest to farmland or exotic
tree plantations has been found to cause a depletion of major nutrients such as organic
matter, total nitrogen, and available phosphorus, while at the same time increasing
soil erosion (Fu 2000; Majule 2003). Deforestation also reduces land cover, which
reduces rainwater infiltration into the soil and affects soil aeration (Majule 2003).
Urban sprawl reduces surface permeability, thus inhibiting infiltration of water
into the soil, and the associated compaction degrades soil structure. Where urban
uses completely cover the soil, as in concrete foundations and asphalt roadways, this
severely impairs soil ecological functions and processes. Moreover, studies in the
United States and China indicate that urban development follows soil resources and
that, in general, the best soils are converted to nonagricultural uses by urban sprawl
(Imhoff et al. 2004; Chen 2002). Urban sprawl and its associated uses also increase
soil contamination by industrial and consumer by-products such as manufacturing
pollutants and residential chemicals (Chen 2002).
7.4 FACTORS AFFECTING LAND MANAGEMENT CHOICES
Land managers make decisions and take actions on the landscape. Their decisions
about investing in conservation, intensifying agricultural production, and converting
land cover are a function of many precursor factors (indirect drivers). Our focus here
is on land managers involved in agricultural production. Our review of the literature
revealed extensive scholarship seeking to explain land management decision-making,
drawing on several key variables: population growth and urbanization, poverty and
access to land and credit, globalization and other economic forces, government poli-
cies, technology, culture and religion, and individual psychological factors.
7.4.1 P oPuLatIon g rowth and u rbanIzatIon
Population growth and distribution both impact activities that degrade soil. As
human populations grow, there is corresponding pressure for shorter fallow periods