Soil Management for Sustaining Ecosystem Services - Principles of Sustainable Soil Management in Agroecosystems - page 523

of soil degradation are biophysical environments such as terrain characteristics; slope
(gradient, length, shape, aspect); landscape position (summit, shoulder slope, foot
slope); parent material; soil type; drainage (surface and profile); climax vegetation;
climate (mean annual rainfall/precipitation, mean annual temperature, evapotrans-
piration); and so forth. Causes of soil degradation may be natural (terrain, parent
material, soil type, groundwater, climate) and anthropogenic. Important among
anthropogenic causes are land use change, deforestation, biomass burning, drain-
age of wetlands/peat soils, farming/cropping systems, soil and crop management,
soil tillage, nutrient management, and the human dimensions. The latter consists of
farm size, farm income, education, social/ethnic/gender equity, land tenure, access
to market and credit, infrastructure, civil strife, and political stability ( Figure 19.1 ).
The extent and severity of soil degradation also depends on the interaction among
processes, factors, and causes.
19.3 SOIL EROSION AND OTHER DEGRADATION PROCESSES
Accelerated soil erosion and the associated land degradation are among the major
environmental and economic issues, especially in subhumid, semiarid, and arid
environments. It aggravates the challenge of achieving food security, alleviating
poverty, and improving the environment. Accelerated erosion is exacerbated by the
tragedy of the common lands (Kabubo-Mariara 2006). About 75 billons tons of soil
may be eroded annually from the world's terrestrial ecosystems, with a rate of soil
loss of 13 to 40 Mg/ha/year from some agroecosystems (Pimentel and Kounang
1998). Because of its preferential removal by erosion, SOC concentration is a reliable
means of monitoring soil degradation by accelerated erosion (Rajan et al. 2011). The
soil erosion hazard is accentuated by increased/excessive and inappropriate mecha-
nization, simplification of crop rotations, reduction in landscape diversity, and loss
of noncrop features. Among several methods of assessment of soil erosion at dif-
ferent scales (Lal 1994, 2001), 137 Cs is a widely used technique (Junge et al. 2010;
Alewell et al. 2009). Using the 137 Cs technique, Junge et al. (2010) estimated rates of
14.4 Mg/ha/year for gross and 13.3 Mg/ha/year for net erosion on cultivated farm-
land in Nigeria. A study conducted in the Lake Victoria Basin in Uganda indicated
that household compounds, unpaved roads, and footpaths are a major source of sedi-
ments into the lake. De Meyer et al. (2011) reported that the average soil loss rate in
compounds could be as much as 107 Mg/ha/year per unit compound and 207 Mg/
ha/year per unit landing site. The mean soil loss rate was estimated at 34 Mg/ha/
year from footpaths and 35 Mg/ha/year on unpaved roads. In the Caribbean island
of St. Lucia, Cox et al. (2006) reported soil loss of 10 Mg/ha/year from agricultural
land and 0.5 Mg/ha/year from forestland.
Accelerated soil erosion remains to be a serious problem in northern Ethiopia
(Tesfahunegn et al. 2012) and is a serious constraint to achieving food scarcity.
In comparison with the traditional management for the Mai-Negus catchment in
northern Ethiopia, Tesfahunegn and colleagues reported that conservation measures
decreased water runoff by 70% (168 vs. 50 mm), erosion by 77% (42,000 vs. 9215
Mg/year), total N loss by 72% (22,400 vs. 6284 kg/year), and total P loss by 75%
(1330 vs. 341 kg/year). In the central rift valley of Ethiopia, which is characterized