Climate Change and Forest Dynamics: A Soils Perspective - Soils and Food Security - page 171

eradication of some native plants is causing dramatic changes in local water
tables, leading to salinisation of the topsoil. Desert expansion, driven by
conversion of grasslands in the Sahel of Africa, Kazakhstan, Uzbekistan and
northern China, has led to wind erosion and dust storms.

d n 1 r 2 n g | 8

4.1.1 Mechanisms of Soil Degradation
Oldeman 181 defined two categories of soil degradation: (1) the displacement of
soil material by water or wind erosion, and (2) in situ soil deterioration through
chemical or physical processes. Erosion or topsoil removal reduces soil fertility
and may reduce crop rooting depths. Deforestation, overgrazing, and
agriculture are the main causes of water erosion, because they expose soil to
the direct impacts of rainfall, and wind erosion is almost always caused by a
decrease in vegetation cover from overgrazing or the removal of vegetation for
another use. Chemical degradation can be from loss of nutrients and organic
matter (insufficient fertilisers, using poor soils, removal of natural vegetation),
salinisation (from poor irrigation practices), acidification (from over applica-
tion of fertilisers), or pollution (in industrialised nations with high population
densities). Physical degradation includes compaction or sealing (from heavy
machinery, low organic matter or high silt), waterlogging (from human
intervention in natural drainage systems), or the subsidence of organic soils
(drainage or oxidation of peat soils).
Soil chemical and physical weathering rates are driven by vegetation,
temperature, and precipitation. 181,183 This can be greatly intensified in areas
where land use accelerates soil denudation, exposing more mineral surface area
and rocks. 184 Bayon et al. 185 showed that chemical weathering of surface minerals
due to intensified human land-use and forest clearing, rather than regional
climate change, may have led to an abrupt vegetation shift from rainforest trees
to savannas in Central Africa 3000 years ago. Records of past vegetation patterns
show a great loss of primary forests as they were replaced by savannas and other
pioneer species between 3000 and 2200 ago. At the same time, archaeological
research shows that Bantu-speaking people migrated into the region and cleared
forest for agriculture and iron smelting. 186-191 This large-scale deforestation event
may still influence current vegetation patterns in African rainforests. 192,193

4.1.2 Soil Degradation Implications for Soil Carbon and Nitrogen
Land erosion plays a significant role in global nutrient cycles. Soil organic
carbon and soil nitrogen are both easily removed by wind and water erosion,
which can lead to feedbacks to the atmosphere. Land cultivation leads to organic
matter losses, directly affecting the soil chemical, physical and biological
properties that affect crop production. 194 Khormali 195 showed that soil organic
carbon and nitrogen in Iran are significantly depleted by increased water erosion
from past deforestation. Vagen et al. 196 showed that some landscapes under
cultivation more than fifty years without organic matter enrichment have