**3.2. Land degradation and soil erosion in humid tropical mountains**

and an interspersed extreme storm rainfall event which had a devastating impact along the

Over the last four decades, deforestation and human interference with the environment have increased in nearly all tropical rain forest environments around the world [39]. The im‐ pact has caused increasing land degradation and is often accompanied by changes in the hy‐ drologic regime, severe soil erosion and a declining productivity of cultivated areas [87, 36]. Recent developments in agricultural techniques, the increased use of agricultural machinery and the replacement of subsistence-orientated agriculture by export-orientated agriculture have resulted in a rapidly increasing and unfavourable change in environmental conditions. Most studies on the role of soil erosion in rain-forest environments indicate that soil erosion in undisturbed rain forests rarely exceeds rates of 1t ha-1 a-1 as the canopy and understorey protect the soil from the impact of raindrops [59, 38]. In rainforests, much of the rainfall is intercepted and evaporated in the canopy and understorey, and permeable litter layers sup‐ port high infiltration rates. Consequently, only a small fraction of the rain water remains available for overland flow [73]. The litter cover on the surface on the other hand, tends to dampen the forces of the impact of heavy raindrops. This cover is highly permeable. The permeability results from macro-pores provided by roots, which reduce the generation of erosive runoff [59]. Under natural conditions, with a continuous cover of litter layers, the water movement occurs as over litter-layer flow and as root litter flow in pores and in shal‐ low subsurface pipes within the root-litter carpet [19]. This water flow is mostly highly dis‐ continuous shallow unconcentrated overland flow with a low erosive power, except in hillslope hollows, where the convergence of surface water flow lines tends to promote a con‐

Disturbance of vegetation in rain forest environments appear to have serious effects on ero‐ sion rates as the spatial variation in the intensity and frequency of large rainfall events tends to be higher than in savannah environments [87]. The loss of ground vegetation and litter reduces the amount of soil organic matter, which diminishes the aggregate stability and increases the vulnerability of the soil to raindrop impact and the likelihood of soil crusting [29, 36]. The destruction of the soil aggregates by raindrop impact and the formation of a fine grained crust on the soil surface tend to impede infiltration. During rain bursts, this causes a rapid increase in overland flow and favours the development of rills and gullies. Soil ero‐ sion and changes in the physical characteristics of the upper soil horizon are not the only effects of vegetation disturbance. The nutrient cycle is markedly changed as nutrients are lost by soil erosion, by leaching of the soil and by the removal of nutrients which were formerly stored in the vegetation [59]. As tropical rainforests are unable to sustain their nutrient base without sufficient vegetation, the combined effects of vegetation destruction and soil erosion tend to result in a marked depletion of the soils and in a reduction in the biodiversity [86]. The complex

south-eastern Brazilian coast.

14 Environmental Change and Sustainability

centrated overland flow.

**3. Soil erosion and land degradation**

**3.1. Soil erosion in humid tropical environments**

Land degradation encompasses various processes ranging from disturbance of the vegeta‐ tion to biodegradation of the humus and litter and the deterioration of soil quality. These changes are functionally associated with the productive capacity of the soils. Measurements of soil erosion rates in tropical environments are often highly variable. Calculated erosion rates range from 0.2 to 10 t ha-1 a-1 for rain forest environments in Guyana, Brazil and the Ivory Coast [34]. In the case of erosion in the Ivory Coast, rates increased on slopes with an inclination of 6% from 0.1 to 90 t ha-1 a-1 (crop cover) and 108 to 170t ha-1 a-1 (barren) [35, p. 113].

However, a quantitative assessment of the on-site and off-site impacts of soil erosion on the landscape remains a challenge because of the wide variety of environments and the relative‐ ly small data basis. Short-term soil erosion measurements from small test-plots do not al‐ ways provide representative rates for hillslopes and the extrapolation of these erosion rates to larger areas is prone to errors as physical properties of the soils, the vegetation cover and parameters such as slope length, slope steepness tend to be highly variable. A further factor is the length of the measurement period. Extreme rainfall events are highly variable in terms of space and time and hence, are often not recorded. Rainfall erosivity modelling, on the other hand, provides information on the likelihood of soil erosion, whilst the calculation of erosion rates is complicated by the high number of interacting variables [74].

Although soil erosion rates imply a continuous loss of soil, the erosive processes are trig‐ gered by separate rainfall events, and the impact of singular rainfall events on soil losses may override all preceding soil erosion rates calculated. The important role of extreme rain‐ fall events on soil losses and on sedimentation rates on the valley floors has been document‐ ed in the drainage basin of the Tubarão river (southern Brazil) [8]. In this area, a total amount of 400mm rainfall (three days) was recorded. This event caused serious soil erosion and resulted in the accumulation of a 30 to 60 cm thick pile of sediment on the valley floor. This implies that meaningful erosion rates can be only deduced when erosion measure‐ ments are supplemented by studies of the sediment balance in the drainage basins [8]. Stud‐ ies on erosion/sedimentation events in drainage basins over a longer range of time (101 to 104 years). On the other hand, are rare and often confronted with the problem of distinguishing between human induced changes and natural environmental changes. The latter applies, in particular, to cases where landforms are polygenetic and are caused by rare, high magni‐ tude events rather than by continuous processes. However, accelerated soil erosion as a re‐ sult of the increased agriculture and the destruction of the vegetation cover has been recorded in the drainage basin of the Ribeira River [9]. The drainage area of the Ribeira river covers an area of 24,200 km2 . Since the 19th century, land use has increased from the equiva‐ lent of a few per cent to an area covering about 5000 km2 in the year 1979 [9]. The Ribeira drainage basin is underlain by deeply weathered metamorphic and plutonic rocks. Most of the lower valley-side slope segments and small hillslope hollows are covered with pedoge‐ netically transformed, clay-rich colluvial sediments of the late Pleistocene and early Holo‐ cene age, which have been deposited above the "in situ" formed saprolite [8, 63]. However, in areas where the original forest has been replaced by shrubs or agricultural land use, the colluvial soils exhibit truncated soil horizons whilst gully incision into the saprolite has giv‐ en rise to the development of shallow hillslope hollows and deeply dissected hillslopes (Fig‐ ure 4, 5). Most of the eroded fine-grained material has been transported to the rivers and on to the flat valley floors [9]. In the drainage basin of the Ribeira River, the high influx of sedi‐ ment into the valleys and onto the flat valley floors has resulted in the accumulation of 5 to 6 m thick clayey sediments, which are rich in organic matter. In the area surrounding the vil‐ lage of Sete Baras, 5.8m thick sediments have been deposited above the river gravel of the Ribeira River [9]. Radiocarbon age determination of the organic matter of these deposits from a layer located just above the river gravel indicates that the material above the river gravel is younger than 300 years [9]. This provides an approximate age for the start of the increased erosion episode resulting from humanly induced disturbance of the vegetation. The geomorphic analysis of erosional forms, of the degradation of the colluvial soils, and of the start of the increased vegetation clearance indicates that soil erosion has contributed to a loss of soil 170m3 ha-1 a-1 or of 235t ha-1 a-1 in the last 130 years [10, p. 65].

**Figure 4.** Accumulation of colluvium in a hillslope hollow in the drainage basin of the Jacupiranga River, which is a tributary of the Ribeira River. The colluvium has been deposited on the saprolite of the mica schists/phyllites. The col‐ luvium was formed in the Pleistocene as a result of less dense vegetation cover and drier climatic conditions. (Photo Römer)

However, the amount of soil loss on the valley-side slopes appears to have varied in differ‐ ent geomorphic settings depending on the relative relief, the physical properties of the rego‐ lith cover and the process domains. Studies on ultramafic rocks in the Jacupiranga Alkaline Complex, which is part of the Ribeira drainage basin provided no evidence of an increase in soil erosion even on steep hillslopes although, mining and cultivation of tea and bananas re‐ sulted in extensive destruction of the original forest cover [57]. Hillslope development in this area is primarily controlled by chemical denudation in the highly permeable weathering mantles and, to a lesser degree, by slow mass movements, whilst surface wash is limited by the lack of a significant overland flow [60]. Nevertheless, the role of the destruction of the vegetation cover cannot be underestimated as leaching processes operate at high rates in the tropics and tend to remove nutrients from the upper soil horizon, possibly reducing the fer‐ tility of the soils.

**Figure 5.** Piping and gully erosion in colluvium resulting from high rainfalls and vegetation disturbance in multiconvex hilly terrain in south-eastern Brazil at Jacupiranga. (Photo Römer)

#### **3.3. Weathering and nutrient cycle**

the lower valley-side slope segments and small hillslope hollows are covered with pedoge‐ netically transformed, clay-rich colluvial sediments of the late Pleistocene and early Holo‐ cene age, which have been deposited above the "in situ" formed saprolite [8, 63]. However, in areas where the original forest has been replaced by shrubs or agricultural land use, the colluvial soils exhibit truncated soil horizons whilst gully incision into the saprolite has giv‐ en rise to the development of shallow hillslope hollows and deeply dissected hillslopes (Fig‐ ure 4, 5). Most of the eroded fine-grained material has been transported to the rivers and on to the flat valley floors [9]. In the drainage basin of the Ribeira River, the high influx of sedi‐ ment into the valleys and onto the flat valley floors has resulted in the accumulation of 5 to 6 m thick clayey sediments, which are rich in organic matter. In the area surrounding the vil‐ lage of Sete Baras, 5.8m thick sediments have been deposited above the river gravel of the Ribeira River [9]. Radiocarbon age determination of the organic matter of these deposits from a layer located just above the river gravel indicates that the material above the river gravel is younger than 300 years [9]. This provides an approximate age for the start of the increased erosion episode resulting from humanly induced disturbance of the vegetation. The geomorphic analysis of erosional forms, of the degradation of the colluvial soils, and of the start of the increased vegetation clearance indicates that soil erosion has contributed to a

ha-1 a-1 or of 235t ha-1 a-1 in the last 130 years [10, p. 65].

**Figure 4.** Accumulation of colluvium in a hillslope hollow in the drainage basin of the Jacupiranga River, which is a tributary of the Ribeira River. The colluvium has been deposited on the saprolite of the mica schists/phyllites. The col‐ luvium was formed in the Pleistocene as a result of less dense vegetation cover and drier climatic conditions. (Photo

However, the amount of soil loss on the valley-side slopes appears to have varied in differ‐ ent geomorphic settings depending on the relative relief, the physical properties of the rego‐ lith cover and the process domains. Studies on ultramafic rocks in the Jacupiranga Alkaline Complex, which is part of the Ribeira drainage basin provided no evidence of an increase in soil erosion even on steep hillslopes although, mining and cultivation of tea and bananas re‐

loss of soil 170m3

16 Environmental Change and Sustainability

Römer)

In tropical rainforests, the biomass above and below the ground contains most of the miner‐ al nutrients. The maintenance of the nutrient level in the soil depends on the continuous cy‐ cling of the nutrients in the canopy and on the rate of decay of organic matter in the litterlayer. The latter is controlled by biological decomposition by invertebrates, and by the physico-chemical processes responsible for the release of nutrients in the upper soil horizons [59, 67]. However, the functional dependencies in the nutrient cycle appear to be stronger in soils with a low nutrient storage and low fertility and weaker in more fertile soils. Once the vegetation cover is destroyed, the supply of organic matter and the formation of the new lit‐ ter on the soil surface is slowed down whilst the breakdown of the organic matter is acceler‐ ated by solar radiation [87, p. 277].

Although the physico-chemical processes controlling the productivity and fertility of the soils and the turnovers of the nutrients are not completely understood, several lines of evi‐ dence suggest that the degree of weathering and textural characteristics of the soil play an important role in the nutrient cycle. High weathering rates result in excessive base-leaching and a low pH, creating a decline in base saturation, loss of major cations and a decrease in the cation-exchange capacity [66]. This promotes the occurrence of free iron and aluminium either in the clay complexes or as amorphous iron and aluminium oxides or hydroxides in the weathering layers. As amorphous iron and aluminium oxides readily absorb, phospho‐ rus tropical soils with a low pH are often characterized by a high phosphorus fixation ca‐ pacity, resulting in a phosphorus deficiency [85, 59]. According to studies in the Amazon of Brazil the fixation of phosphorus rather than the overall nutrient decline appears in many cases to be the cause of the decline in pasture productivity [69].

Soil erosion and intense leaching in soils are responsible for several problems concerning productivity in agricultural land use. In a case study carried out in Rwanda several green farming methods applied to highly degraded soils failed to restore the fertility of the soils [77]. Improved fallow, mulching, green manure and the use of compost and cow dung were not sufficient to maintain the nutrient levels in the soil as the rapid decomposition of the or‐ ganic matter at the start of the rain season resulted in a release and leaching of high amounts of nitrogen and a rapid reduction in the fertility of the soils [77]. From the point of view of the sustainability any agricultural strategies being considered, the materials used for fertiliz‐ ing the soils have to be inexpensive and available from regional or local resources. The im‐ provement of the physico-chemical properties of highly degraded soils, on the other hand, depends on several-site specific and soil-specific factors, and additional information is fre‐ quently required on the dynamics of the soil. Important improvements usually involve in‐ creasing the pH. This reduces phosphorus fixation, the disintegration of chlorite structures and reduces antagonistic effects in cation exchange.

However, any application of material has to maintain the slow dissolution of cations from dissolved minerals and has to inhibit silica dissolution which often involves an increase in pH and results in an increase in the disintegration rate of chlorite structures [77]. In rela‐ tion to the requirements specified in Rwanda, several tests with calcium carbonate, traver‐ tine and volcanic tephra indicated that the combined application of cow dung and tephra represents a measure capable of improving the agricultural capacity of the degraded soils [77]. However, soil erosion, nutrient cycles and soil fertility are highly interrelated and depend often on specific local and regional factors. Although quantitative data of soil ero‐ sion rates and depletion rates are important for the implementation of effective soil conser‐ vation measures, socio-economic factors and the understanding of the traditional/cultural background appear to be of equal importance because many conservation strategies may be impractical or too expensive or are rejected as a result of limited access to the technolo‐ gies required.
