Land Degradation and Pedological Processes in a Changing Climate

316R. Lal / Pedologist (2012) 315-325the importance of the pedological processes affecting landdegradation in a changing climate characterized by a risein temperature, change in precipitation, and an increase inthe frequency of extreme events.2. Basic ConceptsThe processes of land and soil degradation are setin-motionby the loss of equilibrium of a stable soil due tonatural and/or anthropogenic factors. “Land degradation”refers to the loss of climax vegetation and its replacementby secondary vegetation with an adverse impact on soilquality and ecosystem services. In contrast, the term “retrogression”literally means transformation from a complexto a simpler biological form leading to degeneration, abiotrophy,or change from one state to another through naturalor anthropogenic factors, leading to an adverse impacton the quality of the upper soil horizon. The process ofretrogression, caused for example by accelerated erosion,may be partial or total and is initiated by events such asdeforestation, biomass burning, overgrazing, and plowing.The term “desertification” is degradation of land in aridand dry sub-humid areas leading to a decline in ecosystemfunctions and services. Drought is a dominant processdriving desertification and can be one of three types: (i)meteorological drought caused by a long-term decline inprecipitation, (ii) hydrological drought caused by a reductionin surface runoff and fall in the water table, and (iii)pedological drought caused by a decline in soil moistureavailability. Land degradation, especially desertification,affects all three types of drought, but the effects of pedologicaldrought are observed long before those of meteorologicaland hydrological drought. In contrast to landdegradation, the term “soil degradation” refers to the declinein the quality and capacity of soil’s productivity dueboth to natural and anthropogenic factors. Soil degradationis caused by natural and anthropogenic perturbations inthe hydrological cycle, nutrient/elemental cycling, energybudget, and activity and species diversity of soil biota.In the steady state, the pedosphere is in dynamicequilibrium with its environment, which comprises the atmosphere,hydrosphere, biosphere, and lithosphere (Fig.1). This dynamic equilibrium can be disturbed by naturalevents such as volcanic eruptions, wild fires, flash floods,and mass movement. In contrast to natural perturbations,the delicate and dynamic equilibrium is also disturbed byanthropogenic activities such as deforestation, land useconversion, drainage of wetlands, biomass burning, farmingpractices, plowing, and use of agrochemicals. The perturbedpedosphere is prone to accelerated erosion (Lal,2001; Pimentel et al., 1995) and undergoes a change ofstate in response to events such as loss of topsoil, declinein SOM and nutrient content, disruption and changes inelemental and hydrologic cycling and balance, alterationsin energy budget, disruption in intra-solum processes,shift in activity, and species diversity of biota. Thus, thechange in land use and land cover must be undertaken bythose practices which cause the least disturbance to thepedosphere and its environment (Fig. 1; lithosphere, atmosphere,hydrosphere, and biosphere).3. Processes, Factors, and Causes Leading toPerturbation of the Pedosphere and the AttendantLand DegradationProcesses are the mechanisms, drivers, source ofrequired energy, and the principal controls which lead toperturbations through disturbing the delicate and dynamicequilibrium and set-in-motion the land/soil degradationprocess which adversely affects ecosystem functions andservices. Strongly interactive pedological processes whichare impacted by degradation include: physical (weathering,fractionation, sedimentation, translocation, deposition),chemical (dissolution, precipitation, acidification,carbonation), biological (respiration, mineralization, humification),and ecological (shift in climax vegetation, changein land forms and terrain, alterations in drainage densityand patterns). Factors are environmental parameters, attributes,and characteristics, which moderate the directionand magnitude of biophysical processes and alter theparameters leading to predominance of one process overthe others (i.e., physical, chemical, biological, and ecological).Principal factors include climate and terrain, includingslope (length, steepness, shape aspect, regularity,complexity). Causes of land degradation are those whichalter and moderate the effect of factors on pedosphericdegradation and are related to change in land use and landmanagement. The important causes, which also govern