Land Degradation and Pedological Processes in a Changing Climate

R. Lal: Soil degradation processes315Land Degradation and Pedological Processes in a Changing ClimateRattan LAL*Carbon Management and Sequestration Center, The Ohio State University, Columbus, OH 43210Keywords: Soil degradation, Soil quality, Global warming, Food security, DesertificationAbstractLand degradation and retrogression are related processes. Land degradation implies replacement of climax vegetationby secondary vegetation, alteration of humus quantity and composition, and adverse changes in soil quality and relatedecosystem services. Retrogression refers to the loss of the upper soil horizon and reversion to pioneer conditions (i.e.,bare ground). In comparison, soil degradation implies a decline in the quality and capacity of a soil’s productivity throughits misuse. Land and soil degradation are also related to poverty, governance, and political will. When people are povertystricken, desperate, and hungry, they pass on their sufferings to the land. The biophysical processes are driven by social,cultural, economic, and political factors related to human dimensions. These processes can also be traced to human greed,short sightedness, poor planning, and cutting corner for quick economic returns. Land degradation impacts 33% of the Earth’sland surface and affects 2.6 billion people. Pedological processes impacting land degradation include a decline in soil organicmatter (SOM) content, a decrease in the amount and stability of aggregates, crusting, compaction, accelerated erosion,nutrient depletion, elemental imbalance, salinization, waterlogging, a decline in activity and species diversity of soil fauna,and soil contamination. In addition to the decline in productivity and ecosystem services, land degradation also accentuatesthe emission of CO 2 and other greenhouse gases (GHGs) into the atmosphere. It disrupts cycling of C, N, other elements,and water. Understanding of both long-term and short-term C cycles, along with those of N and water, is essential forreversing the degradation trends. Restoring the soil C pool increases soil resilience as well as adaptation to and mitigation ofclimate change. Total C sink capacity of the terrestrial biosphere is 2.55 to 4.96 Pg C/yr, with a projected draw down of about50 ppm of CO 2 by 2100 or 2150. Sequestration of C in soils and the terrestrial biosphere is a win-win strategy; it enhancesagronomic productivity, advances food security, improves the environment, and mitigates climate change.1. IntroductionThe world may be losing 10 ha of arable land everymin to a range of degradation processes (Buringh, 1979),including 3 ha to salinization (Kovoda, 1983). Erosion andsalinization, along with depletion of soil organic matter(SOM) and plant nutrients, are among the principal pedologicalprocesses affecting land and soil degradation.These processes are likely to be exacerbated by climatechange, and the attendant increase in intensity and frequencyof extreme events. With wind and water being theprincipal contributors to climatic erosivity, an increase inthe combined kinetic energy of these two climatic factorscan drastically accelerate soil erosion. Furthermore, water-drivensediment transport can be accentuated by winddrivenrains, especially during extreme events caused byclimate change (Field et al., 2011; Sweet, 1999; Breshearset al., 2003). An increase in temperature, especially in thehigh latitudes, may also accentuate evaporation and exacerbatesalinization in semi-arid and arid regions underlainby parent materials containing high concentrations of solublesalts. Thus, the objective of this article is to describe*Corresponding author: Rattan Lal, E-mail: lal.1@osu.edu, Tel: +614-292-9069, Fax: +614-292-7432Received 22 July 2011; accepted 2 February, 2012

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

R. Lal: Soil degradation processes317Fig. 1. Interactive effects between pedospheric processes and the environment in relation to land degradation.the rate and magnitude of perturbation, are deforestationfarming systems, land use, and specific managementof soil, crops, plants, water, and animals. The biophysicalprocess of land degradation is driven by social, economic,political, and cultural factors. The human dimensions ofland use and management, including demography andgender/ethnic/cultural factors, are important drivers ofthe processes involved (Fig. 2).4. Types of Land DegradationOn the basis of the predominant processes, there areseveral types of land degradation in relation to specific pedospherictransformation (Fig. 3). Physical degradation ofthe pedosphere comprises a reduction in the amount andstability of aggregation, susceptibility to slaking, vulnerabilityto crusting and seal formation, tendency for waterinfiltration capacity to decline, and proneness to increasedrunoff and the attendant susceptibility to erosion by waterand wind. Densification, reduction in total porosity andmacroporosity, and increase in water retention can alsolead to anaerobiosis. Furthermore, accelerated soil erosionis a selective process. It leads to preferential removal ofclay, humus, and other colloidal materials, leaving coarsefractions (sand, gravel) on eroded sites. Reversal of thedownward spiral to restore soil structure, pore continuityand stability, water retention and transmission, gaseousexchange, among others involves adoption of land use andmanagement practices which improve aggregation whilespecifically accentuating the stability of microaggregates.Chemical degradation of the pedosphere encompassesa change in pH (decrease or increase), reduction in cationexchange capacity (CEC), decline in base saturation(Ca +2 , Mg +2 , K + ) and increase in Al +3 , Fe +2 , and Mn +2 , anincrease in salt concentration in the root zone (salinization)with a predominance of Na + on the exchange complex(alkalization), nutrient depletion (N, P, K, Zn, Cu), andelemental imbalance (toxicity of Al +3 , Mn +3 , Fe +2 ) (Fig. 3).Excessive and inappropriate use of water, especially forirrigation in arid and semi-arid regions, is a major causeof secondary salinization. Of several examples, includingthose in South Asia and Australia, degradation of soil andwater resources in the Aral Sea Basin in Central Asia (Qadiret al., 2009) is a specific example of irrigation-induced

318R. Lal / Pedologist (2012) 315-325Fig. 2. Biophysical and anthropogenic processes, factors and causes of land degradation.Fig. 3. Types of land degradation and pedological processes.(SOM: soil organic matter)

R. Lal: Soil degradation processes319degradation of pedospheric processes by salinization.With regard to the threat of climate change, soil can bea source or sink of atmospheric CO 2 and other radiativelyactive gases (e.g., CH 4 , N 2 O). Changes in the SOM poolhave strong implications on the atmospheric abundanceof CO 2 . Depletion of the SOM pool increases the flux ofCO 2 from the soil into the atmosphere, and accretion ofSOM can cause drawdown of CO 2 from the atmosphere. Achange in soil fauna and flora (below ground biodiversity),in conjunction with anaerobiosis and the degree of saturation(wetness), also impact methanogenesis (productionand emission of CH 4 ) and nitrification/denitrification processes(production and emission of N 2 O). Other biologicalprocesses affecting the quality of the pedosphere andits fractions and ecosystem services are the quality andquantity of the labile C pool, microbial biomass C (MBC),depth:distribution ratio (stratification) of the soil C pool,and the relative abundance of soil pathogens vs. predators(Fig. 3).Ecosystem characteristics also impact pedosphericprocesses and their ecological functions and services.Principal ecosystem services adversely impacted by thedegradation of the pedosphere include the quantity andquality of renewable water resources, ecosystem C pool,terrain characteristics and land forms, and below andabove ground biodiversity (Fig. 3). The downward spiralleading to degradation of pedospheric processes is oftenset-in-motion by extractive farming (Fig. 4). Depletion ofthe SOM pool and decline in soil structure exacerbate thedegradation processes which control physical, chemical,biological, and ecological degradation.5. Methods of Assessment of Pedospheric LandDegradationA large number of reports provide statistics on theextent and severity of land degradation, soil degradation,and desertification (e.g., Oldeman and Van Lynden, 1998;Dregne, 1998; Bai et al., 2008). However, the statisticsare often confusing, contradictory, and not comparablebecause the data are based on different terminology andon a wide range of non-standardized methods. The informationis further confounded by the lack of ground truthingand does not relate the severity of degradation to netprimary productivity (NPP), agronomic production, or useefficiency of input.Table 1 outlines some methods used in assessing differenttypes of pedospheric land degradation such as wa-Fig. 4. The downward spiral set-in-motion by land misuse and soil mismanagement, and extractive farming whichcreate a negative C and nutrient budgets.

320R. Lal / Pedologist (2012) 315-325ter erosion, wind erosion, tillage erosion, desertification,salinization, and physical degradation. Other methodsused have been described by Oldeman and Van Lynden(1998), Dregne (1998), and Bai et al. (2008). Because ofthe diverse methods used, results are often not comparable.Thus, there is a strong need to standardize the methodology,and relate it to its impact on the loss of NPP andagronomic productivity.Table 1. Methods of assessment of land degradationIIIIIIIVVVIVIIVIIILand Degradation Process Method ReferenceWater Erosion• Stream bank Mass movement Channel side slope evolution Lohnes (1991)• Cropland erosion Rill/inter-rill Assessment of liquid and Wicherek (1991)solid flow rates• Cropland erosion Rill/inter-rill Erosion rate Jankauskas et al. (2002)• Landscape erosion Decline in rooting depth P movement downslope Kirkby et al. (1997)• Stream flow Riparian zone Paired catchment Scott (1999)• Sediment yieldPedological, hydrogeomorphologicalprocessesRunoff measurement Cammeraat and Kooijman(2009)• Soil erosion severity Landscape processes Genetic soil horizon Jankauskas and Fullen (2002)• Hillslope erosion Rill, inter-rill Modeling Rneard et al. (1991; 1994;1997), Nearing et al. (1994)Wind Erosion• Soil movement Wind erosivity Modeling Skidmore (1994)• Wind:water driven sediment Aeolian and fluvial sediment Climate extreme Field et al. (2011)transportTillage Erosion• Soil movementSoil redistribution, diffusiveprocess137Cs analysis Govers et al. (1996)Desertification• Sand drift Vegetation/soil degradation NDVI from the Red and IR Dhir and Sharma (1991)Bands Soil Brightness index• Soil surface properties Soil surface roughness and Remote sensing Anderson and Croft (2009)moisture• Steppe degradationUnwanted morphological Pedological assessment Meyer et al. (2008)processes• Land cover Ecosystem transformation Modeling (descriptive,empirical, statistical,dynamic)Lambin (1997)Salinization• Human-induced Salinity in the root zone Surface & ground water Ghassemi et al. (1991)models• Productivity Crop tolerance Species adaptation models Rozema and Flowers (2008)• Global land area Processes of salinization Carbon stock Lal (2009)• Secondary salinization Soil structural stability Spatial analysis Odeh and Onus (2008)• Salt affected soils Salt balance Spatial scales Thayalakumaran et al. (2007)Physical Degradation• Decline in soil structure Slaking Permeability Coughlan et al. (1991)• Soil compaction Changes in bulk density Porosity & pore size Horn et al. (1995)distribution• Aggregation Macro- and microaggregates Aggregate stability Boix-Fayos et al. (2001)Vegetation Degradation• Overgrazing Denudation Satellite, Remote sensing Hill et al. (1998)Economic Loss• Cost of land degradation Productivity decline Measure of economic losses Bojö (1996)

R. Lal: Soil degradation processes3216. Cost of Inaction in Addressing Land DegradationFig. 5 outlines a range of costs of inaction in reversingthe degradation trends. Some estimates of the costof inaction have been made by Nkonya et al. (2011). Thecosts may be economic (gross vs. net, financial vs. material),unsustainability (short-term vs. long-term, one timevs. cumulative), decline in productivity (on-site vs. offsite,transient vs. permanent), reduction in profit margin(absolute vs. relative, normal vs. real/actual), and damageto ecosystems and environment (land vs. soil, biodiversityvs. climate, quantity vs. quality of renewable waterresources) (Bojö, 1996). These costs must be assessed atsoilscape, landscape, regional, watershed, national, and internationalscales. Similar to the variability in estimates ofthe area affected, the cost of inaction must also be evaluatedby standardized procedures and must be determinedin relation to on-site and off-site impacts, short-term andlong-term damages, economic and material losses, anddamages to infra-structure and the environment. The latterconsists of the adverse impacts on water resources,biodiversity, and climate.Land degradation by pedospheric processes is asevere problem in developing countries, where soil andnatural resources are already under great stress. Consequently,there is a large yield gap for major grain crops, especiallythose grown in rainfed climates. The data in Table2 provide estimates of actual yield as a percent of attainableyield. In some countries, actual yield under rainfed conditionsis merely 10% of attainable yield. Therefore, restorationof degraded lands and improvement in soil qualitythrough adoption of recommended management practices(RMPs) can quadruple agronomic production. Herein liesthe strategy of advancing food security through improvingTable 2. Actual yield of rainfed grain crops as a percent ofattainable yield (recalculated from Rockström et al.,2010).Country% of AttainableYieldYield ImprovementFactor by RMPsBotswana 27.6 3.6Burkina Faso 23.7 4.2Ethiopia 31.6 3.2India 42.1 2.4Iran 18.4 5.4Iraq 17.1 5.8Jordan 18.4 5.4Kenya 28.9 3.5Morocco 25.0 4.0Niger 26.3 3.4Pakistan 10.5 9.5Syria 18.4 5.4Tanzania 23.7 4.2Thailand 52.6 1.9Uganda 23.7 4.2Vietnam 63.1 1.6Yemen 10.5 9.5Zambia 31.6 3.2Zimbabwe 31.6 3.2RMP: recommended management practicesFig. 5. Economic cost of inaction on land degradation and padological processes.

322R. Lal / Pedologist (2012) 315-325quality of degraded/desertified soils. The loss in productionis not only of immediate concern, but also has cumulativeeffects over time (Fig. 5).7. Climate Change and Pedospheric ProcessesSoils are affected by climate change. Thus, the importanceof including a soil evolution module in modelsfor predicting future climate change is widely recognized(Montagne and Cornu, 2010). Soils affect climate changethrough emission of radiatively active gases (i.e., CO 2 ,CH 4 , N 2 O). Depletion of SOM, through biological degradation,exacerbates gaseous emissions into the atmosphere.Increases in temperature due to global warming may accentuatethe rate of mineralization of SOM, reduce aggregationand aggregate stability, and accelerate soil erosionrisks. An increase in frequency of extreme events, largeand intense rains and high winds, can increase climaticerosivity. Therefore, climate change may accentuateclimate erosivity while increasing soil erodibility. An increasein evaporation can decrease available water capacity,reduce vegetation cover, and decrease the amount ofbiomass-C returned to the soil. Thawing of permafrost(cryosols) and drainage of peat soils can create positivefeedback, accentuating the rate of mineralization of SOMand further exacerbating climate change.Yet, restoration of degraded/desertified soils can increasethe ecosystem C pool and off-set some anthropogenicemissions. The potential of C sequestration is estimatedat 0.2–0.7 Pg C/yr through desertification control,and 0.3–0.7 Pg C/yr through restoration of salt affectedsoils. In addition, adoption of RMPs can sequester 0.4–1.2Pg C/yr in cropland soils, 0.3–0.5 Pg C/yr in grassland/grazing land soils, and 1.4–1.9 Pg C/yr through afforestation/deforestationand the establishment of forest plantations.Thus, the total technical potential of C sequestrationin the terrestrial biosphere is 2.55–4.96 Pg C/yr (Lal,2010).Thus, the threat of human-induced climate changecreates challenges and opportunities. The challenge liesin the increased risks of pedospheric land degradation becauseof higher rates of depletion of SOM, acceleration insoil erosion, and high risks of salinization. Yet, there areopportunities to improve soil quality, and restore degradedsoils by sequestration of C in the terrestrial biosphere.The win-win option is important for restoring degradedsoils, mitigating climate change, improving water qualityand the environment, and advancing food security.8. Technological Options of Restoring DegradedSoil and Desertified LandsThe key strategy of reversing the degradation spiral isrestoration of soil quality (Fig. 6). The goal is to conservewater and nutrients, increase vegetation cover, and improvethe SOM pool. Soil quality is indeed an appropriateindicator of sustainable land management (Herrick, 2000),and the quantity and quality of the SOM pool are importantdeterminants of soil quality. There is a threshold levelof SOM concentration in the root zone (1.9–3.4%), whichdepends on climate, soil type, landscape position, drainage,and profile/solum depth. Rather than concentratedonly in the surface soil, deep SOM storage is also a keyfactor, but the processes governing deep SOM (sub-soilbelow 50 cm depth) placement and dynamics are poorlyunderstood (Lorenz and Lal, 2005; Rumpel and Kogel-Knabner, 2011).There are strong synergies between mitigation of and adaptationto climate change, especially in the context of agriculture(Smith and Oleson, 2010). Biotic sequestration ofatmospheric CO 2 has strong cost-effectiveness. Becauseof the strong interaction between C and water (Evett andTolk, 2009), and between C and N (Lal, 2010), the strategyis to adopt land use and management practices whichaccentuate the use of efficient energy-based input (fertilizers,irrigation, tillage), and create a positive C budget(Lal, 2010). Management of soil fertility, in conjunctionwith that of C and water, is an important factor, especiallyfor restoring degraded soils managed by resource-poorfarmers through extractive farming practices in developingcountries. Land application of organic by-products orco-products (misnomered as waste) is relevant to restoringsoil ecosystem (Odlare et al., 2010). This strategy canbe adopted in conjunction with production of biofuels. Degraded/depletedsoils are prone to drought stress, havelow soil fertility, and are constrained by elemental imbalances(Al +3 toxicity). Such challenges of low soil fertility

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