Land degradation and retrogression are related processes. Land degradation implies replacement of climax vegetation by secondary vegetation, alteration of humus quantity and composition, and adverse changes in soil quality and related ecosystem 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 through its misuse. Land and soil degradation are also related to poverty, governance, and political will. When people are poverty stricken, 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’s land surface and affects 2.6 billion people. Pedological processes impacting land degradation include a decline in soil organic matter (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 accentuates the emission of CO₂ 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 for reversing the degradation trends. Restoring the soil C pool increases soil resilience as well as adaptation to and mitigation of climate change. Total C sink capacity of the terrestrial biosphere is 2.55 to 4.96 Pg C/yr, with a projected draw down of about 50 ppm of CO₂ by 2100 or 2150. Sequestration of C in soils and the terrestrial biosphere is a win-win strategy; it enhances agronomic productivity, advances food security, improves the environment, and mitigates climate change.

The effects after conversion from forest of 30 years of continuous land cultivation on soil quality were investigated by analyzing soil data from three studies carried out in the Sasumua catchment, Kenya during 1977, 1987, and 2007. Soil samples taken at depth of 0–30 cm were analyzed for (1977, 1987, and 2007) soil pH, electrical conductivity, cation exchange capacity, soil organic carbon (3.34%, 2.64%, 3.17%) , total nitrogen (0.30%, 0.2%, 0.38%), exchangeable potassium (K⁺), magnesium (Mg²⁺) and calcium (Ca²⁺). Results of an analysis of variance for the 30-year period showed changes in soil reaction from pH 5.86 to 5.22 (p < 0.005), Mg²⁺ from 3.32 to 1.04 mg/kg (p < 0.001), and K⁺ from 2.89 to 1.11 mg/kg (p < 0.001). Furthermore, Ca²⁺ decreased by 30% and TN increased by 21%. No change was observed in the level of SOC. Factors contributing to the decline of soil pH, Mg²⁺, Ca²⁺, and K⁺ were overgrazing, intensive cultivation of horticultural crops, and soil erosion by water. The application of farmyard manure to the land and the practice of agroforestry have helped to maintaining SOC-levels.

Inner Mongolian is a region located in northern China with a total area of nearly 118.3 million ha. Of that total area, 73% is part of the vast grassland belt that forms the Eurasian Steppe. As a consequence of rapid expansions of both livestock numbers and economic development, 90% of the grassland in this region was degraded by the end of the 20th century. Previous research efforts have focused on vegetation changes in this degraded grassland; however, the soils in this region have not attracted similar attention.

In this study, the characteristics of the vegetation community and soil properties were investigated along with the grassland degradation process for natural grassland utilized for free grazing in the city of Xilinhot, Inner Mongolia. The results showed that during grassland degradation, the aboveground biomass, the diversity and density of the vegetation also significantly decreased. Furthermore, the natural vegetation community of the steppes was replaced by sand-tolerant annual plant species. The invasion of drought-, sand-, and wind-tolerant species reduced the competitive capacity and dominance of native plants, increasing the likelihood of their ultimate disappearance.

In addition, the clay content, aggregate stability and cation exchange capacity of the soil decreased, whereas the pH, electrical conductivity, sand content, calcium saturation percentage and base saturation percentage all increased. These changes in soil properties have led to soil desertification, salinization and erosion in the region. Moreover, carbon and nitrogen were lost from the soil as a result of clay removal and water-stable aggregates breaking down. Thus, the conclusion is drawn that grassland degradation reduces not only grass quantity and quality but also soil quality in the ecosystem.

The environmental problems of soil salinity and erosion represent two major challenges to agriculture in Thailand. Both problems require careful assessment of their in situ status and spatial variation in order to implement appropriate control measures. Here, we review the current state of relevant research on the mapping of these two problems, placing an emphasis on computer-assisted techniques. Salt-affected soils are mapped based on the percentage of salt crusts formed in the area. The techniques used are related to simple digital classification of multispectral remotely sensed data. In soil erosion mapping, the erosion risk caused by water is determined, and most such maps in Thailand are based on values that are derived from various factors through the universal soil loss equation. However, the methods used to determine the levels of each of the factors vary according to different producers. Although computer-assisted mapping of soil salinity and soil erosion has yielded useful results, further improvement is needed. In addition, promising new techniques must be fully tested prior to their adoption in Thailand.

　　Wind erosion is the main cause of desertification in the Sahel region of West Africa. Therefore, it is crucial to evaluate the effects of wind erosion on soil nutrient status for facilitating sustainable agriculture in the Sahel. In previous studies, nutrient loss was estimated by measuring only the soil particle flux above 0.05 m. However, the flux of coarse organic matter (COM; defined as free organic debris with diameter greater than 200 μm) and soil particle flux below 0.05 m should also be measured. This is because COM plays a prominent role in soil nutrient dynamics in the Sahel, and soil particle loss estimated without measuring the near-surface flux is inaccurate. We conducted field experiments between 2005 and 2007 to measure the whole COM and soil particle flux, as well as the associated nitrogen flux, in a cultivated field using the Big Spring Number Eight (BSNE) sampler and an Aeolian Materials Sampler (AMS), which we developed to compensate for the limited capacity of the BSNE sampler. In each year, COM loss from the field was 50–75% of the initial COM existing in the loose surface soil before the occurrence of wind erosion. The soil particle loss was 60–80 Mg ha^-1 y^-1, which corresponded to a 4–5 mm y^-1 loss of topsoil. Consequently, 40–50 kg ha^-1 y^-1 of soil nitrogen was lost from the field; approximately 2–3-fold higher than the annual nitrogen uptake by pearl millet (Pennisetum glaucum), a staple crop in the Sahel. The nitrogen loss estimated by measuring only the soil particle flux at heights of 0.05–1.0 m (conventional method) was 35–45 kg ha^-1 y^-1; an underestimation of 10–20%. Therefore, we conclude that whole COM and soil particle flux should be measured when evaluating the effects of wind erosion on soil nutrient status in the Sahel.

The low productivity of agricultural soils experienced in sub-Saharan Africa (SSA) has been attributed to the catastrophic soil degradation and inherent pedological factors of the region. Soil degradation in this region includes soil erosion by water and wind, extensive leaching due to extreme rainfall, salinity associated with aridity, spillage due to mining and associated problems of deforestation. In areas under traditional management techniques, the recent breakthroughs in biotechnology have not achieved many of the objectives of “Green Revolution”. The soils are highly weathered, low in essential plant nutrient and soil organic matter. The clay minerals in the majority of the soils are mainly oxides of aluminium and iron including non-expansive 1:1 minerals such as kaolinite. However, the region must aspire to achieve, at least in part, the goals of Millennium Development Goals. Sustainable rice production in SSA can be realized through a combination of biotechnology and improvement of the rice ecological environment through ecotechnology. The “sawah”, which is an Indonesian word meaning irrigated lowland is considered to be the missing concept for improving natural resources in the majority of African rice farmers’ fields. A sawah scheme is thus being fashioned to replicate the Japanese “satoyama”. This scheme presents one of the best options to the abundant inland valleys in SSA watersheds for the development of sustainable productivity and for efficient soil forming processes to continue without interruption. Under such schemes agricultural productivity is ensured, while the ecosystem remains stable with little or no degradation. Sawah ecotechnology can improve input efficiency, and when the sawah system has been integrated into improved agronomic practices, such as the system of rice intensification, paddy yields have increased from a low value of 2 t/ha through to 4 t/ha, and presently stands at nearly 10 t/ha. Moreover, sawah rice production has achieved stability of soil degradation, reducing regular soil erosion to within the soil loss tolerance limit that ensures sustainable production in SSA.

Most intensive land degradation processes in Kazakhstan have been developed on rangelands. Although land degradation of rangelands is partly associated with sandy soils and dry climate in deserts, poor rangeland management aggravated during the process of land privatization is a major factor. Land degradation of cropland in rainfed agriculture occurs mostly through soil erosion by both wind and water. Although wind erosion is prominent, much land degradation has been caused by erosion by water as well. Research has shown that one of the major means of addressing the issues of soil erosion on cropland in rainfed agriculture is the elimination of the summer fallow practice on black soils and the adoption of chemical summer fallow concept in dry steppe regions. In the last three years, the area under summer fallow was reduced by 1 million ha. The other measure is No-Till and minimum tillage, which are currently adopted practices on 11.2 million ha of cropland. Degradation of cropland in irrigated agriculture occurs even more intensively because of poor management of soil drainage and irrigation systems. Saline seepage in particular is a major problem in these soils. Therefore, policies should be directed towards rebuilding of all irrigation and drainage constructions.

Ultisols and Oxisols in Malaysia are dominated by kaolinite and sesquioxides, having low pH and exchangeable bases, but high exchangeability for Al; hence, their productivity is low. Cocoa grown in these soils has poor yield. Studies were conducted in Malaysia to determine the effect of applying ground basalt and/or compost on the chemical fertility of the soils and the growth of cocoa. The results showed that basalt application improved the chemical fertility of Ultisols and Oxisols by increasing pH, Ca, Mg, K, and P content, and cation-exchange capacity. The increase in pH precipitated Al as inert Al hydroxides. The pH increase resulted from the hydrolysis of silicates when basalt disintegrated and dissolved in the soils. However, basalt takes time to disintegrate and dissolve completely, and its reaction with the soils resulted in the decrease of pHo, the pH at which the net charge is zero for the variable charge minerals. Because of the presence of variable charge minerals in the soils, the increase in pH and the concomitant decrease in pHo increased the negative charge of the soils, further improving soil productivity. A glasshouse experiment showed that basalt application improved cocoa growth. The improvement in cocoa growth due to basalt application has been confirmed by a field trial, and the best treatment was found to be combined application of basalt and compost.

Soil organic matter (SOM) plays an essential role in the structure and functions of soils as well as nutrient cycling in plant-soil-microbe systems and in the global carbon cycle. Thus, a better understanding of the direct and indirect factors that control SOM properties, and the links between these factors and other ecological processes, is essential for ecosystem management such as mitigation against and rehabilitation of land suffering degradation. We summarize studies that have examined the influence of two of the soil formation factors (climate and parent material) on SOM and soil microbial community composition in the pristine tropical rainforests on the slope of Mount Kinabalu in northeastern Borneo. Here, an emphasis is placed on the ecological links among plants, soils, and microbes. Two sets of sites were established on contrasting parent materials (silicious metasedimentary rock and phosphorus-depleted ultrabasic igneous rock) along a strong altitudinal gradient (700 m to 2700 m), where mean annual temperatures decreased with height from 24 to 12 °C and the rainfall was roughly constant. Across these sites, we showed that the two pedogenic factors have a strong influence on (i) SOM storage in surface horizons and its partitioning between low-density fraction dominated by plant detritus and high-density fraction rich in microbially-processed OM that is strongly associated with the mineral particles, (ii) the relative importance of SOM stabilization mechanisms, and (iii) the soil microbial community structure by using phospholipid fatty acid biomarkers. As a tentative synthesis, we propose a conceptual diagram linking the direct factors controlling SOM, the plant community, and the microbial community with the two pedogenic factors. Finally, we conclude that pedogenic factors provide a useful framework for the understanding of plant–soil–microbe interactions as well as soil formation itself and for the development of better land management strategies.

This paper presents a comparison between the traditional shifting cultivation systems that have been reported in previous studies. The aim is to examine the influence of different burning practices on soil nutrient dynamics under temperate (Japan), tropical monsoon (northern Thailand) and tropical rain forest (the Malaysian state of Sarawak, Borneo) climates. The system in Japan includes cereal cropping continuously for three to four years. The amount of fuel (plant material to be burned) is estimated to be 50 t ha^-1 at most. Ignition is started from an upper slope, and the land is burned slowly and carefully downward. The system used by the Karen people of Thailand consists of a single year cropping of rice followed by 10–15 years of fallow. Burning is carried out at the end of the dry season and a fire is ignited at a lower slope. The system in Sarawak is similar to that employed in Thailand. In each of these two regions, the amount of fuel is around 100 t ha^-1 after 15 years of fallow. The increases in soil temperature from burning are in the order Thailand > Japan > Sarawak, where the differences can be attributed to soil moisture content and the direction of fire ignition. The effect of soil burning is thus more obvious in Thailand and Japan than in Sarawak. After three to four months of burning, the soil mineral N content in many regions decreases to the initial pre-burning level. A comparison between the level of nutrients in ashes and soils suggests that the addition of ash plays an important role in elevating nutrient levels in Japan and Sarawak. However, this may not be the case in Thailand. The increased nutrients return to their initial levels within one year in Sarawak and Thailand and after three or four years in Japan, which could be due to differences in the nutrient-holding abilities of the soils as well as to climatic influences. These differences in soil nutrient dynamics are considered to be reflected by the different cropping systems used in the regions.

In humid regions, soil acidification is a natural process accelerated by acidic deposition and intensive agriculture. Proton budget analysis has revealed that excess cation uptake by plants is the dominant proton source in most forest ecosystems. However, translocation of temporary acids (e.g., organic acids) and distribution of root biomass contribute to the temporal and spatial heterogeneity of proton generation and proton consumption in soils, resulting in different pedogenetic acidification. In Spodosols, protons associated with plant uptake and organic acids released from organic horizons promote acidification of surface horizons. Incipient podzolization is similarly caused by plant uptake and organic acids in highly acidic Inceptisols and Ultisols (soil pH<4.5). In less acidic Andisols and Ultisols, weathering reactions and leaching are caused by carbonic acid dissociation. In glacial till and loess soils exhibiting low acid neutralizing capacity, acidification is accelerated by acidic deposition. In tropical forest soils from highly weathered parent material, timber harvesting leads to depletion of basic cations and soil acidification. In Asia, the conversion of traditional agricultural systems to continuous cropping systems has resulted in soil acidification through nitrification of excess fertilizer nitrogen (esp., ammonium sulfate) within the last few decades. The sensitivity of soils to human impact is greater in soils exhibiting a lower acid neutralizing capacity and soil organic matter (SOM) content. The impact of agriculture on soil acidification may be ameliorated by improving nitrogen fertilization strategies and maintaining SOM levels through proton budget analysis.

Research was conducted in oil palm plantations cultivated on peatlands of West and Central Kalimantan. The research objectives were as follows: (i) to describe organic carbon (C) storage based on 2000–2009 survey data obtained from the Indonesian Center for Agricultural Land Resources Research and Development (ICALRD), Ministry of Agriculture, (ii) to assess carbon storage and CO₂ emission in and from oil palm plantations on peatlands, and (iii) to recommend mitigation options for CO₂ emission. Maps of peatlands based on satellite imagery from 1986, 2002, and 2008 were considered basic information for the following: (a) field observations, (b) peat sampling for analysis of organic C content, bulk density, and ash content for determining organic C storage, and (c) determining the location of oil palm plantations on peatlands for measurement of CO₂ flux. The organic C storage in peats of West and Central Kalimantan was calculated using the parameters of bulk density, peatland area, peat thickness, and organic C content, and was roughly estimated as 3,625 and 6,352 Mt (3.6 and 6.4 Gt), respectively, which is equivalent to 10.7 and 19.0% of total organic C storage in all Indonesian peats. Average CO₂ emission from oil palm plantations on peats varied from 28–69 t ha^-1 yr^-1, depending on the groundwater table, ash content, and understory cover crops. However, peat decomposition and peat thickness had no correlation with CO₂ emission. It was interesting that the ash content of peats >2–3% Fe₂O₃ showed a significant negative correlation with CO₂ emission: Y = –17.12 ln(X) + 58.707 (R²=0.785), where Y is CO₂ emission and X is ash content. Based on the correlation, we estimated that peats containing 6% mineral materials had a relatively low CO₂ emission of about 23 t ha^-1 yr^-1. This emission rate is roughly equal to the CO₂ fixed by oil palm trees for fresh fruit bunch (FFB) production (20–24 t ha^-1 yr^-1). In addition, some emitted CO₂ could be absorbed by understory cover crops, such as Nephrolepis ferns, which often dominate the floors of oil palm plantations. Therefore, recommended management strategies for sustainable cultivation of oil palm plantations on peatlands and for mitigating CO₂ emission are as follows: (i) maintaining a water table no deeper than 40 cm from the peat surface to achieve a balance between the reduction of CO₂ emission and water supply for plant growth, (ii) applying mineral material as an ameliorant wherever the mineral material content of a peat is <6%, and (iii) management of understory cover crops.

Samples were collected from an allophonic Andisol which had a history of the application of fertilizers for more than 60 years, and the vertical distribution and physicochemical form of fallout ¹³⁷Cs were determined. The pH values of the 1:10 water extract in the surface layer soils were 4.1, 4.7 and 6.2 for three different fertilizer treatments: chemical fertilizers only (F), F and compost (FC), and FC and lime (FCL), respectively. The concentration of ¹³⁷Cs at a soil depth of 0-25 cm showed a constant value and then sharply decreased. Below 40 cm, ¹³⁷Cs was not detected in any of the plots. The percentage distribution of ¹³⁷Cs in the exchangeable, organically bound, and strongly bound (residue) fraction was 17, 33 and 50%, respectively, and the physicochemical forms in the surface soil were not different among the three plots. This showed that the ¹³⁷Cs deposited in the agricultural fields was well mixed in the surface soil layers because of plowing, and the distribution of its physicochemical forms in the soil was similar after the long-term application of fertilizers even with soil acidification.

The Rothamsted Carbon model (RothC model) and Chikugo model were applied to 39 soils and corresponding climate data in Ukraine and Kazakhstan to compare (i) carbon input estimated using the RothC model and net primary productivity (NPP) estimated using the Chikugo model, (ii) estimated resistant plant material carbon pool (RPM) using the RothC model and measured light fraction carbon (LFC) content, and (iii) potentially mineralizable carbon (PMC), estimated using the RothC model, as well as measured PMC. Both RPM and PMC estimated using the RothC model correlated well with measured LFC and measured PMC, especially for natural vegetation sites (grassland and forest). For cropland sites, LFC and PMC did not account for the estimated RPM and PMC. These results suggest the utility of LFC and PMC as indices for monitoring carbon dynamics in natural ecosystems. Further studies are required to understand the source of readily mineralizable carbon in agro-ecosystems.

Acid sulfate soils, which have low productivity due to strong acidity, cover approximately 1.6 million ha of the Mekong Delta in Vietnam. In this paper, we show that rice yields decrease with distance from the Bassac River, which is one of branches of the Mekong River. Core boring samples were collected to determine sulfur composition of the soils with depth. The pH (H₂O) of the topsoil at a 10 km site (the closest site to the river) showed that the soil was weakly acid, whereas strong acidity (pH < 4.0) was observed in the topsoil at a 59 km site (the third nearest site to the river). The sedimentation rate at the 10 km site, estimated from the ¹⁴C age of the subsoil, was rapid compared with other sites. Alluvial deposition from the Bassac River affected the thickness of the upper horizons. Buried carbon in the subsoil accumulated at around 6.0 ky BP, when the sea level was at its highest. Pyrite, which was the dominant sulfur in the soil-sulfur compounds, was found to have accumulated in the subsoil, except at the 10 km site. Sites far from the river contained larger amounts of pyrite and this accumulated in thicker horizons. The vertical distribution and content of the pyrite is considered to reflect the supply of sulfur during the transgression and alluvium deposition that occurred with regression of the sea level. The change in rice productivity according to the distance from the river seems to be the result of the soil acidity, which is mainly produced through the oxidation process of the pyrite in the subsoil. This acidification is a symptom of land deterioration by land cover and land use changes.

Actual acid sulfate soils have emerged across large areas of the Mekong Delta due to oxidation of potential acid sulfate soils following land reclamation for mainly agricultural use. Despite adverse conditions for crop production, due to strong acidity and high salinity, suitable water management and chemical fertilizer application enables rice to be grown on agricultural land of actual acid sulfate soils. Changes in the water properties of canals in the double cropping system of a paddy field in areas of actual acid sulfate soils were monitored to evaluate the effect of water management and fertilizer application upon water quality. Canal water adjacent to the paddy field was periodically collected, and general chemical properties were analyzed. The great variation in pH (2.8–6.3) and electric conductivity (EC; 0.1–1.5 dS m^-1) in the drained water seemed to be closely associated with water management of adjacent paddy fields. Canal water collected in the growing season had a low pH and high EC, while a neutral pH and low EC were observed in the fallow season. Values of EC and pH correlated with major components of solutes such as SO₄, Ca, Mg, and Al ions. Al concentration in the acidic water was extremely high, peaking at 260 mg L^-1 in the rice crop season. Inundation and drainage for rice cropping is a major controlling factor for the release of solutes from the soil into drained water. It is important therefore to determine the time lag between initiation of soil management and the consequent changes in the surrounding water system.

In developing methods and land care system in Sahel of Sahel of West Africa, the Hausa farming practice of scattering refuse over the degraded land for improving soil productivity holds promise. When the fields become degraded, the Hausa scatter livestock manure as well as household refuse and sometimes urban refuse over their land. The organic matter improves the soil quality. The author carried out an in situ experiment, using multiple plots scattered with varying amounts of urban refuse over three years, to quantify the soil improvement effect of the refuse scattering practice, which increased termite activity in the soil as well. According to the plant growth observation, the critical amount of urban refuse was at least 20kg/m², approximately 2 cm thick on the ground, for land rehabilitation. The results revealed that the Hausa practice was able to regenerate grassland and to prevent soil erosion and exposure of the sedimentary layer. After two years from refuse input, the plant growth began to deteriorate. In order to maintain plant productivity recovered using urban refuse, it is necessary for continuous input of refuse to compensate for nutrition depletion from the plant remove and soil erosion.