In Depth - Soil Degradation in China

In-depth Analyses
Does China have a soil degradation problem?
Introduction		Tables & Charts
Soils are of vital importance to China's food production capacity. Soil is a natural resource that is not renewable in the short term once it is eroded by water or wind, physically degraded, or chemically depleted. If reclamation is possible at all, it is very costly. In the 1980s and early 1990s various publications painted a grim outlook for world soil resources. Dramatic statements were made predicting that soil erosion would undermine the future prosperity of mankind. China was among the areas where the problems were considered to be the most serious. However, the empirical basis for these predictions was rather weak. While soil-degradation problems could certainly be identified in specific regions and case-study areas, the overall agricultural impact of the problem and its specific geographical distribution on a global or even on a country basis were highly speculative.		Map 1 Map 2 Map 3 Map 4 Map 5 Map 6 Map 7 Map 8 Map 9
	The GLASOD Project Under these circumstances, UNEP commissioned a major study by the International Soil Reference and Information Centre (ISRIC) in the Netherlands. Its objective was a global assessment of the status of human-induced soil degradation (GLASOD). The project began in 1988. The results were published as a world map at an average scale of 1:10 million. There were also additional statistics on the global and continental extent of various types of degradation, including water and wind erosion, and chemical and physical degradation. The results of this study came as a shock to many experts. According to GLASOD, the soils of some 1,964 million ha worldwide were classified as degraded in one way or the other. This was more then 22% of the combined area of agricultural land, pastures, forests, and woodland. Estimates based on agricultural land alone, were, of course, even higher. In Central America, some 75% of the agricultural land was classified as more or less degraded; in Asia the estimate was about 38%. The World Resources Institute compiled many of the GLASOD results in its 1992-1993 World Resources Report.
	A number of scientists have strongly criticized the study. Crosson (1994), for instance, pointed out that, according to GLASOD, most areas in six states of the USA were classified as moderately degraded, which according to the definition should result in greatly reduced agricultural productivity. In reality, however, crop yields in those areas have been rising steadily for the past 40 years (Crosson, 1994). According to the GLASOD map for China, there is almost no cultivated land that is not classified as degraded in one way or the other. For instance, almost all the areas of rice cultivation in the South and Southeast were classified as being affected by high or very high water erosion; large wheat-growing areas southeast of Beijing were classified as suffering from medium levels of chemical deterioration (salinization). In the Northeast, the GLASOD map indicated medium, high, or even very high wind erosion. According to the GLASOD classification, a medium level of soil degradation indicates that "the terrain has greatly reduced agricultural productivity." A high degree of degradation means that "the terrain is nonreclaimable at farm level" and that "original biotic functions are largely destroyed" (Oldeman et al., 1990, p. 15). Yet, under these conditions China was able to increase its wheat production between 1961 and 1996 from about 16 to 110 million tons, its rice production from less than 40 to 130 million tons, and its maize production from under 20 to over 120 million tons. It should be mentioned that the GLASOD results were not based on quantitative data, but on "expert opinions." Also, the project's main objective was to identify the geographical distribution and the type of human-induced soil degradation, and not to assess the impact of these soil problems on agricultural productivity. It was also the first global data set on soil degradation.
	The ASSOD Project To improve on the methodology in the GLASOD approach, and to get a more detailed assessment of Asia, a second major project was started in 1993: the assessment of the status of human-induced soil degradation in South and Southeast Asia (ASSOD). This project was also commission by UNEP from ISRIC. Based on a modified and refined GLASOD methodology, the ASSOD project developed a 1:5 million map of soil degradation in South and Southeast Asia. The map was based on the Soils and Terrain Digital Database, SOTER (see van Lynden and Oldeman, 1997). The information in the ASSOD project was compiled in the form of spatial databases linked to digital GIS maps, which allows detailed statistical analysis. Six major categories were used to classify soil degradation: 1. Types of soil degradation (such as water erosion, chemical deterioration, etc.) 2. Degree of degradation, expressed as the impact on productivity (five classes from negligible to extreme) 3. Extent of soil degradation (as a percentage of the mapping area affected) 4. Rate of soil degradation [from +3 (rapidly increasing) to -3 (rapidly decreasing)] 5. Causative factors (five classes, such as improper management of cultivated land, overgrazing, etc.) 6. Rehabilitation or protective measures (four classes, such as plant management, land management, etc.) The most obvious difference between the GLASOD and ASSOD maps on China is the much greater detail of the ASSOD results. A further improvement was the better differentiation of degradation types. For instance, whereas for South and Southeast Asia GLASOD had indicated that 73% of the degradation was due to water erosion, 20% due to wind, and 7% due to chemical deterioration, ASSOD found only 47% water erosion, but 20% wind erosion, 24% chemical deterioration, and 9% physical deterioration. The most important difference between the two studies, however, was a more realistic assessment of the severity of degradation in ASSOD. While the ASSOD results indicated an even larger area of soil degradation than the GLASOD estimates, the degree of degradation was estimated to be much lower. For instance, whereas for the whole study region of South and Southeast Asia, GLASOD estimated 36.2 million ha of strong and extreme water erosion, the ASSOD project found only 15.9 million ha. On the other hand, GLASOD reported only 62.8 million ha of light water erosion, whereas ASSOD found 175.8 million ha of light water erosion and another 58.7 million ha of negligible water erosion. There were, however, a few exceptions to this trend: GLASOD found much more "terrain deformation," which was defined as an "irregular displacement of soil material by wind action."
Both the GLASOD and ASSOD assessments paint a serious picture of soil degradation. The ASSOD estimates, however, seem to be more realistic in that the degree of degradation is more closely related to its impact on soil productivity. In other words, the estimates of ASSOD take into account that a given amount of soil erosion is a more serious problem on poor, shallow soil than on deep, fertile soil. In the case of China, this leads to a great reduction in extent of strong or extreme soil degradation, a result we believe is more compatible with the history of spectacular crop production increases in China.
ASSOD Estimates of Soil Degradation in China
IIASA's LUC Project analyzed the ASSOD database specifically for China. While the ASSOD assessment is certainly more detailed and seems to be more realistic than the GLASOD maps, we still find that more than 466 million ha (or 50% of the land) in China are affected by one type of soil degradation or another (see Table 1). This is certainly a dramatic figure. However, on closer inspection, we find that the situation is much less serious than it seems at first glance. First, the various degradation types can overlay each other in one mapping unit. The actual size of affected land is therefore certainly smaller than the aggregated total in Table 1. Second, we have to take into account the degree of degradation. According to the ASSOD assessment, there are only two types of soil degradation in China that have an extreme impact on soil productivity: (a) loss of topsoil due to water erosion on some 200,000 ha and (b) loss of topsoil due to wind erosion on between 10,000 and100,000 ha. In other words, extreme soil degradation affects just 5.9% of the total land area of China. On the other hand, 306.9 million ha (or almost 66%) of the degraded land, have only a negligible or light degree of degradation. Third, we must also take into account that the ASSOD estimates are based on total land area; it is not known (at least not from the ASSOD assessment) how much of the degraded land is in cultivated areas. For instance, according to ASSOD estimates, water erosion affects 180 million ha in China to some extent. This is certainly a staggering figure compared with China's total cultivated land, which we have estimated at 132 million ha. However, the number also shows that much of the water erosion in China must be occurring outside cultivated areas. Unfortunately, the ASSOD database does not include information on actual land use. The degradation information is based on SOTER polygons, which include all land areas. We can be quite sure that significantly less then 5% of the cultivated land in China is affected by extreme forms of soil degradation. On the other hand, there can be no doubt that, according to ASSOD, soil degradation is a serious problem in China: 73 million ha of the total land area are affected by moderate degradation and 86 million ha by strong soil degradation. If only 50% of this land is in cultivated areas (which is probably a low estimate), then some 80 million ha (or over 60%) of the cultivated land are affected by some kind of moderate or strong soil degradation.		Table 1
Our Evaluation of the Situation
There is little doubt that China is facing serious soil degradation problems. From studies on specific areas (such as on the Loess Plateau, or on the desertification in Northern China), as well as from global or regional assessments (GLASOD, ASSOD), we have evidence of significant water and wind erosion and chemical deterioration. However, the situation should not be over-dramatized. There are at least three factors that must be taken into account for a balanced assessment of the situation:
	All national or regional assessments of soil degradation in China are based on expert opinion. Quantitative data are available for small local areas and experimental sites. It is extremely difficult to generalize information from specific problem areas in order to characterize the situation in a much larger region. We have to expect that national estimates on soil degradation in China in particular are highly uncertain.
	It is also not clear to what extent the degradation actually affects agriculture. As we have emphasized, the ASSOD estimates are based on total land area. Clearly, additional research is needed to estimate the extent of degradation in cultivated areas.
	We also believe that the methodology for assessing the degree of degradation in the ASSOD assessment is extremely difficult and open to subjective interpretation. In the ASSOD project, local experts assessed the relative impact of a given amount of a certain type of degradation on the productivity of the soil. This approach should address the problem that the impact of degradation is quite different for different soils. For instance, the productivity impact of water erosion depends very much on the thickness of the topsoil layer. When a layer of a few centimeters is washed away from a deep fertile soil (such as in China's Loess Plateau), the impact on productivity is certainly lower than when the same amount of water erosion occors on a poor, shallow soil. This partly explains why China's farmers have been able to cultivate the soils of the Loess Plateau for thousands of years, despite ongoing erosion. In the ASSOD project, experts estimated the extent to which the soil productivity of a particular area was reduced. Obviously, this is an extremely difficult, error-prone task.
	Finally, soil degradation has to be related to the protection potential and to possible rehabilitation measures. These measures greatly depend on (future) technology, the amount of investment capital, the know-how of farmers, and the efficiency of political and administrative measures.
In conclusion, various types of soil degradation are certainly causing serious problems in China. The ongoing measures to stop or diminish loss of topsoil due to water erosion must therefore be intensified, not only to prevent productivity loss in agriculture, but also to minimize hydrological problems in areas downstream. These include, for instance, the serious problems of sedimentation in the middle and lower reaches of the Yellow River, where the rising ground level increases the danger of flooding due to broken dykes. There is also the problem of eutrification, which threatens the ecological balance of rivers and lakes. Wind erosion is a major problem in the arid regions of Northern and Central China, and further reforestation is necessary. This degradation problem primarily affects marginal agricultural areas and grasslands, which currently contribute little to China's overall food supply. The core agricultural regions that produce the bulk of China's crops are largely unaffected by this problem. However, if China wants to increase grassland productivity for livestock, wind erosion problems are of critical relevance. Chemical and physical forms of degradation (such as aridification, sealing, or crusting), on the other hand, are more serious in the major crop areas (see Map 9). However, various forms of vegetation conservation methods and adequate land management practices are available. These practices can obviously reduce the degradation impact considerably, otherwise it would be difficult to explain how China has been able to increase crop production over the past few decades.

Revision 2.0 (First revision published in 1999) - Copyright � 2011 by Gerhard K. Heilig. All rights reserved. (First revision: Copyright � 1999 by IIASA.)