3.1. Density and storage in SOC for the 1980s
Our results indicate that SOC density varies from 2 kg C m−2 in grey–brown desert soils to 45 kg C m−2 in Brown coniferous forest soils (Table 1). The SOC density in forest soils ranges from 10 kg m−2 (with 95% CI: 8, 12) in grey forest soils to 45 kg m−2 (with 95% CI: 38, 52) in brown coniferous forest soils. For mean forest SOC density, our result appears to have larger variation than those reported by previous estimates, ranging from 11 kg m−2 (Wang, 1999) to 19 kg m−2 (Zhou, 1998). This difference is in part because we took into account widely spatial heterogeneity in soil properties across the nation. The large variability in the estimations of SOC density might also result from different data sources and calculation methods.
Table 1. The comparison of SOC density of main soil suborders in the second soil survey
| ||Bulk||Soil carbon density (kg C m−2)|
| ||Samples||Organic||Depth||density|| |
|Soil type||(N)||(%)||(cm)||(g cm−3)||Mean||95% CI|
|Latosolic red earths||30||1.4||112||1.35||12.28||10.97||13.59|
|Dark brown earths||65||3.18||85||1.13||17.55||16.23||18.87|
|Brown coniferous forest soils||9||7.54||79||1.3||44.61||37.56||51.66|
|Dark brown earths||65||3.18||85||1.13||17.55||16.23||18.87|
|Grey forest soils||8||2.3||58||1.28||10.25||8.09||12.41|
|Brown caliche soils||19||0.64||95||1.4||4.86||3.33||6.39|
|Grey desert soils||11||0.6||86||1.25||3.73||0.09||7.37|
|Grey–brown desert soils||10||0.37||84||1.25||2.23||0.12||4.34|
|Frigid calcic soils||8||1.17||87||1.25||7.38||6.82||7.94|
|Cold brown calcic soils||4||5.43||91||1.2||34.39||31.8||36.98|
|Dark felty soils||69||4.03||73||1.2||20.48||18.32||22.64|
|Cold calcic soils||25||1.59||88||1.25||10.14||8.66||11.62|
Both climate and human activities have influenced SOC density across China. SOC density in Alpine soils, mostly located in northeast and southeast of the Tibet altiplano, is also high. In the northeast and southeast Tibet plateau, low temperature and high soil moisture lead to a low rate of decomposition and a relatively high content of soil organic matter. In the sparsely populated western region (e.g. the Qingzang plateau) the natural vegetation is still intact or only slightly impacted (Li and Zhao, 2001). The high productivity of the turf vegetation contributes to the development of a thick A horizon and high SOC. In densely populated east, on the other hand, the land cover is highly fragmented, showing the mosaic of crops, secondary vegetation and natural vegetation (Li and Zhao, 2001). The SOC density in the Loess Plateau and Huanghuaihai Plain, for example, was low, since there is a long history of cultivation. Extensive agricultural land use in the past and the continuing conversion of other land cover to agriculture has strongly disturbed a large part of the native vegetation and reduced the SOC.
To further understand the effect of land use change on SOC storage, we estimated the SOC density under different land use types according to vegetation description of soil profiles in the 1980s. As there is a clear difference in climate, geography, land use and social economy, we divided the whole nation into six large sub-regions: Northeast, North, Northwest, East, South and Southwest (Fig. 2). Some islands were excluded because of their small areas. Based on a vegetation map (1:4 000 000), (Hou, 1982), we classified land use of the whole region into eight categories: dryland, paddy land, forest and woodland, shrubs, steppe and grassland, meadow, wetland and desert. The results of the average SOC content and 95% confidence interval (CI) of the land use types that occur in China are listed in Table 2, and the spatial distribution of SOC density in six sub-regions is shown in Fig. 2.
Figure 2. SOC density for major land use types in the six sub-regions of China. Numbers 1–8 represent the desert, dryland, steppe and grassland, shrubs, meadow, forest and woodland, wetland and paddy land, respectively.
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Table 2. SOC density under land use types of large regions in China
| ||Soil carbon density (kg C m−2)|
| || ||Samples||Depth||Organic||Bulk density|| |
| ||Land use type||(n)||(cm)||(%)||g cm−3||Mean||95% CI|
| ||Steppe and grassland||32||97||1.32||1.37||6.68||2.4||10.96|
| ||Forest and woodland||55||85||2.16||1.33||4.32||3.25||5.39|
| ||Paddy land||20||84||1.4||1.32||2.75||2.14||3.36|
| ||Steppe and grassland||43||103||1.39||1.32||4.5||3.26||5.74|
| ||Forest and woodland||19||100||2.54||1.35||5.81||3.58||8.04|
| ||Paddy land||NA||NA||NA||NA||NA||NA||NA|
| ||Steppe and grassland||41||107||1.56||1.3||14.68||8.25||21.11|
| ||Forest and woodland||31||95||2.66||1.29||19.04||13.85||24.23|
| ||Paddy land||17||128||1.34||1.32||13.4||10.03||16.77|
| ||Steppe and grassland||31||81||1.57||1.38||3.65||2.22||5.08|
| ||Forest and woodland||68||90||1.56||1.33||3.63||3.13||4.13|
| ||Paddy land||213||91||1.42||1.33||3.42||3.02||3.82|
| ||Steppe and grassland||19||118||1.19||1.32||3.12||2.32||3.92|
| ||Forest and woodland||67||98||1.57||1.31||3.95||3.46||4.44|
| ||Paddy land||163||92||1.61||1.29||4.9||4.15||5.65|
| ||Steppe and grassland||24||88||1.08||1.31||2.29||1.74||2.84|
| ||Forest and woodland||51||84||4.02||1.24||7.77||5.81||9.73|
| ||Paddy land||96||81||2.12||1.29||4.6||3.88||5.32|
Our results show that there is a large amount of variability in SOC density among different land uses in the 1980s (Table 2). The SOC density in the wetlands of Southwest China was the highest (45 kg m−2 with 95% CI: 10, 80), followed by meadow soils in the South (26 kg m−2, with 95% CI: 9, 43), forest and woodlands in the Northwest (19 kg m−2, with 95% CI: 14, 24), steppe and grassland in the West (15 kg m−2, with 95% CI: 8, 21), shrubs in the Northwest (12 kg m−2, with 95% CI: 7, 18), paddy lands in the Northwest (13 kg m−2, with 95% CI: 10, 17), and drylands in the Northwest (11 kg m−2, with 95% CI: 10, 12). The desert soils of the Western region ranked the lowest (1 kg m−2, with 95% CI: 0.6, 2).
Wetland had the greatest variability among regions (Northeast, Northwest and Southwest China) as indicated with a range of standard deviation (s.d.) from 33 to 39.There are broad ranges and heterogeneity of types of soil within land use types. This also indicates that the land use types are poor predictors of the amount of SOC content because of the great soil heterogeneity (Kern, 1994).
Of the various patterns of land use in all of China, the SOC density was generally higher in the west than in the east, and there were large variations in the area of Chinese soils under various land uses (Table 2). All types of land use showed considerable variation, but generally the tendency had higher values for wetland, meadow and forest soils. Deserts tended to have low SOC content. The soil units with the lowest SOC content were arid soils (Tables 1 and 2). Soil units from dry climates in western regions tended to have a low SOC content, except for soil units with a mesic climate in southern and eastern regions. The spatial patterns of SOC, as characterized by the land use in Fig. 2 and Table 2, displayed the greatest SOC content in areas of extensive alpine wetland, meadow, mountain forests and poorly drained soils, such as those found in the eastern regions of the Tibet Plateau and Northeast China (Tables 1 and 2). The eastern portion of East and South China, with its extensive cultivation, had a relatively low SOC content. The northern Northeast China had relatively high amounts of SOC, probably because of high amounts of precipitation and cool temperatures (Wang et al., 2002). SOC can be characterized by ecosystem zones on very broad scales. This approach, however, ignores local variations in parent materials (organic materials, coarse fragment content and mineralogy) and soil depth (Kern, 1994; Li and Zhao, 2001). There was a great deal of heterogeneity of soil within land uses, which made this method of data aggregation of limited use.
For the whole of China, our results suggested that, for the 1980s, the average SOC density in China was 10.53 kg Cm−2 (with 95% CI: 10.2, 10.86) and that SOC storage for the nation was 92 Pg C (with 95% CI: 89, 95) for a total soil area of 877.63 × 106 ha2. Estimates of the global SOC storage vary from 1200 to 1600 Pg C (Prentice and Fung, 1990; Sombroke et al., 1993; Post et al., 1982, 1990; Foley, 1995; King et al., 1995; Batjes, 1996). Our analysis indicated that SOC storage in China is about 6–8% of the global soil organic carbon pool, noting that the area of land in China is only 6.4% of the global land area (Fang et al., 1996).
3.2. Historical changes in SOC storage during 1960s–1980s
Our results indicate that SOC storage in the 1960s was 93 Pg C (with 95% CI: 73, 113) for the contiguous China, and that average SOC density was 10.61 kgCm−2 (with 95% CI: 6.21, 15.01) (Fig. 3). Compared to the total SOC storage of 92 Pg (with 95% CI: 89, 95) in the 1980s, our analysis suggests that, during the 1960s–1980s, SOC storage in China decreased by about 1 Pg C (Fig. 3). We used the paired t-test to determine if the difference in SOC storage between first and second soil surveys can be distinguished from zero. The results from the t-test show that there is no significant difference in SOC storage between two soil surveys (with 95% CI).
Figure 3. Differences in bulk density (a), organic (b), carbon density (c) and carbon storage (d) between the first national soil survey in 1960s and the second national soil survey in 1980s. (Histograms and error bar show the mean and 95% confidence interval.)
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Although there is no significant decrease in SOC for the nation during the 1960s–1980s, regional variations in the change of SOC storage still exist. The largest change in SOC storage occurred in Northeast China and the Tibet Plateau. In Northeast China, SOC density in the eastern wetland region decreased from 40–60 to 25–40 kg m−2 during the period from the 1960s to the 1980s. Because primary forests have been protected in the eastern and northern mountains since the 1970s, the SOC content of mountain forests increased by about 5–10 kg m−2. During the 1960s–1970s, many people immigrated into Northeast China for agricultural development forced by the Cultural Revolution. The SOC density of the Songnen Plain decreased from 16–25 to 10–16 kg m−2, most of which occurred in the black soils region. In the western grassland region, the SOC density of grassland decreased by 3–10 kg m−2. In the central plain region, wetland and grassland were converted to farmland and urban developments. These human disturbances reduced the soil fertility. In the southeast region of the Tibet Plateau, the SOC density of alpine meadow decreased by 10–20 kg m−2. Following changes in agrarian reform policies in 1959, Tibet entered a new era of rapid agricultural development. Alpine steppe and meadow were converted to farmland in the southeast region. As a result, the SOC content of the southeast region was reduced.
In the northern and southern regions, the SOC density was reduced by 0–3 kg m−2 due to the expansion of agricultural land. In the grassland region of Inner Mongolia, however, the SOC density was reduced largely from 2.15–5 to 0–2.15 kg m−2due to desert expansion and soil degradation. In the large areas of semi-arid and arid regions of West and Northwest China, SOC storage was reduced by about 0–3 kg m−2, in part because the climate was drier and vegetation could not survive. In the mountain region, the SOC density of the steppe decreased because of the development of animal husbandry.
Our estimates of soil carbon storage and its change are comparable to other studies. Houghton (1995; 1999) estimated that, due to deforestation and other land-use changes, the total net flux of carbon to the atmosphere from terrestrial ecosystems in China was about 9.0 Pg C for the time period from 1850 to 1980 (Houghton, 1995) and 9.4 Pg C from 1850 to 1990 (Houghton, 1999), and that the annual flux was about 0.07 Pg C from 1950 to 1980, including the loss of vegetation and soil carbon. From 1900 to 1994, Tian et al. (1999) estimated that the natural ecosystems in the coterminous US lost a total of 4.8 Pg SOC due to cropland expansion and urbanization, about 0.051 PgC yr−1. The land areas of China and the US are almost identical, indicating that the loss of the Chinese SOC pool was small relative to the US and global SOC loss for the study period.
Much uncertainty still exists in the assessment of SOC storage. Uncertainty arises from a variety of sources, including different methods, unreliable data, and missing and incomplete data (Lal et al., 1995b). Moreover, the heterogeneity of SOC concentration and its dynamic nature make it unfeasible to obtain estimates of SOC changes on annual or finer time scales from direct measurement (Post et al., 1998). For example, our study was based on soil sample data derived from different research projects. These projects adopted different criteria for soil classification, mapping scales and degree of representatives of soil profiles between two national soil surveys. The lack of standardized sampling methods is also a source of sampling error. Furthermore, the lack of sufficient data on bulk density of soil horizons, climate and land-use/land-cover, insufficient depth of soil profiles (<1 m) and the limited number of soil pedons on natural soils (as compared to cultivated soils) also limits the accuracy of the results (Eswaran et al., 1993; Kern, 1994; Li and Zhao, 2001). In addition, although large numbers of soil profile data in the 1980s can make a reliable estimate for contemporary SOC storage, the small numbers of soil profiles in the 1960s can bias our estimate of SOC for that time period and hence the change in SOC during the 1960s– 1980s.
Some classes in soil taxonomy have been changed in China since 1978. The 1978 version of soil taxonomy was used in this study for the first national soil survey and the 1994 version of soil taxonomy was used for the second national soil survey (National Soil Survey Office, 1998). The integration of soil and vegetation maps and soil survey data provides a basis to estimate regional SOC storage and distribution (Kern, 1994; Li and Zhao, 2001). A better framework and a good compromise for the level of detail, because of aggregation at a more specific level such as subgroup or family, would require extremely large pedon databases (Kern, 1994). In this study, based on the simple classification of land use for soil profiles, we analyzed the SOC spatial characteristics in six sub-regions under different land uses. However, the method could produce a relatively large variation similar to the ecosystem complexes method documented by Kern (1994).
Traditionally, we estimate soil organic carbon reservoir based on organic content to a depth of 1 m (Post et al., 1982; Foley, 1995; Lal, 1999). The subsoil below 1 m has a lot of organic and inorganic carbon, especially in tropical and subtropical soils (Sombroke et al., 1993). Estimating SOC for the entire soil profile provides a more accurate estimate of nation-wide SOC than extrapolating SOC from a 1 m sampling profile. How to extrapolate site data to regions is the critical question in the accurate estimation of soil carbon. There is an urgent need to develop robust, science-based, flexible and practical protocols for monitoring and verifying temporal changes in soil carbon (Post et al., 1998). It is obvious that there is still a need to further develop methodology to derive more accurate estimates of soil carbon. Nevertheless, this study has presented a more accurate estimate of SOC by considering the variations of different soil types, soil depth, soil horizons and their corresponding soil organic matter content, which are based on a large number of sampled soil profiles across the country. Establishing a global database on soil profile samples would certainly help to produce more realistic and representative results in soil carbon studies (Eswaran et al., 1993; Kern 1994; Post and Kwon, 2000; Li and Zhao, 2001).