total carbon nitrogen in the soils of the world carbon and nitrogen in the soils of the world 153...

European Journal of Soil Science, June 1996, 47, 15 1 - 163

Total carbon and nitrogen in the soils of the world

N.H. BATJES International Soil Reference and Information Centre (ISRIC), PO Box 353, 6700 AJ Wageningen, The Netherlands

Summary

The soil is important in sequestering atmospheric C02 and in emitting trace gases (e.g. C02, C€L, and N20) that are radiatively active and enhance the ‘greenhouse’ effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases.

A discrepancy of approximately 350 x loi5 g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo-referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a %” latitude by W longitude version of the corrected and digitized 1 : 5 M FAO-UNESCO Soil Map of the World.

Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157-2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684-724 Pg of C in the upper 30 cm, 1462-1548 Pg of C in the upper 100 cm, and 2376-2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695-748 Pg of carbonate-C is held in the upper 100 cm of the world’s soils. Mean C : N ratios of soil organic matter range from 9.9 for arid Yennosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133-140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric COz and the predicted associated rise in temperature are discussed.

Introduction Soil organic matter is a key component of any terrestrial ecosystem, and any variation in its abundance and composition has important effects on many of the processes that occur within the system. Nonetheless, the size and dynamics of the carbon and nitrogen pools in the soils of the world are still poorly known (IPCC, 1990; Legros et al., 1994). Three main reservoirs regulate the carbon cycle on earth (IPCC, 1990): the Oceans ~ 3 9 0 0 0 x lOI5 g (or Pg) of C; the atmosphere ( m 750 Pg C), and terrestrial systems ( x 2200 Pg C). Although the soil-vegetation carbon pool is small compared with that of the oceans, potentially it is much more labile in the short term. The carbon balance of terrestrial ecosystems can be changed markedly by the direct impact of human activities-including deforestation, biomass burning, land use change, and environmental pollution-which release trace gases that enhance the ‘greenhouse effect’ (Bolin, 1981; Trabalka & Reichle, 1986; IPCC, 1990).

Received 10 April 1995; revised version accepted 12 December 1995

Organic matter amounts in the soil are regulated essentially by net primary production, the distribution of photosynthates into ‘roots’ and ‘shoots’, and the rate at which these various organic compounds decompose. Plant residues (litter) that fall on the soil are gradually altered through physical fragmenta- tion, faunal and microfloral interactions, mineralization and humus formation. Litter is not included in the calculation of soil organic carbon mass (Buringh, 1984; Kimble et al., 1990; Sombroek et al., 1993), although amounts of carbon stored in the litter layers of many virgin and forested soils can be considerable.

The soil is the largest terrestrial pool of organic carbon, with global estimates ranging from 1115 to 2200 Pg of C (see Batjes, 1992), 1576 Pg of C (Eswaran et al., 1995), and 1220 Pg of C (Sombroek el al., 1993), respectively. Most methods for determining soil organic carbon do not account for resistant forms such as charcoal (Skjemstad et al., 1990; Sanford et al., 1985); thus, it remains difficult to quantify this source of organic carbon in global budgets. Reserves of inorganic carbon (as carbonate) stored in soils have been estimated to be 780-930 Pg of C by Schlesinger (1982), and

0 1996 Blackwell Science Ltd. 151

152 N.H. Barjes

720 Pg of C by Sombroek et al. (1993), which can be released by weathering.

Global calculations of the pool of carbon and nitrogen in the soil are complicated by a number of factors, notably: (1) the still limited knowledge of the extent of different kinds of soil; (b) the limited availability of reliable, complete and uniform data for these soils; (c) the considerable spatial variation in carbon and nitrogen content, stoniness and bulk density of soils that have been classified similarly; and, (d) the confound- ing effects of climate, relief, parent material, vegetation and land use.

The discrepancy of approximately 350 Pg of C in global amounts of soil organic carbon between the estimates of Sombroek et al. (1993) and Eswaran et al. (1993) is addressed here. New estimates of the soil carbonate carbon and nitrogen pools are also presented, and possible changes in soil C and N dynamics induced by enhanced atmospheric COz concentrations and predicted climatic change are discussed. The study is based on the global soil database, World Inventory of Soil Emission Potentials (WISE), developed at the International Soil Reference and Information Centre (Batjes & Bridges, 1994).

Materials and methods

Source of soil data

The most appropriate way to study the organic and carbonate carbon contents of soil is on a unit area base, for a specified depth interval, which requires information of the spatial distribution of different types of soil, soil carbon, bulk density, and stoniness with depth. The relevant data were derived from the WISE database which consists of two main components (Fig. 1) . The spatial component is a %" by %" version of the edited and corrected 1 : 5 M Soil Map of the World (FAO, 1991). For each grid cell the typology and relative extent of the

soil units are known. Thus, the FAO-UNESCO (1974) legend can be used to aggregate available soil profile data and to link derived interpretations of soil properties with the soil units on the grid map. The profile or attribute data component of the WISE database currently holds 4353 profile descriptions, corresponding to 19 222 horizons, of which 86% have data for organic carbon, 45% for nitrogen, 32% for carbonate, 3 1 % for bulk density, and 21% for the amount of fragments > 2 mm. The geographical distribution of the profiles is: Africa (1799); South West and North Asia (522); South East Asia (553); Australia and the Pacific Islands (122); Europe (492); North America (266); and, South America and the Caribbean (599).

Most profiles in the WISE database came from five main sources: (a) ISRIC's Soil Information System, ISIS (Van de Ven & Tempel, 1994); (b) the Soil Database System (SDB) of the Food and Agricultural Organization (FAO, 1989); (c) digital soil data compiled by the National Soil Conservation Service of the United States of America (NRCS); (d) profiles obtained by WISE project staff from national soil survey organizations which supplied descriptions and analyses of profiles which were representative of the units of the World Soil Map present in their individual countries; and (e) suitable profiles gathered from ISRIC's library collection. Special attention was given to the way that the original profile data were collected and also to the way in which this information was compiled from records for WISE. The original laboratory methods were recorded in WISE.

The profiles held in ISIS were compiled specifically to represent the map units of the Soil Map of the World, with special emphasis on the Subtropics and Tropics in view of ISRIC's international mandate. They have all been described using the Guidelines for Soil Description (FAO-ISRIC, 1990) and analysed by a common methodology in the ISRIC laboratory (Van Reeuwijk, 1992). The profiles in the NRCS set originate from the USA and 41 other countries of the world.

................................................................................................................................................................................ WISE SOIL DATABASE AUXILIARY SYSTEMS Fig. 1 Schematic representation of the WISE

database.

0 1996 Blackwell Science Ltd, European Journal of Soil Science, 47, 151-163

Total carbon and nitrogen in the soils of the world 153

They are described according to the Soil Survey Manual (USDA, 1993), and were analysed at the Lincoln laboratory (USDA, 1984). These analytical methods compare well with those used at ISRIC (Kimble and Van Reeuwijk, personal communication). Profiles from the SDB database (FAO, 1989) were described using the same guidelines as ISRIC, but the chemical and physical analyses were done in different labora- tories as was the case for profiles obtained directly from various national soil survey organizations. Therefore, Vogel (1994) camed out a comparison of the different soil analytical met hods.

As many countries could not respond to the WISE request for profiles, more use had to be made of soil profile information in ISRIC’s library. It contains many soil monographs from many parts of the world that were commissioned for different reasons. Although the associated profile descriptions vary in completeness and quality, it was possible to extract about 1900 profiles from this source. In all cases the source of the data has been stored in WISE, and the inferred quality of the profile data was coded.

Analytical methods

Soil carbon and nitrogen. Sources of uncertainty in soil carbon and nitrogen data, related to differences in sampling methodology, season of sampling, land use history, and laboratory methods, are well documented. Organic carbon is univer- sally determined by oxidation to C02, and is directly measured as CO2 or by weight loss of the sample or by back-titration of the excess of the added oxidant. Carbon values determined by dry and wet combustion are comparable because they all recover 100% of the organic C (Vogel, 1994). Indirect combustion methods, such as that of Kurmies and Tinsley, can recover most of the organic C, and the results are comparable with those obtained by direct methods. The Walkley-Black method, however, gives variable recovery of soil organic C. Nonetheless, standard conversion factors of 1.33 for incom- plete oxidation and of 58% for ‘the carbon : organic matter’ ratio are commonly used to convert Walkley-Black carbon to the total organic-C content, even though the true factors vary greatly between and within soils because of differences in the nature of organic matter with soil depth and vegetation type (Grewal et al., 1991). Therefore, the Walkley-Black method -which was used for 85% of the profiles in WISE-gives only an approximation of soil organic carbon content. This is a common problem in global studies of soil carbon stocks (Schlesinger, 1977).

Both volumetric and gravimetric methods have been used to determine the soil carbonate values in the WISE database. Their results are considered to be comparable, particularly for the larger carbonate contents (Van Reeuwijk, personal communication).

Soil nitrogen content of about 93% of the profiles in WISE was determined by the Kjeldahl method. This method

determines all soil nitrogen, including absorbed NH;, except that in nitrates.

Bulk density. Bulk density is critical for converting organic carbon percentage by weight to content by volume (e.g. kg m-2 to 1-m depth), but it varies with the structural condition of the soil, in particular the mineralogy, water content and packing. In general, bulk density determined by the core sampling method is comparable with values obtained by the clod method (Vogel, 1994). However, bulk density measurements for gravelly to extremely gravelly soils are difficult to compare because results vary significantly with sample volume (Vincent & Chadwick, 1994).

Bulk density is not determined in most routine analyses, so values have to be determined using pedotransfer functions or rules (Bouma & Van Lanen, 1987). Linear regressions of bulk density against combinations of the controlling variables described earlier often give rather small coefficients for linear determination (?) (e.g. Post et al., 1982), which restricts their predictive use. An alternative is to use pedotransfer rules based on expert judgment. Firstly, the mean bulk density was computed for each profile; this mean was then used for all horizons with missing bulk density data. All substituted values were flagged in the ‘derived’ database to differentiate them from the measured data. Secondly, the mean bulk density was calculated for each FAO-UNESCO soil subunit. Finally, if there were no bulk density data for a particular soil profile, the mean bulk density for the corresponding subunit was used in the ‘derived’ database.

Computation of soil carbon and nitrogen stocks

General procedure. There are three stages in determining the size of the organic carbon pool. First, the amount of organic carbon is determined for individual soil profiles. Second, this information is analysed on the basis of the FAO-UNESCO (1974) soil subunit. Finally, the results of these analyses are combined with information on the spatial extent of the various soil subunits in the world to estimate the global size of the organic carbon pool. A similar approach was also used to calculate the size of the soil carbonate carbon and soil nitrogen pools.

Calculation for individual projles. with k layers, the total organic carbon by volume is:

For an individual profile

k

where Td is the total amount of organic carbon (in Mg m-2) over depth, d , pi is the bulk density (Mg m-3) of layer i , Pi is the proportion of organic carbon (g C g-l) in layer i , Di is the thickness of this layer (m), and Si is the volume of the fraction of fragments > 2 mm. In the absence of measured data, Si was approximated by using the median concentration

0 1996 Blackwell Science Ltd, European Journal ofSoil Science, 47, 151-163

154 N.H. Batjes

of fragments > 2 mm of the particular FAO-UNESCO soil subunit.

Calculations per FAO- UNESCO soil subunit. Total organic carbon mass, M, for the land area of the world was determined by :

n

j = I

where k f d is the total mass of organic carbon (Pg C) held in the upper d cm of the soil, n is the total number of %" by %" grid cells (n = 259 200), A , is the area of soil unit i in grid cell j , Tijd is the mean organic carbon content of individual soil subunits i of grid cell j over the depth interval d. It is assumed that each soil profile in WlSE represents the corresponding FAO-UNESCO subunit uniformly.

In computing the world soil carbon and nitrogen pools it was also important to include the miscellaneous units considered on Soil Map of the World (FAO, 1991). To this end expert estimates of organic carbon, carbonate carbon and soil nitrogen content were assigned to the 'salt flats', areas of 'rock debris and desert detritus', 'dune sands and shifting sands' and areas termed 'not determined' in the 'derived' data base (by depth zone).

Results Soil Map of the World consists of 26 major soil units, differ- entiated at the highest level on the basis of effects of different soil forming processes, insofar as these are reflected in observable and measurable properties. Although the original analyses in this study were carried out for the 106 subunits (see Table 2), the results presented are mainly for the soil unit level. This has been done largely to enhance the legibility of the tabular output.

Ranges observed per FAO-UNESCO unit

Bulk density. Table 1 lists the means and extremes of bulk density. For mineral soils, except those formed on volcanic ash (Andosols), the mean ranges from 1.26 Mg m-3 for Ferralsols to 1.67 Mg m-3 for Vertisols. Andosols have a mean bulk density of 0.73 Mg m-3. The variation of measured bulk density for the mineral soil units is moderate, with coefficients of variation of 6% to 28%. Histosols have a mean bulk density of 0.31 Mg m-3 with a large coefficient of variation (80%), which is attributed to differences in fibre content and degree of humification of the peat soils considered. In other global studies a mean bulk density of 0.15 to 0.25 Mg m-3 has been used for organic soil (Buringh, 1984; Sombroek et al., 1993).

Soil carbon content. In most studies, soil organic carbon content has been calculated to a depth of 100 cm, except for the shallow Lithosols, Rankers and Rendzinas, with special reference to the upper 50 cm. The latter encompasses the

Table 1 Mean and range in bulk density by FAO-UNESCO soil unita (Mg m-3)

Soil unita N Min. Max. Mean CVb

Acrisols Cambisols Chernozems Podzoluvisols Rendzinas Ferralsols Gleysols Phaeozems Lithosols Fluvisols Kastanozems Luvisols Greyzems Nitosols Histosols Podzols Arenosols Regosols Solonetz Andosols Rankers Vertisols Planosols Xerosols Yermosols Solonchaks

990 1219

120 48 34

724 486 500

1 496

48 1030

5 219 106 20 1 103 167 173 353

2 636

67 253 I 75 150

0.54 0.53 1.07 1.15 0.53 0.69 0.54 0.80 1.42 0.55 1.02 0.60 1.34 0.58 0.03 0.52 1.22 0.91 0.95 0.28 1.26 0.90 0.60 0.90 0.5 1 0.63

I .98 2.47 2.34 1.88 1.94 2.00 2.17 2.31 1.42 2.02 2.25 2.45 1.58 1.85 0.94 2.16 1.90 2.13 2.01 0.99 1.70 2.5 1 2.00 2.02 2.27 2.01

1.41 1.36 1.45 1.65 1.34 1.26 1.38 I .46 1.42 1.40 1.55 1.54 1 S O 1.43 0.31 1.32 1.61 1.51 1.64 0.73 1.48 I .67 1.57 1.49 1.51 1.48

15 20 17 9

28 18 24 16

17 21 14 6

17 80 24

8 14 12 22 15 16 16 12 14 19

-

asoil units are listed in alphabetical order, based on the first letter of the FAO-UNESCO (1974) legend code (e.g. A, Acrisols; B, Cambisols, and so on). bCV is the coefficient of variation (%).

depths that are most directly involved in interactions with the atmosphere, and that are most sensitive to land use and environmental changes (see reviews by Bouwman, 1990; Batjes, 1992). In this study, depth intervals of 0-30 and 0-50 cm are used to enable a comparison with the results of other studies. Generally, fewer samples were taken from the deeper layers than from the superficial layers. This difference in the number of samples at the various depths must be kept in mind when interpreting Table 2, as it implies that the results are less reliable for the deeper layers.

Large amounts of soil organic carbon lie below 100 cm in both mineral and organic soils (Sombroek et al., 1993; Tarnocai, 1994). So far, available data have often precluded worldwide calculations of these less dynamic reserves of soil carbon. However, the WISE data set has made it feasible to suggest estimates to a depth of 200 cm for those soil units and areas where such depths are applicable. These are mainly deep soils in the Tropics and Subtropics, such as Acrisols, Ferralsols and Nitosols, as well as some organic and alluvial soils. In other cases, such as Lithosols, Rendzinas and Rankers, a shallower depth has always been used in the calculations.

0 1996 Blackwell Science Ltd, European Journal of Soil Science, 47, 151- 163


Table 2 Mean organic carbon contents for four depth intervals by FAO-UNESCO soil unitslkg m-’

0-200 cm 0-30 cm 0-50 cm 0-100 cm

Soil unit Mean CV n Mean CV n Mean CV n Mean CV n

Acrisols Femc Gleyic Humic Orthic Plinthic

Carnbisols Chromic Dystric Eutric Ferralic Gleyic Humic Calcic Vertic Gelic

Chernozems Glossic Haplic Calcic Luvic

Podzoluvisols Dystric Eutric Gleyic

Rendzinas

Ferralsols Acric Humic Orthic Plinthic Rhodic Xanthic

Gleysols Calcaric Dystric Eutric Humic Mollic Plinthic Gelic

Phaeozems Calcaric Gleyic Haplic Luvic

Lithosols

5. I 3.7 6.2

10.6 3.7 5.1

5.0 4.4 7.6 4.4 4.2 5.2

11.6 3.0 4.6 6.6

6.0

8.0 4.6 6.9

5.6 5.9 6.0 3.2

13.3

5.7 5.4 9.3 5.4 5.3 4.9 4.3

7.7 4.8 6.8 5.8

15.8 10.3 8.5

10.5

7.7 7.0 7.2 7.8 7.8

3.6

-

83 65 97 54 52 64

91 62 82 97 51 67 59 89 59 56

60

42 58 69

65 65 71

-

-

1 I4

60 50 49 51 33 53 53

109 90

146 74 80 55 66

103

53 56 68 61 42

128

309 122 19 71 63 34

53 I 30 85

124 44 47 45

100 42 14

64 0

21 30 13

9 4 4 I

19

256 22 50 83 8

43 50

243 14 57 86 31 45 3 7

202 24 17 70 91

4

6.7 4.8 7.9

14.1 5.0 6.8

6.9 6.0 9.5 6.3 5.5 6.8

16.1 4.3 6.4 9.7

8.6

11.1 6.8 9.6

5.9 6.9 5.3 3.7

-

-

17.6 7.4

13.2 7.0 6.9 6.4 5.9

9.7 4.2 9.4 7.1

19.4 13.1 10.6 13.4

10.5 9.7 9.3

10.6 11.0

-

84 59 96 57 46 63

82 62 73 68 50 61 60 78 55 55

56

42 59 63

52 54 24

-

-

-

61 51 49 51 32 54 44

100 57

125 66 76 45 65 45

48 51 62 56 37

-

302 120 18 70 60 34

48 1 30 82 99 43 45 42 90 36 14

61 0

21 28 12

7 4 2 1

0

25 1 22 49 82 8

42 48

21 1 9

50 78 28 39 3 4

194 24 17 64 89

0

9.4 6.7 9.0

20.3 7.1 9.2

9.6 8.2

12.5 8.8 7.3 9.0

21.1 7.1 9.5

12.4

12.5

16.1 10.3 12.1

7.3 8.6 6.3 4.8

-

-

10.7 10.2 19.0 9.6

10.1 9.1 8.2

13.1 5.0

12.6 9.7

29.3 16.8 12.6 20.4

14.6 12.8 11.7 15.0 15.6

-

82 49 60 57 43 59

77 58 47 63 49 60 68 70 53 80

60

54 57 60

43 42 21

-

-

-

63 44 52 53 30 50 39

109 69 68 65 99 70 59 -

47 51 60 57 31

-

269 104 16 63 55 31

332 18 59 68 35 26 31 67 25 3

44 0

16 20 8

7 4 2 1

0

228 21 47 72 4

39 45

142 5

33 54 18 28 3 1

147 20 13 50 64

0

10.4 6.8

11.5 29.3 7.3 6.5

15.7 15.8 19.5 12.0 10.9 19.5 45.6 11.5 - -

19.6

18.1 21.3

-

-

7.8 8.7 6.3 -

-

16.9 14.5 26.0 16.2 15.1 11.7 12.2

19.9

16.9 9.8

86.6 51.5 21.4

-

-

21.6

20.4 21.6 21.8

-

-

113 49

64 40 82

92

43 50 40 45 66 37

-

-

- -

18

2 21

-

-

31 31 -

-_

-

61 40 53 40

45 23

212

56 34

126

5

-

-_

-

-

54 - - 91 21

-

56 23

1 15 12 5

36 1 7 6 8 3 4 7 0 0

6 0 3 3 0

3 2 1 0

0

79 12 26 15 1

14 11

14 0 4 5 2 1 2 0

15 0 1 4

10

0

Table continued on next page


156 N.H. Batjes

Table 2 (Continued)

0-30 cm 0-50 cm 0-100 cm 0-200 cm


Fluvisols Calcaric Dystric Eutric Thionic

Kastanozems Haplic Calcic Luvic

Luvisols Albic Chromic Ferric Gleyic Calcic Orthic Plinthic Vertic

Greyzems Gleyic Orthic

Nitosols Dystric Eutric Humic

Histosols Dystric Eutric Gelic

Podzols Ferric Gleyic Humic Leptic Orthic Placic

Arenosols Albic Cambic Ferralic Luvic

Regosols Calcaric Dystric Eutric Gelic

3.8 2.4 6.2 4.2 8.3

5.4 7.2 4.4 4.5

3.1 3.4 3.9 2.6 3.7 2.0 3.6 2.7 4.6

10.8 10.2 11.0

4. I 3.8 3.1

10.0

28.3 25.1 34.2 40.6

13.6 17.6 9.8

10.3 12.8 17.8 16.9

1.3 2.5 1.2 1.6 1 .O

3.1 1.6 5.2 2.8

11.8

114 86 52

124 69

52 46 48 -

I03 I43 76 83

1 I9 63 95 60 45

49

57

85 66 89 50

47 54 39 1 1

101 45

143 89 71 96 81

108 119 111 85 49

122 81 89

142 48

-

300 121 29

127 23

22 9

12 1

604 23

100 103 84

128 139

1 1 16

4 1 3

77 23 41 13

42 28 10 4

82 2

14 18 10 28 10

262 1 1

145 75 31

86 22 26 36 2

5.6 3.7 8.3 6.2

12.5

7.5 9.7 6.5 6.8

4.3 5.0 5.5 3.7 4.7 3 .O 4.7 3.7 7. I

13.6 16.9 12.6

5.6 5.2 4.3

13.5

46.4 42.5 51.0 66.7

17.3 22.2 14.4 12.0 17.7 21.9 21.9

1.9 3.4 1.8 2.3 1.5

4.0 2.1 6. I 3.6

19.7

122 77 53

134 77

55 58 43 -

85 122 67 75

101 65 77 53 40

53

67

80 59 80 47

47 53 41

7

92 29

123 80 65 88 73

93 97 97 81 42

114 90 88

127 45

-

278 115 27

113 23

19 7

11 1

555 19 92 98 77

116 127

1 1 15

4 1 3

74 22 39 13

42 28 10 4

75 2

14 17 7

25 10

237 10

129 71 27

66 15 20 29 2

9.3 6.3

12.2 10.9 23.2

9.6 13.8 7.1

10.3

6.5 5.7 8.0 5.8 7.0 5.1 7.1 5.3

11.1

19.7 28.6 16.3

8.4 8.1 6.5

18.2

77.6 72.9 72.4 125

24.2 26.4 24.4 17.8 11.9 38.2 22.4

3.1 3.4 2.9 3.7 2.6

5.0 4.5 5 .O 4.6 -

136 56 76

140 89

50 44 35 -

78 97 51 59

1 I9 64 70 51 25

53

78

72 49 70 47

47 50 54 12

94 33 90 79 75 82 71

77 29 76 80 38

133 75 83

122

-

-

200 87 18 78 17

8 3 4 1

377 13 56 79 52 72 86 8

11

3 1 2

67 20 35 12

34 21 9 4

43 2 9

13 3

13 3

166 7

88 53 18

42 11 9

21 0

16.1 12.8 11.5 16.7 41.6

- - - -

9.9

13.2 7.4 9.0

10.7 11.8 5.3

15.8

23.3 47.1 11.3

11.3 12.3 10.0 26.1

218 123

264

59.1

-

-

- -

43.0

111 -

-

5.5

4.4 6.7 6.2

7.0 8.4 6.2 8.2

-

-

172 67

191 43

-

- - - -

56

33 78

47 51 23

-

-

-

87 - -

47 22 53 -

31 - -

6

60 - - 75

17 -

-

58

28 59

-

-

48 59 55 45 -

18 8 1 7 2

0 0 0 0

42 0 7

12 1 5

13 3 1

2 1 1

20 7

12 1

4 1 0 3

6 0 0 4 0 2 0

14 0 6 7 1

9 2 5 2 0

Table continued on next page



Table 2 (Continued)

0-30 cm 0-50 cm 0-100 cm 0-200 cm


Solonetz Gleyic Mollic Orthic

Andosols Humic Mollic Ochric Vitric

Rankers

Vertisols Chromic Pellic

Planosols Dystric Eutric Humic Mollic Solodic Gelic

Xerosols Haplic Calcic Luvic Gypsic

Y ermosols Haplic Calcic Luvic Takyric Gypsic

Solonchaks Gleyic Mollic Orthic Takyric

3.2 4.7 7.7 2.3

11.4 13.3 9.2

12.2 8.2

15.9

4.5 3.8 5.5

3.9 5.4 2.8 8.8 8. I 3 .O -

2.0 2.8 3.2 1.6 2.8

1.3 1.2 1.2 1.9 0.9 0.8

1.8 1.8 5.1 1.6 3.3

92 70 71 82

69 63 52 82 77

153

87 61 91

99 114 73

16 58

-

-

64 34 47 68 62

121 85 67

120

59

73 82 38 59

-

-

59 17 5

37

160 89 27 15 29

6

267 146 121

54 10 18

1 8

17 0

113 16 14 75 8

44 7

10 15

1 1 1

63 19 3

40 1

4.2 6.5 5.9 3.3

16.5 19.1 13.2 15.7 12.7

-

6.7 5.8 8.0

5.2 6.3 4.1

10.2 10.2 3.9 -

2.8 3.9 4.1 2.3 3.9

1.8 1.7 1.9 2.4 1.5 1.1

2.6 2.5 7.0 2.3 5.2

78 67 33 73

65 60 56 83 72

-

71 53 73

86 111 60

18 53

-

-

61 30 46 65 65

93 71 56 98

36

67 73 37 51

-

-

53 16 3

34

154 88 27 14 25

0

254 137 117

48 9

16 1 8

14 0

I03 15 12 68

8

37 7 7

13 1 9

59 19 3

36 1

6.2 9.4 7.4 4.8

25.4 29.4 20.3 16.3 20.7

-

11.1 9.5

13.2

7.7 6.6 5.7

12.1 13.8 6.5 -

4.8 5.9 6.0 4.2 6.2

3.0 3.1 3.5 3.4

2.0

4.2 4.3

10.1 3.8 8.9

-

83 75 26 71

69 62 72 78 88

- 58 46 59

56 60 54

25 70

-

-

53 31 45 60 54

44 56 41 31

30

67 81 44 52

-

-

39 12 3

24

120 75 23

5 17

0

205 110 95

28 3 9 1 7 8 0

73 14 8

44 7

24 5 6 8 0 5

42 13 2

26 1

5.1 5.7

4.9

31.0 29.9 35.3

24.1

-

-

-

19.1 15.0 25.6

16.9

14.3

18.4 17.6

-

-

- 8.7 8.9

12.8 7.4 - 6.6 7.0

5.9 -

- - 5.7

15.6

3.2 -

-

4 1 0 3

13 8 4 0 1

0

29 16 13

4 0 1 0 1 2 0

8 4 1 3 0

3 2 0 1 0 0

3 1 0 2 0

CV is coefficient of variation (%); n is the number of observation by depth interval. Depth interval for Lithosols (I) is 0-10 cm.

The data in Table 2 provide mean figures for global assess- ments of soil carbon and nitrogen pools that could not be applied satisfactorily in national studies, as regional differences in microclimate, parent material and land use for soils of a particular FAO-UNESCO subunit were not taken into account. Additionally, there are no data to indicate that the available profiles are statistically representative of the world wide distribution of these soil units; this problem is commonly encountered and recognized (Kimble et al., 1990; Sombroek et al., 1993).

Mean soil organic carbon content in the upper 100 cm of the various FAO-UNESCO soil unit ranges from 3.1 kg C m-’ for sandy Arenosois to 77.6 kg C m-’ for Histosols. The large values for the latter are due to the slow decomposition of organic matter under water saturated conditions, particularly when mean soil temperatures are low (see Gelic Histosols). Small amounts of organic carbon are encountered in Xerosols (4.2-6.2 kg C m-*) and Yermosols (3.1-3.4 kg C m-’) from the arid regions where plant growth is limited. Humic subunits typically contain large amounts of organic carbon, ranging

0 1996 Blackwell Science Ltd, European Journal of Soil Science. 47, 151-163

158 N.H. Batjes

from 18.2 kg C m-2 for well drained Humic Nitosols to 29.3 kg C m-2 for poorly drained Humic Gleysols. Similarly large amounts (23.3 kg C mP2) are encountered in poorly drained Thionic Fluvisols which form in brackish environ- ments rich in organic matter. The large value for Andosols (25.4 kg C m-2) can be explained by the protection of organic carbon by allophane (Mizota & Van Reeuwijk, 1989). Gener- ally, the stabilizing effect of clay particles on soil organic matter decreases in the sequence: allophane > amorphous and poorly crystalline Al-silicates > smectite > illite > kaolinite (Van Breemen & Feijtel, 1990).

Changes in the relative distribution of soil organic carbon with depth have been studied only on those profiles with complete data to a depth of 100 cm (Table 3). On average, 39-70% of the total organic carbon in the upper 100 cm of mineral soil is held in the first 30 cm, and 58-81% in the first 50 cm. These figures illustrate the potentially large amounts of COf that can be released when tropical soils are deforested, with changes in land use, or increased oxidation of superficial peat layers on drainage (Detwiler, 1986; Veldkamp, 1993).

Table 3 Relative distribution of organic carbon as a function of depth by FAO-UNESCO soil units (except Rendzinas, Rankers and Lithosols)

A“ Ba

Soil unit n Mean CV Mean CV

Acrisols Cambisols Chernozems Podzoluvisols Ferralsols Gleysols Phaeozems Fluvisols Kastanozems Luvisols Greyzems Ni tosols His t o s o 1 s Podzols Arenosols Regosols Solonetz Andosols Vertisols Planosols Xerosols Yermosols Solonchaks

269 332 44

7 228 142 147 200

8 311

3 61 34 43

166 42 39

120 205

28 73 24 42

54 52 50 I 0 53 59 53 44 40 41 57 49 31 59 44 49 50 48 40 52 42 39 44

20 29 31 19 18 29 20 36 27 28 33 24 41 31 29 33 33 29 28 22 24 31 33

I1 70 69 81 71 75 72 62 62 66 13 67 58 14 62 66 68 67 60 69 62 58 64

13 18 20 1 1 1 1 18 13 23 15 18 18 15 24 25 18 19 20 19 16 15 15 17 21

aA stands for the ratio of organic carbon of 0-30 cm divided by that in the 0-100 cm zone, and B for the ratio of the 0-50 cm divided by that in the 0- 100 cm zone. Both the mean and coefficient of variation (CV) are given as percentages.

Soil nitrogen density. The N-content for each soil unit to a depth of 100 cm is given in Table 4. Because the analyses were based on sites with varying types of land use or vegetation the figures provide only a global impression of total soil N. The general figure observed for the upper 100 cm of the soil is from 0.37 kg N m-* for arid Yermosols to 4.01 kg N mW2 for Histosols. The figures in Table 4 are similar to those given for similar mineral soils in the Amazon basin (Moraes et al., 1995).

Soil C : N ratios. The C : N ratio is a good indicator of the degree of decomposition and quality of the organic matter held in the soil. However, ratios are prone to considerable variation resulting from errors in determining both variables. Therefore, the data were edited before analysing the C : N ratios. All horizons with C : N ratios smaller than 2 and greater than 70 were considered to be outliers and were omitted from the analyses. The mean C : N ratio across the soil units ranged from 9.9 for Yermosols to 29.8 for Histosols. The general trend of the values in Table 5 suggests a decrease in C : N ratio with depth, which reflects a greater degree of breakdown and older age of the humus stored in the lower parts of the profile. The mean C : N ratio for the 30-50 cm and 50- 100 cm depth interval is identical for the Vertisols; this reflects the intense mixing or churning typical of these deep cracking-and- swelling clay soils.

Estimates of world soil C- and N-pools

Organic carbon. In the following text, two estimates are given for the soil carbon pool of the world (by depth range). The first value is based on the median stone content and the second is for so-called ‘stone-free’ soils. The total mass of organic carbon stored in the upper 100 cm of the soils of the world is 1462-1548 Pg of C (Table 6). The stone-free value of 1548 Pg C agrees well with the 1576 Pg of C published by Eswaran et al., (1993), who based their study on a map of the ‘Major Soil Regions of the World’ which uses the USDA Soil Taxonomy (USDA, 1975) to group the soils. This means that comparable results for the soil carbon pool in the upper 100 cm have now been obtained using different approaches to soil classification and mapping. Sombroek et al., (1993), however, obtained a somewhat smaller estimate of 1220 Pg C in the top 100 cm using the digital Soil Map ofthe World (FAO, 1991) and about 400 profile descriptions.

The total content of carbon in organic soil varies with peat fibre and ash contents. Reserves of carbon in the top 100 cm of the world’s peat soils have been estimated variously at 300 Pg of C (Sjors, 1980), 202-377 Pg of C (Adams et al., 1990), and 357 Pg of C (Eswaran et al., 1993). The figures of Sjijrs (1980) and Eswaran et al., (1993) are comparable with the 330 Pg of C found in the current study. Estimates for the other depth increments are 120 Pg of C for the first 30 cm and 679 Pg of C for the upper 200 cm, respectively.



Table 4 Distribution of soil nitrogen content for three depth intervals by FAO-UNESCO soil unit (kg N m-’)

0-30 cm 0-50 cm 0-100 cm

Soil unit Mean cv n Mean cv n Mean cv n

Acrisols Cambisols Chernozems Podzoluvisols Rendzinas Ferralsols Gleysols Phaeozems Lithosols Fluvisols Kastanozems Luvisols Greyzems Nitosols Hisosols Podzol Arenosols Regosols Solonetz Andosols Rankers Vertisols Planosols Xerosols Yermosols Solonchaks

0.48 0.58 0.88 0.54 1.05 0.46 0.75 0.71 0.42 0.50 0.68 0.45 0.96 0.49 1.61 0.8 1 0.22 0.45 0.45 0.91 2.18 0.50 0.41 0.33 0.15 0.27

82 84 41 44

101 57 78 61

132 304 40 66 57 50 67 84 71

100 79 54

I22 58 57 45 88 68

237 353 32

5 16

198 141 157

4 I84 17

213 3

34 26 61 35 49 35

124 4

147 33 29 26 31

0.66 0.77 1.22 0.55

0.64 0.97 0.98

0.72 0.98 0.63 1.32 0.68 2.38 1.01 0.33 0.57 0.67 1.31

0.75 0.55 0.43 0.25 0.44

-

-

-

78 71 39 20

54 13 54

339 41 65 52 50 63 76 73 95 64 54

50 54 43 82 62

-

-

-

205 286 31 4 0

187 91

130 0

158 14

175 3

32 26 52 27 38 25

1 1 1 0

118 28 21 16 25

1.10 1.12 1.70 0.16

0.97 1.34 1.51

1.23 1.78 1.03 1.92 1 .oo 4.01 1.39 0.52 0.70 1.11 1.99

1.23 1 .oo 0.58 0.37 0.75

-

-

-

74 137 71 142 48 17

1 0

48 126 102 51 53 68

0 399 83 28 2 60 83 40 3 51 26 55 19 76 29 67 20 72 19 51 7 57 66

0 42 55 47 12 54 6

103 3 58 15

- -

-

-

CV is coefficient of variation (%); n is the number of observation per depth interval; depth interval for Lithosols is 0- 10 cm.

Various authors have estimated the amount of organic carbon in the soil of the tropics. Estimates for the first 100 cm depth are 496 Pg of C (Kimble et al., 1990) and 506 Pg of C (Eswaran et al., 1993). Based on the current study, soil organic carbon in the tropics is 201-213 Pg C for 0-30-cm, 384-403 Pg C for 0-100-cm, and 616-640 Pg of C for the 0-200-cm depth range, respectively. The values for 0-30 cm illustrate again the large amount of C that might be mobilized following deforestation and conversion to grass- lands, as is currently the case in the Amazon Basin (Detwiler, 1986; Veldkamp, 1993). Fisher et al. (1994) drew attention to the important role of deep tropical soils and tropical land use in the global carbon cycle.

Carbonate carbon. Inorganic carbon in the soil occurs largely in carbonate minerals, such as calcium carbonate (CaC03) and dolomite (MgC03). Some types of soil, particularly the acid and strongly weathered ones, do not contain appreciable amounts of inorganic carbon because the carbonates originally present in the parent material have been dissolved. Large carbonate concentrations are common in the soil of dry areas, as well as in soil formed over calcareous parent materials such as the Rendzinas and some Lithosols.

The mean carbonate-C content (assuming 12% C in CaC03) of the individual soil subunits has been used to calculate the world’s soil carbonate carbon stocks, thereby accounting for observed differences in carbonate-C content between the various subunits of a particular FAO-UNESCO soil unit (e.g. calcic versus dystric Cambisols). The total carbonate-C pool estimated with this approach is 222-245 Pg of C for the upper 30 cm, and 695-748 Pg of C for the upper 100 cm. The latter figure accords with earlier findings of 780-930 Pg (Schlesinger, 1982) and 720 Pg of carbonate-C (Sombroek et al., 1993). No attempt has been made to estimate the amount of carbonate-C held in deeper soil layers (100-200 cm) because of the limited data.

Total carbon. Total carbon in soil, defined as being the sum of both the organic and carbonate carbon, is estimated to be 2 157-2293 Pg of C for the upper 100 cm. This is about 300 Pg of C more than the amount estimated by Sombroek et al. (1993), which is largely the result of Sombroek et al.’s smaller estimate of soil organic carbon.

Soil nitrogen. The methodology used to estimate carbon pools was also used for soil nitrogen, giving global estimates


160 N.H. Batjes

Table 5 Distribution of C:N ratios as a function of depth by FAO-UNESCO soil units

0-30 cm 30-50 cm 50- 100 cm

Soil unit Mean cv n Mean cv n Mean cv n

Acrisols Cambisols Chernozems Podzoluvisols Rendzinas Ferralsols Gleysols Phaeozems Lithosols Fluvisols Kastanozems Luvisols Greyzems Nitosols Histosols Podzols Arenosols Regosols Solonetz Andosols Rankers Vertisols Planosols Xerosols Yermosols Solonchaks

13.2 11.5 10.8 13.6 11.2 14.3 12.6 11.4 1 1 . 1 11.2 10.6 11.6 8.9

12.6 25.8 23.8 14.2 13.5 12.2 13.3 17.1 13.3 11.5 9.9

1 1 . 1 11.7

42 44 28 39 37 40 45 25 33 52 60 42 36 54 59 47 54 51 45 48 42 34 44 30 39 54

604 836 73 15 48

48 1 364 347

6 39 1 46

537 17 84 57

198 97

141 96

274 11

34 1 99 90 84 77

10.1 9.7

10.7 7.4

12.6 11.2 10.0

11.3 8.8 9.9

11.0 9.8

29.8 21.5 12.6 9.6

10.5 13.8

12.5 10.3 9.2

10.5 9.2

-

-

-

44 46 37 39

49 56 29

61 1 1 54

49 46 59 70 36 65 61

41 71 42 30 41

-

-

-

-

149 209 26 3

138 79

122

95 10

135 1

24 21 42 18 23 20 91

54 22 14 10 19

-

-

-

8.9 9.0 9.4 7.5

11.8 10.4 8.9

10.4 8.6 9.4 8.6 8.6

22.3 24.5 9.9

10.2 8.8

14.3

12.5 7.9 7.0

10.9 8.5

-

-

-

52 62 49 30

58 65 44

67 17 52 35 69 42 52 69 75 51 63

42 40 31 60 46

-

-

-

205 230 29 5

183 93

139

127 6

140 3

37 30 55 22 19 18

I I9

100 21 7 6

22

-

-

-

of 63-67 Pg of N to a depth of 30 cm, and 133- 140 Pg of N to a depth of 100 cm. The latter values are somewhat greater than the 92- 117 Pg of N calculated using an ecosystems approach (Zinke et al., 1984), possibly because most profile descriptions in WISE originated from agricultural soils which may have been amended with N fertilizers. By comparison, about 10 Pg of N is held in the plant biomass and about 2 Pg N in the microbial biomass (Davidson, 1994).

Discussion and conclusions

Revised estimates for the world soil carbon and nitrogen pools are presented using the WISE database. The estimated soil organic carbon content of 1462-1548 Pg of C in the upper 0-100 cm compares well with the findings of Eswaran et al. (1993). Large amounts of organic carbon, which are not yet considered in most global C-budgets (see IPCC, 1992), are also stored between depths of 100 and 200 cm. Much of this deeper carbon occurs in fairly stable forms, and therefore will not contribute much to current gaseous emissions.

Various sources of uncertainty in making global calculations of soil carbon and nitrogen pools, such as representativeness, quality and reliability of the various data sources, have been

discussed. Enhanced statistical confidence in the results will require more profiles per soil subunit, stratified per agro- ecological zone, and a probabilistic basis for sampling. The information (FAO-UNESCO, 197 1 - 198 1) on the extent of different soil types in the world also needs to be updated (Sombroek, 1990; Oldeman & Van Engelen, 1993). This type of information is becoming increasingly critical to enable changes in soil properties induced by land use changes, atmospheric pollution, and predicted climate change (Arnold et al., 1990; Stigliani et al., 1991) to be determined.

Over long periods of time, carbon storage in the soil varies mainly as a result of climatic, geological and soil-forming factors (Adams et al., 1990), whilst over shorter periods of time it is mainly vegetation disturbances or succession, and changes in land use patterns that affect storage. Because the carbon and nitrogen data in the WISE database were compiled from field samples collected over the last 20-30 years, they do not represent the C- and N-content of the world's soil at any single point in time. Rather, they provide a global estimate of C- and N-pools.

At present, changes from forest to grassland and agriculture especially have a marked effect on the oxidation of superficial soil carbon stocks, enhancing emissions of COz and other trace

0 1996 Blackwell Science Ltd. European Journal of Soil Science. 47, 151-163

Total carbon and nitrogen in the soils ofthe world 161

Table 6 World soil carbon and nitrogen pools (in Pg of C and N. respectively)

Depth range/cm

Region 0-30 0-100 0-200

Tropical regionsa Soil carbon

Organic-C Carbonate-C Total

Soil Nitrogen

Other regions Soil carbon


Soil Nitrogen

World Soil carbon


Soil Nitrogen

201-213 72-79

273-292 20-22

483-51 1 150- 166 633-677 43-45

684-724 222-245 906-969 63-67

384-403 616-640 203-218 - 587-621 - 42-44 -

1078-1145 1760-1816 492 - 5 30 -

1570- 1675 - 91-96 -

1462-1548 2376-2456 695-748 -

2 1 57 -2296 - 133-140 -

"The tropics have been defined as the region bounded by latitude 23.5"N and 23.53. The first estimate for the mineral soils is 'without' stones.

gases to the atmosphere (IPCC, 1992). One effect of the predicted global warming will be to accelerate the decomposition of soil organic matter at an overall rate of 1 1-34 Pg of C per degree Celsius of warming (Schimel, 1995), thereby releasing COz to the atmosphere which will further enhance the warming trend. Studies on the C02 'fertilization effect' and the associated physiological decrease in the transpiration of crops (Bazzaz & Fajer, 1992) suggest that net primary production may increase. This would lead to more carbon being returned to the soil and so thereby curb the increase in atmospheric COz content caused by fossil fuel combustion and biomass burning (Goudriaan & Unsworth, 1990; Sombroek et al., 1993); a phenomenon for which experimental evidence now exists (Francey et al., 1995).

An increased concentration of C in the soil will accelerate microbial processes, as will warmer and moister soil (Davidson, 1994). However, it appears that concentrations of N and other major nutrients in woody and herbaceous species might decrease as the concentration of atmospheric COz increases (Overdieck, 1990; CoOteaux et al., 1995). This might result in a gradual decrease in the degradability of plant residues by microbes (Lekkerkerk et al., 1990) and possibly to nutrient stress in natural ecosystems. Consequently, the overall quality and degradability of plant organic matter might decrease (Bradbury & Powlson, 1994; CoOteaux et al., 1995), which would then lead to an accumulation of organic matter in soil (Lekkerkerk et al., 1990). Changes in N-fixation,

N-mineralization, denitrification and cation leaching associated with eutrophication, acidification and toxification as well as effects of depletion in the ozone layer and enhanced ultraviolet-B radiation on flora and fauna are also important in this context (Brookes & McGrath, 1984; Davidson, 1994; Caldwell et al., 1995). In view of all these interrelations, which are strongly influenced by changes in socio-economic, techno- logical and environmental factors, the reliability of any model prediction of the dynamics and evolution of soil organic matter pools remains open to debate. Land-atmosphere models (Mellilo, 1994; Goldewijk et al., 1994; Schimel, 1995), however, are critical to study the possible effects of different scenarios of land use and climatic change on soil carbon and nitrogen pools.

Acknowledgements

The WISE database has been developed at ISRIC in the framework of the Dutch National Research Programme on Global Air Pollution and Climate Change (Project 851039). The assistance of staff members of the USDA Soil Conser- vation Service (NRCS), Food and Agriculture Organization (FAO), ISRIC, and a wide range of national soil survey organizations in providing descriptions of representatative soil profiles for the WISE database is gratefully acknowledged.

The contributions of Dr E. M. Bridges in compiling the profile database and of Dr F. 0. Nachtergaele (FAO) in developing the gridding procedure for the spatial component of the WISE database are specially acknowledged.

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