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DIVISION S-5—PEDOLOGY

Brazil's Soil Carbon Stocks

^a Institut de Recherche pour le Développement (IRD), Biogeoquimica Ambiental, CP 96, BR-13400-970, Piracicaba, SP, Brazil
^b Centro de Energia Nuclear na Agricultura (CENA), Biogeoquimica Ambiental, CP 96, BR-13400-970, Piracicaba, SP, Brazil

* Corresponding author (mbernoux{at}cena.usp.br)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Soil organic carbon (C) stocks to a depth of 30 cm were estimated for Brazil on the basis of a map of different soil-vegetation associations (SVA) combined with results from a soil database. The SVA map was derived by intersection of soil and vegetation maps. The original soil and vegetation classification were simplified to six soil and 15 vegetation categories. The SVA map comprised 21 111 map units (MU) that were spread into 75 different SVA categories. Mean representative C stocks (RCS) of the SVA categories ranged from 1.51 to 41.78 kg C m^-2. More than three fourth of all SVA areas had an associated RCS varying between 3 and 6 kg C m^-2. Because these data represent sites with native vegetation in the absence of significant disturbances, they represent a valuable baseline for evaluating the effect of land-use change on soil C stocks for Brazil. In total, about 36.4 ± 3.4 Pg C was stored in the 0- to 30-cm soil layer.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS AND DISCUSSION REFERENCES

 
INTEREST IN THE GLOBAL CONSEQUENCES of climatic alterations due to the greenhouse effect has led scientists to study the global carbon (C) cycle. The world's mineral soils represent a large reservoir of C, with estimates ranging from 1115 to 2200 Pg C (1 Pg = 10¹⁵ g, or 1 Pg = 1 billion tonnes) in the first meter (Post et al., 1982; Eswaran et al., 1993; Batjes, 1996). The biomass of plants is estimated to comprise between 560 and 835 Pg C (Whittaker and Likens, 1975; Bouwman, 1990). The tropical forest ecosystems account for 20 to 25% of the world terrestrial (soil and vegetation) C (Brown and Lugo, 1982; Dixon et al., 1994).

Models of the C cycle require accurate estimations of the masses in the different reservoirs. Regarding the soil compartment, global C pools are difficult to estimate because of still limited knowledge about specific properties of soil types (Sombroek et al., 1993; Batjes, 1996), the high spatial variability of soil C even within one soil map unit (Cerri et al., 2000), and the different effects of the factors controlling the soil organic C cycle (Pastor and Post, 1986; Parton et al., 1987). Thus, regional studies are necessary to refine global estimations obtained by aggregation of regional estimates, mainly at country scale.

For Brazil, it is important to assess the pools under native vegetation for several reasons. First of all, C storage in a native ecosystem reflects that ecosystem's capacity to sequester C. Also, the importance of an understanding of the national C pool levels is reinforced by the statements of the United Nations Framework Convention on Climate Change (UNFCCC) signed at Rio de Janeiro in 1992. Moreover, the IPCC/UNEP/OECD/IEA Program on National Greenhouse Gas Inventories recognized that a "C stocks" (CS) approach based on measurements of CS in the compartments (biomass, litter, soil) could be a supplement to the current IPCC methodology, because "Direct measurements of changes in CS are extremely difficult" (in IPCC/UNEP/OECD/IEA, 1997; Volume 3, chapter 5.1.2. The proposed approach).

Second, estimates of the stored C in soils of Brazil at national level are scarce. Schroeder and Winjum (1995), and more recently, Turner et al. (1998) reported that 72 Pg were stored in the top meter of soils in Brazil for the IPCC year-base 1990. The objective of this paper is to give reliable values and a distribution map, for the 0- to 30-cm reference layer used by IPCC/UNEP/OECD/IEA (1997), of the Brazilian soil CS under undisturbed vegetation. This would provide a valuable baseline for evaluating the effect of land-use change on soil CS and for improving overall estimates of current CS in Brazil.

	MATERIAL AND METHODS

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS AND DISCUSSION REFERENCES

 
In this study, the following steps were performed: elaboration of a map of representative SVA for Brazil; organization of a soil profiles data base gathering information such as C concentration, bulk density, soil type, and native vegetation; calculation of RCS values for each SVA category and subsequent production of the map of their distribution.

The SVA map was derived by combination of the Brazilian soil (EMBRAPA, 1981) and vegetation (IBGE, 1988) maps at the 1:5 000 000 scale. These maps are distributed in digital form by FAO (These data were acquired from the UNEP/GRID World Wide Web site at the U.S. Geologic Survey EROS Data Center).

Elaboration of a Soil-Vegetation Association (SVA) Map
Soil Map
The original soil map (EMBRAPA, 1981) was built up from 2698 map units (MU, in this paper the term "map unit" will refer to a single-part polygon of a digital map). These MU were split into 69 soil types (from 18 major orders of the Brazilian soil classification systems used). A simplified soil classification was considered (Table 1) on the basis of criteria recommended by IPCC/UNEP/OECD/IEA (1997, p. 5.38) such as soil texture, base saturation, and soil water status. The IPCC/UNEP/OECD/IEA (1997) proposed six categories that are characterized as high activity clay (HAC) mineral soils, low activity clay (LAC) mineral soil, sandy soils, volcanic soils, wet soils, and organic soils. Definitions of the Brazilian soil classification system were taken into account to separate HAC (soils with cation-exchange capacity 24 cmol_c kg^-1 clay) from LAC (soils with cation-exchange capacity <24 cmol_c kg^-1 clay). The original Brazilian soil map possesses no soil type corresponding to volcanic soils and organic soils. On the basis of these observations, soil types could theoretically be split into HAC soils, LAC soils, sandy soils and wet soils. But, the Brazilian order named "Latossolos," that pertains to LAC soils, covers alone 38.8% of the country (see Fig. 1) . The Brazilian Latossolos correspond to well drained Oxisols in the U.S. Soil Taxonomy and to Ferralsols in the FAO-UNESCO (FAO-UNESCO) soil map legend. The LAC soils were thus divided into LAC-Latossolos (S2, 689 MU) and a LAC-non-Latossolos (S3, 1008 MU). The other soil categories were named HAC soils (S1, 208 MU), sandy soils (S4, 257 MU), and wet soils (S5, 368 MU). An additional category (Table 1 and Fig. 1) contains all the soils that did not correspond to one of the other groups, and were marked as "other soils" (S6, 168 MU).

View larger version (53K): [in this window] [in a new window]   Fig. 1. Soil and vegetation maps of Brazil with simplified legends (soil and vegetation categories are detailed in Tables 1 and 2).

SVA Map
The SVA map was derived by intersection of the soil and vegetation maps. Each MU of the output map was characterized by combining the information derived from the soil (S categories in Table 1) and vegetation maps (V categories in Table 2). The MU of the SVA map that corresponded to a MU characterized as water in the vegetation map and/or in the soil map, were classified as "water."

Soil Profiles Database Elaboration
A digitized soil profiles database was elaborated from information already published in soil inventories at regional or national scale. Most of the soil profile information came from description of soils pits surveyed by the RADAMBRASIL project (Ministério das Minas e Energia, 1973 to 1983; Fundação Instituto Brasileiro de Geografia e Estatística, 1986–1987) and by the EMBRAPA projects at the state level (e.g., EMBRAPA/SNLCS, 1978–1980). The soil survey of the RADAMBRASIL project was initially carried out in the Brazilian Amazon basin and then extended to the entirety of Brazil. These soil surveys were performed to identify, to delimit, and to localize the occurrence of various soil classes. The following information was considered for each soil profile: localization (geographic coordinates and/or corresponding city and state), soil type, present vegetation, limits of the analyzed horizons, C concentration (Walkley-Black type-methods, Walkley and Black, 1934), bulk density (BD), and clay content (pipette method, Day, 1965). In total, 3969 soil profiles (10 457 soil horizons) were entered into the database.

Calculation of the Individual (Profile by Profile) C Stocks
Calculations (0–30 cm) were done according to the classical way of calculating C stock (C mass per area) for a given depth, by summing C stock of the successive horizons in the respective profile. Carbon stock in a soil horizon was determined as a product of BD, C concentration, and horizon thickness. In the surveys, the soil profiles were divided into horizons (A, B, C horizons). In most cases, the calculations concerned two horizons where the first horizon typically was entirely above 30 cm, and the second crossed this 30-cm limit. When a horizon crossed the 30-cm limit, simply the portion of the horizon that was above that depth calculate its CS.

For most of the soil profiles, no BD was reported. In those cases, the soil BD was estimated for each horizon from the results of a previous study of the entire Brazilian Amazon Basin for similar soils (Bernoux et al., 1998). In this study, the multilinear regressions used for BD estimations, for horizon with clay contents 20% (sandy horizon) were:

[1]

In the case of clay contents >20%, the calculation was made according to the soil category:

[2]

[3]

[4]

CLAY represents the clay content, after dispersion with sodium hexametaphosphate, in percentage (w/w) of the soil fraction <2 mm, and OC the organic C by dichromate oxidation (Walkley and Black, 1934), in percentage (w/w) of the soil fraction <2 mm.

In some cases (six horizons involved in CS calculation), when C concentrations were higher than 15% and the estimated BD showed values below 0.5 g.cm^-3, the BD were set to 0.8 g.cm^-3. This value may seem arbitrary, but it takes into account personal observations of the concerned soils and the fact that the mean clay content for these six horizons is 39.5%.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS AND DISCUSSION REFERENCES

 
SVA Map Structure
The elaboration of new soil and vegetation legends were necessary to reduce the original number of soil and vegetation groups to a reasonable number of categories. The SVA correspond to natural Brazilian regions because they indirectly take into account the geography of the country, including the direct effects of the climate on vegetation. The map totaled 21 111 MU with 17 945 MU (8 257 846 km²) corresponding to a specific SVA category, and 3166 MU (252 143 km²) were considered water. The MU were spread into 75 different SVA categories of the 90 theoretically possible cases (six soil categories crossed with 15 vegetation categories). The surface covered by each SVA category is given in Table 3. The smallest SVA category (75 km²) corresponded to the "S1, HAC soil—V15, Woody Oligotrophic Vegetation" association, whereas the largest (956 196 km²) corresponded to the "S2, LAC Latossolos—V2, Dense Amazon Forest" association. More than half of the SVA categories (45 of the total) had an area smaller than 50000 km², but covered 654 660 km².

View this table: [in this window] [in a new window]   Table 3. Number (N) of map units and corresponding area (km2) of the SVA categories.

Representative Carbon Stocks values
The RCS ranged from 1.5 kg C m^-2 (S4 sandy soil—V11 Northeast Steppe) to 41.8 kg C m^-2 (S6 Other Soils—V3 Atlantic Forest). More than three fourth of all SVA categories area were associated with a RCS ranging from 3 to 6 kg C m^-2, and 41% of the extent covered by SVA showed RCS ranging from 4 to 5 kg C m^-2. These stocks are consistent with data for the world level (Batjes, 1996) derived from the WISE (World Inventory of Soil Emission Potentials) soil database. Batjes (1996) reported worldwide mean CS values for the 0- to 30-cm layer of 1.3, 3.1, 4.1, 5.1, and 5.7 kg C m^-2 for Arenosols, Luvisols, Nitosols, Acrisols, and Ferralsols, respectively. Their humic sublevels exhibit values around 10 kg C m^-2. In a recent paper, Batjes and Dijkshoorn (1999) calculated for the soils of the whole amazon basin slightly higher values for Arenosols, Luvisols, and Nitosols (2.1, 4.7, and 5.7 kg C m^-2, respectively) and lower values for Acrisols and Ferralsols (4.4 and 5.1 kg C m^-2, respectively).

Total C Stored at the Country Level
The potential total CS of Brazilian soils under native vegetation for the 0- to 30-cm layer (Fig. 2) was obtained by combining the table of the RCS with the SVA map. We calculated that a total of 36.4 Pg C was stored in 8 257 846 km² of the SVA. On the basis of the standard error (SE) calculated for each SVA category (with an arbitrary level of 25% of the representative value when it is derived from only one CS value) as a mean to estimate the accuracy of the total CS, the entire error would be 3.4 Pg C, i.e., 9.4% of the total. The accuracy of the all CS and the associated uncertainties are discussed later.

View larger version (80K): [in this window] [in a new window]   Fig. 2. Map of the soil carbon stocks (0–30 cm) in soils of Brazil under native vegatation.

Some estimates exist for the Brazilian Legal Amazon (BLA) basin that represents nearly 5 000 000 km², i.e., 59% of the whole of Brazil. Moraes et al. (1995), Bernoux et al. (2001b), and Cerri et al. (2000) presented the first reliable estimations of CS in soils for the BLA basin in different depths. Cerri et al. (2000) estimated a CS of 41 Pg C for the 0 to 100 cm, with 23.4 Pg C (i.e., 57%) stored in the top 0.3 m.

Adding the stocks calculated from this study for the BLA, a total of 22.7 ± 2.3 Pg C were estimated. This result can be considered as the most reliable estimation of C in the 0- to 30-cm soil layer of the BLA. Unlike to Cerri et al. (2000), this CS took into account the variability of the vegetation.

Distribution Map of the Total CS
Figure 2 shows that the soil and the vegetation have different influences on the CS distribution, depending of the geographical localization. For instance, the regions with the highest CS has a soil influence marked by the presence of wet soils (Western region—Pantanal, Central region—Bananal, and Northwestern Amazon Basin). On the other hand, the southern part of Brazil had high CS mainly because of the colder climatic influence (that influence directly the vegetation and the soil organic matter decomposition). The northeastern region (known as "Sertão") is characterized by low CS and showed an important climatic influence (semiarid). In the Amazon region, the dense forest presented higher CS than the open forest. In the Cerrado region, the soil type had a great influence on the CS.

Accuracy of the Results and Sources of Uncertainties
Several sources of uncertainties could be identified. First, the information from the soil database stem from different sources. Even if most of the teams followed similar soil and vegetation classifications, discrepancies may arise because each soil survey performed used different methods of defining vegetation and soil types. For example, a vegetation type can be classified by one author as "steppic savanna," and by another as "savanna." The influence of this type of uncertainty is very difficult to quantify.

The lack of soil bulk densities, and the subsequent use of multilinear regression equation to estimate them, represents another source of uncertainty. Bernoux et al. (1998) reported that the SE associated with the predicted soil bulk densities for Latossolos or Podzólicos soils, which cover 66% of Brazil, were between 0.06 and 0.11 g cm^-3, i.e., 5 to 10% of the values. Therefore, the error due to BD estimation that could be expected on calculated CS at point locations would be <10% of the final results. For the C content, the methodology used for the analyses may have also varied among different laboratories. But, the most important source of uncertainties may be the fact that methodologies based on dichromatic oxidation were used to determine C concentrations. Methods to estimate soil OC are numerous; the traditional method in soil science for many years has been the Walkley and Black (1934) procedure, which is rapid but unfortunately not completely accurate. This method shows variable recovery factors ranging from 60 to 86%, with an average recommended factor for subsequent correction of 76% (USDA, 1996). The method sections of the various soil surveys never specified the factor used (or not used) for subsequent corrections. Several authors (Nelson and Sommers, 1982; Tiessen and Moir, 1993; Bowman, 1998) discussed advantages and disadvantages of the Walkley and Black (1934) procedure. However, only this method was used for the data "production," and is certainly the most considerable source of uncertainty.

Finally, concerning the validity over the time, the total CS calculated in this paper is the CS that should be in soils under native vegetation at the construction date of the vegetation map (1970s and 1980s) that correspond also to the execution of the soil surveys. The data used to derive the CS map are based on sites with native vegetation in the absence of significant disturbances; thus, the map represents the baseline of the potential CS distribution under native conditions. Even with the problem of the lack of total accuracy of historical soil data, this study allows us to puts reasonable limits on total organic C stored in Brazilian soils and its repartition. It also provides useful baseline data for future studies dealing with land use changes and the impact on C dynamics at regional or continental scales. For example, using the map of the RCS distribution for the 0- to 30-cm soil depth, Bernoux et al. (2001a) calculated the first approximation of CO₂ fluxes from soils in Brazil for the 20-yr periods 1970 to 1990 and 1975 to 1995. They showed that the annual fluxes for Brazil indicate a net emission of CO₂ to the atmosphere, which decreased from 93.3 Tg CO₂ (1 Tg = 10¹² g) for the period 1970 to 1990 to 46.4 Tg CO₂ for the period 1975 to 1995. Intermediary calculation to derive these annual fluxes estimated that 34.4 Pg C was stored in Brazilian soil for the year 1995, i.e., 2 Pg C below what would be expected under native conditions.

Brazilian soils corresponded roughly to 5% the world's C stock estimated to be 684 Pg C in the upper 30 cm (Batjes, 1996). This fact alone is important because it shows that only one nation's policies can greatly influence the world C soil budget.

	ACKNOWLEDGMENTS

 
The authors acknowledge the Climate Change Program of the Brazilian Ministry of Science and Technology for the support. They also gratefully thank the contribution of V. Eschenbrenner, U. Herpin, and N. Paciornik to this paper. Research support was provided by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) with grants 98/10499-0 and 00/02439-0.

Received for publication July 28, 2000.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (34) Citing Articles via Google Scholar Google Scholar Articles by Bernoux, M. Articles by Cerri, C. C. Search for Related Content PubMed Articles by Bernoux, M. Articles by Cerri, C. C. GeoRef GeoRef Citation Agricola Articles by Bernoux, M. Articles by Cerri, C. C. Related Collections Soil Classification and Mapping