Estimations of amounts of soil organic carbon and fine root carbon in land use and land cover classes, and soil types of Chiapas highlands, Mexico

Forest Ecology and Management 177 (2003) 191–206

Estimations of amounts of soil organic carbon and fine root carbon in land use and land cover classes, and soil types of Chiapas highlands, Mexico Jorge Mendoza-Vega*, Erik Karltun, Mats Olsson Department of Forest Soils, Swedish University of Agricultural Sciences, Box 7001, 750 07 Uppsala, Sweden Received 8 May 2001; received in revised form 21 May 2002; accepted 16 June 2002

Abstract Amounts of organic carbon in the mineral soil (SOC) and fine-root (<5 mm) carbon (RC) were quantified, when possible down to 1 m depth, in 150 soil profiles from 39 sites in the highlands of Chiapas, Mexico. The sampling sites were classified and grouped according to the following soil type: Leptosols, weathered soils (Ferralsols, Acrisols, Lixisols and Nitosols), Luvisols and Cambisols/Phaeozems. Likewise, they were classified in the following groups with respect to land use/land cover (LU/LC): oak–evergreen cloud forest, fragmented forest, pine and pine–oak forest, and open land (cultivated land and pasture). No clear influence of soil-type grouping on amounts of SOC was revealed, between soil-type groups (soil type). The weathered soils had higher amounts of SOC and RC in the upper 50 cm than the other soil types and the lowest amounts of SOC and RC in the 50– 100 cm layers. The LU/LC groups showed marked differences in amounts of SOC. Open land had between 20 and 60% less soil carbon than the different types of forests. Oak–evergreen cloud forest had the largest amounts of SOC at all depths. The differences in amounts of carbon (SOC and RC) between oak–evergreen cloud forest and the other LU/LC classes were proportionally larger in the 50–100 cm layer. This suggests that the amounts of SOC in deep soil layers may be influenced by changes in LU/LC. The considerably smaller amounts of SOC and RC, 174 and 38.5 Mg ha
1, and 31 and 24.8 Mg ha
1, exhibited in open land compared to oak–evergreen cloud forest and pine and pine–oak forests, respectively, suggest that conversion of forests to open land reduces SOC and RC. Despite the magnitude of the differences, evaluation with ANOVA (nested design) did not reveal a statistically significant influence of LU/LC. The large spatial variability in amounts of soil carbon prevented precise estimates. Changes induced by a gradient in altitude among the plots may have contributed to the large variability between the plots. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Land use; Land cover; Soil type; Soil organic carbon; Chiapas highlands; Mexico

1. Introduction There are strong concerns that the increase in the so-called ‘‘greenhouse’’ gases in the atmosphere, of *

Corresponding author. Tel.: þ46-18-67-22-12; fax: þ46-18-67-34-70. E-mail address: [email protected] (J. Mendoza-Vega).

which CO2 is the most abundant, is influencing the climate of the earth. It is estimated that 1.6 pg C is released to the atmosphere annually due to deforestation of tropical rainforest and other changes in land use (Lal et al., 1995). This is about 30% of the annual C released from fuel fossil burning and 23% of the total annual C released by anthropogenic activities in the world. It is about 10 times as much as the annual C

0378-1127/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 3 9 - 5

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released due to changes in land use in temperate regions (Lal et al., 1995). The importance of monitoring and verification of greenhouse gas-fluxes from forestry projects and landuse changes has frequently been stressed (Dixon and Turner, 1991; Pinard, 1996). In particular, C pools vulnerable to significant change should be monitored, such as soil organic carbon (SOC) in soils subjected to land-use changes. The extent to which soils and vegetation act as CO2 sinks or sources depends largely on land-use management (Lal, 1994). Some of the land-use activities that affect carbon fluxes in soils are deforestation, afforestation, biomass burning, cultivation, crop residue management and application of inorganic fertilisers and organic manure (Lal, 1994). Increased oxidation of SOC resulting in a release of CO2 to the atmosphere not only affects the global carbon balance, but also soil fertility. Maintenance of soil organic matter is particularly important for plant nutrient retention and supply in the tropics where the use of chemical fertilisers is limited, and highly weathered soils are common. Further, it contributes to the maintenance of soil structure and water holding capacity (Sanchez et al., 1989). In the literature a variety of data is found concerning SOC status after forest conversion to agriculture. The loss of SOC due to forest conversion to crops and grassland varied between 20 and 50% (Buringh, 1984; Detwiler, 1986; Schlesinger, 1986). In mechanised agriculture with fertiliser inputs a decrease in SOC levels has been observed, probably due to a limited return of crop residues and an increase in organic matter oxidation (Davidson and Ackerman, 1993; Flach et al., 1997; Buyanovski and Wagner, 1998). In shifting cultivation a loss of SOC has been observed immediately after the burning of the slashed vegetation (Garcia-Oliva et al., 1999). Shifting cultivation with short fallow results in a fast decline of SOC (Detwiler, 1986). In some cases where improved management has been implemented after forest conversion, SOC has reached levels similar to those of the original vegetation (Bridges and de Bakker, 1998; Gregorich et al., 1995). According to Chone´ et al. (1991), SOC levels in forest converted to pasture recovered after 8 years. Similarly, Koutika et al. (1997) found that forest converted to pasture resulted in a decline in SOC and thereafter an increment with pasture age. In some long-term experiments with manure addition, Johnston (1991) found that SOC levels

constantly increased with the manure inputs; however, the soil organic matter rapidly started to decline when the manure applications stopped. In some other cases of manure additions the SOC build-up in the soil was more permanent (Sanford et al., 1985). Mexico ranks among the top 10 countries regarding carbon releases from forest clearing (Detwiler and Hall, 1988) and occupies the 14th place in the world as a contributor to fossil fuel C emissions (95 Tg C per year). Estimated deforestation rates for Mexico range from 0.4 to 1.5 million ha per year (Masera et al., 1992). Estimates of the C flux between Mexico’s terrestrial biosphere and the atmosphere range from 14 to 71 Tg C per year (Masera et al., 1997). The uncertainties about deforestation rates, forest degradation and associated biomass reductions make studies of land-use changes in relation to the overall carbon balance in the country an imperative (Masera et al., 1997). In order to estimate the potential of terrestrial ecosystem processes in C sequestration, it is necessary to calculate net annual C flux and total C of these ecosystems, including vegetation, soils and post-harvest products (Cairns et al., 1996). In Mexico there are representative examples of the pathways of forest conversion to other forms of land use in the tropics. The traditional shifting cultivation system whereby the fields are cleared, usually with the help of fire, and cultivated for shorter periods than they lie fallow (Conklin, 1957) has gradually changed into a more intensive agricultural system. In the highlands of Chiapas deforestation takes place due to conversion of the land to agropastoral systems, tree harvesting and wildfires (de Jong and Montoya-Go´ mez, 1994). In a previously published paper, de Jong et al. (1999) estimated the influence of the LU/LC on the carbon flux between the 1970s and 1990s using C data for above-ground biomass, roots and soil in the highlands of Chiapas. In this paper we use data from the same plots in a closer analysis of the carbon distribution within the soil profile and differences in the amount of SOC between LU/LC systems. The possible effect of differences in soil type is also evaluated.

2. Material and methods The highlands of Chiapas state (Fig. 1) are located in southeast Mexico, 92–938W and 168300 –178N. The

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193

Fig. 1. Map showing the study area, the highlands of Chiapas.

highlands consist of a high plateau oriented north– south with altitudes ranging from 1200 to 2600 m a.s.l. Its bedrock originates from the Cenozoic era (Late Tertiary), and consists predominantly of calcareous sedimentary rocks, lutite and sandy rocks. The regional climate is subtropical to temperate subhumid (Miranda, 1952). In the study area mean annual rainfall and mean annual temperature varies between 1000 and 1700 mm, and between 13 and 19 8C, respectively (Dı´az-Herna´ ndez et al., 2000). Rainfall occurs mainly from May to October, with winter droughts. The experimental plots are located in altitudes that range from 1620 to 2561 m a.s.l. Holdridge (1967) classified the life zones in the study area as tropical lower montane and premontane moist and subtropical lower

montane and montane wet zones. The most important LU/LC classes are pine forests, mixed pine–oak forests, oak and evergreen cloud forests, tree and shrub fallow fields (hereafter referred to as fragmented forests), grassland (induced pasture, promoted native species and introduced pasture, exotic species), and cultivated or cropped land. Representative tree and plant species for each LU/LC are presented in Table 1. On the mountainous and rolling landscapes of the Chiapas highlands, shallow and poorly developed soils are found such as Leptosols and Cambisols associated with Phaeozems and Regosols. Most of these soils have a neutral to alkaline pH owing to their calcareous parent material. More acid and clayey soils such as Luvisols, and weathered, acid and clayey soils such

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Table 1 Representative tree and plant species of the LU/LC classesa Pine forest

Pine–oak forest

Oak–evergreen cloud forest

Fragmented forest

Open land (grassland and cropped land)

Pinus maximinoi P. montezumae P. oaxacana P. oocarpa P. pseudostrobus

Pinus ayacahuite P. maximinoi P. montezumae P. oaxacana P. oocarpa Quercus candicans Q. crassifolia Q. crispipilis Q. rugosa Q. segoviensis

Quercus benthamii Q. crassifolia Q. laurina Q. rugosa Alnus acuminata ssp. Liquidambar styraciclua Pinus ayacahuite Acer negundo Clethra macrophylla Magnolia sharpii

Bacharis vacciniodesb Cestrum angyrisb Eupatorium spp.b

Festuca amplissimac Eragrostis mexicanac Stipa ichuc Panicum laxiflorumc Cinodon dactylond Pennisetum clandestinumd

a

Source: Gonza´ lez-Espinosa et al. (1997). Characterised by the tree species typical of the mature forests and also shrubs. c Pasture, native species. d Introduced pastures. b

as Acrisols are also found associated with Nitisols and Lixisols (INEGI, 1985). 2.1. Land use in the highlands of Chiapas Agriculture and agropastoral systems are the main economic activities in the study area. Owing to the large population increase over the last decades, the farming units have become very small, mostly between 0.5 and 2 ha (Parra-Va´ zquez et al., 1989), and are intensively cultivated. The increase in population has also put pressure on forests, and at present the landscape is a complex mosaic of cultivated land, secondary forest, disturbed forest and grazing land (Gonza´ lez-Espinosa et al., 1991). The forests have been disturbed by selective harvesting of pine trees for local timber production, oak trees for fuel and charcoal, and extensive sheep grazing (de Jong and Montoya-Go´ mez, 1994). 2.2. LU/LC classes and sampling design Six LU/LC classes were identified as representative of the study area and were defined according to the following criteria: (1) oak and evergreen cloud forests were forests in which oak trees constituted over 80% of the basal area; (2) pine–oak forests were forests in which pine and oak trees together constituted over 80% of the basal area; (3) pine forests were forests in

which pine trees constituted over 80% of the basal area; (4) fragmented forests included open forests and shrub land (10–40% canopy cover); (5) cultivated land included seasonal and permanent agricultural systems; (6) pasture included induced (promoted native species) and introduced pasture (exotic species) and grasslands. Total canopy cover of the first three classes was over 40% (de Jong et al., 1999). Plots of 60 m  90 m were established in all the LU/ LC classes. The number of plots chosen for each LU/ LC class was based on its distribution in the study area and the expected variability in the amount of C. The location of the plots was subjectively chosen. For pine–oak forest and fragmented forest, which were the most common LU/LC classes and those expected to show high variability in the amount of C, a larger number of plots (n ¼ 11 and 10, respectively) were selected. For cultivated land and pasture, which were expected to show lower variability in the amount of C, fewer plots (n ¼ 3 for each group) were chosen. The plots were divided into six subplots of 30 m  30 m, 20 m  20 m and 10 m  10 m. The design of the plots and subplots followed the methodology for aboveground inventory and biomass sampling, as described by de Jong et al. (1999). The below-ground biomass and mineral-soil material were sampled from four 50 cm  50 cm and 1 m deep pits, hereafter referred to as soil profiles, and separated into layers 0–10, 10–20, 20–30, 30–50 and 50–100 cm deep or down to the

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2.4. Soil type and LU/LC grouping

bedrock if found, or to a highly calcareous material. Soil samples were taken from each profile and layer. Roots no larger than 5 mm in diameter were separated with a 2 mm mesh sieve from each layer, weighed fresh and then oven-dried at a temperature of 90 8C for 12 h to constant weight. Fresh/dry weight ratio was determined for each sample. Organic carbon content in root and soil was determined on three replicates using the Walkley and Black (1934) method. This method oxidises only the organic carbon, carbonates do not interfere (Hesse, 1971). Soil bulk density was determined on clod soil samples by the wax procedure (Smith and Mullins, 1991). The waxed clod is weighed in air and in water, the wax coating must subsequently be removed and weighed.

Both soil types and LU/LC classes were grouped into new soil types and LU/LC classes in order to obtain a more balanced statistical design. Thus all LU/ LC classes were represented for each soil type (Table 3). Ferralsols, Acrisols, Lixisols and Nitisols were grouped in a soil type called ‘‘weathered soils’’. All of these are soils that have undergone a long weathering process. This means they contain no more than traces of weatherable minerals and have low activity clays. They are usually deep and fine textured. Luvisols became a type on their own. They are also fine textured soils, but differ from weathered soils in that they still have weatherable minerals, a high CEC and a high base saturation. Cambisols were grouped together with Phaeozems and conformed to the soil type ‘‘Cambisols/Phaeozems’’ since both Phaeozems and Cambisols found in the area have a Mollic A horizon (A horizon rich in organic matter) and are moderately developed. Leptosols were excluded from the statistical analysis when LU/LC classes were compared since they are shallow soils (30 cm), limited in depth by calcareous material. However, they were compared to other soil types down to a depth of 30 cm. Pine and pine–oak forests were grouped together in a class called ‘‘pine and pine–oak forest’’ since pine– oak forest had more in common with pine forests concerning the amount of above-ground C than with oak–evergreen cloud forest (Table 4). Cultivated land and pastureland were grouped as ‘‘open land’’. Both land types had the smallest amounts of above-ground C (Table 4). Oak–evergreen cloud forests formed a class of their own.

2.3. Soil classification In order to reduce dissimilarities caused by inherent differences in the development of different soil types when comparing LU/LC classes, soils were classified in each plot according to the FAO system (FAO, 1988). They were then included in the statistical analysis as a control used to reveal possible bias caused by differences between soil types. Eight soil types were identified in the study area: Luvisols, Cambisols, Nitisols, Leptosols, Phaeozems, Lixisols, Acrisols and Ferralsols. The most common soil types were Cambisols and Luvisols, with 32 and 30 profiles, respectively; followed by Ferralsols, Acrisols, Leptosols and Phaeozems, with 20, 19, 17 and 16 profiles, respectively; and the least number of profiles were found for Nitisols and Lixisols with eight profiles each. Table 2 shows the original association between LU/LC classes and soil types.

Table 2 Associations between LU/LC classes and soil types (total number of profiles) LU/LC class

Soil type Luvisols

Total Ferralsols

Cambisols

Nitisols

Leptosols

Phaeozems

Lixisols

Acrisols

Pine–oak forest Pine forest Oak–evergreen cloud forest Fragmented forest Cultivated land Pasture

15 4 3 4 4 –

4 8 – 8 – –

12 – 4 4 4 8

– – – 4 4 –

2 4 4 7 – –

– – 8 4 – 4

4 4 – – – –

4 – 7 8 – –

41 20 26 39 12 12

Total

30

20

32

8

17

16

8

19

150

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Table 3 Analysis design for total SOC associations (soil type versus LU/LC class)a LU/LC class

Soil type

Total

Weathered soils

Luvisols

Cambisols/Phaeozems

Pine and pine–oak forest Oak–evergreen cloud forest Fragmented forest Open land

24 7 20 4

19 3 4 4

12 12 8 16

55 22 32 24

Total

55

30

48

133

a

Total number of profiles (Leptosols are not included).

Table 4 Mean amounts of above-ground carbon (Mg ha
1) by original and modified LU/LC classesa Original LU/LC class

Above-ground C amount

Grouped LU/LC

Above-ground C amount

Oak–evergreen cloud forest Pine oak forest Pine forest Fragmented forest Cultivated land Pasture

189.0 135.0 120.0 29.0 6.0 18.0

Oak–evergreen cloud forest Pine and pine–oak forest Fragmented forest Open land

189.0 127.5 29.0 12.0

a

Modified from de Jong et al. (1999).

2.5. Soil depth and statistical analysis The mean amount of SOC for any specific soil depth was calculated as the average for all soil profiles of that depth or deeper, i.e. shallower soils were excluded. Soils that were only 50 cm deep, for example, were not included in the comparison of deeper soils. The distribution of SOC and root carbon (RC) in the profile was calculated by only taking into account those soil pits that were 100 cm deep. Profiles were nested within plots and the GLM procedure (SAS, 1989) was run to test if differences in amounts of SOC and RC between LU/LC classes, soil types and LU/LC–soil type interactions were significant. Differences in amounts of SOC and RC between LU/LC classes and soil types with depth were also analysed. 2.6. Climate and SOC amounts In order to identify a possible influence of climate on amounts of SOC, and taking into account the lack

of climatic data (temperature and precipitation), we made a correlation analysis between the altitude above sea level, available for each plot, and the amounts of SOC, both excluding and including the plots with the Leptosols.

3. Results Differences in amounts of SOC and RC between some LU/LC classes and some soil types were large and they showed a pattern. Leptosols, which are shallow soils not deeper than 30 cm, had the largest amount of SOC of all soil types at that depth (Table 5). Comparing amounts of SOC between weathered soils, Luvisols and Cambisols/Phaeozems there was a consistent pattern for amounts of SOC at all intervals down to 50 cm depth, increasing in the following order: Cambisols/Phaeozems < Luvisols < weathered soils. In the interval between 50 and 100 cm the order was reversed, i.e., weathered soils < Luvisols < Cambisols/Phaeozems. The amount of SOC in the

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197

Table 5 Mean amounts of SOC (Mg ha
1) and (95% confidence interval) for the studied soil types Soil type

N

0–10 cmN

0–20 cmN

0–30 cmN

0–50 cmN

50–100 cm

N

0–100 cm

Leptosols Weathered soils Luvisols Cambisols/Phaeozems

17 54 30 48

83 65 60 52

124 107 97 87

153 129 124 108

156 (18)51 144 (30)19 131 (19)34

32 (6) 49 (18) 57 (26)

51 19 34

190 (21) 192 (46) 187 (40)

(10) (7) (10) (6)

16 54 29 47

(16)13 (11)53 (18)27 (10)45

(26) (14)53 (24)23 (12)41

Fig. 2. Distribution of SOC by soil type and depth.

entire depth down to 100 cm was almost identical for the three soil types. Weathered soils had most of their SOC in the upper 20 cm layer (56%) and a considerably smaller portion in the layer between 50 and 100 cm depths (17%, Fig. 2). Luvisols and Cambisols/Phaeozems had 45% of their SOC in the top 20 cm and 26 and 30%, respectively, in the interval between 50 and 100 cm. None of the soil types differed significantly from the others regarding the amount of RC. However, weathered soils had the largest amount of RC at all depths, except between 50 and 100 cm where they had the smallest (Table 6). The high values and wide con-

fidence intervals for weathered soils were caused by two profiles with much higher RC values than other soil profiles in the same group. If these are omitted, the difference in the amount of RC between weathered soils and the other two soil types was negligible in the upper part of the profile. Leptosols had the least amount of root C at all depths down to 30 cm, in contrast to the results for SOC (Table 5). The distribution of RC in the soil profile did not differ much between soil types for the upper 10 cm as opposed to the lower 50 cm (Fig. 3). Weathered soils had only 6% of the total amount of RC in the 50–100 cm layer, while Cambisols/Phaeozems and Luvisols had

Table 6 Mean amounts of RC (Mg ha
1) and (95% confidence interval) for the studied soil types Soil type

N

0–10 cm

N

0–20 cm

N

0–30 cm

N

0–50 cm

N

50–100 cm

N

0–100 cm

Leptosols Weathered soils Luvisols Cambisols/Phaeozems

17 54 30 48

4.8 13.7 7.5 9.3

16 54 29 47

9.3 22.9 20.1 13.5

13 53 27 45

12.7 29.3 28.0 18.9

53 23 41

34.0 (19.1) 20.0 (8.7) 20.2 (7.9)

51 19 34

2.3 (0.6) 2.8 (1.4) 3.2 (2.3)

51 19 34

36.9 (20.1) 23.9 (10.5) 23.3 (10.1)

(1.6) (5.8) (2.3) (4.5)

(2.9) (13.0) (12.0) (5.3)

(3.6) (16.8) (17.0) (8.4)

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Fig. 3. Distribution of RC by soil type and depth.

14 and 12%, respectively, in the same layer. Again, the two extreme values for RC in the weathered soils influence the figures. In absolute amounts the differences are negligible (Table 6). When comparing amounts of SOC between LU/LC classes (Leptosols omitted), open land had the smallest and oak–evergreen cloud forests the largest amount of SOC at all depths (Table 7). In the 0– 30 cm layer open land had a lower amount of SOC than all other LU/LC classes, whereas oak–evergreen cloud forest, fragmented forest, and pine and pine–oak forest did not differ notably from one another. At the 0–50 cm depth oak–evergreen cloud forest had a higher amount of SOC amount than pine and pine– oak forest and open land, and fragmented forest had a higher amount of SOC than open land. At 50–100 cm depth, oak–evergreen cloud forest had more SOC than the other three LU/LC classes. Open land and pine and pine–oak forest had only about a third of the amount of SOC found in the oak– evergreen cloud forest. Also at 0–100 cm depth, oak– evergreen cloud forest had a higher amount of SOC

than the other three LU/LC classes, and fragmented forest had a higher amount of SOC than pine and pine– oak forest (29.5% more) and open land (59% more, Table 7). Oak–evergreen cloud forest had 30% of the total amount of SOC in the interval between 50 and 100 cm, while pine and pine–oak forest, fragmented forest and open land had 20, 23 and 23%, respectively, of their SOC at the same depth (Fig. 4). All LU/LC classes, except oak–evergreen cloud forest, contained higher amounts of SOC in the 0–10 cm layer than in the 50–100 cm layers. At 0–10 cm depth oak–evergreen cloud forest had a higher amount of RC than the other LU/LC classes, fragmented forest, pine and pine–oak forest, and open land. In the 0–20 and 0–50 cm layers fragmented forest had the highest amounts of root C. At 50– 100 cm depth, oak–evergreen cloud forest had approximately 2.5, 4 and 38 times higher amounts of RC than pine and pine–oak forest, fragmented forest and open land, respectively (Table 8). In all LU/LC classes there was a pronounced decrease in the amount of RC with depth (Fig. 5). In open land 70% of

Table 7 Mean amounts of SOC (Mg ha
1) and (95% confidence interval) for the studied LU/LC classes LU/LC class

N

0–10 cm N

0–20 cm

N

0–30 cm

N

0–50 cm

N

50–100 cm

N

0–100 cm

Oak–evergreen cloud forest Fragmented forest Pine and pine–oak forest Open land

22 32 55 24

71 63 60 42

123 106 96 69

18 32 53 22

149 133 118 84

15 31 48 22

201 164 134 105

11 28 43 19

92 48 33 32

11 28 43 19

309 215 166 135

(13) (9) (6) (4)

21 32 53 24

(18) (16) (11) (8)

(20) (20) (15) (10)

(32) (23) (19) (14)

(52) (19) (9) (14)

(74) (29) (26) (26)

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199

Fig. 4. Distribution of SOC by LU/LC and depth.

Table 8 Mean amounts of RC (Mg ha
1) and (95% confidence interval) for the studied LU/LC classes LU/LC class

N

0–10 cm

N

0–20 cm

N

0–30 cm

N

0–50 cm

N

50–100 cm N

0–100 cm

Oak–evergreen cloud forest Fragmented forest Pine and pine–oak forest Open land

22 32 55 24

16.5 14.8 9.6 2.5

21 32 53 24

22.3 26.7 20.0 3.0

18 32 53 22

31.8 33.5 27.3 3.2

15 31 48 22

30.2 39.6 26.5 3.5

11 28 43 19

7.7 1.8 3.1 0.2

42.7 (34.5) 43.1 (34.7) 29.0 (6.2) 4.2 (2.3)

(9.8) (9.5) (1.8) (1.5)

(10.7) (21.9) (6.8) (1.6)

the amount of root C down to 1 m was contained in the upper 10 cm. In the same depth interval fragmented forest and pine and pine–oak forest held 36–38%, respectively, and oak–evergreen cloud forest 58% of the RC down to 1 m.

(17.7) (28.1) (8.9) (1.9)

(15.0) (32.1) (5.5) (2.0)

(6.2) (0.7) (0.8) (0.2)

11 28 43 19

The result of the correlation analysis run between the altitude above sea level from the experimental plots and the amounts of SOC are presented in Fig. 6a (Leptosol excluded) and Fig. 6b (Leptosols included). When the Leptosol plots were excluded it resulted in a

Fig. 5. Distribution of RC by LU/LC and depth.

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Fig. 6. Correlation between SOC amounts and altitude (m a.s.l.): (a) Leptosols excluded and (b) Leptosols included.

higher correlation coefficient and higher significance, R ¼ 0:4, p ¼ 0:016 compared to R ¼ 0:29, p ¼ 0:072, for all plots. The table in Appendix A gives mean values of SOC content and bulk density for the LU/ LC–soil type associations.

4. Discussion SOC accumulation is a function of organic litter input rate and SOM mineralisation rate. The climate and vegetation are driving forces in this process while topography and soil type (soil properties that protect organic matter against decomposition) may modify the rate of the processes and hence the magnitude of SOM accumulation. Changes in SOC induced by LU/ LC changes are often connected to a change in vegetation cover that modifies the litter-input rate, and changes in SOM mineralisation rate due to soil disturbance. The complex interaction of the factors involved in determining the SOC content in the soil makes it difficult to investigate and predict changes in amounts of SOC with LU/LC changes. The high spatial variability in carbon content within the same soil order has also added to the difficulty of making reliable estimations of amounts of soil carbon per unit area (Kimble et al., 1990; Eswaran et al., 1995;

Greenland, 1995). In studies conducted by Eswaran et al. (1995) and Kimble et al. (1990) the variance of amount of soil carbon within the same soil type did not decrease with an increasing number of samples. These authors also found that vegetation and land use have a confounding effect on soil-carbon content determinations. In our study, despite there being substantial differences in amounts of SOC between LU/LC classes, the differences were not statistically significant. However, the effect of LU/LC consistently showed lower probability values than soil type effect. Unfortunately, information about the time elapsed since the change in land use was not available in our investigation. This may have contributed to the variability between the plots since many soils may still be in a transient state with respect to changes in SOC content. Another contributing factor is that the plot design chosen, originally adapted for biomass measurements on the same plots, proved ineffective for evaluating the soil data. However, our results indicate that LU/LC has an impact on amounts of SOC in the studied area. Open land (cultivated land and pasture) had SOC quantities that were considerably lower than those of forests, 20 to nearly 60% lower, depending on the forest type. The soil types did not differ notably in SOC, with the exception of Leptosols, which had higher amounts of

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SOC in all soil layers compared to the other three soil types, weathered soils, Cambisols/Phaeozems and Luvisols down to 30 cm depth. We assume that the high SOC concentration in Leptosols is due to their calcareous origin. The Leptosols of the present study are of the rendzina type; these are soils that contain at least 40% calcium carbonate. The high content of calcareous material acts as a stabiliser for the soil organic matter, protecting it from microbial biodegradation (Duchaufour, 1982). It is also expected that the below-ground organic matter concentration will be located in the available soil layer. In other words, Leptosols, which are shallow soils, will have high SOC concentrations but often less total SOC per unit area, whereas deeper soils will have lower SOC concentrations than Leptosols but more total SOC per unit area. Further, Leptosols were not chosen for agricultural purposes because of their shallowness. This may also contribute to the comparatively high amounts of SOC in the Leptosol sites. Conversely, the Leptosols had a lower amount of RC than the other soil types to the compared depth (30 cm), despite the fact that they were not used in agriculture. This supports the hypothesis that the high calcium carbonate content of these soils protects the organic matter from biodegradation, i.e. low amount of RC but high amount of SOC. The highly weathered and low activity clay soils in the group of weathered soils had the highest amounts of SOC down to 50 cm depth and the lowest from 50 to 100 cm depth compared to other soil types (Table 5). The SOC was more evenly distributed in Luvisols and particularly in Cambisols/Phaeozems. In accordance with our results, Legger (1991) found that in Ferralsols and Acrisols, soils that make the bulk of the weathered soils (Ferralsols, Nitisols, Lixisols and Acrisols) of our study (Table 2), most of the living plant roots concentrate in the upper 10–50 cm of the soil. One reason for root concentration in the upper 50 cm of the soil profile could be the low pH and hence Al toxicity in the subsoil of these soils that hinder root development. The amounts of RC showed differences between the soils that were consistent with differences in SOC, except for Leptosols, as noted above. In addition to Leptosols, there were other soil profiles that were not dug to 100 cm depth due to physical impediment. In de Jong et al. (1999) the amounts of SOC were estimated to a 100 cm depth independent of the real soil depth. This approach allowed an estimation to be made of

201

amounts of SOC per LU/LC-unit area and SOC fluxes in time with land-use changes. In the present work the influence of LU/LC on amounts of SOC was of major importance: when comparing LU/LC classes the mean amount of SOC for any specific soil depth was calculated as the average for all soil profiles down to that depth. Soils with a lithic contact at a more shallow depth were excluded in these comparisons. Oak–evergreen cloud forest sites have a higher amount of SOC compared to the other three LU/LC classes. This is probably due to the larger amount of above-ground carbon (Table 4), resulting in greater litter input. The differences between oak–evergreen cloud forest and the other LU/LC classes in amounts of SOC and RC are clearer at the 50–100 cm depth intervals. A higher amount of RC at deeper levels for oak–evergreen cloud forests results in higher root litter input, which can explain the differences in the amount of SOC in deeper layers. In a study conducted by Lugo (1992) in the tropics, broadleaf evergreen forests had greater detrital inputs from root turnover than coniferous forest; Vogt et al. (1995) found the same pattern in soil organic matter. The fragmented forest sites had the second largest SOC (Table 7) and the largest RC quantities (Table 8), despite the fact that they had the next smallest amount of above-ground C of all LU/LC classes (Table 4). This could probably be explained by the gradual conversion of forest into fragmented land. Oak trees are used for fuel and for making charcoal. Peasants do not completely clear the forest for this purpose, but they continuously remove individual trees leaving behind organic debris that is incorporated into the soil. Pine trees, on the other hand, are used for timber production. Harvesting is sometimes carried out as selective cutting. Openings are created in the forests that are used for grazing or partial cropping. This disturbance is less severe than when the forest is felled completely and used for extensive grazing or cropping. When slash and burn methods are used there is sometimes an increase in SOC after the burning. Nye and Greenland (1960) investigating the effects of shifting cultivation on soils in fields in Ghana, Liberia and Nigeria detected an increase in soil carbon after the cleared and slashed vegetation was burned. Depending on variations in the fire intensity, various amounts of organic matter are left behind and subsequently incorporated into the soil. Formation of recalcitrant substances upon

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burning may also inhibit SOM decomposition (Skjemstad et al., 1990). In the cut, slash and burn system some trees and bushes are left standing, thus when the site is left fallow forest recovery is relatively fast. The secondary vegetation produces a strong growth of roots: it has been estimated that forest regrowth can have an annual production of root material as high as 2.5–5.0 Mg ha
1 (Mouttapa, 1973). Down to 50 cm depth fragmented forest had the largest amount of root C of all LU/LC classes, but the variation was also large (Table 8). One reason for the large variation in amounts of root C was the large variation in the C content of the roots; it has been shown that it is difficult to differentiate between dead and living roots (Anderson and Ingram, 1993). An estimation of root amount in forest ecosystems can be made using allometric relationships between root and above-ground biomass obtained from forests, as in Cairns et al. (1996). This approach is designed to decrease the risk of strong bias due to high amounts of roots close to trees. However, in fragmented forests, the aboveground biomass is drastically reduced although the trees are often kept alive, keeping the root system alive, resulting in an increased root/shoot ratio. Thus, for this type of land the cited allometric relationships will underestimate the amount of roots considerably. Land-use history may also play an important role for the amount of SOC in fragmented land. Farmers usually choose the most fertile forest sites with more SOC for growing crops. SOC status in a fallow field or secondary forest will also depend on how long and to what extent the sites were cropped-grazed, and the amendments, if any, that were applied during cultivation. Conversion of forest to grassland has some times resulted in an increased of SOC content (Fisher et al., 1994; Trumbmore et al., 1995), but this seemed not to be the case in the highlands of Chiapas. The species grown in our study region did not belong to the deeprooted grasses found in other regions of tropical America, e.g. the Amazon region (Trumbmore et al., 1995). Post et al. (1982) found that there is a correlation between climate and SOC content, particularly between precipitation and temperature, and SOC content. SOC commonly increases with increasing precipitation and with decreasing temperature for any particular level of precipitation. The significant positive correlation coefficient found between altitude above sea level and the amounts of SOC in our study

may suggest that a proportion of the variation in the amounts of SOC is explained by the climate, or is due to the influence of predominant tree and plant species related to an altitudinal gradient. A source of error in estimating amounts of SOC could be the analytical methods used for soil bulk density and organic carbon content. For instance, the clod method we used for determination of the soil bulk density does not take into consideration the inter-clod or inter-aggregate spaces, resulting in higher bulkdensity values compared to the core method (Smith and Mullins, 1991). Mendoza (unpublished data, 2002) obtained higher bulk-density values with the clod method compared to the core method in the same soils. The Walkley and Black or wet oxidation method for the determination of SOC is one of the methods most commonly used worldwide for this purpose. It has the advantage, compared to the dry combustion method, that it only takes into consideration the organic carbon. This is especially important when analysing soils rich in calcium carbonate, as in our study. However, this method may underestimate the organic carbon if the temperature (125 8C) required for the oxidation of the organic carbon is not reached during the analysis.

5. Conclusion Regional studies for measuring soil carbon stocks, and detailed soil resource assessments for estimations of SOC for global appraisals are needed (Eswaran et al., 1995). These studies are also required to ascertain the best performance of models and to validate them (Paustian et al., 1997). However, they are scarce and most of them were intended for soil survey purposes and not for SOC appraisals (Greenland, 1995). Even though the differences in amounts of SOC between LU/LC classes were not statistically significant, the results indicate that LU/LC is a more important variable for determining amounts of SOC in soils compared to soil type. Regional assessments of soil carbon stocks should therefore include information about LU/LC. Adjustment of the SOC pool is a slow process that for many of the soils studied would not have reached completion. Therefore, the estimated losses as a result of land-use conversion are probably rather conservative.

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The results of this study illustrate the problems of studying the long-term effects of changes in land-use on amounts of SOC. The problem of controlling all factors other than LU/LC and soil type, the unknown original SOC content of the soil before changes in LU/ LC and the considerable short-range spatial variability of SOC within the studied plots all contribute to the difficulty of interpreting data. Ideally, the long-term effect of LU/LC on the amount of soil carbon should be studied in a series of permanent field experiments with different land-use treatments.

203

Acknowledgements The authors thank Ben H.J. de Jong and the field staff from El Colegio de la Frontera Sur for their important participation in the collection of the soil and root data. The US Environmental Protection Agency has funded the information in this document under Cooperative Agreement CR922200 with El Colegio de la Frontera Sur, Interagency for International Development/Mexico, and Contract No. 68-W5-0065 with OAO Corporation.

Appendix A Mean values of SOC content and dry bulk density for the LU/LC–soil type associations is given in the following table Soil depth LU/LC–soil type associations (cm) FFa–WSb FF–CPhc SOC content (%) 0–10 10–20 20–30 30–50 50–100

8.1 5.5 3.1 1.8 0.8

SOC content (%)

Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

0.9 0.9 1.0 1.0 1.0

6.5 2.9 2.1 1.2 1.3

1.0 1.3 1.1 1.2 1.2

10.4 3.6 2.1

0.9 1.1 1.1

6.3 3.6 2.3 1.2 0.7

0.9 1.0 1.0 1.0 1.0

OF–CPh Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

10.7 6.0 3.1 1.9 1.5

0.8 1.0 1.0 1.0 1.0

8.3 6.8 4.7 3.5 2.9

0.8 0.9 1.0 1.0 1.0

PPOFg–WS

0–10 10–20 20–30 30–50 50–100

OF–Lep

SOC content (%)

SOC content (%) 7.1 3.7 2.0 1.0 0.4

FF–Luve

Dry bulk density (Mg m
3)

OFf–WS

0–10 10–20 20–30 30–50 50–100

FF–Lepd

PPOF–CPh

SOC content (%) 7.7 3.6 2.4

OF–Luv Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

0.9 1.1 1.3

7.3 2.4 1.6 1.0 0.6

0.9 1.0 1.0 0.9 1.0

PPOF–Lep

Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

0.9 1.0 1.0 1.0 1.1

6.4 2.8 1.7 0.8 0.6

0.9 1.0 1.0 0.9 0.9

SOC content (%) 6.5 2.7 1.7

PPOF–Luv Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

0.7 0.8 1.1

8.9 5.6 3.3 1.5 1.0

0.9 1.1 1.1 1.1 1.0

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Appendix A. (Continued ) Openh–WS SOC content (%) 0–10 10–20 20–30 30–50 50–100

5.3 3.9 2.0 1.2 0.6

Open–CPh

Open–Lep

Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

1.0 1.0 1.0 1.1 1.0

3.4 2.2 1.4 0.8 0.5

1.1 1.2 1.2 1.2 1.2

SOC content (%)

Open–Luv Dry bulk density (Mg m
3)

SOC content (%)

Dry bulk density (Mg m
3)

4.6 1.8 0.8 1.0 0.5

1.0 1.1 1.0 1.1 1.0

a

Fragmented forest. Weathered soils. c Cambisols/Phaeozems. d Leptosols. e Luvisols. f Oak forest. g Pine and pine–oak forest. h Open land. b

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