Effects of different land use on soil chemical properties, decomposition rate and earthworm communities in tropical Mexico

ARTICLE IN PRESS Pedobiologia 53 (2009) 75—86

www.elsevier.de/pedobi

Effects of different land use on soil chemical properties, decomposition rate and earthworm communities in tropical Mexico Violette Geissena,, Karina Pen ˜a-Pen ˜ab, Esperanza Huertaa a

El Colegio de la Frontera Sur, Administracio ´n de Correos No. 2, 86100 Villahermosa, Tabasco, Me ´xico INRES Universita ¨t Bonn, Nussalle 13, 53115 Bonn, Germany

b

Received 3 November 2008; received in revised form 25 March 2009; accepted 26 March 2009

KEYWORDS Tropical soils; Land use; Litter decomposition; Earthworms

Summary The effects of land use on soil chemical properties were evaluated, and earthworm communities and the decomposition rate of three typical land use systems in tropical Mexico, namely banana plantations (B), agroforestry systems (AF) and a successional forest (S) were compared. The study was carried out from November 2005 to April 2006. A completely randomized sampling design was established in six sites (B1, B2, AF1, AF2, S1 and S2). Soil properties and chemical characteristics (texture, pH, organic carbon (Corg), nutrients, and available Zn and Mn), earthworm communities and the decomposition of Bravaisia integerrima and Musa acuminata litter were analyzed over a period of 8 weeks. All soils were loamy clays with a medium to high content of nutrients. Three principal clusters were generated with the soil chemical properties: a first cluster for forest soils with high Corg and Ntot and low available Zn content, a second cluster for AF1 and a third cluster for B1, B2 and A2. The decomposition of B. integerrima litter was significantly faster (half-life time: 1.8 (AF2)–3.1 (B1) weeks) than that of M. acuminata (4.1 (AF2)–5.8 (S2) weeks). However, the decomposition rates did not differ significantly among the different sites. The greatest earthworm diversities were observed in AF2 and B1. Native species were dominant in the forest soils, whereas exotic species dominated in AF and in the banana plantations. The abundance and biomass of certain earthworm species were correlated to physical and chemical soil parameters. However, litter decomposition rates were not correlated with any of the soil physical–chemical parameters.

Corresponding author. Tel.: +52 993 3136110x3405; fax: +52 993 3136110x3001.

E-mail address: [email protected] (V. Geissen). 0031-4056/$ - see front matter & 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.pedobi.2009.03.004

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V. Geissen et al. While none of the land use systems studied led to a decrease in nutrient status, earthworm biodiversity and abundance, or in litter decomposition rate, they did result in a change in earthworm species composition. & 2009 Elsevier GmbH. All rights reserved.

Introduction The conversion of natural tropical ecosystems to agricultural systems may accelerate the degradation of the soil. This implies not only a change in soil chemical properties, but also a reduction in biodiversity (Lal 1988; Lavelle et al. 1992; Decae ¨ns et al. 2001; Hairiah et al. 2001). Furthermore, soil fertility decreases rapidly following land clearing and this is accelerated by erosion of the exposed soil, an increase in the mineralisation of soil organic matter and litter due to elevated temperatures, the leaching of nutrients, and the deterioration of the soil structure as a consequence of decreased activity of soil macro-organisms (Lavelle et al. 1992). Unfortunately, the effects of land use change and agro-industrial production on the quality of the soil and soil biota in the tropics are poorly known (Brown et al. 2001; Decae ¨ns et al. 2001; Hairiah et al. 2001). Adequate management of agricultural land may favour the environmental conditions required by certain groups of soil flora and fauna. The importance of soil fauna is due to its influence on soil physical and chemical properties, and on the production potential of soils (Tian et al. 1997; Brown et al. 2001). Mexico has lost more than 95% of its original forests in the past 60 years (Durand and Lazos 2004). The lack of non-land-based economic opportunities has resulted in a steady increase in land clearing for agricultural and pasture purposes, which in turn has led to a loss of forest and habitat for species of flora and fauna. At present, some efforts have been made to stop the process of deforestation, although with very limited success. In the area of this study, Tabasco, drastic land use changes have occurred over the past 60 years, and the forest cover has decreased from 58% to 5% (Zavala-Cruz and Castillo-Acosta 2002). Nowadays, agriculture – mainly banana and other fruit plantations – and pastures occupy around 66% of the state’s land area (INEGI 2005). This severe land use change has caused a significant alteration in soil fertility and soil fauna (Palma-Lo ´pez et al. 2002; Geissen and Morales Guzman 2006). The goals of this study were to evaluate the effects of land use on soil chemical properties and to compare earthworm communities and litter decomposition rates in three typical land use

systems in SE Mexico, namely banana plantations, agroforestry systems and a successional forest. Furthermore, special attention was paid to the relationships between soil physical and chemical parameters, earthworm communities and litter decomposition.

Methods Study Area The banana-producing municipality of Teapa is located in the sierra region of the state of Tabasco, SE Mexico. It has a surface area of 680 km2 (INEGI 2005) and is part of the watershed of the Sierra River. It is strongly affected by flooding of the rivers Teapa and Puyacatengo (Figure 1). The climate is generally warm and humid with an average annual precipitation of 3862 mm. The average annual temperature is 25.4 1C (INEGI 2005) and three seasons – low rainfall (April–May), tropical rainfall (June–October) and moderate rainfall (November–March) – characterize the region. The predominant soils in the area are Gleysols and Fluvisols with a clay texture, which are inundated when the rivers overflow during the high rainfall season. The parent material of all the soils in the region is homogeneous and is characterized by loamy or clayey alluvial material. Thus, the composition of the parent material did not differ among the selected sampling sites. The original vegetation in the Teapa region is tropical rainforest, evergreen seasonal forest and lowland riparian forest. Nowadays, banana cultivation dominates the region, with an open ditch drainage system that consists of principal, secondary and tertiary drains.

Study sites Two banana plantations (Musa acuminata, 25 and 30 years of cultivation, 1800 plants ha1) (B1, B2), two agroforestry systems (with 12 years of cultivation, M. acuminata, 200 plants ha1) intercropped with Spanish cedar (Cedrela odorata, 1200 plants ha1) (AF1, AF2) and two successional forests (S1, S2) were studied (Figure 1). All sites were originally used as riparian primary forest. Banana and

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Figure 1. Location of the study area (B1, B2: banana sites, AF1, AF2: agroforestry sites, S1, S2: succession forest sites).

agroforestry plantations are intensively managed with NPK fertilization and pesticides. Fungicides such as bithiocarbamates against Black Sigatoca have been applied weekly by air at doses of 2.5 kg ha1 since 1995. All cultivated areas are drained and therefore suffer fewer flooding events. S2 and S1 are 15- and 20-year-old lowland riparian, periodically flooded successional forests, dominated by the middle stratum tree Bravaisia integerrima (Daniel, 1988) that reaches heights of 15–20 m. The understory is composed of shade-

tolerant palms, succulent herbs and broad-leaved Heliconias. Woody lianas, vines and strangling figs are also common. These forests do not have a drainage system.

Experimental design, sampling and analysis The study was carried out from November 2005 to April 2006. A completely randomized experimental design was established within each of the

ARTICLE IN PRESS 78 six sites (B1, B2, AF1, AF2, S1 and S2) with 10 plots of 4 m2 per site and a distance of 50–200 m between each plot. Each site covered 1 ha. Areas close to trees, banana plants and/or drains were avoided. It was considered that soils under the same land use system, resulting from natural conditions, were similar. However, natural conditions also limited the desired statistically optimal experimental design. Therefore, the results obtained for each site were analyzed separately and all sites were compared. Earthworms were collected from each of the 60 plots. For financial reasons, only half of the plots (30) were sampled for soil analyses. Earthworm and soil sampling took place in December 2005. In addition, a leaf decomposition experiment using two different leaf species (B. integerrima and M. acuminata), the two dominant vegetation types in the region, was carried out in the same plots over a period of 8 weeks, from February to April 2006.

Soil physical and chemical parameters Soil samples were taken for soil analysis at two different depths from five of the ten plots. The samples were taken from the undisturbed walls of the monolith pits at 0–10 and 10–30 cm (Swift and Bignell 2001). The soil analyses followed the specifications of the Mexican soil analysis norm (SEMARNAT 2000). The following parameters were analyzed: soil texture (Bouyoucos method), bulk density (BD) (cylinder), pH (KCl), Corg (Walkley and Black), Ntot (Micro-Kjeldahl), available P (Pavail) (Olsen), exchangeable K (Kex), and CEC that was determined with the ammonium acetate extraction method at pH 7 (Etchevers and Barra 1992). Available Mn and Zn (Mnavail, Znavail) were analyzed with a DTPA extraction.

Earthworms The technique employed was based on the methods proposed by Anderson and Ingram (1993). At each sampling plot, earthworms were extracted by hand from a 25  25  30 cm monolith at depths of 0–10 and 10–30 cm. All earthworms were collected and preserved in 4% formaldehyde for future identification. Local experts were involved in the identification of species due to the lack of a taxonomic key for Latin American earthworms. Unidentified individuals were classified as ‘other species’. Earthworms were washed to remove the formaldehyde, dried with absorbent paper and weighed

V. Geissen et al. individually. The weights were corrected for loss by multiplying by 1.13. This factor was estimated in experiments that calculated the weight differences for 20 fresh and preserved earthworms. The earthworm communities at each site were characterized by population density (individuals m2), biomass (g m2), species richness and diversity. The earthworm diversity recorded for the different sites was compared using the Shannon index (Moreno 2001). The indices were calculated based on the earthworm density per monolith.

Litter decomposition Litter decomposition was evaluated using the litterbag technique (Swift et al. 1979). Litterbags (15  17 cm) with a 5 mm mesh size were used. This mesh size allows access by all groups of soil animals (micro-, meso- and in part macrofauna) and minimizes the loss of material – in comparison with a larger mesh – during transportation. This coarse mesh size allowed the determination of the actual decomposition rate including the activity of all the soil biota. B. integerrima and M. acuminata leaves were used in order to compare the decomposition rates of a native and a cultivated species. Initial C/N ratios were 17 and 22 for B. integerrima and M. acuminata, respectively. In January 2006, fresh Bravaisia leaves were harvested directly from various adult trees in S1 and S2. Musa leaves were cut from adult plants – outside the banana region – to avoid introducing a bias into the experiment due to fungicides. The collected leaves were oven dried at 105 1C for 24 h to eradicate all bacteria and fungi (Geissen 2000). Dry leaves were cut into small 2cm-long and 1-cm-wide pieces to fit in the bags. After drying, the litterbags were filled with 5 g of banana leaves or with 3 g of Bravaisia leaves. Site S1 was not included in the study because of a long period of flooding. In February, 25 litterbags per species were placed in the upper 5 cm of the soil in five plots, resulting in five replicates, with five collection dates (2, 4, 5, 6 and 8 weeks exposure). After collection, the bag contents were rinsed with water in a tray to remove soil residues. Leaf remains were oven dried at 105 1C for 24 h after which weight loss was recorded. The content of Ntot and Ctot of the remaining leaves was analyzed at the beginning and end of the experiment, using two mixed samples per site, by dry combustion in a Leco CHN 1000 combustion analyser (LECO 1988).

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0.36a70.10 0.28a70.02 1.41b70.33 1.61b70.38 1.31b70.89 1.04b70.27 44.8e76.94 3.1b70.75 8.5c76.56 19.6d74.79 14.3d77.58 1.9a70.73 47.7b73.76 48.4b71.31 43.6b74.65 33.2a72.89 32.8a73.31 34.1a74.97 0.56b70.06 0.50b70.11 2.13d70.82 1.23c70.58 0.44b70.15 0.23a70.04 12.5b70.55 11.6a73.06 12.0b71.47 8.3a71.65 b 11.0 71.40 26.9c70.98 10.4b71.08 18.6b76.88 10.2ab72.97 19.6b74.56 8.5a71.17 9.8a76.72 6.8c70.18 3.53c70.95 5.5b70.11 2.35bc70.33 5.9bc70.97 2.19b70.77 4.4a70.24 1.27a70.26 5.0ab70.87 1.27a70.30 6.9c70.07 1.75ab70.87

0.29c70.05 0.24bc70.02 0.20b70.02 0.12a70.02 0.13a70.04 0.21b70.05

Znavail (mg kg1) Mnavail (mg kg1) CEC (cmol kg1) Kex (mg kg1) Pavail (mg kg1) C/N (ratio) Ntot (g 100 g1) Corg (g 100 g1)

S1 S2 AF1 AF2 B1 B2

Only the soil physical–chemical parameters for the 0–10 cm depth are presented, as 80% of the

pH (KCl)

Physical–chemical soil properties

Site

Results

Table 1.

An analysis of variance (ANOVA) was performed to test for significant differences between the study sites, when the normality assumption (Kolmogorov–Smirnov test) was met. Prior to the ANOVA the data were inspected for homogeneity of variance (Levene test). After obtaining a statistical significance with the ANOVA, a post-hoc test (Dunnett’s T3) was used to determine significant differences (po0.05) between single sites. In the cases of nonnormal distributions of the data, the Mann–Whitney U-test was carried out. All tests were conducted using the software SPSS v. 12.0. Soil properties are interpreted following the criteria established in the Mexican norm (SEMARNAT 2000). Litter decomposition was estimated for each site and species. The remaining weights (%) were plotted against time and fitted to a negative exponential function: Mt ¼ M0 ekt where Mt is the remaining mass at time t, M0 is the mass weight at time 0 or at the start of the experiment, and k is the decomposition constant. This curve implies the loss of a constant fraction of weight over successive intervals of time (Swift et al. 1979). Functions were calculated using the software CFIT (Helfrich 1996). The differences calculated for the equation slopes were tested using multiple t-tests. Cluster analyses were carried out following STATISTICA v. 6.0. For agglomerative cluster analyses, Euclidean distances were chosen as distance measures and average linkages as the clustering algorithm. A cluster analysis of the sites (as cases) was applied including the chemical and physical parameters as variables (n ¼ 13). A Canonical Correspondence Analysis (CCA) was used to explore correlations between the physical and chemical soil parameters for the 0–10 cm depth, the densities of the single earthworm species and the decomposition rates. A Monte Carlo permutation was conducted to statistically test whether the species were significantly correlated with the environmental variables. A CCA bi-plot was employed to visually represent the environmental conditions at the sites and the distribution of the earthworm communities, using the software CANOCO v. 4.5. Site S1 was not included in the analysis, as no litter decomposition experiment was carried out there.

Physical–chemical soil properties of the different sites (BD ¼ bulk density) (mean and standard deviation; significant differences (po0.05: aoboc)).

Statistical analyses

0.90a70.02 1.00b70.02 0.94a70.06 1.08b70.05 1.04b70.04 1.09b70.10

79 BD (g cm3)

Effects of different land use in tropical Mexico

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V. Geissen et al.

S1 S2 AF1 AF2 B1 B2

1.5

2.0

2.5

3.0 3.5 4.0 Euclidean distances

4.5

5.0

Figure 2. Cluster dendrogram (Euclidian distances) performed by physical–chemical soil properties (n ¼ 6 sites).

earthworms were found in this upper soil layer and litter decomposition was tested there as well (Table 1, Figure 2). The dominant textural class in these soils was clay. S1 and S2 had clay-textured soils, AF1 silty clay, AF2 and B1 clay loam, and B2 silty clay loam. Soil bulk densities varied between 0.9 (S1) and 1.09 g cm3 (S2). Three main clusters were identified, with two (I), one (II) and three (II) sites, respectively (Figure 2). Cluster I consisted of successional forests, cluster II represented AF1, and cluster III included banana plantations and AF2. Notwithstanding that dissimilarities (Euclidean distance of 3.7) were found within forests S1 and S2, these constitute a group that is independent from the managed systems with greater Corg (3.53 and 2.35 g 100 g1) and Ntot (0.29 and 0.24 g 100 g1), lower available Zn (0.36 and 0.28 mg kg1), and a greater CEC (47.7 and 48.4 cmol kg1), in comparison with the other sites (Table 1). The dendrogram indicated dissimilarities between AF1 and the other managed systems AF2, B1 and B2 (Euclidean distance of 4.7) and a high similarity between AF2 and B1 (Euclidean distance of 1.9) (Figure 2). AF2 and B1 were both acidic with pH values of 4.4 and 5.0, respectively, with the lowest Corg and Ntot, and the highest available Zn content (Table 1). B1 and AF1 showed the highest available P content (Table 1). The highest available Mn content was recorded for S1, followed by AF2 and B1.

Earthworms Eleven earthworm species identified in the study sites belong to the five families Megascolecidae

(4 species), Acanthodrilidae (1 species), Ocnerodrilidae (2 species), Moniligastridae (1 species) and Glossoscolecidae (3 species) (Table 2). More than 80% of the earthworms were found in the 0–10 cm deep soil layer. In this study, nearly all the identified species were endogeic, with the exception of the epigeic Dichogaster bolaui (Fragoso 2005). Six species were found in the successional forests: Lavellodrilus bonampakensis, Diplotrema papillata, Phoenicodrilus taste, Ilgenia sp., Balanteodrilus pearsei and Pontoscolex sp. Differences in species number were detected between the forests, as only 2 species were present at S1: L. bonampakensis and D. papillata, and 4 species were present in S2. The native species L. bonampakensis, D. papillata and P. taste were only found in the forests. Six exotic species (Polypheritima elongata, D. bolaui, Drawida barwelli, Periscolex brachycystis, Pontoscolex sp. and Pontoscolex brachycystis) were collected in the banana plantations, as well as the native species B. pearsei (Table 2). P. elongata, D. bolaui, D. barwelli and P. corethurus were present exclusively in the banana plantations and agroforestry systems and did not occur in the forest sites. The highest Shannon index was found in site B1 0.54 (Table 3). B. pearsei had the widest distribution, as it occurred in five sites with the highest abundance in B2 (46 ind m2, Table 2). Earthworm abundance in the banana plantations and the agroforestry systems was not significantly different from those of the forest soils (Table 2). However, the biomass of all earthworms was significantly lower in the successional forest soils than in the banana plantations and the agroforestry systems. This may be due to the fact that the large mesohumic earthworm species P. elongata, P. corethrurus, B. pearsei and Pontoscolex sp. were found mainly in the B1, B2 and AF2 managed systems, whereas the small polyhumic species were predominant in the forests.

Litter decomposition Litter decomposition was fitted to a negative exponential model (Figure 3). The litter decomposition of B. integerrima was significantly faster than that of M. acuminata. After 8 weeks of exposure, 8–20% of B. integerrima litter and 4–40% of M. acuminata litter remained (Figure 3). The half-life time of B. integerrima ranged from 1.8 (AF2) to 3.1 (B1) weeks, whereas the half-life time of M. acuminata varied from 4.1 (AF2) to 5.8 (S2) weeks (Table 4). The decomposition rates did

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Table 2. a, b: Earthworm density (ind m2) (a) and biomass (g m2) (b) (median); significant differences between the sites (po0.05): aobocod (N: native species, E: exotic species) (M: Megascolecidae, A: Acanthodrilidae, O: Ocnerodrilidae, MO: Moniligastridae, G: Glossoscolecidae).

(a) Balanteodrilus pearsei (N, M) Diplotrema papillata (N, M) Lavellodrilus bonampakensis (N, M) Polypheretima elongata (E, M) Dichogaster bolaui (E, A) Phoenicodrilus taste (N, O) Ilyogenia sp. (O) Drawida barwelli (E, Mo) Periscolex brachycystis (E, G) Pontoscolex corethrurus (E, G) Pontoscolex sp. (E, G) Juveniles not identified Mean total density

S1

S2

AF1

AF2

B1

B2

0a 75b 5 0a 0 0 0 0a 0 0a 0 0 80

2ab 10b 0 0a 0 22 3 0a 0 0a 2 2 41

35cb 0a 0 27b 5 0 0 11ab 0 5a 8 3 94

16bc 0a 0 16ab 3 0 0 21ab 0 69b 0 0 125

5b 0a 0 19ab 0 0 0 22ab 0 67b 3 0 116

46c 0a 0 11ab 3 0 0 45b 5 0a 2 5 117

(b) Balanteodrilus pearsei (N, M) Diplotrema papillata (N, M) Lavellodrilus bonampakensis (N, M) Polypheretima elongata (E, M) Dichogaster bolaui (E, A) Phoenicodrilus taste (N, O) Ilyogenia sp. (O) Drawida barwelli (E, Mo) Periscolex brachycystis (E, G) Pontoscolex corethrurus (E, G) Pontoscolex sp. (E, G) Juveniles not identified Mean total biomass

0a 4.7b 3.8 0a 0 0 0 0a 0 0a 0 0 8.55a

0.02ab 0.57a 0 0a 0 0.2 0.07 0a 0 0a 0.04 1.3 2.02a

Table 3. Shannon-index for earthworms and species number in the different study sites.

5.4d 0a 0 32b 0.05 0 0 0.29ab 0 0.87a 1.7 0.05 40.4b

5.1cd 0a 0 8.6ab 0.05 0 0 0.5ab 0 21.3b 0 0 35.6b

1.2 b 0a 0 1.7ab 0 0 0 0.85b 0 16.3b 0.69 0 20.8ab

3.1 c 0a 0 0.7a 0.05 0 0 1.4b 0.1 0a 6.4 0.05 11.8ab

not vary significantly between the different sites. The C/N ratio also declined with the time of decomposition. The mean initial C/N ratios of B. integerrima and M. acuminata of 17 and 22 decreased to 12 and 14 at the end of the experiment.

(clay, C/N, CEC, Ntot, Corg), and the second axis is closely related to the pH value. Earthworm species strongly correlated to the humus component were D. papillata (Dpa), Ilyogenia sp. (Ily), P. taste (Pta), Pontoscolex sp. (Pon). D. bolaui (Dbo), P. elongata (Pel) and B. pearsei (Bpe) had a strong positive correlation with silt and K content and are related to the two main components. The last group of earthworm species is represented by D. barwelli (Dba) and P corethrurus (Pco) and had a strong positive correlation with the content of Mnavail, Znavail and BD. The litter decomposition rate was not correlated with any physical or chemical soil property (Figure 4).

Relationship between soil properties, earthworm populations and litter decomposition

Discussion

Shannon index Species number

S1

S2

AF1

AF2

B1

B2

0.06 2

0.24 6

0.49 7

0.38 5

0.54 5

0.41 7

The earthworm distribution in the different study sites was clearly delimited and correlated to the physical and chemical soil parameters (Figure 4). The first axis represents the humus component

Soil chemical properties All soils had a high clay content that explains their marked water-holding capacity, their

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V. Geissen et al. 100

Remaining weight (%)

SF2

90

AF1

80

AF2 B1 B2

70 60 50 40 30 20 10 0 0

1

2

3

4 Weeks

5

6

7

8

100 SF2 AF1 AF2 B1 B2

90

Remaining weight (%)

80 70 60 50 40 30 20 10 0 0

1

2

3

4 Weeks

5

6

7

8

Figure 3. (a, b) Litter decomposition (remaining weight (%)) of B. integerrima (a) and M. acuminate (b) in the sites B1 and B2, AF1, AF2, S1, S2 (no significant differences between the study sites at po0.05). Table 4. Half-life time (d) of the leaf litter in the different sites (columns with subscripts indicate significant differences between two leaf species at the same site (po0.05: aob). Leaf-type

SF1

AF1

AF2

B1

B2

Bravaisia integerrima Musa acuminata

2.9a 5.8b

2.1a 4.3b

1.8a 4.1b

2.8a 5.3b

3.1 4.9

No significant differences were determined between the sites.

tendency to be waterlogged (as was observed mainly in S1) and their high to very high CEC in terms of the Mexican soil evaluation system

(SEMARNAT 2000). The clusters of forest soil properties and banana/agroforestry soil properties were clearly defined, which indicated that the chemical soil properties, especially Corg, Ntot and available Zn content, were markedly different between the successional forest and the managed system. These differences may be explained by the humic conditions in the undrained forests, which could, in turn, lead to a higher humification and reduced mineralization rate. Furthermore, the agroforestry (AF2) and monoculture banana systems (B1) that showed the greatest similarities were strongly acidic with an intermediate Ntot and Corg and a high available P content, according to

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Figure 4. Relationship between physical–chemical soil properties, earthworm community and litter decomposition. (B. pearsei (Bpe), D. papillata (Dpa), P. elongate (Pel), D. bolaui (Dbo), P. taste (Pta), IlyIlyogenia sp. (I), D. barwelli (Dba), P. corethrurus (Pco), Pontoscolex sp. (Pon); bulk density (BD), Musa, Bravaisia (litter decomposition rates), CEC (cation exchange capacity), C/N (C/ N ratio in soil), biomass (W).

SEMARNAT (2000), and were located in the same farmland. This suggests that despite the intercropping of AF2 with cedars for 12 years, soil properties had not changed significantly. The similarity seems to be influenced more by spatial distribution than by management practices. According to Hornburg and Bru ¨mmer (1993), the availabilities of Mn and Zn increase with decreasing pH values (o5.5). That observation agrees with the results of the present study. AF2, the site with the lowest pH (4.4), had significantly higher amounts of available Zn and high available Mn content. However, S1 (pH 6.8) had the highest available Mn content (44.8 mg kg1), probably due to reducing conditions that resulted from flooding whereby Mn was mobilized from oxides (Ahn 1993).

Earthworms In Mexico, 93 earthworm species have been described of which 46 are native and 47 are exotic (Fragoso 2001). The presence or absence of native species may be an indicator of soil perturbation. Competition from exotic species or changes in habitat (e.g., vegetation cover), even without soil disturbance might also be important. The native species collected in this study were found only in forests, whereas mainly exotic species were present in the banana and agroforestry ecosystems. B. pearsei, a large native species, was found in both, the forest and managed systems. Apparently, this

83 species is well adapted to perturbation, as Huerta et al. (2006) previously described. S1, with the longest flooding periods, had the lowest species richness, followed by the periodically flooded and non-drained forest site S2 that supported 5 species. However, earthworm density in S1 (80 ind m2) was comparable to that recorded by Liu and Zou (2002) for a damp forest in Puerto Rico (89 ind m2). The low species number in S1 and S2 can be explained by the periodic flooding that may prevent earthworms that are not tolerant to flooding from inhabiting these soils (Fragoso and Lavelle 1992; Dechaine et al. 2005). It was assumed that the low species number in S1 was a consequence of the extremely wet conditions at the site. Geissen and Morales Guzman (2006) found a two-fold higher earthworm abundance in non-flooded successional forests of the same region. Another unmeasured factor such as the nature of food supply may also influence the difference in the earthworm species composition in the forest sites and the managed systems. Earthworm density and biomass in the agroforestry systems and banana plantations in this study were also low in comparison with other studies (Araujo and Lo ´pez-Herna ´ndez 1999; Lapied and Lavelle 2003; Huerta et al. 2005). However, Geissen and Morales Guzman (2006) found similar abundances in pastureland in the same region. The earthworms showed an aggregative distribution and were concentrated mainly in the upper 10 cm of the soil (ratio 4:1 for 0–10 cm soil compared to 10–30 cm soil). Previous studies have shown that this distribution of earthworms in soil is common in tropical ecosystems, and is explained by the concentration of nutrients in the upper layers (Fragoso and Lavelle 1992; Araujo and Lo ´pezHerna ´ndez 1999; Sa ´nchez de Le´on et al. 2003; Dechaine et al. 2005; Huerta et al. 2005). We conclude that the installation of drainage systems influenced earthworm communities more than pesticide or fertilizer applications in the banana and agroforestry plantations. The high application of bithiocarbamate fungicides in the banana plantations and the agroforestry systems did not lead to an accumulation of this pesticide in soils (Melgar Valdes et al. 2008), which is likely why earthworm abundance and diversity were not strongly affected.

Litter decomposition The decomposition rates did not vary among the different sites probably due to the high spatial heterogeneity of the soil characteristics, such as

ARTICLE IN PRESS 84 soil moisture and temperature that affect the biological activity of the soil (Swift et al. 1979). It was clearly shown that banana leaves were less susceptible to microbial and/or soil animals activity compared to B. integerrima leaves, which is attributable to the litter quality and chemical composition of the leaves. Many authors have shown that litter quality is one of the determining factors in the decomposition process (Anderson 1975; Swift et al. 1979; Liu and Zou 2002; XulucTolosa et al. 2003; Vasconselos and Laurance 2005). The higher initial C/N ratio of banana leaves, as well as their high content of lignin (6.7–17.2%), hemicellulose and cellulose (52%) (Lekasi et al. 2001) are assumed to be responsible for the slower decay rate of M. acuminata (Swift et al. 1979; Tian et al. 1995). Similar short half-life times were reported by Geissen and Morales Guzman (2006) who attributed the fast decomposition (half-life time 1.2–3.8 weeks) of leguminous litter in Teapa to the low initial C/N ratio of the leaves (12). Decomposition experiments carried out by Lekasi et al. (2001) reported a similar initial C/N ratio (20) for banana leaves, however, the half-life time of the leaves was 60 days (about 8.5 weeks) which is slightly above our findings (4.1–5.8 weeks). Although the experiment here was not designed to quantify faunal effects on decomposition, it was assumed that the soil fauna contributed to both the physical and chemical degradation of plant residues. It was observed during the collection of litterbags that some of them contained earthworm casts, which may indicate a contribution of earthworms during the decomposition process (Anderson 1975; Swift et al. 1979).

Relationships between soil properties, earthworms and litter decomposition In contrast with Geissen and Morales Guzman (2006), no correlation was observed between the physical–chemical soil properties, and the litter decomposition rate, which suggests that the microclimate and the water content at the sites are the predominant factors in decomposition, as has been suggested in other studies (Tian et al. 1997; Baker et al. 2001). D. papillata is a native and polyhumic species whose distribution is correlated with organic matter and Ntot content, as has been previously recorded for other polyhumic species by Huerta et al. (2007) in tropical forests. The exotic species P. corethrurus, D. barwelli and P. elongata present in the banana and agroforestry systems in this study

V. Geissen et al. are known to have wide environmental tolerance ranges and a high adaptability to different ecosystems (Fragoso and Lavelle 1992; Talavera-Sosa 1992; Barois et al. 1999; Fragoso et al. 1999; Lapied and Lavelle 2003).

Conclusions We conclude that the intensive management of banana monoculture and agroforestry systems does not lead to a decrease in nutrient content or in litter decomposition rate in comparison with successional forests. Earthworm populations changed from native to exotic species. However, the diversity and species richness were higher in managed soils than in successional forests. We also suggest that water restrictions in the study area are more important to the earthworm populations than the application of pesticides or fertilizers to the banana and agroforestry plantations. More studies are required, however, to test the effect of these pesticides on earthworm populations.

Acknowledgements We are grateful for the financial support provided by the Ministry of Agriculture of Tabasco (SEDAFOP). We also thank Andrea Macbeth for language editing.

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