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Fertility of tropical soils under different land use systems—a case study of soils in Tabasco, Mexico

Conference Paper

Fertility of tropical soils under different land use systems—a case study of soils in Tabasco, Mexico

Fertility of tropical soils under different land use
systems—a case study of soils in Tabasco, Mexico
V. Geissen *, G. Morales Guzman
El Colegio de la Frontera Sur, Administracio
´n Correos 2, Apartado Postal 1042, 86100 Villahermosa, Tabasco, Mexico
Received 15 March 2004; accepted 18 February 2005
Abstract
Increasing deforestation in Mexico in the past 40 years has led to significant land use changes. It is important to establish land
use systems that allow for the necessities of an increasing population and the conservation ofsoil fertility in the long term. In this
study, we investigated the influence of different land use forms on soil fertility in Tabasco, SE Mexico. We chose two different
commonly used pastures (Cynodon plectostachyus and Brachiaria decumbens) and a succession forest. We characterised soil
fertility by physico-chemical parameters (texture, density, pH, P, Corg., Ntot., cation exchange capacity (CEC)) as well as by
biological parameters, such as litter decomposition, microbial biomass and earthworm community. To estimate litter
decomposition we used leaves of Gliricidia sepium, a common fodder tree in the region. The three land use systems had
very similar soil chemical characteristics. All three can be characterised as acidic (pH between 4.1 and 5.3) with a high content of
organic matter and total nitrogen. However, the three land use systems differed significantly with respect to their soil biological
characteristics. Earthworm density as well as litter decomposition were significantly lower under B. decumbens than in the other
soils. In all land use systems, the participation of macrofauna and mesofauna accelerated litter decomposition rate significantly
as compared with decomposition with microfauna and microflora alone.
We extracted two components of the pool of data by main component analysis. The acidity component explained mainly the
microbial litter decomposition rate. The rate of litter decomposition – with participation of soil meso- and macrofauna – could be
explained by the humus component. We assume that biological parameters were more suitable to characterise differences
between the different land use systems. The use of C. plectostachyus and succession forest showed a positive effect on soil
fertility.
#2005 Elsevier B.V. All rights reserved.
Keywords: Litter decomposition; Soil fertility; Tropical soils; Earthworms; Land use systems
1. Introduction
In the last 60 years, Tabasco, Mexico, as in many
other tropical areas, has had a high rate of deforesta-
tion. Along with the deforestation, there has been a
drastic change in land use. In 1940, 49.1% of Tabasco
www.elsevier.com/locate/apsoil
Applied Soil Ecology 31 (2006) 169–178
* Corresponding author. Tel.: +52 993 3136110x3405;
fax: +52 993 3136110.
E-mail address: vgeissen@web.de (V. Geissen).
0929-1393/$ – see front matter #2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsoil.2005.02.012
was used as forestland and only 20.7% as grassland
(Palma Lo
´pez and Triano Sa
´nchez, 2002). Today only
13.6% of the area of Tabasco is covered by forestland,
while 42.4% is grassland that is cultivated for cattle
management. This change of land use has caused
problems, such as soil erosion and decreasing soil
fertility in Tabasco (Ortiz et al., 1994). It is important
to establish land use systems that allow for the
demands of increasing population while conserving
the soil fertility in the long term. First efforts have
been made to establish pasture-forestry land use
systems in the region. Very few investigations about
actual soil fertility under the influence of different
pastures in comparison of forestland in Tabasco have
been made (Palma Lo
´pez and Triano Sa
´nchez, 2002).
Furthermore, soil fertility often only is defined by
chemical and physical parameters of the soil (Prado
Wildner and da Veiga, 1993).
The aim of this study is to compare the influence
of the two commonly used pastures (Cynodon
plectostachyus and Brachiaria decumbens)anda
succession forest on soil fertility in Tabasco. We
chose these systems because these two pastures
cover a high percentage of Tabasco. In some areas,
farmers have begun to integrate succession forest in
grassland as a new form of land use systems. Based
on the results of this study, we make a recommenda-
tion of a land use system that conserves soil fertility.
Besidessoilchemicalandphysicalparameters,we
also included soil biological parameters, such as
decomposition rate and earthworm community to
compare soil fertility in different land use systems.
We chose earthworms because they are one of the
most important macro-decomposer groups in the
tropics (Swift et al., 1979). Furthermore, we studied
the relationships between the different parameters
investigated.
2. Materials and methods
2.1. Site description
The investigations took place in the region of
Teapa, Tabasco, in southeast Mexico. The site is
characterised by warm and humid tropical climate
with precipitation throughout the year. The average
temperature is 25.4 8C and the mean annual pre-
cipitation is 3862.5 mm. The maximal precipitation
occurs in September with 600 mm, and the minimal
precipitation in April with 150 mm (INEGI, 2000).
The test site is located in an area about 600 m above
sea level. The test sites had an inclination of between
30 and 40%. Typical soils in this region are humic
Acrisols above sandstone. The soil depth varies
between 0.60 and 1.00 m. The depth of the organic
A horizon is about 15 cm. Typical land use systems in
this region are pastures with B. decumbens and
pastures with C. plectostachyus.B. decumbens is
described as very resistant to diseases and supportive
of low soil fertility, with a low protein content of about
3% (Renrun et al., 2003). C. plectostachyus is used by
the farmers to increase fodder quality because its
protein content is very high, between 5 and 15%
(Ramos Jua
´rez et al., 1995). In all experimental sites,
former vegetation was Panicum maximum on humic
Acrisols, i.e. the differences found between the sites
was caused by the present land use forms. In 1988 B.
decumbens and C. plectostachyus were seeded. The
succession forest was planted in 1998. The vegetation
in the forest is very dense with a high variety of
tropical trees and undergrowth vegetation. Dominant
species of vegetation are: Vernonia ssp., Hampea ssp.,
Heliocarpus ssp., Acacia ssp., and Cassia ssp.
Fertilizers were not applied under any of the land
use systems. The mean vegetation cover in pastureland
differed from 95% under C. plectostachyus and the
succession forest, to 80% under B. decumbens.
2.2. Experimental design
The investigations were carried out from July to
November 2002. We installed three replication plots in
each land use form, each with a size of 100 m
2
. The
whole study area had a size of about 5 ha.
2.3. Soil chemical and physical parameters
In July 2002, we took three soil samples in each
plot in three depths (0–5 cm, 5–10 cm, and 10–
20 cm) to determine soil chemical and physical
properties (Tab le 1), i.e. nine samples per depth and
land use system, 27 samples in total in each land use
system. We selected soil physical and chemical
parameters due to the land evaluation system of
Landon (1984): soil texture and density as important
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178170
for soil water and air conditions, pH that char-
acterises soil acidity and is strongly correlated with
base saturation, Corg., Ntot., Pdisponible and cation
exchange capacity (CEC) that are important para-
meters to characterise soil fertility for plant
production (Landon, 1984).
We used ANOVA followed by the Scheffe test to
estimate significant differences between the different
land use systems. The probability was corrected by the
Sidak procedure for multiple comparisons.
2.4. Soil biological parameters
2.4.1. Microbial biomass
To quantify microbial biomass we estimated the
basic respiration by the Isermeyer technique (Schinner
et al., 1996) and calculated biomass from the data
obtained. We analysed the microbial biomass from the
same samples we used for soil chemical analysis in the
depths 0–5 cm and 5–10 cm from the July sampling.
In October we repeated the sampling to estimate
microbial biomass in two different seasons. We used
ANOVA followed by the Scheffe test to estimate
significant differences between the different land use
systems. The probability for multiple comparisons
was corrected by the Sidak procedure.
2.4.2. Decomposition rate
To quantify the decomposition rate of leaf litter in
the different land use systems we applied a modifica-
tion of the litterbag test described by Swift et al.
(1979). We used 10 cm 10 cm litterbags and filled
each litterbag with 4 g of dried leaves of Gliricidia
sepium which is a leguminous tree commonly used in
the region to feed cattle. Its C/N ratio is described as
12 (Abunyewa et al., 2004). For the litterbags we used
mesh sizes of 5, 2 and 0.02 mm. The use of different
mesh sizes allowed us to estimate the role of the
different groups of the soil biota (macro-, meso-,
microfauna and microflora) in the process of decom-
position. In the litterbags with 5 mm mesh size, all
groups of the soil biota can enter, the litterbags with
2 mm mesh exclude the macrofauna and in the
litterbags with 0.02 mesh, only the microfauna and
microflora have access.
Since the decomposition rate in tropical countries
in very high, especially if the C/N ratio of the leaf litter
is low, the time of investigation was only 10 weeks. On
the 25th of July, we inserted the litterbags into the
upper 5 cm of the soils, the layer with the highest
biological activity. We inserted three replications for
each date of collection, i.e. 15 litterbags of each mesh
size per plot. Every 2 weeks, we collected the
litterbags and estimated the weight loss of the leaves
(five samplings in total).
Mass losses of the leaves were fitted to negative
exponential function (y=e
ax
). Differences between
the regression slopes of the different land use systems
were tested by multiple t-test using the Sidak
procedure for correcting probability levels.
2.4.3. Earthworms
We investigated the earthworm communities in
July and October 2002. We extracted endogeic and
anecic earthworms in each plot in three areas of
50 cm 50 cm, i.e. nine replications in each land use
system by a combination of formalin extraction and
hand sorting (Schinner et al., 1996). We only
investigated endogeic and anecic species because
epigeic species normally do not appear in pastureland.
We determined abundance and biomass of the
earthworms following Borges (1988). We compared
abundance and biomass of the earthworm species in
the different land use systems using the U-test. The
probability for multiple comparisons was calculated
according to the Sidak procedure.
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178 171
Table 1
Analysis of soil chemical and physical parameters
Parameters Methods of analysis
pH Extraction in KCl
P(extractable) Olsen and Sommers (1982)
Cation exchange capacity Secretarı
´a de Medio Ambiente y Recursos Naturales (2000)
Corg. Walkley and Black (1934)
Ntot. Kjeldahl (Chapman and Pratt, 2000)
Texture, density Secretarı
´a de Medio Ambiente y Recursos Naturales (2000)
2.4.4. Relation between the different parameters
We applied bivariate correlations using the Pearson
correlation coefficient to describe significant correla-
tions between the parameters investigated. For this ana-
lysis,weusedsoilchemicaldatafromthe depth 0 to 5 cm
and microbial and earthworm data from the October
sampling.Furthermore,weappliedprincipalcomponent
analysis to describe relations between selected para-
meters. Using main component analysis we extracted
two principal components from the data of the October
sampling from all nine investigation plots. We included
dates of 27 sampling points in the data analysis.
3. Results
3.1. Soil chemical and physical parameters
The three land use systems showed few differences
concerning their soil chemical characteristics. In all
three land use systems, we found the same soil texture:
the upper 10 cm was a sandy clay loam and at the
depth of 10–20 cm, a clay loam. The apparent density
of the soils did not differ in the different land use
systems, with ranges of 1.1–1.3 cm g
3
in all depths
(p(Sidak) <0.016) (Table 2a). All soils were very
acidic with pH values of 4.7–5.3 (Table 2b). Their
interchange capacity varied between 26.1 and
34.6 cmol kg
1
and did not show significant differ-
ences between the land use systems ( p(Sidak) 0.016).
Organic matter content decreased in all three
systems with increasing soil depth. Only in the upper
depth of 0–5 cm did the organic matter content differ
significantly between the land use systems ( p(Sidak)
<0.016). It was significantly higher under C.
plectostachyus (5.8%) than under B. decumbens
(3.9%) The organic matter content in this layer of
the succession forest did not show significant
differences with any of the pasturelands.
The content of extractable phosphorus in the upper
5 cm of the soil was significantly higher under C.
plectostachyus (12.4 mg kg
1
) than in the soils of the
other land use systems (4.8, 6.1 mg kg
1
)(p(Sidak)
<0.016) (Table 2b).
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178172
Table 2
Physical and chemical parameters of the soils under the use of the pastures C. plectostachyus and B. decumbens and succession forest (significant
differences: p(Sidak) <0.016: a <b); x: mean, s: standard deviation
Land use system Depth (cm) Sand (%) Silt (%) Clay (%) Density (g cm
3
) Texture
xsxs xsx s
(a) Physical parameters
C. plectostachyus 0–5 57.5 8.51 20.7 9.00 21.8 2.85 1.1 0.05 Sandy clay loam
B. decumbens 0–5 41.1 6.74 22.0 4.36 36.9 4.06 1.2 0.05
Succession forest 0–5 46.8 3.43 22.0 2.00 31.2 3.13 1.2 0.06
C. plectostachyus 5–10 47.5 9.67 18.7 4.58 33.8 7.49 1.2 0.08 Sandy clay loam
B. decumbens 5–10 41.9 7.55 21.6 1.67 36.5 7.77 1.2 0.03
Succession forest 5–10 45.5 7.90 21.6 5.73 32.9 6.89 1.2 0.04
C. plectostachyus 10–20 34.0 5.85 19.0 3.02 47.0 5.70 1.2 0.05 Loamy clay
B. decumbens 10–20 35.1 5.04 20.2 1.86 44.7 4.90 1.2 0.09
Succession forest 10–20 37.1 7.10 23.3 2.24 39.6 6.57 1.2 0.06
pH (KCl) Organic mater (%) Ntot. (%) P(mg kg
1
) CEC (cmol kg
1
)
xs x s xs x s x s
(b) Chemical parameters
C. plectostachyus 0–5 4.9 0.20 5.8 b 1.04 0.4 0.07 12.4 b 5.62 28.1 2.1
B. decumbens 0–5 4.9 0.53 3.9 a 1.19 0.3 0.09 4.8 a 3.69 24.6 1.8
Succession forest 0–5 5.3 0.26 4.7 ab 0.57 0.3 0.05 6.1 a 2.45 26.6 3.1
C. plectostachyus 5–10 4.8 0.26 2.4 1.55 0.2 0.11 5.0 1.43 25.6 3.5
B. decumbens 5–10 5.0 0.41 3.6 0.73 0.3 0.06 3.0 1.92 26.1 2.7
Succession forest 5–10 5.1 0.11 3.7 0.63 0.3 0.05 4.9 2.52 26.6 2.0
C. plectostachyus 10–20 4.7 0.24 2.5 0.78 0.2 0.06 3.0 1.14 29.1 3.1
B. decumbens 10–20 4.7 0.36 2.8 1.04 0.2 0.08 2.6 1.59 30.1 2.6
Succession forest 10–20 5.1 0.09 2.8 0.48 0.2 0.04 3.8 2.29 28.1 3.0
3.2. Microbial biomass
The different land use forms did not show any
differences with reference to the microbial biomass
(p(Sidak) <0.016)). In all land use forms microbial
biomass were significantly higher in July than in
October ( p<0.05). In July, microbial biomass ranged
from 194 to 245 mg biomass C g
1
dry soil, in October
from 117 to 139 mg biomass C g
1
dry soil. There
were no significant differences between the microbial
biomass of the two depths, 0–5 cm and 5–10 cm, both
depths located in the Ah horizon ( p<0.05).
3.3. Earthworms
In all three land use systems, the diversity of
lumbricids was low, with only one species of
earthworms found, Pontoscolex corethurus (Glosso-
colecidae). In both July and October, the highest
density of earthworms was found in the succession
forest (195 and 148 individuals m
2
), and the lowest
in the soils under B. decumbens (81 and 45
individuals m
2
)(Fig. 1). Whereas in July the density
in the succession forest was significantly higher than
in both pasture lands, in October abundance and
biomass of earthworms under B. decumbens were
significantly lower than in the other two land use
systems ( p(Sidak) <0.016) (Fig. 1). In all three land
use systems, summer and autumn population did not
show significant differences ( p<0.05).
3.4. Decomposition rate
The half-life time of G. sepium differed from
between 1.2 (decomposition with all groups of soil
biota) and 3.8 weeks (only microbial and microfaunal
decomposition). In all land use systems, the decom-
position rate was significant lower in those cases
where the soil fauna was excluded ( p(Sidak) <0.016)
(Table 3). The shortest half-life time was found when
all groups of the soil biota were included into the
process of decomposition. However, the exclusion of
only the macrofauna did not lead to significant
changes in the decomposition rate as compared to
litterbags with a mesh size of 2 mm ( p(Sidak)
<0.016) (Table 3).
The three land use systems differed significantly
concerning the velocity of the leaf litter decomposi-
tion in those cases when the soil meso- or macrofauna
was participating in the process of decomposition
(p(Sidak) <0.016) (Fig. 2a–c). Under B. decumbens
the decomposition rate with participation of soil fauna
was significantly slower than in the other land use
systems. However, decomposition with participation
of microfauna and microflora did not differ between
the different land use systems ( p(Sidak) <0.016).
3.5. Relation between the different parameters
Significant correlations were found between the
content of Corg. and Ntot. (r= 0.99, p<0.0001) and
between the decomposition with participation of all
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178 173
Fig. 1. Biomass (g m
2
) and abundance (individual m
2
) (mean
and standard deviation) of Pontoscolex corethurus in the different
land use systems in July and October 2002 (significant differences:
p(Sidak) <0.016: a <b).
Table 3
Half-life time of the leaves of Gliricidia sepium (weeks) in the
different land use systems under participation of the different groups
of the soil biota (significant differences between the different groups
of soil biota: ( p(Sidak) <0.016): a <b)
Land use form All soil
biota
Soil biota
<2mm
Only
microflora
C. plectostachyus 1.2 a 1.5 a 3.5 b
B. decumbens 2.3 a 2.7 ab 3.5 b
Successional forest 1.4 a 2.0 a 3.8 b
fauna and decomposition with participation of soil
biota <2mm(r= 0.82, p= 0.006). Furthermore, there
were significant positive correlations between the
content of C (N) and decomposition with participation
of the soil fauna (r= 0.88(0.96), p= 0.002(0.0001)).
The decomposition rate with participation of micro-
fauna and microflora was significantly negative
correlated with the pH value (0.75, p= 0.021).
Microbial biomass and earthworm biomass and the P-
content were not significantly correlated with one of
the other parameters.
To describe further relations between the different
parameters we extracted two principal components
from the following variables: pH, Corg., earthworm
biomass, decomposition with participation of all soil
fauna and decomposition with participation of
microfauna and microflora. 84.6% of the variance
of the parameters: were explained by the components
extracted: organic matter component explained 48.2%
of the variance, soil acidity 36.4% of the variance
(Fig. 3). Abundance and biomass of P. corethurus was
partly explained by the organic matter component and
partly by acidity. Decreasing acidity and increasing
organic matter content was related with increasing
abundance of P. corethurus. Decomposition of
microflora and microfauna was mainly explained by
soil acidity factor. As the soil pH increased,
decomposition of microflora and microfauna
decreased. Litter decomposition with participation
of soil animals was mainly explained by the organic
matter component. High content of Corg. coincided
with a high decomposition rate (Fig. 3).
4. Discussion
4.1. Soil chemical parameters
The majority of the soils of the three land use
systems were very acidic (SEMARNAT, 2000) with
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178174
Fig. 2. Calculated decomposition of leaf litter in C. plectostachyus,
B. decumbens and succession forest under the participation of
different groups of soil biota: (a) only microflora and microfauna;
(b) soil biota <2 mm; (c) all groups of the soil biota (significant
differences ( p(Sidak) <0.016): a <b).
Fig. 3. Principal components extracted from the data of October
sampling (EB: earthworm biomass, C: Corg., dm: decomposition
with participation of microflora and microfauna, df: decomposition
with participation of all soil biota).
pH values <5.0 in all depths of 0–20 cm. Since the
soils never have been treated with fertilizer, low pH
value is result of natural acidification. The different
land use systems did not influence soil acidity.
The content of organic matter in the upper 5 cm was
high (3.9–5.8%) in all systems (Secretaria de Medio
Ambiente y Recursos Naturales, 2000). The signifi-
cantly higher organic matter content of C. plectosta-
chyus is probably caused by a higher amount of leaf
litter and dead roots entering the system during the year.
All soils showed a medium content of organic matter
down to the investigated depth of 20 cm (Secretaria de
Medio Ambiente y Recursos Naturales, 2000).
The content of Ntot. was intermediate in the upper
20 cm of the soils in all ecosystems. The different land
uses influenced neither the content of organic matter
nor the content of Ntot. This was a surprising result
because the raw protein content of the vegetation of
the succession forest (mean of 20% raw protein in the
leaves, Go
¨l, 1975) and of C. plectostachyus (5–15%
raw protein in leaves (Mele
´ndez et al., 1980), 10–12%
raw protein in leaves (Ramos Jua
´rez et al., 1995), is
higher than Bracheria decumbens (5.4–6% raw
protein in leaves (Vela
´zquez and Cuesta, 1990).
As well as the content of Corg., the content of
exchangeable Pin the upper soil under C. plectos-
tachyus was high, while the upper soil under B.
decumbens was characterised by a lack of P. The P-
content in soil under succession forest was char-
acterised as medium (Secretaria de Medio Ambiente y
Recursos Naturales, 2000). The high content of plant
available Punder C. plectostachyus can be explained
by the high organic matter content. Especially in
tropical countries, where Pis strongly adsorbed by Fe
and Al oxides, increased humus content leads to a P
mobilization (Szott and Melendez (2001).
The cation exchange capacity of all soils was high
(Secretaria de Medio Ambiente y Recursos Naturales,
2000) due to their loamy texture and the high humus
content.
The evaluation of soil chemical parameters
indicates that C. plectostachyus had the best effect
on soil fertility described by soil chemical parameters
in the humic Acrisols investigated. This may be
because the succession forest was established only 4
years ago and this is likely an insufficient time for the
leguminous trees, such as Acacia ssp., Cassia ssp. to
have a positive effect on soil fertility.
4.2. Soil biological parameters
4.2.1. Microbial biomass
Microbial biomass was low in the organic layer
with values from 129 to 249 mgg
1
dry soil in
comparison with other studies in tropical soils (Singh
et al., 1989; Ghani et al., 2003). In contradiction to
other authors we did not find any effect of land use
forms on microbial biomass. Powlson et al. (1987),
Anderson and Domsch (1989) and Ghani et al. (1999)
found microbial biomass to be very sensitive to short
term changes in soil management. However, Ghani
et al. (1996) and Bolinder et al. (1999) described that
changes of microbial biomass occur gradually and,
therefore, are difficult to detect in the short or medium
term. This may be an explanation for the lack of
influence of the different land use systems in microbial
biomass.
4.2.2. Earthworms
In all land use systems only one species of
earthworms was found: the endogeic, tropical earth-
worm Pontoscolex corethurus (Glossoscoleidae)
(Trigo et al., 1999). The appearance of this species
in Tabasco has already been described by Fragoso
(2001). This may be due to the way the earthworms
were extracted from the mineral soils. Therefore,
epigeic species were not considered. However, in
grassland we would not expect epigeic species. Ortiz
(2000) found in succession forests three to four species
and in grassland two to nine species. The abundance of
the earthworms in our study differed from 45 to 195
individuals m
2
. These abundances correspond to the
average abundances and biomass as found by Ortiz
(2000) and Fragoso and Lavelle (1992) in succession
forest and Ortiz (2000) and Fragoso (2001) in
grasslands of Mexico. Under C. plectostachyus and
under the succession forest the population of
Pontoscolex corethurus was significantly higher than
under B. decumbens. This is probably caused by the
strong relation between population development and
soil humidity and temperature as described by Lavelle
et al. (1987) and Fragoso and Lozano (1992). The
vegetation cover under B. decumbens (80%) was less
dense than under the other land use forms (95%).
Therefore, the soils under C. plectostachyus and
succession forest were more protected against dryness
and high temperatures than under B. decumbens. This
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178 175
fact may explain the significantly lower abundance of
Pontoscolex corethurus under B. decumbens.
4.2.3. Decomposition rate
The presence of soil fauna accelerated the
decomposition rate in all three land use systems.
However, inclusion or exclusion of only macrofauna
did not affect decomposition rate. This leads to the
conclusion that meso and microfauna play an
important role in the process of decomposition of
these tropical soils. Heneghan et al. (1998),Seta
¨la
¨
et al. (1991) and others also described the importance
of soil micro-arthropods in leaf litter decomposition in
European forest soils.
The half-life time of leaf litter of 1.2–3.8 weeks
was very short. Abunyewa et al. (2004) described a
similar half-life time of 22 days for leaves of G.
sepium. The fast decomposition of these leaves is due
to the fact that the C/N ratio of G. sepium is very small
with values of 12 (Abunyewa et al., 2004). Zhang and
Zak (1995),Geissen and Bru
¨mmer (1999),Moro and
Domingo (2000) and others confirm the strong
correlation between C/N ratio of leaves and decom-
position rate. Furthermore, temperature and soil
moisture have large effects on litter decomposition
(Berg et al., 2000). That means that under tropical
conditions with an average temperature of 25.4 8C and
annual mean precipitation of nearly 4000 mm
decomposition is accelerated.
Whereas the decomposition with participation of
soil fauna was significantly higher in the soil under C.
plectostachyus the sites did not differ concerning litter
decay with participation of microfauna and micro-
flora. This may be because there was no significant
difference of soil acidity in the different land use
systems, which seems to be the factor of main
importance in decomposition by microfauna and
microflora.
However, litter decay with participation of soil
meso and macrofauna differed significantly between
the land use forms. Under Bracheria decumbens litter
decay with participation of all groups of the soil biota
was significantly slower than under secondary forest
and C. plectostachyus.
In this study, the influence of soil meso- and
microfauna on the process of decomposition is
obvious. In all three land use systems, decomposition
rate was significantly higher with the participation of
the mesofauna than without its participation. How-
ever, inclusion or exclusion of macrofauna did not
influence the decomposition rate. Positive effects of
soil fauna on litter decomposition – especially of litter
with a high N content – was already described by Swift
et al. (1979),Wise and Schaefer (1994) and others. It is
surprising that in contrast to other studies of litter
decomposition in tropical soils (Fragoso and Lavelle,
1992; Pashnasi et al., 1994) where earthworms are
described as the group of soil fauna which is the most
important for the process of litter decomposition,
macrofauna in our study did not have any impact on
decomposition rate. The importance of microarthro-
pods in the process of decomposition in wet tropical
forests has been confirmed by Heneghan et al. (1999)
and Gonza
´lez and Seastedt (2001).
4.3. Relation between the different parameters
Biological parameters, such as density of earth-
worms and their biomass and decomposition rate were
strongly related with soil chemical parameters. Muys
and Granval (1991) also demonstrated that in
temperate climate the nutrient status of soils is the
main determinant of earthworm communities. Several
authors confirm this relation in forests of temperate
climate (Makeschin, 1991; Piol and Josens, 1995;
Geissen, 2000).
Decomposition with participation of microfauna
and microflora in our study increased with increasing
acidity of the soils. This may be because in tropical
soils fungi whose biomass increases with decreasing
pH are most important for microbial decomposition.
Litter decomposition with participation of soil
fauna was positively related with organic matter
content. Geissen (2000) also described density and
activity of soil fauna being positively influenced by
organic matter that serves as a nutrient recourse and
improves the water holding capacity of soils.
5. Conclusions
Organic matter and soil acidity are the soil
chemical parameters that show the most important
influence in soil biota and the process of decomposi-
tion. Litter decomposition and earthworm commu-
nities are sensitive indicators for the characterisation
V. Geissen, G.M. Guzman / Applied Soil Ecology 31 (2006) 169–178176
of soil fertility under tropical conditions, whereas
microbial biomass as a sum parameter is not adequate
to describe effects. The use of C. plectostachyus or
succession forest in the humic Acrisols increases soil
fertility in comparison to the use of B. decumbens.
Based on the results of our study we conclude that C.
plectostachyus in monoculture or in combination with
succession forest is adequate for sustainable land use
in the study area.
Acknowledgements
We are grateful to Fundacio
´n Produce, Tabasco,
Mexico for financial support. We thank Concecio
´n
Casango and Atila Herna
´ndez de la Cruz for support in
field and laboratory work. We thank Regino Gomez
Alvarez for helpful advices.
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