Soil Organic Matter and Biological Changes in a Natural-to Planted
Forest Succession: Terminalia superba Plantations Grown on Deforested
Plots in Congo
A comparative study was carried out in Mayombe, between
the soil of natural forest and the soil under four Terminalia superba
plantations of 7, 12, 32 and 48 year old. In each plantation type and
natural forest composite soil samples were taken in 0-10, 10-20, 20-30,
30-40 and 40-50 cm layers. The goal was to assess the impact of reforestation
on soil organic matter and its biological characteristics. Statistically
differences between the sites were tested using the Analysis of Variance
(ANOVA). The results showed that there was a general decrease of soil
organic matter in the profile of all sites except in the 48 year old plantation
for total nitrogen. In the surface soil the carbon content and total nitrogen
were respectively, 22.2 mg g-1 and 1.56 μg g-1
in the forest. The carbon content was between 14.9 and 23.5 mg g-1
while total nitrogen was between 1.31 and 2.24 μg g-1
in the plantations. The microbial biomass carbon (Cmic) was 732 μg
g-1 soil in the forest, it varied between 461 and 740 μg
g-1 in the plantations. The metabolic quotient (qCO2)
was between 3.6 mg C-CO2 mg-1 Cmic day-1
and 4.8 mg C-CO2 mg-1 Cmic day-1 in plantations.
The qCO2 in forest was 3.6 mg C-CO2 mg-1
Tree plantations cover an area of more than 50,000 ha in the littoral
and Mayombe regions in the South of Republic of Congo. They are planted
with both exotic and indigenous species and are generally monospecific
(Goma-Tchimbakala and Bernhard-Reversat, 2006). Terminalia superba
(T. superba) is the main native species used for reforestation
in the Mayombe. Studies in the tropics have shown significant changes
in soil organic matter following conversion of natural forest into cultivation
and these changes have been shown to affect soil fertility (Dominy et
al., 2002; Yimer et al., 2007). The changes that occurred after
deforestation and subsequent cropping are decreases in plant available
nutrients (Lu et al., 2002), decreases in microbial activity (Sahani
and Behera, 2001), increases in bulk density, soil erosion and runoff
(Rasiah and Kay, 1995) and decreases in porosity, infiltration and water-holding
capacity (Lu et al., 2002; Sahani and Behera, 2001). However, when
some of these degraded soil were brought under well managed pasture, significant
improvements in degraded properties occurred after the introduction of
pasture (Rasiah and Kay, 1995; Rasiah et al., 2004). In south of
Republic of Congo, it is not known if similar improvements would occur
under T. superba plantations which were grown on deforested soil.
In agreement with Rasiah et al. (2004) such information is necessary
for several reasons. First, if natural forest regrowth is poor after abandonment
for substantially long period of time, then the information on soil properties
may help, at least partially, to identify the factor (s) controlling regrowth
or reestablishment. Second, this information may be useful to assess the
resiliency of soils in different tropical environment to recover from
man induced changes and finally to determine the time required for substantial
recovery to occur. The latter information is crucial if monetary benefits
are considered for abandonment and subsequent reforestation. Long-term
impacts of afforestation and reforestation on the distribution of soil
organic matter and biological activities have not been reported for the
South of Republic of Congo. Research regarding the effects of reforestation
on soil organic matter dynamics and biological properties with T.
superba plantation aging will help to determine the potential of recovery
of these soils for carbon sequestration and biological properties.
The purpose of this study was to assess changes in soil organic matter
and biological properties due to reforestation with T. superba in
Mayombe in comparison with natural forest soil characteristics.
MATERIALS AND METHODS
Site description: The sites were previously described by Goma-Tchimbakala
and Bernhard-Reversat (2006). Briefly they are located at Bilala in the
Mayombe at 4°31`S, 12°4`E and above mean sea level 300 m altitude.
Mean annual rainfall is 1250 mm. Relative humidity is high all over the
year close to 85%. The temperature ranges from 28°C in the rainy season
to 18°C in the dry season. The soil is desaturated ferralitic and
lies on sandy-clayey or clayey-sandy material resulting from the weathering
of cretaceous green rock and dolomitic limestone. In this study the clay
content has narrow range from 27 to 29% in the surface soil (Table
1). T. superba plantations were established by the National
Reforestation Service (SNR) after clear cutting the forest and burning
all plants residues. The tree spacing is 10x10 m. Four plantations of
7, 12, 32 and 48 years old were chosen for this study (Fig.
1). The undergrowth vegetation was well developed in the planted plots
and the Fabaceae, Annonaceae, Euphorbiaceae, Moraceae, Marantaceae and
Menispe-rmaceae were the most numerous plant families.
||Average particle size distribution of the surface soil
under T. superba plantations and natural forest
The nearby natural
forest was used for comparison with the planted plots. The most represented
tree families in the natural forest are the Cesalpiniaceae, followed by
the Rubiaceae and the Euphorbiaceae. All the stems larger than 10 cm Diameter
at Breast Height (DBH) were listed together with the plants of the understory
(Goma-Tchimbakala and Bernhard-Reversat, 2006).
Soil sampling: Soil samples were collected from ten systematically
located squares (1 m2 for each square) in each plantation type
and natural forest. In each square two composite soil samples were taken
in 0-10, 10-20, 20-30, 30-40 and 40-50 cm layers. Each composite soil
sample was combined from four cores from a layer. Samples for chemical
analysis were air-dried and then were passed through a 2 mm mesh soil
Microbial activity and microbial biomass: For the measurement
of basal respiration, 100 g of a moist soil sample was weighed into 1000
mL jar, adjusted to 55% of its water-holding capacity and incubated during
7 days at 28°C in the dark. The CO2 produced was absorbed
in 50 mL 0.2 M NaOH solution and determined by titration of the excess
NaOH with 0.2 M HCl. The CO2 respired was analysed periodically
at 2, 4 and 7 days.
||Situation of the plantations and natural forest (the
plantation plots are not drawn at the scale)
Potential Basal Respiration (BR), expressed in mg
CO2-C g-1 soil day-1, was calculated
when microbial respiration had stabilized, i.e., during the last 3 days
of incubation. After 7 days` incubation, 4 soil sub-samples (25 g each)
were used to determine the microbial biomass. Microbial biomass carbon
(Cmic) and microbial biomass nitrogen (Nmic) were estimated by the chloroform
fumigation-extraction method (Vance et al., 1987). Two portions
were fumigated with ethanol-free chloroform during 24 h. After removal
of CHCl3, C was extracted from fumigated and unfumigated soil
samples with 0.5M K2SO4 with a soil: extractant
ratio of 1:5 (w:v) for 30 min on a rotating shaker. After shaking and
centrifugation at 5100 rpm during 10 min, the soil suspension was filtered
with a 0.22 μm filter (Millipore Corporation, Belford, MA 01730)
and the filtrate was collected. Dissolved Organic Carbon (DOC) was determined
on both the fumigated and unfumigated soil extracts. The Cmic was calculated
according to the equation: Cmic = EC/KEC (Vance et al., 1987), where Ec was the difference between extractable
C from fumigated samples and unfumigated samples and KEC =
0.38. The Nmic was calculated as Nmic = EN/KEN,
where EN was the difference between extractable N from fumigated
and unfumigated samples and KEN = 0.54 (Brookes et al.,
1985; Joergensen and Mueller, 1996). The metabolic quotient (qCO2)
is the ratio of basal respiration to Cmic of soil, expressed in mg CO2-C(mgCmic)-1 day-1 . The microbial quotient (qCmic), which is the ratio
of Cmic to Corg of a soil, is expressed in mg Cmic(g(Corg))-1.
Chemical analysis: The analysis were carried out in the laboratory
of IRD (Institute of Research for Development) at Pointe-Noire with standard
methods. Carbon was analyzed by the modified Walkey and Black method.
Total nitrogen was determined using the Kjeldahl procedure. The extractable
C for microbial biomass (DOC) was analyzed by the HachTM methods (Jirka
and Carter, 1975): Two millimeters of filtered DOC were heated for 2 h
with potassium dichromate, the organic matter reducing the dichromate
ion (CrO7-2) to green chromic ion (Cr3+).
The amount of Cr3+ produced was determined with colorimetric
method using DR/890 colorimeter. The extractable N was measured after
mineralization of the N to NH4+. Two milliliters
of concentrated H2SO4 and 9 drops of H2O2
were added to 2 mL of the filtered soil extracts and heated at 200°C
during 24 h. After the mineralization process, the N was analyzed by the
Data analysis: We assessed changes in soils under plantations by computing
the difference between mean values of individual soil properties under different
T. superba plantations and the values of same soil properties under natural
forest and expressed as a percentage of the value under the natural forest.
These percent changes were used as references of soil degradation or improvement
across the investigated layers.
Statistically significant differences were tested using the Analysis
of Variance (ANOVA). The Protected Least Significant Difference (PLSD)
Fisher test was used for mean separation when the analysis of variance
showed statistically differences. All statistical analysis were performed
using Stat View 5.0 (SAS Inst. Inc., 1998).
Organic carbon and total nitrogen in the profile: The results
showed that deforestation affected contents of soil organic carbon and
total nitrogen significantly (p<0.001; Table 2, 3).
The soil surface in the 7 year old plantation had the weakest soil organic
content than all other plantations (Table 2). Soil organic
carbon content increased with the aging of plantations and reached 23.5
mg g-1 under the mature plantation (48 years old). Comparison
of mean differences among sites revealed that the 48 year old plantation
had similar soil organic carbon with the natural forest (Table
||Depth distribution of soil organic carbon, total
nitrogen and C:N ratio under T. superba plantations and natural
||ANOVA of soil organic carbon and total nitrogen
|*** p<0.0001; ** p<0.001; * p<0.05
Soil organic content in the 0-10 cm layer in these two sites was
higher than in the plantations between 7 and 38 year old (Table
2). Total nitrogen showed the same trend as soil organic content in
the plantations. The 7 year old plantation had significantly low total
nitrogen content while the 48 year old plantation had the highest content.
However comparison between the sites showed that the soil of natural forest
had low nitrogen content than plantation of 32 and 48 year old. However
total nitrogen in natural forest was not different with the 12 year old
plantation. The low total nitrogen content recorded under natural forest
induced a higher C/N ratio than in plantations (Table 2).
The vertical distribution of organic carbon and total nitrogen appeared
to differ in soils under plantations and between the soil under plantations
and natural forest (Table 2). In the depth 10-20 cm
the mean of soil organic carbon were significantly higher (p < 0.05)
in the 32 and 48 year old plantations than in the 7-12 year old plantations
and the natural forest. On other hand differences were not significant
between the 7 and the 12 year old plantation. After, organic carbon decreased
regularly in all other layers until 50 cm in depth. Comparison between
the sites showed that the 7 year old plantation had the lower organic
carbon content and the higher content was in natural forest.
The distribution of total nitrogen in the profile was more complex (Table
2). Total nitrogen was similar under plantations in the l0-20 cm layer
and the content was significantly high than in natural forest (Table
3). The same trend was observed in 20-30 cm concerning the plantations
of 7, 12, 32 years old and natural forest, whereas the 48 year old plantation
had highest total nitrogen content (Table 4). In the
two last layers the natural forest and the 48 year old plantation had
high total nitrogen content than all other plantations. The C:N ratio
decreased in general from soil surface to 30-40 cm in depth. This decline
was followed by an increase in 40-50 cm layer. However C:N ratios did
not differ significantly in plantations and forest.
Microbial biomass and microbial activity: The microbial biomass
carbon (Cmic) varied between 461 and 740 μg g-1 soil whereas
the microbial biomass nitrogen (Nmic) was between 87 and 114 μg g-1
soil (Table 5). Comparable Cmic and Nmic were
recorded in natural forest, semi mature and mature plantations (Table
5). On the other hand the 7 year old plantation and the 12 year old
plantation had similar and significantly lower Cmic and Nmic
than in the 48 year old plantation and the natural forest (p<0.0001,
Table 5, 6). The lower Cmic to
Nmic ratio was in the 7 year old plantation while the higher ratio
was observed in the 48 year old plantation.
||Comparison of mean differences between the sites
|***p<0.0001; **p<0.001; *p<0.05; ns: non significant
||Microbial activity, microbial biomass and microbial
indices under T. superba plantations and natural forest soils
(mean and standard deviation)
|Cmic: microbial biomass carbon; Nmic: microbial biomass
nitrogen; C: organic carbon; The number with the same letter in the
column were not different at level 0.05
There were significant differences
between all the other plantations (p<0.0001, Table 5).
The Cmic expressed versus soil organic C (qCmic) showed no clear trend
and varied in narrow range of 3.0-3.9% (Table 5). The
mineralized C was higher in the natural forest and the 48 year old plantation
than in the 7-12 year old plantations, (p<0.0001, Table
5). Expressed as percent of soil organic C, the mineralized C was
significantly higher in the soils under the 7-12 year old plantations
than under the natural forest and the 48 year old plantation (Table
Percent changes under T. superba plantations: Table
7 showed that loss of organic carbon was between 20.2 and 44.8% in
the 7-12 year old plantations. It was between 13.7 and 27.1% in the 32
year old plantation. In these 3 plantations the losses occurred in all
layers, except in the 10-20 cm layer in the 32 year old plantation where
the increase was observed. In the 48 year old plantation there was an
increase of organic carbon in the 0-10 and 10-20 cm layers while the loss
taken place in the 20-30 cm, 30-40 cm and 40-50 cm layers. Comparison
between plantation types revealed that loss of organic carbon decrease
with the increase of the plantation age. Apart in the 10-20 cm layers,
the loss of total nitrogen occurred in the soil profile in the 7-12 year
old plantations (Table 7). In these 2 plantations the
strong loss was between 33.9 and 50% in the 30-40 and 40-50 cm in depth.
The surface soil presented positive changes (7.1 to 25.6%) in the 32 year
old plantations while in the deep layers loss of total nitrogen varied
between 1.7 and 33.3%.
||ANOVA of microbial biomass
|*** p<0.0001; ** p<0.001; * p<0.05; ns: non
||Percent changes in soil carbon and nitrogen under
T. superba plantations (differences between mean of soil
carbon and nitrogen under plantations and value under forest expressed
as a percentage of value under forest)
|+ Increase; -Loss
Except in the layer 40-50 cm there was a positive
change in the profile of the 48 year old plantation, the gain varied between
1.8 and 43.6%. Comparison between plantation types showed that loss of
nitrogen in the surface soil occurred only in the 7-12 year old plantations
while in the 32-48 year old plantations nitrogen content increased.
Soil organic carbon and total nitrogen: The findings indicated
that surface soil under the 48 year old plantation had significantly higher
organic carbon and total N than in soils under the 32, 7 and 12 year old
plantations, which might be a result of higher organic matter accumulation
due to increased of litter fall and root biomass and reduced litter decomposition
rates (Goma-Tchimbakala and Bernhard-Reversat, 2006; Saik et al.,
1998; Yimer et al., 2007). Apart in the soil under the 48 year
old plantation, the amount of organic matter under plantations was lower
than in the natural forest due to the lack of litter return, high mineralization
of organic matter following deforestation, losses of organic matter by
water erosion (Paul et al., 2002; Jaiyeoba, 2003; Yimer et al.,
2007). Deforestation accelerates soil organic matter loss by changing
microclimate and exposure to microbial decomposition (Harmand and Pity,
2001). The amount of organic carbon in the surface soil under young and
semi-mature plantations was between 17.1 and 32.8% less important compared
to original amount in natural forest. The depletion of total N was in
the range between 6.4 and 16.0%. In these two cases the loss of soil organic
matter was more pronounced under the 7 year old plantation. The same phenomena
had observed by others during cultivation or reforestation (Kotto Same
et al., 1997; Saik et al., 1998; Xu et al., 2000). In
the studies of these authors the loss reached respectively, 31% of organic
carbon and 11% of total nitrogen. In ferralitic soils of Sierra Leone,
two years after deforestation and planting Brams (1971) noted a loss of
40% of total nitrogen. Harmand (1997) observed, under planted cassia and
eucalypts, that fast loss of carbon and nitrogen occurred only up to 40
cm in depth and that variations of organic matter became very small beyond
this limit. In the depth, the distribution of soil organic carbon and
total N differed between the plantation ages. Soil organic carbon and
total N decreased with each layer from 0 to 50 cm in two plantations 7
and 12 years old. The most pronounced decrease occurred for 20-30 cm layer
in the 7 year old plantation and for 10-20 cm layer in the 12 year old
plantation. In the semi-mature and mature plantations there was improve
of carbon content in the subsurface due to the increase of root biomass
and decrease of oxidation. The C:N ratio in the 0-10 cm layers was almost
similar in all the planted stands. This constancy compared to natural
forest may indicate that the soil organic matter quality was more affected
during the succession than the organic mater quantity. Finally the level
of organic matter reached that of the natural forest after several decades.
The low nitrogen content of organic matter in the forest soil could result
from its higher mineralization (Goma-Tchimbakala, 2003).
Microbial biomass and organic matter quality: Changes due to reforestation
also concerned organic matter quality (Joergensen et al., 1995).
The results of microbial biomass fall in the range of some tropical forests
0.27-0.79 mg Cmic g-1 soil. The disturbance caused by the reforestation
disappeared with the aging and the microbial indices reached the same
level as natural forest. This may due to in part by changes in soil organic
matter induced by litter diversity input of mature plantation and natural
forest than in young plantations with lower litter diversity. This is
in agreement with the finding of Bardgett and Shine (1999). The microbial
quotient (qCmic) gives an indication of the stability of an ecosystem.
It provides early warning of soil quality deterioration or environmental
change (Anderson, 2003; Yan et al., 2003; Freschet et al.,
2008). In this study, qCmic values indicated that the microbial biomass
of the 32 and 48 year old plantations was closely to of the forest and
was related to the amount of soil organic carbon. This finding was supported
by the high microbial biomass founded under forest, semi mature and mature
plantations where the soil organic content was high. No significant differences
in qCmic were observed between the plots, which may indicate that there
was little variation in soil organic matter quality with the age of the
organic matter input (Freschet et al., 2008). The qCmic expressed
as a percentage were in the range, between 1 and 5%, reported in numerous
studies for tropical soil (Mao et al., 1992; Priess and Folster,
2001; Salamanca et al, 2002). The CO2 production expressed
the microbial activities. It was followed the same evolution as the microbial
biomass for similar reasons. The metabolic quotient (qCO2)
has been used as bioindicator for ecosystem succession (Insam and Haselwander,
1989) and environmental stress (Anderson and Domsch, 1993; Joergensen
et al., 1995; Priess and Fölster, 2001). The metabolic quotient
reflects the efficiency of the use of organic carbon by microorganisms
(Knoepp et al., 2000). Many researchers had hypothezed that environmental
stress like low soil organic matter quality or nutrient deficiency reduces
organic carbon efficiency use increasing the mineralized carbon needed
for the ecosystem (Wolters and Joergensen, 1991; Kassim et al.,
1992; Agnelli et al., 2001). The decrease of qCO2 with
the age indicated the presence of microbial populations with more capability
to use carbon compounds. The low qCO2 recorded in the 48 year
old plantation and forest suggested less adverse environmental conditions
and higher efficiency use of the organic resources than in the 7-12 year
old plantations. According with Agnelli et al. (2001), a more efficient
microbial community evolves in a more complex system with the heterogeneous
organic matter input. According to Anderson and Domsch (1985) it may indicate
that microbial biomass has recover after disturbance and become more stable
in mature plantation.
The results of this research show that a decrease in soil organic matter
occurred more in the surface soil. The qualitative and quantitative changes
in organic matter and microbial characteristics with aging led to conditions
close to the initial forest. The general trends suggested that Terminalia
superba plantations might not degrade and rather improve soil fertility
for further cultivation or plantation if they are logged when 48 year
old as was planed. Plant diversity in the undergrowth, multistrate structure,
high nutrient turn over and high productivity could be involved in restoring
fertility after forest clearing.
The International Foundation of Science (IFS) provided financial support
for this research in form of research grant (D/2542-1). The National Reforestation
Service (Service National de Reboisement, SNR) is acknowledged for welcoming
the fieldwork in their plantations. Kokolo A. and A.N.S. Mboussou Kimbangou
M. Ndoundou-Hockemba are acknowledged for their assistance during the
soils sampling. Soil analysis were carried out at the analysis laboratory
of the Institute of Research for Development of Pointe-Noire. We thank
Dr. Victor Mamonekene who checked the English of the first manuscript.
We also thank the four anonymous reviewers which helped us to improve
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