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Savannas on longterm nutrient availability in the soils
Jean-Paul Laclau, Philippe Deleporte, Jean-Pierre Bouillet, Jacques Ranger
To cite this version:
Jean-Paul Laclau, Philippe Deleporte, Jean-Pierre Bouillet, Jacques Ranger. Influence of afforestation with eucalypts in Congolese Savannas on longterm nutrient availability in the soils. 1. International symposium on the Management of tropical sandy soils for sustainable agriculture, Nov 2005, Khon Kaen, Thailand. FAO Regional Office for Asia and the Pacific, 524 p., 2005, Management of Tropical Sandy Soils for Sustainable Agriculture. ”A holistic approach for sustainable development of problem soils in the tropics”. 27th November - 2nd December 2005. Khon Kaen, Thailand. Proceedings. �hal-01352687�
Influence of afforestation with eucalypts in Congolese savannas
on long-term nutrient availability in the soils
Laclau, J.-P.
1; P. Deleporte
2; J.-P. Bouillet
1and J. Ranger
3Keywords: Eucalyptus, savanna, biogeochemical cycles, nutrient budget, sustainability, Congo
Abstract
Fast growing forest plantations managed in short rotations in order to maximize biomass production are likely to deplete soil nutrient reserves. The effects of Eucalyptus stands on long-term nutrient availability in sandy soils were studied in the coastal plains of Congo, using a biogeochemical cycle approach. Atmospheric deposition, canopy exchange and transfer through the soil were estimated on the whole rooting depth (6 m) over three years, in an experimental design installed in a native savanna and an adjacent 6-year-old Eucalyptus plantation. Complementary measurements during 3 years after planting the same Eucalyptus clone in the experimental savanna made it possible to establish input-output budgets of nutrients for the whole rotation and to compare them with the native savanna ecosystem.
Even if the nutrient fluxes in the savanna ecosystem were affected by afforestation, the biogeochemical cycles remained highly conservative after planting eucalypts. The main outputs of nutrients from the soil occurred during burning in savanna and with biomass removal at the harvest in the Eucalyptus stand. Both ecosystems were efficient in preventing losses by deep drainage (<5 kg ha-1 year-1 for N, P, K, Ca, and Mg).
After afforestation, weeding in the Eucalyptus stands eliminated the leguminous species responsible for N input by symbiotic fixation of about 20 kg ha-1 year-1. Whereas the budgets of P, K, Ca and Mg were roughly
balanced, the current silviculture led to a deficit of about 140 kg N ha-1 in the soil, throughout a 7-year
rotation. This deficit was large relative to the pool of total N in the upper soil layer (0-50 cm), which was about 2 t ha-1. The nutrient budgets were consistent with field trials on fertilization, showing that the
sustainability of Congolese plantations will require an increase in N fertilizer inputs over successive rotations.
1 CIRAD, UR80, département Forêts, Programme Arbres
et Plantations, TA 10/C, 34398 Montpellier Cedex 5. Fax: (33) 4 67 59 37 33 E-mail: laclau@cirad.fr; jpbouillet @cirad.fr
2 CIRAD/UR2PI, BP 1291, Pointe-Noire, République du
Congo. Fax: (242) 94 47 95 E-mail: deleporte@cirad.fr
3 INRA, Biogéochimie des écosystèmes forestiers,
54280 Seichamps, France. Fax: (33) 5 83 39 40 68 E-mail: ranger@inra.fr
Introduction
Large areas of native vegetation have been replaced by monospecific Eucalyptus stands for several decades, and this genus is nowadays the most represented in tropical plantation forests. The ecological impact of Eucalyptus plantations has been widely discussed around the world (Cossalter and Pye-Smith, 2003). This issue has been studied in littoral plains of Congo, after afforestation with Eucalyptus clones in native savannas (Loumeto and Huttel, 1997; Mboukou-Kimbatsa et al., 1998; Bernhard-Reversat et al., 2001; Ranger et al., 2004).
Paired comparisons of soil chemical properties in the top soil under savannas and adjacent Eucalyptus stands showed that 20 years of short-rotation silviculture did not modify carbon stocks but decreased N and Ca contents (Bouillet et al., 2001). To gain insight into the processes governing the mineral functioning of these ecosystems, the biogeochemical cycles of nutrients have been compared in a clonal Eucalyptus stand and an adjacent savanna over three years: modifications of the chemical composition of solutions throughout their transfer were investigated in the two ecosystems (Laclau et al., 2003a, b), as well as the dynamics of biomass and nutrient accumulation in the plants (Laclau et al., 2002; Laclau et al., 2003c). The flux dynamics is an important parameter to consider, because nutrient availability and demand should coincide both in time and space under conditions of high rainfall and permeable soils.
The present study aimed at establishing input-output nutrient budgets for the whole Eucalyptus rotation in order to assess the long-term effects of
silvicultural practices on nutrient availability and to elaborate sustainable management practices.
Material and methods
1. Site characteristics
The study site was located on a plateau at an elevation of about 80 m and a distance from the sea of 10 km (4ºS 12ºE). The mean annual rainfall over the last 50 years was 1,200 mm, with a dry season between May and September, and the mean temperature was 25ºC with seasonal variations of approximately 5ºC. The geological bedrock is composed of thick detritic formations of continental origin, dated from plio-pleistocene. An experimental design monitoring the biogeochemical cycles was installed in 1997 in a Eucalyptus stand and a native savanna. The area was flat and the distance between the two designs was about 500 m. Soils were ferralic arenosols (FAO classification), acidic (pHH2O ≈ 5) and characterized by a sandy texture (sand content >85% down to a depth >12 m) and a low amount of available nutrients (CEC <0.8 cmolc kg-1, even in the upper layers). The organic
matter content of soils decreased in both stands from about 1.3% in the surface layer (0-5 cm) to about 0.15% below a depth of 2 m. Concentrations of exchangeable elements were of the same order of magnitude in the two stands. Nevertheless, in the topsoil (0-5 cm layer), the concentration of Ca in the eucalyptus stand was lower than in the savanna and the concentrations of available P and Al were higher (Laclau et al., 2005).
2. Plant material
The Eucalyptus clone studied here comes from natural crosses in Congo between a few individuals of Eucalyptus alba Reinw. ex Blume (mother tree) and a group of poorly identified Eucalyptus hybrids (father tree). The stand was planted in January 1992 on savanna, at a stocking of 530 trees per hectare, and a starter fertilization (150 g per cutting of NPK 13:13:21) was applied. Manual weeding in the planting row and chemical weeding in the inter-row were made in the early stages of stand development. The stand was 6 years old at the onset of the study, with a mean height of 26 m and a mean over-bark volume of 158 m3 ha-1.
Other studies dealing with the dynamics of nutrient fluxes throughout a 7-year rotation were performed in this area, using a chronosequence approach for the same clone (Laclau et al., 2003c).
The grass Loudetia arundinacea (Hochst.) Stend represented 80% of the total aerial biomass of the
savanna, which reached about 5 t ha-1 of dry matter at
the end of the rainy season. This savanna was burnt every dry season (August) like most savannas in Congo (Laclau et al., 2002).
3. Methodology
Climatic data, soil moisture and solution chemistry were measured in both ecosystems from January 1998 to December 2000. Methods used to assess input-output fluxes were described by Laclau et al. (2005). In brief:
Atmospheric depositions. Lack of reliable measurement methods for dry deposition made it necessary to use a calculation approach based on the comparison of nutrient fluxes in an open area and beneath the canopy. We considered Na+ as a tracer,
assuming that canopy exchange was negligible for this element compared with dry deposition of marine aerosols in this coastal area (Parker, 1983). Dry deposition of nutrients was considered proportional to that of Na+.
Weathering of soil minerals. Soil minerals were considered to be external to the available soil nutrient reservoir where trees take up their nutrients. Mineralogy of the different particle size fractions and mineral bearing nutrients were quantified using identification of mineral by X ray diffraction, total and selective chemical analysis, thermogravimetric analysis and normative calculation. The geochemical Profile model developed by Sverdrup and Warvfinge (1988) was calibrated for the site to estimate the magnitude of this flux, overlooking weathering processes for the very stable accessory minerals.
Accumulation in the aerial ligneous biomass. Twelve trees were sampled in the Eucalyptus stand at age 6.5 years and 10 trees at age 8 years. Selected trees were cut down, and the major components were isolated: stemwood, living and dead branches, stembark, and leaves. The fresh stem weight was measured and disks of wood and bark of constant thickness were taken every 3 meters. Samples were dried (65ºC) and the dry biomass of the components in each tree was calculated proportionally. All the branches and leaves were collected and samples of these components were dried. One composite sample for each component of each tree was ground, homogenized and sent to the laboratory for chemical analyses. Tables were established and applied to the stand inventories to quantify the stand biomass and nutrient content per hectare.
Nutrient losses by drainage. Nutrient fluxes were calculated multiplying the water fluxes at each soil depth (assessed from hydrological model) by the mean concentration of nutrients in gravitational solutions. Run off solutions were collected from 4 replicates of 1 m2 collectors in the two ecosystems.
Soil solutions were sampled in both ecosystems from 4 replicates of ceramic cup lysimeters installed at various depths between 15 cm and 6 m and connected to a vacuum pump maintained manually (daily checking) at a constant suction of about – 60 kPa. A complete description of the lysimetry design was presented by Laclau et al. (2003b). Three to five replicates of TDR probe (Trase System I) were installed at the depths of 15, 50, 100, 200, 300, 400 and 500 cm in the Eucalyptus stand, and at the same depths down to 3 m in the savanna. Volumetric water content was measured automatically every 3 hours from July 1998 to December 2000 in the Eucalyptus stand and once or twice a week in the savanna. A model based on the Richard’s equation for simulating one-dimensional water flows (Hydrus 1D) was calibrated in the Eucalyptus ecosystem and in the experimental savanna to quantify water flows at the depths where lysimeters were installed (Laclau et al., 2005). All the equations used in the model were established from measurements performed in 1998 and 1999. The ability of the model to predict the fluxes was checked during the year 2000.
Chemical analyses
Once a week, after volume measurements and sampling, the solutions were collected and carried to the laboratory where they were kept at +4ºC. Pooled volume weighted samples were made every 4 weeks for chemical analyses. Each replicate of soil solution collected was analyzed separately. The solutions were filtered (0.45 µm) in the Congo and measurements of pH (HI 9321) and SO42- by colorimetry (ANA 8
Prolabo) were performed as quickly as possible. The samples were then acidified with H2SO4, sent
to the CIRAD laboratory in France where nitrate and ammonium were measured by colorimetry (INTEGRAL PLUS – Alliance instruments). Total P, K, Ca, Mg were determined by ICP emission spectroscopy (JY 50).
In plant samples, N was determined by thermal conductivity after combustion (FP-428) and P, K, Ca, Mg, by a sequential spectrometer ICP (JY 24) after digestion by hydrofluoric acid and double calcination.
Input-output budgets
Current annual and seasonal input-output budgets were established between 1998 and 2000 from the measurements performed, considering losses by deep drainage at a depth of 4 m under savanna and 6 m under Eucalyptus. Indeed, previous studies showed the absence of roots beyond the depth of 3 m under savanna, and extremely low densities in this Eucalyptus stand (Laclau et al., 2002). The nutrient uptake by plants considered in the current budgets was the permanent uptake: immobilization in stemwood for the Eucalyptus stand and transfers to the atmosphere during burning in the savanna.
The calculation of nutrient budgets over the whole rotation required assessing the nutrient fluxes during the first years after afforestation. To measure losses by drainage during the early growth of the stand, the experimental savanna was planted with the same Eucalyptus clone in May 2001. Soil solutions were collected and analyzed for 2 years with the same methodology. Losses were assessed every 4 weeks, multiplying the average concentration in soil solutions at a depth of 4 meters by the simulated water flow (Unpublished data). Dry deposition was adjusted proportionally to the foliar biomass of the stand throughout stand rotation (Laclau et al., 2000). From 2 years onward, annual fluxes in the Eucalyptus ecosystem were considered identical to values measured at the end of the rotation. Indeed, a chronosequence approach showed that the main fluxes of the biological cycle are roughly constant (Laclau et al., 2003c).
Results and discussion
1. Main changes in nutrient cycling after afforestation
Planting Eucalyptus in the native savanna modified the inputs of nutrients to soil through the following processes:
A filter effect of the canopy at the end of the Eucalyptus rotation led to dry depositions of nutrients of the same order of magnitude as wet deposition, whereas the pattern of Na+ concentration in wet
depositions and throughfall suggested that this flux was negligible in the savanna (Laclau et al., 2003a). Atmospheric deposition was in the range of values given in the literature for forest stands. However, this flux was determined with high degree of uncertainty
which influence the accuracy of determination of input-output budgets at the ecosystem level (Laclau et al., 2005).
Fertilizers are usually applied in Eucalyptus plantations. This flux was considered in the budgets calculated for the whole rotation (Figure 1) but not in Table 1, because fertilizers were applied only at planting in the stand studied.
Biological N2 fixation is necessary to explain the
sustainability of savannas in littoral areas of Congo for about 3,000 years showed by isotopic studies (Trouvé, 1992), despite large losses of N during annual fires. The input of N by symbiotic fixation, assessed roughly to balance the N budget in savanna, amounted to about 22 kg ha-1 year-1 (Table 1). The legume species
Eriosema erici-rosenii R.E. Fries (Papilionoideae) was found in all the savannas of the region. Chemical weeding during the early growth of the Eucalyptus stand led to the disappearance of this flux.
Calculations performed with the Profile model suggested that the amounts of P, Ca and Mg released by the weathering processes were negligible in this Ferralic Arenosol. The amount of K released on a soil depth of 6 m in the Eucalyptus ecosystem was about 0.3 kg ha-1 year-1 and half on a depth of 3 m under
savanna. This flux was consistent with the extremely low nutrient concentrations in soil solutions sampled
Figure 1. Input-output budgets (kg ha-1) of N, P, K, Ca
and Mg for the whole Eucalyptus rotation, and for different harvesting scenarios
Scenario 1: de-barked pulpwood harvest
Scenario 2: de-barked pulpwood and firewood harvest Scenario 3: pulpwood with bark harvest
Scenario 4: whole tree harvest
Table 1. Mean input-output fluxes of nutrients in the soil under the Eucalyptus stand at the end of the rotation, and
under the savanna, from 1998 to 2000 (kg ha-1 year-1)
Savanna Eucalyptus stand
N P K Ca Mg N P K Ca Mg ANNUAL FLUXES Wet deposition 4.8 0.3 2.7 3.3 1.4 4.8 0.3 2.7 3.3 1.4 Dry deposition 0.0 0.0 0.0 0.0 0.0 6.5 0.3 3.8 4.5 1.8 Symbiotic Fixation1 21.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Weathering 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.3 0.0 0.0 Total inputs 26.4 0.3 2.8 3.3 1.4 11.4 0.6 6.8 7.8 3.2 Surface runoff 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.2 0.4 0.1 Deep drainage 3.0 0.1 0.6 0.4 0.2 4.3 0.3 2.1 1.1 1.2 Immobilization2 – – – – – 32.7 3.7 4.8 3.9 2.5 Burning 23.4 1.5 2.4 2.6 2.9 0.0 0.0 0.0 0.0 0.0 Total outputs 26.4 1.6 3.0 3.0 3.1 37.0 4.1 7.1 5.4 3.8 ANNUAL BUDGETS
Mean over 3 years 0.0 -1.3 -0.2 0.3 -1.7 -25.6 -3.5 -0.3 2.4 -0.6 Inter-annual range:
Min 0.0 -1.4 -0.4 -0.1 -1.8 -29.7 -3.9 -2.4 1.6 -0.3
Max 0.0 -1.2 0.0 0.5 -1.6 -21.6 -3.0 1.3 3.4 -0.7
1 Calculated to balance the nitrogen budget in the savanna, 2 Nutrient immobilization in stemwood.
by tension lysimeters below the rooting zone (Laclau et al., 2003b).
Outputs of nutrients also were modified by afforestation:
Transfers of plant nutrients to the atmosphere during burning were a major output of nutrients in this
savanna. They amounted to 23.4, 1.5, 2.4, 2.6 and 2.9 kg ha-1 year-1 for N, P, K, Ca and Mg, respectively
(Laclau et al., 2002). They represented, respectively, 85%, 25%, 39%, 21%, and 28% of the N, P, K, Ca and Mg contents in the above-ground biomass before the fire. This output of nutrients was avoided after afforestation.
Nutrients immobilized in stemwood were exported from the ecosystem at the harvest. They amounted to about 30 kg ha-1 year-1 of N, and from 3
to 5 kg ha-1 year-1 of P, K, Ca and Mg, at the end of
stand rotation (Table 1). Nutrient exportation with biomass removal does not occur in most of Congolese savannas owing to the absence of cattle.
Losses of nutrients by surface runoff were negligible in savanna and represented very low amounts in the Eucalyptus stand (<0.5 kg ha-1 year-1).
At the end of the rotation, the Eucalyptus stand was as efficient as the native ecosystem of savanna to prevent losses of nutrients by deep drainage. The losses of N, P, K, Ca and Mg amounted to 4.3, 0.3, 2.1, 1.1 and 1.2 kg ha-1 year-1, in the Eucalyptus stand, respectively,
and 3.0, 0.1, 0.6, 0.4 and 0.2 kg ha-1 year-1 in savanna
(Table 1). Losses of nutrients by deep drainage observed over the first two years after afforestation of the experimental savanna remained low. Although the mineralization of savanna residues occurred when the root system of the trees was not completely established, the fluxes of N, P, K, Ca and Mg at a depth of 4 m were lower than 3 kg ha-1 year-1 on average
(Unpublished data). Rainfall amounts were much lower during this period than from 1998 to 2000 which may explain the very low losses by drainage observed. Therefore, it cannot be excluded that nutrient outputs by drainage would be higher, and consequently the N budget more unbalanced, if afforestation occurred during a period with a more normal precipitation distribution.
Moreover, afforestation led to great changes in the internal nutrient cycling within the ecosystem (Laclau et al., 2003a, c). Foliar leaching of cations observed in savanna decreased after planting eucalypts. Plant uptake of nutrients from the soil increased sharply after afforestation. Internal retranslocations of nutrients occurred in the savanna but were not quantified. This process supplied about 30% of the annual requirements of N and P in the Eucalyptus plantation from 2 years of age onwards, and about 50% of K requirements. Litter fall and litter decomposition were negligible in the savanna ecosystem and became important nutrient fluxes from age 2 years onwards, in the Eucalyptus stands.
The biogeochemical cycle of N was the most affected by afforestation. In particular, both mineralisation and nitrification rates permanently increased after planting eucalypts (Ranger et al., 2004). The immediate increase in nitrification rate observed after the destruction of the savanna was interpreted as a drastic change in the control of the nitrifying populations. Savanna vegetation was known to inhibit the activity of nitrifiers by allelopathic processes not yet completely elucidated. Investigations made in Ivory Coast showed the role of grass ecotypes on the control of nitrifiers (Abbadie et al., 2000). Allelochemicals responsible for the inhibition of nitrifiers were not yet identified in the savannas, contrarily to other specific forest situations (Paavolainen et al., 1998).
2. Input-output budgets
Harvesting method had a great influence on the nutrient budgets (Figure 1). The range of variation between the most conservative harvesting method (scenario 1) and the most costly in nutrients (scenario 4) was about 180 kg ha-1 for N, 25 kg ha-1 for P, 55 kg
ha-1 for K, Ca, and 30 kg ha-1 for Mg. De-barking the
stems on site retained at the soil surface 31, 9, 21, 28 and 16 kg ha-1 of N, P, K, Ca and Mg, respectively.
These values represented about 10% of the amount of N accumulated in the above-ground part of the trees at harvest, 20% of that of P and K, and 35% of that of Ca and Mg. The removal of firewood for surrounding populations in Congo (scenario 2) led to further losses of 50, 8, 20, 9 and 7 kg ha-1 of N, P, K, Ca and Mg,
respectively, relative to the most conservative method where only de-barked pulpwood is harvested. The current silviculture in Congo led to a deficit of 144 kg ha-1 of N for the first rotation after afforestation. This
deficit represented about 7% of the initial amount of total N in the A1 horizon (0-50 cm) under savanna.
In highly-weathered sandy soils, the long-term sustainability of these plantations is therefore greatly dependent on the reliability of fertilization practices.
Even if certain fluxes were assessed with large uncertainty, input-output budgets demonstrate clearly that Eucalyptus plantations take advantage, during the first rotation after afforestation, of a N soil fertility inherited from the previous vegetation of savanna. Unfavourable qualitative changes add further to the quantitative deficit of the N budget: savanna organic matter is progressively replaced by Eucalyptus organic matter poor in N (Trouvé et al., 1994), and whose chemical composition (tannins, lignin, polyphenols) leads to a slower mineralization (Bernhard-Reversat et al., 2001). For the other elements, the budgets for the whole rotation were well balanced relative to the
amounts of available elements in the soil. This behaviour is consistent with fertilizer field trials in this area, which show that tree responses to N inputs increase over successive rotations, whereas no response to P and K inputs is observed, even in replanted sites 20 years after savanna conversion (Bouillet et al., 2003).
3. Consequences for sustainable silivicultural practices
Low amounts of nutrients in the soils of this area (P excepted), and the high cost of fertilizer inputs make it essential to strictly limit nutrient losses throughout stand rotation. Several modifications in silvicultural practices were proposed to achieve this goal:
■ Field trials of fertilization. The quantitative data from the budgets show that field trials should focus on N fertilization. Future plantations, with much more productive clones of the hybrid E. urophylla × E. grandis, might lead to unbalanced budgets of K, Ca and Mg in the soils. It would then be important to determine whether these elements become limiting after several rotations.
■ Soil preparation and weed control. Minimum
cultivation is recommended to limit nutrient losses by erosion and planting must occur as quickly as possible after harvesting to reduce nutrient losses by drainage. Moreover, weed control must be planned to take advantage of the temporary fixation of nutrients in the biomass of weeds during the early growth of the stands (Nambiar and Sands, 1993).
■ Harvesting method. The effects of various harvesting scenarios on nutrient budgets were quantified. They show that current practices including de-barking on site is fundamental owing to the chemical paucity of the soil. This feature was confirmed by an experiment dealing with organic matter management in Congolese Eucalyptus plantations (Nzila et al., 2002).
■ Fire prevention. Nutrient budgets provide new light on the negative effects of fires in these plantations. Large losses of N by volatilization during burning have clearly detrimental consequences on the long-term production in this area where N is the first nutritional limiting factor. Effective fire prevention is therefore crucial for the sustainability of these plantations.
■ Introduction of a legume understorey. Numerous studies show that mixed plantations between Eucalyptus and legume species can have beneficial effects on soil N fertility (e.g. Binkley et al., 2003). In the Congo, mixed plantations cannot be established because the growth of N fixing trees is much lower than that of Eucalyptus. In this case, experiments were set up recently introducing an understorey of Acacia in Eucalyptus stands to enhance soil fertility, through inputs of organic matter and atmospheric N.
Conclusion
Quantification of nutrient fluxes throughout a rotation of Eucalyptus in the Congo demonstrated that the influence of silvicultural practices varied greatly according to the elements. Whereas the amounts of P, K, Ca and Mg in the soil were roughly stable throughout stand rotation, current silvicultural practices led to a deficit of about 140 kg ha-1 of N in
the soil. The budgets were strongly dependent on the harvesting method because this period accounted for the major output flux from the system. Input-output budgets suggested that Eucalyptus stands benefit from a N fertility inherited from the previous ecosystem of the savanna. Weeding destroyed a legume species responsible for N input in the savanna ecosystem estimated at around 20 kg ha-1 year-1.
Therefore, the sustainability of Eucalyptus plantations in this area will require an increase in N fertilizer inputs over successive rotations. Another option to improve the N status in these soils might be to introduce a biological nitrogen fixing species, compensating for the destruction of the native legume species in the savanna. Several experiments have been set up recently in the Congo to assess the influence of various Acacia species introduced as understorey in Eucalyptus stands. Further research is necessary to investigate silvicultural practices providing a positive influence of a leguminous understorey on soil N availability in the long term, without competing significantly with eucalypts during the early growth of the stands.
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