SOIL ORGANIC CARBON DYNAMICS UNDER A MOSAIC FORESTSAVANNAH IN THE PLATEAU TEKE
S.A. Ifo
INTRODUCTION
In Congo Brazzaville, the forests occupy 64.6 % of territory (FAO, 2005). They extend from north until the southern of the country on nearly 22 106 ha distributed in
three major zones: the forest of the North’s Congo (68%), the forest of Chaillu (16%) and the forest of Mayombe (7%). The remaining 9% (about 22 106 ha) are found as
mosaic of forests and savannah, mostly gallery and patches on secondary forests as in the plateaux teke (14°E, 4°S).
Currently characterized by a positive dynamics for at least five or six centuries, these natural forests could play an important role in the mitigation of CO2 in the
atmosphere. The objective of this work: i)-quantifying carbon stock in forests and savannah; (ii) following deep inputs of organic matter originated from savannah and
forest.
DESCRIPTION OF THE EXPERIMENTAL SITE
Annual rainfall trends (mm)
This study was conducted in the site of Iboubikro (3°26’S-15°23E). The site is localized at140 km
North East of Brazzaville on the plateau teke. The average annual rainfall is 2100 mm, (figure 1).
and average temperature of 26°C. The soil of the region is a deep acidic sandy arenosol (Schwartz
and Namri, 2002). The two studied forest were a secondary forest (SF) with Macaranga barteri
[Euphorbiaceae] and Musanga cecropioides R. Brown. [Cecropiaceae] and a gallery forest (GF)
with Colletoecema dewevrei (De Wild.) and Eriocoelum microspermum Gilg ex Radlk. [Sapindaceae].
Three plots of 40m x 40m were delimited in each site.
500
400
300
200
100
0
J F MAM J J A S O N D J FMAMJ J A S ON D J F MAM J J A S O ND
2006
2007
2008
figure 1
METHODES
3 soils core per site (GF, site 1 and SF, site 2) and 2 soils core in the savannah were collected up to one meter deep and was separated by 20 cm layer. After treatments
Soils were analyzed for total C and total nitrogen content with an elemental microanalyser (NCS 2500). With the mass spectrometry (figure 2) 13C in soil were
quantified for each horizon, thus we determined the C originated from forest and those originated from savannah and could follow the impact of the progression of
forest in the C total. The C stock Qi (KgC m-2) in a soil layer i with a thickness of Ei (m) was calculated from (1). The C content Ci (Kg C g-1) of the layer and its bulk
density Di (Kg m-3). The total C stock at a given depth was calculating by summing Qi. (2). The C originated from forest was estimated from equation (3).
Equation 1
Equation 2
Equation 3
δ13Ct = Carbon 13 of organic matter of soil samples ;
δ13Cs = Carbon 13 of organic matter derived from
savannah
δ 13Cf = Carbon 13 of organic matter derived from
savannah
Ct= total carbon.
RESULTS AND DISCUSSION
The results obtained showed that the total carbon up to one meter is more important in the gallery and secondary forest than under the savannah. We obtained up to
one metter 6.4kg.m-2 in the GF, 6.2 kg.m-2 in SF and 4.6 kg.m-2 in the savannah. The 3 means of total C are different between sites (ANOVA, p=0.0001). Studying of
the variation of δ13C with profile depth showed that in the forest, the δ13C values were close to those of C3 (-26‰), δ13C decrease with depth untill -19‰ in the
deepest at -100 cm. Under savannah, the δ13C values are close to those of C4 vegetation (-13,6‰ ). In all the profile under savannah, we observed that the δ13C values
are constant. We concluded that there clearly a mixing between old organic matter from savannah and the new organic matter from trees. The result suggest the net
effect of the growing of forest on the old savannah vegetation due to climate change in this area.
Total carbon soil between GF,SF and Sa (KgC.m-2)
Ctotcum020
8.00
pourcentage 13C
Ctotcum80100
-30
a
-25
-20
-15
1.00
2.00
3.00
0
0.00
4.00
1.00
2.00
3.00
4.00
-5
-20
b
-20
-20
7.00
-40
4.00
Profondeur (cm)
a
b
3.00
c
FG
-60
FS
SA
-40
Depth (cm)
-40
Depth (cm)
c
6.00
5.00
0
0.00
0
-10
Evolution of Cfor in comparison with Ctotal
FS under secondary forest (KgC.m-2)
Evolution of Cfor in comparison with Ctotal FG
under gallery forest (KgC.m-2)
Distribution verticale du signal
isotopique 13C dans les 3 sites.
-60
-60
-80
-80
-100
-100
-80
2.00
Cfor FS
1.00
-100
-120
0.00
CforFG
-120
Ctot FS
Ctot FG
FG
FS
-120
SA
Ifo_suspense@yahoo.fr
BP 69 Université Marien Ngouabi
Faculté des Sciences
Tél: (242) 068029720.
Brazzaville-CONGO
This research was funded by the
International Foundation for Science
(IFS)