T. Desjardins et al., ORGANIC-CARBON AND C-13 CONTENTS IN SOILS AND SOIL SIZE-FRACTIONS, AND THEIR CHANGES DUE TO DEFORESTATION AND PASTURE INSTALLATION IN EASTERN AMAZONIA, Geoderma, 61(1-2), 1994, pp. 103-118
In soils of the eastern Amazonian forest, modifications in soil organi
c matter (SOM) contents as a consequence of deforestation and pasture
installation were investigated. Profile distribution of total organic
carbon (C) and nitrogen (N), and of C-13 isotope abundance (expressed
in delta(13)C parts per thousand units) were compared. The two soils,
one under native forest and the other one after ten years under pastur
e of Pennisetum purpureum had similar C/N values, which slightly decre
ased with increasing depth, from 13.6 to 11.9-12.7 within the first 40
cm. In the pasture soil, the C content was slightly lower than in the
forest soil, and reached 29 t ha(-1) compared with 31 t ha(-1), and 1
5 t ha(-1) compared with 16 t ha(-1), in the 0-20 and 20-40 cm layers,
respectively. The delta(13)C values reflected the origin of SOM, from
either the forest (which had a C3 photosynthetic cycle), or the pastu
re (which had a C4 cycle). They were higher by at least 6.5 and 1.5 un
its, in the respective 0-10 and 10-20 cm layers of the pasture soil, t
han in the corresponding layers of the forest soil. These delta(13)C v
alues were used to estimate the proportions of C derived from the fore
st (Cdf) and from the pasture (Cdp). The calculations clearly indicate
d a strong input of Cdp, which reached 46-49% and 21-24% of total C in
the respective layers. Particle size fractionation showed that SOM ch
anges were predominant in the upper soil layer (0-10 cm), and vanished
with increasing depth: in the pasture soil, the coarse organic residu
es accumulated, whereas the amount of C in the finest fractions decrea
sed, which suggested a slowing down in humification processes. In the
forest soil, the delta(13)C values were lower in the coarsest than in
the finest fractions, the latter being less affected than the coarsest
ones by the vegetation change; in the 0-10 cm layer, these values inc
reased by about 7-10 units in the sand-size fraction, and only 4-5 uni
ts in the clay-size fraction. Thus, the replacement of Cdf by Cdp was
greater in the sand-size fraction ( 55-65% of total C), than in the cl
ay-size fraction (34-45% of total C). Based on C-14 dating, the oldest
poor (i.e. that of mean age 5000 years) of forest SOM was calculated
at each depth. In the soil surface, it was still smaller than the more
labile fraction of Cdf, and represented about 17% and 26% of total C
in the 0-10 cm and 0-20 cm layers, respectively.