Some typical features of soil organic matter dynamics and soil texture were
studied to discuss the particular spatial pattern of tiger bush in Niger a
nd its dynamics. The soil texture through silt and clay contents showed a h
igh variability in the vegetation are as well as in the bare area. These va
riations were clearly linked to water/wind erosion and termite activity. Ti
ger bush soils showed a high capacity to store soil organic matter despite
a moderate primary production, even in the bare area where the input of pla
nt debris has been nil for many decades. The carbon content was higher with
in the vegetation are (0.93%) than within the bare area (0.45%). Additional
ly, potential carbon mineralisation significantly varied in relation to the
total carbon content and thus to primary production. Then, the vegetation
arcs can be viewed as 'fertility islands' as in many arid ecosystems. The m
easurements of delta(13)C showed a dominant contribution of C-3 plants to t
he soil organic matter pool. Nevertheless, the contribution of C-4 plants w
as not negligible. Two hypotheses could be proposed: a different mineralisa
tion rate between C-3 and C-4 plants; or (ii) a better physical protection
of C-4 compounds against biodegradation. The soil variables depending total
ly or partly on biological factors, such as carbon and nitrogen contents, c
arbon isotopic composition, carbon potential mineralisation, did not show a
ny symmetry in their variations along the studied transects. It was expecte
d in the vegetation are because the vegetation cover does not show symmetry
in its specific composition and spatial structure. In the bare area, a cle
ar asymmetry was observed on some of the variables: carbon content, fine ma
terial content and natural abundance of C-13. This supports the hypothesis
that the vegetation arcs move upslope, and weakens the hypothesis of the al
ternance of contraction and spreading periods of the vegetation cover. (C)
Elsevier, Paris.