Soil organic matter dynamics in tiger bush (Niamey, Niger). Preliminary results

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.