LAUSR

Abstract The purpose of this study was to evaluate soil organic carbon, nitrogen stocks and physico-chemical soil parameters in eight provinces, various land use/ land cover systems and soils type in Gabon. 701 data of physico-chemical properties (particules size distributions, organic and nitrogen contents, C/N ratio and pH-water) of arable soil layers (0–30 cm) were collected from the ORSTOM's (Office de la Recherche Scientifique et Technique d'Outre-Mer) reports resulting from a sampling campaign in 8 out of the 9 provinces of Gabon 40 years ago. We compared soil pH, texture, bulk density, SOM contents and SOC and SON stocks in eight provinces under forests (primary and secondary) savannahs (herbaceous and shrubs) and under cropland and fallow forests and in main soils type. Soil bulk density ranged between 0.9 (G7) and 1.2 g.cm−3 (G1) and was significantly lower in G2, G4 and G9. Soils were acidic and no significant difference in soil pH among provinces were observed. However, soils under forest was more acidic than in savannahs. The soil textural show predominance of sand and clay and there was no difference in soil texture. According to soil type, SOC stocks were 5.5 kgC.m−2, 17.3 kgC.m−2, 18.7 kgC.m−2, 21.8 kgC.m−2, 22.3 kgC.m−2, 26.0 kgC.m−2 and 27.8 kgC.m−2 respectively in leached ferralitic soil, rejuvaneted ferralitic, lithic rough ferralitic, indured yellow ferralitic, hydromorphic soil, reworked soil and modal in ferralitic soil. SON stocks were higher in hydromorphic ferralitic soils (1.9 kgN.m−2) and reworked ferralitic soil (1.8 kgN.m−2) compared to others type soil. SOC and SON stocks were lower in center (14.2 kgC.m−2 and 1.4 kgN.m−2 in G3) and southern (16.1 kgC.m−2 and 1.4 kgN.m−2 in G5) of Gabon. Under various land uses system, SOC and SON were higher in cropland (27.0 kgC.m−2 and 2.8 kgN.m−2) compared to forests and savannahs, suggesting that burning forest did not affect SOC and SON stocks in different provinces.