Effect of original vegetation on nutrient loss patterns from Oxisol cropland in forests and adjacent savannas of Cameroon

Forest-savanna mosaic is widespread in tropical Africa mainly occurring on nutrient-poor Oxisols. Though sustainable agriculture is a major concern in this region, little is known about the effects of original vegetation (i.e., forest vs savanna) on nutrient losses from cropland. Hence, we evaluated basic cation losses and nutrient balance of Oxisol cropland cultivated over two years in the Cameroonian forest-savanna mosaic. Solute fluxes at 30-cm depth in maize croplands derived from forest (CRFR) and savanna (CRSV) were compared with those in adjacent forest (FR) and savanna (SV) ecosystems. Nutrient inputs by rainfall, outputs from solute leaching and cropland grain removal, and soil nutrient stocks measured at depths between 0 and 30 cm were investigated. The main anion present in FR soil solutions was NO3- (0.16-0.19 mmol(c) L-1), while it was present in negligible amounts in SV. The 2-year NO3- flux in CRFR (156 kg N ha(-1)) was double that in CRSV (78 kg N ha(-1)), leading to greater 2-year K+ leaching in CRFR (118 kg K ha(-1)) than in CRSV (37 kg K ha(-1)). The ratio of 2-year nutrient losses to total soil stocks was the greatest for Ca both in CRFR (5%) and CRSV (4%), while K loss also reached 5% in combination with lower solution pH in CRFR. In conclusion, cultivation of former forest land substantially increased NO3- leaching, resulting in depletion of both K and Ca; whereas, cultivation of former savanna results in mainly Ca depletion.