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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 (CR FR ) and savanna (CR SV ) 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 NO 3 − (0.16–0.19 mmol c  L −1 ), while it was present in negligible amounts in SV. The 2-year NO 3 − flux in CR FR (156 kg N ha −1 ) was double that in CR SV (78 kg N ha −1 ), leading to greater 2-year K + leaching in CR FR (118 kg K ha −1 ) than in CR SV (37 kg K ha −1 ). The ratio of 2-year nutrient losses to total soil stocks was the greatest for Ca both in CR FR (5%) and CR SV (4%), while K loss also reached 5% in combination with lower solution pH in CR FR . In conclusion, cultivation of former forest land substantially increased NO 3 − leaching, resulting in depletion of both K and Ca; whereas, cultivation of former savanna results in mainly Ca depletion. [ABSTRACT FROM AUTHOR]

We lack an understanding of nitrogen (N) cycles in tropical forests of Africa, although the environmental conditions in this region, such as soil type, vegetation, and climate, are distinct when compared with other tropical forests. Herein, we simultaneously quantified N fluxes through precipitation, throughfall, and 0-, 15-, and 30-cm soil solutions, as well as litterfall, in two forests with different soil acidity (Ultisols at the MV village (exchangeable Al3+in 0–30 cm, 126 kmolcha–1) and Oxisols at the AD village (exchangeable Al3+in 0–30 cm, 59.8 kmolcha–1)) over 2 years in Cameroon. The N fluxes to the O horizon via litterfall plus throughfall were similar for both sites (MV and AD, 243 and 273 kg N ha–1 yr–1, respectively). Those values were remarkably large relative to other tropical forests, reflecting the dominance of legumes in this region. The total dissolved N flux from the O horizon at the MV was 28 kg N ha–1 yr–1, while it was 127 kg N ha–1 yr–1 mainly as NO3–-N (~80%) at the AD. The distinctly different pattern of N cycles could be caused by stronger soil acidity at the MV, which was considered to promote a superficial root mat formation in the O horizon despite the marked dry season (fine root biomass in the O horizon and its proportion to the 1-m-soil profile: 1.5 Mg ha–1and 31% at the MV; 0.3 Mg ha–1and 9% at the AD). Combined with the published data for N fluxes in tropical forests, we have shown that Oxisols, in combination with N-fixing species, have large N fluxes from the O horizon; meanwhile, Ultisols do not have large fluxes because of plant uptake through the root mat in the O horizon. Consequently, our results suggest that soil type can be a major factor influencing the pattern of N fluxes from the O horizon via the effects of soil acidity, thereby determining the contrasting plant–soil N cycles in the tropical forests of Africa. [ABSTRACT FROM AUTHOR]

Soil water erosion is a major agricultural concern in tropical Africa with high precipitation and low soil fertility where Oxisols are generally distributed. A field experiment was conducted in east Cameroon during the rainy season in 2013 to investigate the effects of surface mulching with the residues of Imperata cylindrica on soil losses and surface runoff water generation in a cassava cropland on an Oxisol. Three treatments were established using two small plots for each treatment: bare plot (BA), cassava plot (CA) and cassava with mulch plot (CM). Soil loss and surface runoff water were measured, and water budgets of rainfall, surface runoff and soil moisture within rainfall events were measured in all treatment plots. Runoff coefficients in all treatment plots were suppressed below 8·0% because the large volume of large pores of surface soil of Oxisols contributed to the high drainage capacity over a rainy season even under wet soil moisture conditions. Total soil loss in CM was decreased by 49% compared with that in BA and CA, despite there not being a large difference in runoff water among treatments. These results suggest that surface mulching with the residues of I. cylindrica can substantially suppress soil losses caused by particle detachment by raindrops, while it maintain soil surface with originally high permeability in cassava croplands on Oxisols. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Land use intensity in the Cameroon plateaus, where Ferralsols and the associated highly weathered soils are dominant, has been steadily increasing, so that soil fertility status and its controlling factors on these plateaus should be elucidated to establish long-term land management strategies. Although it is known that Ferralsols have small, but variable CEC values, it has not been fully determined which clay components are the main source of CEC clay in Ferralsols. Here, soils in three Cameroon plateau regions of different altitude (the lower South Cameroon (SC) Plateau with an altitude of 300–600 m, SC L ; the upper SC Plateau with an altitude of 600–900 m, SC H ; and the Adamawa Plateau with an altitude above 900 m, ADM) were collected, and their physicochemical and mineralogical properties were investigated. The relative abundance of illitic minerals, which are a minor but the most promising source of CEC clay in Ferralsols, was determined by using three indicators: the relative abundances of 1.0 nm-thick structures as represented by XRD peak intensity (XRD 1.0 nm ), structural K in illitic minerals as represented by clay-K content, and the quantity of intermediate interlayer sites between illite and vermiculite structures (i.e., frayed edge sites) as represented by the radiocesium interception potential (RIP). The relative abundances of two low-activity clay components, kaolinite and gibbsite, were determined by differential thermal analysis, and the degree of Al-substitution in goethite was determined by differential x-ray diffraction analysis. All three quantitative indicators for illitic minerals decreased along the altitude gradient, SC L > SC H > ADM, and were positively correlated with CEC clay . While kaolinite also showed a decreasing trend at higher altitude, it did not show any significant correlation with CEC clay . In contrast to the decrease in these Si-bearing minerals, gibbsite and Al-substituted goethite increased along the same altitude gradient. These associative changes strongly indicate that illitic minerals were the clay component largely controlling CEC clay and that de-silication or aluminization may proceed at higher altitude or on stable surfaces with older formation ages in the Cameroon plateau regions. Thus, mineralogical properties well explained the distribution of Ferralsols; they were marginal in the SC L region, while those with relatively high CEC clay were dominant in the SC H region and those with lower CEC clay were dominant in the ADM region. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) dynamics after slash-burn agriculture are poorly understood in African tropical forest, though recent studies have revealed C4 grass invasion as a forest understory influences SOC dynamics after deforestation. This study aimed to quantify the relative SOC contribution of C4 and C3 plants separately through the sequential fallow periods of forest (cropland, or 4-7, 20-30, or >50 years of fallow forest) in the tropical forest of eastern Cameroon. We evaluated the SOC stock and natural 13 C abundance for each layer. The SOC stock was largest in 4-7 years fallow forest (136.6 ± 8.8 Mg C ha -1 ; 100 cm depth, and C4:C3 = 58:42), and decreased with increasing fallow period. SOC from C4 plants was larger in the 4-7 and 20-30 years fallow forests (57.2-60.4 ± 5.8 Mg C ha -1 ; 100 cm depth), while it clearly decreased in >50 years fallow forest (35.0 ± 4.1 Mg C ha -1 ; 100 cm depth), resulting in the smallest SOC in this mature forest (106.4 ± 12.9 Mg C ha -1 ; 100 cm depth). These findings indicate that C4 grass understories contributed to the SOC restoration during early fallow succession in the tropical forest of eastern Cameroon.