Makoto Shibata

Effect of vegetation on soil C, N, P and other minerals in Oxisols at the forest-savanna transition zone of central Africa

Abstract The forest-savanna transition zone, which evolves as a result of past climate change, is widely distributed in central Africa. Because nutrient-poor soils (Oxisols) are widely distributed in this area, it is necessary to understand the characteristics of soil nutrients in relation to the vegetation. We collected 52 soil samples from five pits each for two different vegetation types (forest and savanna) in this area and evaluated the effect of vegetation type on soil physicochemical properties [pH, soil texture, cation-exchange capacity, bulk density, crystalline and non-crystalline aluminum (Al) and iron (Fe)] and nutrient status [carbon (C), nitrogen (N), phosphorus (P) and other soil minerals]. We also evaluated the fractionated P. Whereas most physicochemical properties were similar between the two vegetation types throughout the soil profile (0–80 cm depth), clay content, bulk density and soil pH clearly differed between the vegetations at the surface layer (0–10 cm). At 80 cm soil depth, soil C, N and P were 87.9, 7.7 and 3.7 Mg ha−1, respectively, in forest, and 98.6, 7.1 and 3.1 Mg ha−1, respectively, in savanna. Although there was no clear difference between the amounts of soil C, N and P, the upper-soil (0–40 cm) C:N ratio was clearly lower in forest (11.0–12.0) compared with savanna (13.0–15.7), because the main plant species in the forest can fix N effectively. We also found a smaller ratio of sodium hydroxide (NaOH)-extractable inorganic P to total soil P in forest compared with savanna. Because the content of crystalline and non-crystalline Al and Fe in forest soil was similar to that of savanna, the different soil C:N ratio would cause different availability of P between the vegetation types, although the mechanism is unclear. These results indicate that savanna vegetation is N-limited and forest vegetation is N-saturated (and possibly P-limited) in this zone. We also found that, at 20 cm soil depth, total soil potassium (K) in forest was 1590 kg ha−1, which was 930 kg ha−1 less than that in savanna (2520 kg ha−1; P < 0.05), although a similar difference was not measured for Na, Ca, and magnesium (Mg). Because we observed lower soil pH in forest, not only plant K uptake but also K leaching loss would contribute to lower soil K in forest.

Nitrogen flux patterns through Oxisols and Ultisols in tropical forests of Cameroon, Central Africa

ABSTRACT 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 kmolc ha–1) and Oxisols at the AD village (exchangeable Al3+ in 0–30 cm, 59.8 kmolc ha–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–1 and 31% at the MV; 0.3 Mg ha–1 and 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.

Symbiotic N2-Fixation Estimated by the 15N Tracer Technique and Growth of Pueraria phaseoloides (Roxb.) Benth. Inoculated with Bradyrhizobium Strain in Field Conditions

This field experiment was established in Eastern Cameroon to examine the effect of selected rhizobial inoculation on N2-fixation and growth of Pueraria phaseoloides. Treatments consisted of noninoculated and Bradyrhizobium yuanmingense S3-4-inoculated Pueraria with three replications each. Ipomoea batatas as a non-N2-fixing reference was interspersed in each Pueraria plot. All the twelve plots received 2 gN/m2 of 15N ammonium sulfate 10% atom excess. At harvest, dry matter yields and the nitrogen derived from atmospheric N2-fixation (%Ndfa) of inoculated Pueraria were significantly (P < 0.05) higher (81% and 10.83%, resp.) than those of noninoculated Pueraria. The inoculation enhanced nodule dry weight 2.44-fold. Consequently, the harvested N significantly (P < 0.05) increased by 83% in inoculated Pueraria, resulting from the increase in N2-fixation and soil N uptake. A loss of 55 to 60% of the N fertilizer was reported, and 36 to 40% of it was immobilized in soil. Here, we demonstrated that both N2-fixing potential of P. phaseoloides and soil N uptake are improved through field inoculations using efficient bradyrhizobial species. In practice, the inoculation contributes to maximize N input in soils by the cover crops biomass and represent a good strategy to improve soil fertility for subsequent cultivation.

Ecosystem Processes of Ferralsols and Acrisols in Forest-Soil Systems of Cameroon

Tropical African forests are dominated by Ferralsols and leguminous species, while Southeast Asian forests are dominated by Acrisols/Alisols and Dipterocarpaceae. Hence, their ecological processes can differ, depending on soil acidity and nitrogen (N) availability. To provide an overview of the carbon (C) and N dynamics, as well as soil acidification processes on Ferralsols and Acrisols, in tropical African forests, we quantified soil respiration and element fluxes through different flow paths (as precipitation, throughfall, litterfall, litter leachate, and soil solutions) and analyzed proton budgets in two secondary forested sites in Cameroon. Our results demonstrate that at Mvam Village (MV; Acrisols), N was mostly taken up within the O horizon, which has a dense root mat, while half of the input N leached down to the mineral horizon at the Andom Village (AD; Ferralsols) site. Nitrification was the main proton-generating process in the canopy and the O horizon of AD, and it caused a large amount of cation leaching, which resulted in the accumulation of basic cations because of the high proton consumption rates in the A horizon. In contrast, because of the dense root mat at MV, the excess cation uptake by plants in the O horizon made the largest contribution to proton generation, which resulted in intensive acidification of the surface soil. Our results suggest that ecosystem processes differ depending on soil type (i.e., soil acidity). Thus, legumes growing on Ferralsols in tropical African forests have unique plant-soil interactions via active nitrification in the O horizon.

Changes in Elemental Dynamics After Reclamation of Forest and Savanna in Cameroon and Comparison with the Case in Southeast Asia

To understand the effects of original vegetation (forest or savanna) on changes of solute leaching and proton budgets after reclamation of Oxisols in Cameroon, we quantified the soil nutrient fluxes in forest, adjacent savanna, and each adjacent maize cropland and compared with the case in Southeast Asia. In forest plot, excess cation accumulation in wood has contributed to soil acidification in the entire soil profile, while soil acidification rates were much lower in savanna plot because of limited plant uptake. As a result of cultivation, NO3− fluxes were substantially increased and nitrification was the main process of soil acidification in both croplands. Reflecting the nutrient flux pattern of original vegetation, protons generated by nitrification in forest cropland plot (9.4–10.1 kmolc ha−1 year−1) were significantly higher than that in savanna cropland (3.4–4.5 kmolc ha−1 year−1). The rate in savanna cropland was comparable to that in Thailand Ultisol (5.0 kmolc ha−1 year−1) with moderate soil pH and higher than that in Indonesian Ultisol (1.5 kmolc ha−1 year−1) with low pH. Despite low pH of bulk Oxisols of Cameroon, they would provide favorable habitat for nitrifiers with physically well-structured microaggregates, allowing active nitrification in the plots of Cameroon. High rate of nitrification suggests the risk of nutrient deficiency in cropland is more serious in nutrient-poor Oxisols. The effects of reclamation on soil acidification processes would depend on the original vegetation and also on soil pH and physical structure, which affect the nitrification activity. Since the ratio of K+ concentrations to sum of Mg2+ and Ca2+ concentrations increased with decreasing soil solution pH, the lower solution pH, which could stem from cultivation, might promote K leaching from cropland.

Effects of 3-year cultivation on the soil nutrient status in a tropical forest and savanna of Central Africa, as determined by the microbial responses to substrate addition

ABSTRACT The forest–savanna transition zone is widely distributed on nutrient-poor Oxisols in Central Africa, and a population explosion has led to the rapid cultivation of these vegetation types in this zone. To reveal and compare the effects of short-term (3 years) cultivation on the soil nutrient status of the forest and savanna vegetation in this area, we evaluated microbial nutrient limitation and availability by conducting hourly measurements of soil microbial respiration after the addition of glucose in combination with nitrogen (N) and/or phosphorus (P) to soils that were collected from a forest site (FOR), a savanna site (SAV), as well as cropland for 3 years derived from a forest (Crop-F) and a savanna (Crop-S), in eastern Cameroon. The N addition had little effect on the pattern of microbial respiration rate for the FOR and Crop-F sites, indicating N rich for microbes. In contrast, N addition resulted in the increases in maximal respiration rates after the exponential increase for the SAV and Crop-S sites, indicating microbial N limitation, and cultivation accelerated the soil N depletion. Furthermore, we observed that P addition resulted in the increase in the maximal respiration rates, indicating microbial P limitation for all sites, except for FOR site. Since the cultivation significantly affected the microbial properties only in the forest ecosystem, such as the increase in the microbial specific growth rate and the decreased microbial C:N and C:P ratios, these changes would induce the P limitation for Crop-F. These results indicate that (1) the FOR site was a N-rich ecosystem for soil microbes, and 3 years of cultivation in the Crop-F site did not alter the high soil N status but induced microbial P limitation, with the changes in the microbial properties, and that (2) the SAV site was N and P limited for soil microbes, and 3 years of cultivation clearly decreased the soil N availability.

Plant–soil interactions maintain biodiversity and functions of tropical forest ecosystems

Tropical forests are characterized by high biodiversity and aboveground biomass growing on strongly weathered soils. However, the distribution of plant species and soils are highly variable even within a tropical region. This paper reviews existing and novel knowledge on soil genesis, plant and microbial physiology, and biogeochemistry. Typically, forests in Southeast Asia are dominated by dipterocarps growing on acidic Ultisols from relatively young parent material. In the Neotropics and Africa, forests contain abundant legume trees growing on Oxisols developed in the older parent materials on stable continental shields. In Southeast Asia, the removal of base cations from the surface soil due to leaching and uptake by dipterocarp trees result in intensive acidification and accumulation of exchangeable Al3+, which is toxic to most plants. Nutrient mining by ectomycorrhizal fungi and efficient allocation within tree organs can supply phosphorus (P) for reproduction (e.g., mast fruiting) even on P-limited soils. In the Neotropics and Africa, nitrogen (N) fixation by legume trees can ameliorate N or P limitation but excess N can promote acidification through nitrification. Biological weathering [e.g., plant silicon (Si) cycling] and leaching can lead to loss of Si from soil. The resulting accumulation of Al and Fe oxides in Oxisols that can reduce P solubility through sorption and lead to limitation of P relative to N. Thus, geographical variation in geology and plant species drives patterns of soil weathering and niche differentiation at the global scale in tropical forests.

Soil phosphorus of stable fraction differentially associate with carbon in the tropical forest and savanna of eastern Cameroon

ABSTRACT The forest–savanna transition zone, which contains nutrient-poor soils (Oxisols), is found throughout central Africa. To evaluate the effect of deforestation on soil phosphorus dynamics, which regulate the plant growth in this area, we quantified the relationship between phosphorus (P) and carbon (C) in different fractions and compared their relationship to forest and savanna (deforested vegetation) in eastern Cameroon. We analyzed the P, C, and nitrogen (N) contents of soil using the physical fractionation method (0.25–2.0 mm as macro-particulate organic matter [M-POM]; 0.053–0.25 mm as micro-POM; and <0.053 mm as Clay+silt) in different land management (young and old forests and annual and perennial grass savannas at 100-cm soil depth). We found larger soil P stock in forests (4.7–4.9 Mg P ha−1) than that in savannas (3.4–4.0 Mg P ha−1), though soil C and N stocks were similar between the vegetation. We also observed lower soil P stock in the active fraction (M-POM) with its higher C:P and lower C:N ratio in forest surface layer (0–10 cm), indicating that forests have lower available soil P. By using the regression analysis, we found a clear relationship between P and C in the stable fraction (Clay+silt) of the upper layer (0–40 cm) for each land management, and the coefficient of the regression was clearly different between the forest and savanna. It indicates that a more chemically complex organic P form of the stable fraction exists in forest soil than in savanna soil. These results indicate that the deforestation (savannazation) affect the active and stable P dynamics and it should cause the lower soil P stock of the upper layer in savanna than in forest.