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Organic acid secretion from the roots enables plants to acquire phosphorus (P) which is poorly soluble in soil. We previously reported that when soybeans were cultivated in vermiculite in the presence of insoluble calcium phosphate, as a phosphorus source, sulfur (S) fertilization increased organic acid secretion from the roots and improved P acquisition in soybeans. In the present study, we confirmed that S fertilization increased secretion of organic acids such as citric acid when soybeans were cultivated in Andosols having a strong P fixation capacity. In contrast, concentration of citric acid in soybean roots did not increase by S fertilization. Therefore, the relationship between S nutrition and gene expression of citric acid exporters was investigated to understand the mechanisms of induction of citric acid secretion by S. Further, we verified whether the expression of citric acid transporter genes, GmMATE13 and GmMATE47, is involved in the induction of citric acid secretion from the roots by S fertilization. The expression level of GmMATE13 in roots was significantly increased by S fertilization compared to that without S fertilization. Therefore, our results suggest that S nutrition is involved in inducing GmMATE13 expression and contributes to the excretion of citric acid from the soybean roots. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fixation by rhizobia plays an important role in N assimilation of soybean (Glycine max). Nitrogenase, the enzyme responsible for N fixation in rhizobia, contains a large amount of sulfur (S) molecules and requires phosphorus (P) in adenosine triphosphate. Thus, S and P are essential for plant growth and N fixation by rhizobia. However, P in soils forms insoluble PO43 – salts with metals. Plant roots solubilize these PO43- salts by secreting organic acids and absorb solubilized PO43-. In this study, the effects of S application on P solubilization by soybean roots and the involvement of rhizobia were evaluated. Soybean plants inoculated with Bradyrhizobium diazoefficiens USDA110 were grown in vermiculite, and Ca3(PO4)2 was added as the sole P source. A hydroponic solution supplemented with three S concentrations (0, 20, and 1000 µM) was applied, and plants were grown for 5 weeks. Root fresh weight, the soil-plant analysis development (SPAD) values, and shoot and root S concentrations increased significantly with S application, but root nodule numbers and nitrogenase activity were not significantly affected. Organic acid secretion by roots was drastically increased by S application, and shoot P concentration and content also increased significantly. These results indicated that S application can promote P solubilization by increasing the secretion of organic acids by roots and increase in N fixation is not involved in this promotion at least in early vegetative growth period. [ABSTRACT FROM AUTHOR]

Intercropping with cereal and phosphorus (P)-mobilizing leguminous crops is a traditional agricultural practice in Sub-Saharan Africa. It is essential to evaluate the effect of intercropping (i.e., rhizosphere sharing) on fractionated rhizosphere soil P dynamics under the various soil P conditions and legumes, to improve the soil-plant P dynamics in this region. This study aimed to identify the effect of rhizosphere sharing of maize (MZ)/P-mobilizing legumes intercropping on fractionated rhizosphere soil P dynamics and plant P uptake under different P fertilization rates in strongly weathered cropland soil of Tanzania. We conducted 60-day MZ and P-mobilizing legumes intercropping pot experiments under different P-fertilized conditions (0, 50, and 100 kg P ha−1) with two legumes (pigeon pea (PP) and groundnut (GN)) by using three different root-separation treatments (no separation (NS) for rhizosphere sharing, nylon mesh separation (MS), and complete separation (CS)), to compare the effect of rhizosphere sharing in one pot. We evaluated the Hedley-fractionated P of rhizosphere soil and plant P uptake in each pot. We found that both P-efficient legumes did not clearly decrease the less labile P (NaOH-P) of rhizosphere soil in no P treatment, while these clearly decreased the less labile P in 100 kg P ha−1 treatment. It indicates that P-efficient legumes can solubilize the fertilized P but not native less labile P of the soil. The NS treatment increased the labile P of rhizosphere compared with CS by solubilizing the fertilized P, except for MZ/GN intercropping at 50 kg P ha−1. Relative to CS, MZ/PP intercropping increased the total plant P uptake per pot (20%–40%) under P-fertilized conditions, whereas MZ/GN intercropping decreased it (17%) at 50 kg P ha−1 or showed no change at 100 kg P ha−1. MZ/PP intercropping improved P fertilizer recovery efficiency, whereas MZ/GN intercropping did not. Enhanced P use efficiency through intercropping with P-mobilizing legumes is dependent on P-solubilizing characteristics of each P-mobilizing legume. [ABSTRACT FROM AUTHOR]

The article discusses the use of biochar, a carbonated biomass generated through pyrolysis, for improving agricultural productivity and carbon sequestration. The benefits of biochar have been observed in regions such as East and Southeast Asia, where there is a long history of biochar utilization. The article highlights the importance of sharing information on biochar in order to effectively use it in other countries. The special section in the journal Soil Science & Plant Nutrition serves as a starting point for research on biochar utilization for soil sustainability from Asian agronomical and ecological perspectives. [Extracted from the article]

Sulfur (S) and Nitrogen (N) metabolisms in plants are interacted and it is known that S deficiency decrease N absorption and metabolism. In leguminous plants S deficiency also decreases N2 fixation by rhizobia in the nodules. Deep placement of a controlled-release N fertilizer is a good method to provide nitrogen to soybean without inhibiting N2 fixation; thus, it was hypothesized that this method is able to provide nitrogen effectively to sulfur-deficient soybean plants. In this study effects of deep placement of coated urea on S-N physicological interaction, growth and productivity in soybean plants were examined using pot experiments. Soybean plants were grown with sulfate concentrations of 30, 100, or 1000 μM, with or without deep placement of coated urea. Shoot weights at the developing stage were not affected by S deficiency. SPAD values of leaves during the flowering stage decreased with S deficiency and increased with the deep placement of coated urea. S deficiency decreased seed weight per plant at the harvesting stage, but this decrease was attenuated by the deep placement of coated urea. N and S content in shoots at the developing stage increased with the deep placement of coated urea, whereas in seeds, only the N content increased. N2 fixation activity based on the relative ureide-N content in xylem sap indicated that the deep placement of coated urea did not inhibit N2 fixation activity at the early flowering stage. Without deep placement of coated urea, the relative ureide-N content decreased under S deficiency at the seed filling stage. These results suggest that the deep placement of coated urea is an efficient method to supply N to support soybean yield under S deficiency. Abbreviations: Deep+: with deep placement of coated urea; Deep–: without deep placement of coated urea [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Soil phosphorus (P) forms have been practically defined as chemically fractionated pools. A knowledge of the abundance and diversity of P forms in soil, and the factors affecting them, will lead to better soil management. However, little is known about the differences in P forms among soils with different geological properties in tropical Africa. The aim of this study was to investigate the P forms in soils with different physicochemical properties formed under different geological conditions in southern Tanzania and to identify the factors affecting the P forms in these soils. In total, 37 surface soil samples were collected from three geological groups; the plutonic (mainly granite) rock (PL) group, the sedimentary and metamorphic rock (SM) group, and the volcanic ash (V) group. Soil P was sequentially extracted by NH4Cl, NH4F, NaHCO3, NaOH + NaCl, and HCl, and inorganic (Pi) and organic P (Po) in each fraction were determined. The lowest total P was in the PL group (average, 360 mg P kg-1) because of the high sand content. Iron (Fe)-P (NaOH-Pi) was the major form in this group, accounting for 8.4% of total P. In the SM group (average total P, 860 mg P kg-1), Fe-P was the major form in most, accounting for 7.8% of total P. Soils in the SM group occasionally had high calcium (Ca)-P due to application of chemical fertilizer at the collection site. The V group had the highest total P (average, 1600 mg P kg-1) and its major P form was Ca-P, which was possibly derived from primary minerals (i.e., apatite), accounting for 14% of total P. In addition, the high oxalate-extractable Al possibly caused the accumulation of Al-P in the V group. Oxalate-extractable Fe generally increased with increasing Fe-Pi, while oxalate-extractable Al increased with increasing organic P and Al-Pi in soils in all three geological groups. These results demonstrate that the soil P forms differ greatly among sites in southern Tanzania with different geological conditions and associated soil properties. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) dynamics in croplands need to be improved for the increase of food production in sub-Saharan Africa. Our objective was to investigate the crop residue management in terms of quality and application method to improve soil N dynamics during the period of crop growth in maize croplands with special reference to soil textures in dry tropical agroecosystems. Field experiments were conducted during the rainy seasons of 2012 and 2013 at clayey and sandy sites in Tanzania. Five treatment plots were established: plots with no residue applied (Ctrl plot), chemical fertilizer applied (F plot), maize residue (high C:N ratio = 60) incorporated (M-In plot) or mulched (M-On plot), and cowpea residue (low C:N ratio = 21) incorporated (P-In plot). In each plot, we regularly collected surface soil and crop samples and measured inorganic N (NO3−–N and NH4+–N), the soil respiration rate, and crop N uptake. Rainfall and soil moisture content were also continuously monitored. Our results showed that cowpea residue with high biodegradability caused prompt increases in the soil respiration rate and soil inorganic N because decomposition of the applied residue occurred within ca. 2 weeks after the application at both the clayey and sandy sites. It resulted in high N use efficiency in the early growing period and 18–73% higher total crop N uptake than that in the maize residue plots at both sites. The application methods (incorporation and mulching) showed no significant difference in soil N dynamics and crop yield, although higher soil respiration was observed in the M-In plot than the M-On plot at the clayey site. However, at the sandy site, mulching of maize residue resulted in the increased soil inorganic N content in the M-On plot by 18–26% compared with that in the M-In plot during the cropping season because of the higher soil moisture content. Consequently, crop N uptake in the mulching plot resulted in 26–59% higher than that in the incorporation plot. [ABSTRACT FROM PUBLISHER]

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]

The wet–dry cycles of soil primarily drive carbon (C) dynamics in dry croplands that mainly experience sporadic rainfall events. We evaluated thein situshort-term (hourly) dynamics of soil carbon dioxide (CO2) efflux and microbial biomass, to compare the significance of a single rainfall event with/without C substrate to reveal the effects of a single rainfall on the soil C dynamics in clayey dry croplands in four different climates and ecosystems. The experiments were conducted on four clayey dry croplands as follows: Thailand (TH) and Tanzania (TZ) in tropical climates, and Kazakhstan (KZ) and Hungary (HG) in continental climates. Hourly measurements of soil CO2efflux,in situmicrobial biomass (MB) andin situmicrobial activity (qCO2) were conducted after the application of simulated rainfall (W plots) and rainfall/glucose (WG plots) treatments. We also evaluated the easily mineralizable carbon (EMC) by incubation. The rainfall treatment caused an increase in the qCO2but not in MB, causing a clear but short C flush in all W plots (10–37 h), while the WG treatment caused an increase both of qCO2and MB, resulting in substantially longer and larger C flush in the WG plots (ca. 100 h). The ratio of the cumulative soil CO2flux caused by rainfall treatment to EMC was larger in TH-W and TZ-W plots (8.2 and 4.9%, respectively) than in the KZ-W and HG-W plots (2.9 and 1.1%, respectively). In addition, applied glucose was more heavily mineralized in the TH-WG and TZ-WG plots (15.0 and 9.7%, respectively) than in the KZ-WG and HG-WG plots (6.4 and 3.4%, respectively), because of the different MB increment patterns for the first 24 h, i.e., immediate and large MB increments in TH and TZ, but not in KZ and HG. These results reveal a possible mechanism that causes the rapid decomposition of soil organic carbon and applied organic matter in the dry tropical cropland. [ABSTRACT FROM PUBLISHER]

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−1less 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. [ABSTRACT FROM PUBLISHER]

The wetting-drying cycles of soil primarily drive carbon (C) and nitrogen (N) dynamics in tropical monsoon climates. We evaluated the in situ short-term C and N dynamics and the effect of soil texture during a wetting-drying cycle in relation to hourly microbial dynamics. Two croplands of differing soil textures (clay [THc] and sand [THs]) in Thailand were used for the experiment. Hourly measurements of soil CO efflux and inorganic nitrogen (Inorg-N) were conducted and we determined fluctuations in the in situ microbial biomass ( In-situ-MB) and in situ microbial activity ( In-situ-qCO) after the application of a simulated rainfall (W plot) and a rainfall/glucose (WG plot) treatment. The rewetting of dry soil led to a C flush, which finished within 50-120 h because of rapid soil drying at both sites. Comparing the microbial dynamics in the THc-W and THs-W plots, it is clear that the rainfall treatment predominantly increased In-situ-qCO in the THc-W plot, whereas it increased In-situ-MB in the THs-W plot. These different microbial dynamics resulted in different C and N dynamics; that is, the cumulative soil CO efflux for the first 100 h after the treatment was effectively greater in the THs-W plot (3.4 g C m) than in the THc-W plot (2.8 g C m). In addition, distinct N-mineralization associated with a decreasing In-situ-MB was observed only in the THs-W plot, although this was not the case in the THc-W plot. Hence, we concluded that rainfall events should play a more important role in the C and N dynamics in sandy soils compared with clayey soils because of different microbial dynamics after rewetting of a dry soil. [ABSTRACT FROM AUTHOR]

Soil microbes are considered to be an important N pool in dry tropical croplands, which are nutrient poor. To evaluate the N contribution of soil microbes to plant growth in a dry tropical cropland, we conducted a maize cultivation experiment in Tanzania using different land management treatments (no input, plant residue application, fertilizer application, plant residue and fertilizer application, and non-cultivated plots). Over 104 experimental days, we periodically evaluated the microbial biomass N and C, plant N uptake, microbial respiration in situ and inorganic N in the soil. A significant amount of inorganic N was lost in all of the treatment plots as a result of leaching during the initial 60 days and inorganic N remained low thereafter (∼20–35 kg N ha−1 : 0–15 cm), whereas soil microbial respiration substantially decreased because of soil drying after 60 days (grain-forming stage). During the grain-forming stage (60–104 days), we found a distinct effect of plant N uptake on soil microbial dynamics, although we did not observe an obvious effect of plant residue and/or fertilizer application; microbial biomass N decreased drastically from 63–71 to 18–33 kg N ha−1 and the microbial biomass C : N ratio simultaneously increased (>10-fold) in all maize-cultivated plots; these features were not observed in the non-cultivated plot. Plant N uptake over the same period was 26.6–55.2 kg N ha−1, which was roughly consistent with the decrease in microbial biomass N. These results indicate that strong competition for N occurred between soil microbes and plants over this period and N uptake by plants prevented microbial growth. Thus, we concluded that soil microbes contribute to plant growth by serving as a N source during the grain-forming stage in dry tropical cropland. [ABSTRACT FROM AUTHOR]

Geophagy is defined as the practice of eating soil and is observed worldwide. In Tanzania, edible soil sticks called pemba are consumed mainly by pregnant women, but the physiological function of eating pemba has not yet been elucidated. The objectives of the present study were to evaluate the physicochemical properties of edible soil sticks compared with ordinary soils in Tanzania and to evaluate the function of geophagy in terms of micronutrient supply and the adsorption capacity of materials such as toxins. The color of eight pemba samples collected from various markets was reddish or whitish and their shape was generally columnar with an average length, width and weight of 6.1 cm, 1.8 cm and 22 g, respectively. Pemba had a more clayey texture (48% clay on average) than the ordinary soils investigated for comparison, and the clay composition was generally dominated by kaolinite. The concentrations of available nutrients in pemba, extracted with 0.1 mol L−1 NaCl (pH 2), were 391 mg Ca kg−1, 234 mg Mg kg−1, 17 mg Mn kg−1, 6.5 mg Fe kg−1, 4.9 mg Cu kg−1, 2.8 mg Co kg−1, 1.9 mg Zn kg−1 and 1.1 mg Ni kg−1, and extraction with reductant drastically increased the amounts of Fe and Mn released. The amount of available nutrients supplied by pemba consumption at a rate of 50 g day−1 amounted to 99% of Mn and 13% of Fe in the case of reddish pemba and 75% of Cu in the case of whitish pemba of the daily requirement by pregnant women, suggesting the potential of pemba to supply these micronutrients. A moderate cation exchange capacity (CEC) level (11.1 cmolc kg−1) and lower ratio of CEC to clay content for the pemba compared with the soils indicated that the adsorption capacity was not the main criteria for choosing soil materials and instead fine-textured soils with kaolinitic clay mineralogy were deliberately chosen for pemba. In conclusion, the main function of eating pemba in Tanzania, and probably the original function of geophagy, is the supply of micronutrients, such as Mn, Cu and Fe, rather than the adsorption of toxic materials. [ABSTRACT FROM AUTHOR]