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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]

Crop residue management strategies must be adapted for improving carbon (C) balance and soil C stock in agroecosystems in sub‐Saharan Africa with consideration of the crop residue availability and site‐specific soil characteristics. We conducted field experiments to determine the effects of crop residue application method (incorporation/mulching) and quality (maize/cowpea) and N fertilizer application on the soil respiration rate and soil C stock in the surface soil layer (0–15 cm) in maize croplands with contrasting soil textures (clay/sandy) over 2 years from 2012 to 2014 in Tanzania. At the clay site, the incorporation of maize residues showed a 38% increase in CO2 flux compared to mulching, whereas, at the sandy site, mulching showed a 16% increase compared to the incorporation. At the sandy site, mulching practice retained soil moisture content and apparently enhanced the decomposition of the original soil organic C in the surface layer. It is, therefore, suggested that mulching practice may accelerate a long‐term depletion of soil C stock at the sandy site. The cowpea residue incorporation led to rapid decomposition because of its high biodegradability at both sites. The N fertilizer application stimulated the decomposition of labile soil organic matter. The soil C stock in the surface layer did not significantly change after the 2‐year experiment, irrespective of crop residue treatment and soil type. In conclusion, adequate crop residue management in terms of suppressing CO2 flux during a cropping season depends on soil type, but the long‐term effect on soil C stock is unclear. [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]

Abstract: Evaluation of carbon dynamics is of great concern worldwide in terms of climate change and soil fertility. However, the annual CO2 flux and the effect of land management on the carbon budget are poorly understood in Sub-Saharan Africa, owing to the relative dearth of data for in situ CO2 fluxes. Here, we evaluated seasonal variations in CO2 efflux rate with hourly climate data in two dry tropical croplands in Tanzania at two sites with contrasting soil textures, viz. clayey or sandy, over four consecutive crop-cultivation periods of 40 months. We then: (1) estimated the annual CO2 flux, and (2) evaluated the effect of land management (control plot, plant residue treatment plot, fertilizer treatment plot, and plant residue and fertilizer treatment plot) on the CO2 flux and soil carbon stock at both sites. Estimated annual CO2 fluxes were 1.0–2.2 and 0.9–1.9 Mg C ha−1 yr−1 for the clayey and sandy sites, respectively. At the end of the experiment, crop cultivation had decreased the surface soil carbon stocks by 2.4 and 3.0 Mg C ha−1 (soil depth 0–15 cm) at the clayey and sandy sites, respectively. On the other hand, plant residue application (7.5 Mg C ha−1 yr−1) significantly increased the surface soil carbon stocks, i.e., 3.5–3.8 and 1.7–2.1 Mg C ha−1 (soil depth 0–15 cm) at the clayey and sandy sites, respectively, while it also increased the annual CO2 fluxes substantially, i.e., 2.5–4.0 and 2.4–3.4 Mg C ha−1 yr−1 for the clayey and sandy soils, respectively. Our results indicate that these dry tropical croplands at least may act as a carbon sink, though the efficiency of carbon accumulation was substantially lower in sandy soil (6.8–8.4%) compared to clayey soil (14.0–15.2%), possibly owing to higher carbon loss by leaching and macro-faunal activity. [Copyright &y& Elsevier]

Abstract: In Sub-Saharan Africa, conservation of available soil N during early crop growth, when N loss by leaching generally occurs, is important to improve crop productivity. In a dry tropical cropland in Tanzania, we assessed the potential role of soil microbes as a temporal N sink–source to conserve the available soil N until later crop growth, which generally requires substantial crop N uptake. We evaluated the effect of land management [i.e., no input, plant residue application before planting (P plot) with or without fertilizer application, fertilizer application alone, and non-cultivated plots] on the relationship between soil N pool [microbial biomass N (MBN) and inorganic N] and crop N uptake throughout the ∼120-d crop growth period in two consecutive years. In the P plot, MBN clearly increased (∼14.6–29.6kgNha−1) early in the crop growth period in both years because of immobilization of potentially leachable N, and it conserved a larger soil N pool (∼10.5–21.2kgNha−1) than in the control plot. Especially in one year in which N leaching was critical, increased MBN maintained a larger soil N pool in the P plot throughout the experimental period, and a delay of increased MB C:N ratio and a substantial decrease in MBN was observed, indicating better soil microbial N supply for crop N uptake during later crop growth. Therefore, plant residue application before planting should enhance the role of soil microbes as a temporal N sink–source, leading to the conservation of potentially leachable N until later phase of crop growth, especially in years in which N leaching is relatively severe. Although further studies are necessary, our results suggest that plant residue application before planting is a promising option to achieve better N synchronization. [Copyright &y& Elsevier]

Abstract: Soil microbes are an essential component of most terrestrial ecosystems; as decomposers they are responsible for regulating nutrient dynamics, and they also serve as a highly labile nutrient pool. Here, we evaluated seasonal variations in microbial biomass carbon (MBC) and nitrogen (MBN) as well as microbial activity (as qCO2) for 16 months with respect to several factors relating to soil moisture and nutrients under different land management practices (plant residue application, fertilizer application) in both clayey (38% clay) and sandy (4% clay) croplands in Tanzania. We observed that MBC and MBN tended to decrease during the rainy season whereas they tended to increase and remain at high levels during the dry season in all treatment plots at both of our test sites, although soil moisture did not correlate with MBC or MBN. qCO2 correlated with soil moisture in all treatment plots at both sites, and hence soil microbes act as decomposers mainly during the rainy season. Although the effect of seasonal variation of soil moisture on the dynamics of MBC, MBN, and qCO2 was certainly greater than that attributable to plant residue application, fertilizer application, or soil texture, plant residue application early in the rainy season clearly increased MBC and MBN in both clayey and sandy soils. This suggests that plant residue application can help to not only counter the N loss caused by leaching but also synchronize crop N uptake and N release from soil microbes by utilizing these microbes as an ephemeral nutrient pool during the early crop growth period. We also found substantially large seasonal variations in MBC and MBN, continuously high qCO2, and rapid turnover of soil microbes in sandy soil compared to clayey soil. Taken together, our results indicate that soil microbes, acting as both a nutrient pool and decomposers, have a more substantial impact on tropical sandy soil than on clayey soil. [Copyright &y& Elsevier]

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]