In the context of major global environmental challenges such as food security, climate change, fresh water scarcity and biodiversity loss, the protection and the sustainable management of soil resources in Africa are of paramount importance. To raise the awareness of the general public, stakeholders, policy makers and the science community to the importance of soil in Africa, the Joint Research Centre of the European Commission has produced the Soil Atlas of Africa. To that end, a new harmonised soil map at the continental scale has been produced. The steps of the construction of the new area-class map are presented, the basic information being derived from the Harmonized World Soil Database (HWSD). We show how the original data were updated and modified according to the World Reference Base for Soil Resources classification system. The corrections concerned boundary issues, areas with no information, soil patterns, river and drainage networks, and dynamic features such as sand dunes, water bodies and coastlines. In comparison to the initial map derived from HWSD, the new map represents a correction of 13% of the soil data for the continent. The map is available for downloading.

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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.

Water erosion is one of the main concerns driving land degradation in mountainous areas throughout the world, and its characteristics change widely with soil and climate conditions in different locations. To investigate the effects of soil and rainfall properties on surface runoff and soil loss, we installed runoff plots (width 0·8 m × slope length 2·4 m) enclosed by corrugated iron sheets, and evaluated water budgets for rainfall, surface runoff and soil moisture for every rainfall event over a rainy season at four sites (designated NY, TA, SO and MA) in the Uluguru Mountains of Tanzania. Two of the sites, NY and TA, were located in mountainous areas and had steep slopes and higher rainfall amounts, while the two foothill sites, SO and MA, were flatter and with coarser textures. Runoff amount was related to rainfall amount, but also to infiltration capacity. Runoff amounts in mountainous areas were higher than in foothill sites while runoff ratio was low at sites with high permeability in the surface layer, TA and MA. Sediment concentrations and soil loss were basically enhanced by high rainfall amount and intensity at mountainous sites. Soil texture also controlled the erodibility of soils; MA had twice the soil loss of SO owing to sandy soil despite similar rainfall amounts at MA and SO. Our results show that the high rainfall amounts in the mountainous areas and high susceptibility of sandy soils to erosion enhanced soil loss by water erosion in this study.

Nutrient deficiency, high rate of evapotranspiration, and insufficient and erratic rainfall are the critical challenges for crop production in the dryland areas (DLAs) of Sub-Saharan Africa, including Tanzania, where 61% of arable land is prone to drought. In addressing these challenges, field trials were conducted in central Tanzania to evaluate the interactive effects of ripping and tie-ridges with organic (FYM) and inorganic fertilizers (N) on the mitigation of the critical period of soil moisture stress (CPSMS) for sorghum yield performance. Both in situ rainwater harvesting techniques (IRWHT) and flat-cultivated land were integrated with 8 Mg FYM ha-1 , 70 Kg N ha-1 , and a combination of 35 Kg N ha-1 and 4 Mg FYM ha-1 (N+ FYM). Among the IRWHT, tie-ridges stored a significant water volume of 577 and 457 m 3 ha-1 , which mitigated the CPSMS by the maximum of 95% and 37% for the above-average rainfall and below-average rainfall season, respectively. However, it only registered the highest grain yield (2.02 Mg ha-1) and biomass (3.46 Mg ha-1) in a belowaverage rainfall season. The highest overall grain yield (5.73 Mg ha-) and biomass (12.09 Mg ha-1) were harvested in ripping with combined fertilizer treatments in an above-average rainfall season, while the lowest grain yield (0.5 Mg ha-1) and biomass (1.2 Mg ha-1) were registered in the flat-cultivation control in the belowaverage rainfall season. In the latter season, IRWHT increased the mitigation potential in the order; flat cultivation < ripping < tie-ridges; and sorghum yield, highly correlated with drought mitigation index. The results showed that sorghum grain yield and final biomass performance depend on the influence of IRWHT applied, rainfall amount, soil moisture level, integrated fertilizer, and length of the CPSMS. In the aboveaverage rainfall seasons, fertilizers mask the influence of the IRWHT. The opposite is true in the below-average rainfall season. Although ripping-N+ FYM resulted in the highest overall yield, the study recommends practicing tie-ridges integrated with N+ FYM due to regular occurrences of low and unreliable rainfall in the dryland areas.

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