Additional file 18. Table S7. Number of samples allocated for each country, and number of samples collected.

Background: Top-soil microbiomes make a vital contribution to the Earth's ecology and harbor an extraordinarily high biodiversity. They are also key players in many ecosystem services, particularly in arid regions of the globe such as the African continent. While several recent studies have documented patterns in global soil microbial ecology, these are largely biased towards widely studied regions and rely on models to interpolate the microbial diversity of other regions where there is low data coverage. This is the case for sub-Saharan Africa, where the number of regional microbial studies is very low in comparison to other continents. Results: The aim of this study was to conduct an extensive biogeographical survey of sub-Saharan Africa's top-soil microbiomes, with a specific focus on investigating the environmental drivers of microbial ecology across the region. In this study, we sampled 810 sample sites across 9 sub-Saharan African countries and used taxonomic barcoding to profile the microbial ecology of these regions. Our results showed that the sub-Saharan nations included in the study harbor qualitatively distinguishable soil microbiomes. In addition, using soil chemistry and climatic data extracted from the same sites, we demonstrated that the top-soil microbiome is shaped by a broad range of environmental factors, most notably pH, precipitation, and temperature. Through the use of structural equation modeling, we also developed a model to predict how soil microbial biodiversity in sub-Saharan Africa might be affected by future climate change scenarios. This model predicted that the soil microbial biodiversity of countries such as Kenya will be negatively affected by increased temperatures and decreased precipitation, while the fungal biodiversity of Benin will benefit from the increase in annual precipitation. Conclusion: This study represents the most extensive biogeographical survey of sub-Saharan top-soil microbiomes to date. Importantly, this study has allowed us to identify countries in sub-Saharan Africa that might be particularly vulnerable to losses in soil microbial ecology and productivity due to climate change. Considering the reliance of many economies in the region on rain-fed agriculture, this study provides crucial information to support conservation efforts in the countries that will be most heavily impacted by climate change.

Contaminated land in South Africa is regulated through the National Environmental Management Waste Act (Act 59 of 2008) (NEMWA) and the National Norms and Standards for the Remediation of Contaminated Land and Soil Quality (NSCLA) (GN R.331 of 2014). These standards were obtained from the Framework for the Management of Contaminated Land. A soil screening value (SSV1) for the protection of groundwater resources is proposed which is based on a 2-phase (stage) equilibrium partitioning and dilution model which includes a dilution factor and partitioning coefficient (K-d), converting the water quality guideline to a total soil screening value. The appropriateness of the screening values has been questioned because of the uncertainties surrounding the K-d values used by the Framework. This paper investigates the K-d values of Cu, Pb, and V for selected South African diagnostic soil horizons to evaluate the reliability of the current K-d values used by the Framework during Phase 1 screening. The Kd values of Cu for the 10 horizons ranged between 13 and 19 044 e.kg(-1), all exceeding the value of 10 e.kg(-1) provided by the Framework. For Pb the values ranged from 25 to >252 294 e.kg(-1) as compared to the Framework's 100 e.kg(-1). Similarly, the Kd value of 200 e.kg(-1) for V recommended by the Framework is higher than the measured K-d value of 15 to 173 e.kg(-1) for all 10 diagnostic horizons. This study demonstrated that the observed wide K-d value ranges for each element were related to the variation in basic soil properties such as soil pH, organic carbon, clay, Fe, and Al content. Therefore, the K-d values for Cu, Pb, and V currently used by the Framework are not representative of typical South African diagnostic soil horizons. Linear regression models were developed for the prediction of Cu, Pb, and V K-d values from measured soil properties, which could be used to generate soil-specific K-d values.

The use of dialysis membrane tubes filled with hydrous ferric oxide (DMT-HFO) solution has recently been reported as an effective way to characterize phosphorus (P) desorption over a long term in laboratory studies. However, the DMT-HFO method, similar to other soil tests, exploits 100% of the sample volume, which is much more than what the plant roots can exploit under natural conditions. One possible solution to mimic the root P uptake better by this method could be to modify the shaking procedure using different shaking periods. The objectives of this study were to investigate the influence of variable shaking times on the rate of P desorbed by the DMT-HFO method and to relate the desorption indices generated with maize yield in a diferential P fertilizer trial. The effect of varying shaking options on the extractable DMT-HFO-Pi for the different P treatments showed a significant difference only for treatment MNPK. Significant correlations were obtained between the labile pool rate coefficients [kA1 (0.92**), kA2 (0.99**), kA3 (0.92**), and kA4 (0.92**)] and maize grain yield for shaking options 1, 2, 3, and 4, respectively. The only rate coefficient from the less labile pool, kB, which showed a significant but moderate correlation (r = 0.78*) with maize grain yield, was kB1. The cumulative amount of P (mgkg-1) extracted by DMT-HFO showed no statistically significant correlations with maize grain yield in all the options considered. Judging from the r values, the rate coefficients appeared to be better indices of plant availability than the amount of P extracted by DMT-HFO, and based on the r values, option 2 seemed relatively better than the others because it showed the strongest correlation in both cases.

Knowledge on the availability of residual P in soils is of great importance for fertilization management. The use of dialysis membrane tubes filled with hydrous ferric oxide solution has recently been reported as an effective way to characterize P desorption over long-term laboratory studies. However, there is relatively little information relating the desorption indices of this method to plant parameters. The objective of this research was to relate the kinetic data generated using the DMT-HFO method to shoot dry matter yield of maize grown under greenhouse conditions. Accordingly, the labile pool rate coefficient (kA) showed a highly significant correlation with both shoot dry matter yield (r = −0.994**) and plant P uptake (r = −0.982**). The less labile rate coefficient (kB) also showed a significant correlation with both shoot dry matter yield (r = −0.856*) and P uptake (r = −0.893**). The correlation between the cumulative P extracted and shoot dry matter yield was highly significant. A strongly significant correlation was also observed between Bray 1P and shoot dry matter yield. Judging from the r-val-ues, both the kinetic parameters and the cumulative amount of P desorbed could serve as reliable indices of plant available P revealing the effectiveness of this method in estimating the availability of residual P in soils. However, assessment of the reliability of this method at field level is important. Data from a wider range of soils is necessary to evaluate the universality of this method.

In an attempt to characterize the phosphorus (P)-supplying capacity of a soil and to understand the dynamics of soil P, a procedure was followed whereby consecutive extraction procedures were carried out on a soil sample, first by dialysis membrane tubes filled with hydrous ferric oxide (DMT-HFO), followed by subsequent P fractionation procedure. However, this combined method is lengthy and time-consuming, and an approach to shorten these P desorption studies in soils was important. The major objective of this article, therefore, was to present a shortcut method as an alternative approach to the combined fractionation method. Comparison of the sum of DMT-HFO-P-i, sodium bicarbonate (NaHCO3)-P-i, sodium hydroxide (NaOH)-P-i, D/hydrochloric acid (HCl)-P-i, and C/HCl-P-i extracted by a conventional step-by-step method with the sum of DMT-HFO-P-i and a single D/HCl-P-i extraction as a shortcut approach for all extraction periods resulted in a very strong and significant correlations. Both these methods were correlated with maize grain yield, and it was found to be highly significant. This study revealed that this shortcut approach could be a simplified and economically viable option to study the P dynamics of soils especially for soils where the P pool acting as a source in replenishing the labile portion of P is already identified.

ABSTRACT Many agricultural fields that have received long-term applications of phosphorus (P) often contain levels of P exceeding those required for optimal crop production. Knowledge of the effect of the P remaining in the soil (residual effect) is of great importance for fertilization management. Plant P availability of residual P in soils is usually estimated using successive cropping experiments carried out in field or greenhouse studies. As this approach is very expensive and time consuming, more rapid soil test methods that can approximate this biological measure are required. The objective of this paper was to use a different approach to evaluate P availability (desorption) over a long period of time instead of the classical means of extraction. Thus, a modified sequential P extraction procedure using dialysis membrane tube filled with ferric hydrate solution (DMT-HFO) was used on the long-term P fertilized soils that received differential P treatments (PoLo, P1L1 and P2L1) to determine the changes in the different P pools and to relate these P fractions with maize yield. In this study, the contribution of both the labile and non-labile Pi fractions in replenishing the solution Pi was significant where as the organic fractions appeared to have limited contributions in replenishing the solution P. Highly significant correlations were observed between dry matter yield and the P pools extracted by HFO-Pi (0.997*), HCO3-Pi (r = 0.994**), OH-Pi (r = 0.969**), OH-Po (r = 0.944**), D/HCl-Pi (0.991**), and C/HCl-Pi (r = 0.997**). Strongly significant correlations were also observed between the different P fractions and plant P uptake. The C/HCl-Pi was the fraction that decreased most especially for the high P treatments indicating that this fraction contributed significantly to the P extracted by DMT-HFO. This suggested that this fraction might be a buffer to more labile P fractions. The combined method employed here could act as an analytical tool to approximate successive cropping experiments carried out under green house condition. But the applicability of this method at a field level should also be assessed. Data from a wider range of soils is also needed to evaluate the universality of this method.

We propose a rate equations model for the description of a mul tilongitudinal mode semi- conductor laser. We show how usually disregarded effects su ch as parametric interaction, carrier-diffusion and interference between the fields of thedifferent longitudinal modes play an important role in the final operating characteristic s of the laser. Parametric inter- action induces a non-trivial distribution of spectral poweramong different cavity modes. Interference results in nonlinear gain terms in the equatio ns, which favour longer wave- lengths versus shorter ones. Carrier-diffusion is regulati ng the magnitude of such effects. In the limit of very strong diffusion, previous rate equatio ns models are recovered.

Growing input cost in extensive and intensive agriculture production systems, as well as growing concern about environmental pollution in intensive production systems (e.g., the growing cut flower industry in southern africa) will force farmers in developing countries to reevaluate conventional approach to fertilization programs. Phosphate (P) desorption characterisation of soils under cultivation, to determine the time frame of applied P release, can be used to optimize fertilizer P programs. The desorption kinetics of residual and applied P to an acid sandy clay soil were investigated over 56 days using hydrous ferric oxide in dialysis tubes as a specific P sink, followed by a sequential P fractionation. P desorption kinetics were described with a two-component first-order model. Assumptions made were that two discrete P "pools", (a labile P pool [SPA] and less labile P pool [SPB]), participated in the desorption process and that P release from the two soil pools follow first-order kinetics. Soil P desorption kinetics were described (R-2 = 0.97 for the control and R-2 = 0.95 and R-2 = 0.99 for the P treatments of 75 (R75) and 150 mg kg(-1) (R 150), respectively) by the two-component first-order model. The P treatments reached no desorption maximum in the 56 days. P released from the control treatment in the 56 days was 10 times more than the B ray-ex tractable P. Only 8% of P applied in R150 and 6% in R75 were recovered in the 56 days. P applied increased desorption rate of P from SPA and the contribution from SPA to total P released in the 56 days. P applied had less of an impact on desorption rate of P from SPB. However, the 150 mg kg(-1) P treatment increased the half-life of SPB of the studied soil with +/- 13.5 days.

The chemical transformations of applied and residual soil phosphor-us (P) into different pools in two soils [Alfisols], a red sandy clay soil [Haplo-Palcustalfs] from Rustenburg, which is a high P fixing, and a red sandy loam soil [Pale-Xerults] from Loskop, a low P fixing were examined after treatments with different P rates (0, 25, 50, 100, 150, and 200 mg kg(-1)) and incubation periods (1, 60, 120, 180, and 240 days) under laboratory conditions. A sequential P extraction procedure was carried out on each treatment combination to determine the changes in 1) soluble and labile, 2) adsorbed, and 3) occluded and residual P pools in each soil. By fractionating soil P into different pools, it was possible to observe the transformation and distribution of added and initial soil P into different pools after each incubation period. Although solution and labile P decreased with time of incubation, there were corresponding increases in adsorbed, occluded, and residual P. Loskop soil had more marked increases in solution and labile P than the Rustenburg soil, which showed higher values for the adsorbed, occluded, and residual P forms. These differences could explain the reportedly higher levels of P fixation (adsorption and/or precipitation) by the Rustenburg than by the Loskop soil. Approximately 30-60% of the added P was transformed into less labile P pools in 1 day and 80-90% of the added P was transformed to the less labile P pools after 60 days of incubation. This transformation was very well marked in the higher than in the lower P fixing soil. A major proportion of the P transformation was to the NaOH-extractable P pool (adsorbed/nonlabile pool). The added P desorbed from this pool was fairly constant (about 30%) for the different incubation periods. Thus, to maximize fertilizer P efficiency, especially in the Rustenburg soil, band placement at planting time should be recommended, because the findings show that after 1 day of incubation 60% of the added P was transformed into less labile P. When band placed in the soil, the soil in the vicinity of the band will be saturated with P, and some P will remain in more labile forms and thus available for a longer period of time. The use of plants with well-developed root systems could also be recommended to ensure that the roots explore the soil volume more effectively. It may also be noted that although the added fertilizer P is transformed to more stable (immobile) P forms, it could be seen as long-term residual P pools for plants.