Southern African Soils Research Articles

Selenium Status of Southern Africa.

Selenium is an essential trace element that exists in inorganic forms (selenite and selenates) and organic forms (selenoamino acids, seleno peptides, and selenoproteins). Selenium is known to aid in the function of the immune system for populations where human immunodeficiency virus (HIV) is endemic, as studies suggest that a lack of selenium is associated with a higher risk of mortality among those with HIV. In a recent study conducted in Zambia, adults had a median plasma selenium concentration of 0.27 μmol/L (IQR 0.14-0.43). Concentrations consistent with deficiency (<0.63 μmol/L) were found in 83% of adults. With these results, it can be clearly seen that selenium levels in Southern Africa should be investigated to ensure the good health of both livestock and humans. The recommended selenium dietary requirement of most domesticated livestock is 0.3 mg Se/kg, and in humans above 19 years, anRDA (recommended daily allowance) of 55 mcg Se/per dayisis recommended, but most of the research findings of Southern African countries have recorded low levels. With research findings showing alarming low levels of selenium in soils, humans, and raw feed materials in Southern Africa, further research will be vital in answering questions on how best to improve the selenium status of Southern African soils and plants for livestock and humans to attain sufficient quantities.

Heavy Metal Threats to Plants and Soil Life in Southern Africa: Present Knowledge and Consequences for Ecological Risk Assessment.

In recent times there has been remarkable development in the field of soil ecotoxicology and risk assessment (RA) models. It is, however, debatable if these RA models are robust representatives for worldwide relevance. In order to investigate this, the current overview aims to address heavy metal threats to soil life in southern Africa by investigating present knowledge and consequences for RA using research in southern Africa as a case. To this end, the focus is on southern African soils, soil life and living conditions. To critically discuss these issues, we report on extensive research conducted in the southern African context and looked how comparable these findings are to RA models employed in the western world. This is done by providing an inventory of selected studies focused on the ecotoxicity of metals towards soil life. It is concluded that there is a dearth of information on southern African soil life, most of which are laboratory-based studies carried out by a handful of researchers. Future research incorporating the available information into a soil ecosystem assessment procedure is paramount. It is recommended that a starting point to tackle this might be the development of holistic sight-specific guidelines for ecological risk assessment at larger spatial scales (km2) which takes into cognizance landscapes, vegetation and faunal characteristics.

Hydrogen isotope fractionation of leaf wax n-alkanes in southern African soils

The hydrogen isotope composition of plant leaf wax (δDwax) has been found to record the isotope composition of precipitation (δDp). Hence, δDwax is increasingly used for palaeohydrological reconstruction. It is, however, also affected by secondary factors, such as vegetation type, evapotranspiration and environmental conditions, complicating its direct application as a quantitative palaeohydrological proxy. Here, we present δDwax data from soils along vegetation gradients and climatic transects in southern Africa to investigate the impact of different environmental factors on δDwax. We found that δDwax correlated significantly with annual δDp (obtained from the interpolated Online Isotopes in Precipitation Calculator data set) throughout eastern and central South Africa, where the majority of the mean annual precipitation falls during the summer. We found evidence for the effect of evapotranspiration on δDwax, while vegetation change was of minor importance. In contrast, we found that δDwax did not correlate with annual δDp in western and southwestern South Africa, where most of the annual precipitation falls during winter. Wide microclimatic variability in this topographically variable region, including distinct vegetation communities and high vegetation diversity between biomes as well as a potential influence of summer rain in some locals, likely compromised identification of a clear relationship between δDwax and δDp in this region. Our findings have implications for palaeoenvironmental investigations using δDwax in southern Africa. In the summer rain dominated eastern and central region, δDwax should serve well as a qualitative palaeohydrological recorder. In contrast, the processes influencing δDwax in the winter rain- dominated western and southwestern South Africa remain unclear and, pending further analyses, potentially constrain its use as palaeohydrological proxy in this region.

Oxidative ratio (OR) of Southern African soils and vegetation: Updating the global OR estimate

Oxidative ratio (OR) is the ratio of moles O2 released per mole CO2 sequestered through photosynthesis in the terrestrial biosphere. The lower the OR value the more CO2 an environment can potentially sequester. It is this property of the organic matter that plays a role in models of CO2 partitioning between the atmosphere and the biosphere. Recent studies have shown that the accepted value of OR (1.1±0.05) may not be appropriate but that there are a number of research gaps before a full account of global OR values can be carried out.This study aims to fill some of the research gaps by carrying out a targeted sampling campaign in southern Africa. Vegetation, litter and soil samples were taken from a range of soil orders and biomes across a series of locations in South Africa and Swaziland. From these samples this study has been able to update a recent meta-analysis and show that although there were significant differences between some sites and vegetation types, there was no significant difference between the soil orders or biomes sampled. This study has also been able to update the global ORterra estimate to 1.06±0.06.

Determinants of fertiliser use by smallholder maize farmers in the Chinyanja Triangle in Malawi, Mozambique and Zambia

Farm surveys in Malawi, Zambia and Mozambique were carried out to assess the determinants of fertiliser use given continued low yields, low organic matter and general poor soil health in southern African soils. Regression modelling showed that fertiliser use was influenced by household and farm characteristics. In addition, it was also influenced by social and human capital and farmers’ perceptions of the effect of fertilisers on soil fertility. Farmers who perceived fertilisers as bad for their soil were less likely to adopt their use. This is a key result, as the emerging discussions on a green revolution for Africa, as well as the continued food crisis discussion, are prompting increased fertiliser use as an immediate intervention for increasing nutrient inputs into agriculture in the developing world. Increased policy efforts should be placed not only on increasing access to fertilisers but also on evolving farmers’ perceptions and attitudes towards fertiliser use.

Cultivation-Induced Effects on the Organic Matter in Degraded Southern African Soils

We studied quantitative and qualitative changes in soil organic matter (SOM) due to different land uses (reference woodland versus cultivated) on six soils from Tanzania (Mkindo and Mafiga), Zimbabwe (Domboshawa and Chickwaka), and South Africa (Hertzog and Guquka). Structural characteristics of the humic acids (HAs) were measured by Curie-point pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and solid-state 13C nuclear magnetic resonance (CPMAS 13C NMR) spectroscopy. Significant changes in concentration and composition of SOM were observed between land uses. Losses of organic carbon after cultivation ranged from 35% to 50%. Virgin soils showed large proportions of colloidal humus fractions: humic acids (HAs) and fulvic acids (FAs) but negligible amounts of not-yet decomposed organic residues. The change in land use produced a contrasting effect on the composition of the HAs: a noteworthy “alkyl enhancement” in Mkindo soil and “alkyl depletion” in Chikwaka and to a lesser extent in Domwoshawa. The remaining soils displayed only minor alterations.

Biofertilization of Degraded Southern African Soils with Cyanobacteria Affects Organic Matter Content and Quality

We studied, under laboratory conditions, the effects of Nostoc 9v (cyanobacteria) on the soil organic matter (SOM) content and quality of different soils from Tanzania, Zimbabwe, and South Africa. Soils were inoculated with Nostoc 9v at a rate of 5 mg dry biomass g−1 soil and incubated for 3 months at 27°C under 22 W m−2 illumination for 16 h and 8 h under darkness. Moisture content was maintained at 60% of field capacity. The composition of the SOM and the structural features of the humic acid (HA) fraction were studied by visible and infrared (IR) spectroscopies. Nostoc 9v proliferated and colonized the surfaces of all soils very quickly. The results obtained showed significant changes in the quantitative and qualitative characteristics of the SOM due to cyanobacterial growth. Inoculation and further incubation with Nostoc 9v promoted increases in organic carbon (OC) that ranged from 0.4 g C kg−1 soil to 9.0 g C kg−1 soil. Cyanobacterial growth also affected SOM characteristics through the incorporation of free, extractable, or particulate biomass with a predominantly aliphatic character. Important descriptors of the extent to which cyanobacterial metabolism modified the characteristics of the native SOM were the changes in the observed amounts of the two humin types, the increase in lipid concentration, and the changes in the optical density of the HAs.

Nitrogen isotope composition of soils, C 3 and C 4 plants along land use gradients in southern Africa

This paper provides values for nitrogen isotopic abundances of southern African soils and plants along land use gradients of varying aridity. The δ 15N values of soils and plants were generally higher in sites with greater land use intensity, except in the most arid site where δ 15N decreased with land use intensity. The enrichment in 15N with land use intensity agrees with the expected effects of grazing and cultivation on N cycling processes, including increased volatilization of ammonium, exports of plant material, and decreased N 2 fixation by the destruction of cyanobacterial soil crusts. Gross mineralization and nitrification rates were more affected by local heterogeneity in the soils than by aridity or land use. In general, C 3 plants had significantly higher δ 15N than C 4 plants from the same location, suggesting different N use by the two plant types. This study suggests that land use intensity affects N cycling processes that may result in different and opposite changes of ecosystem δ 15N, as those observed between the most arid, and the other semi arid sites analyzed. In addition to the 15N enrichment caused by the loss of gaseous and plant 14N, changes in tree and grass cover may affect soil δ 15N by the differential uptake of soil N with different isotopic abundances.

The genus &lt;i&gt;Eupodes&lt;/i&gt; Koch, 1835 (Acari: Prostigmata: Eupodidae) from southern African soil and vegetation. Part 1. Characterisation of the genus, designation of the type species and descriptions of three new species

Based on a study of the Eupodoidea of southern Africa, the genus Eupodes is redefined, with the incorporation of additional features, to distinguish it from other genera. E. striola Koch, 1835 is conceived as type species for the genus. Three new species, E. lutatus, E. indentatus and E. hamatus are described and figured. Special attention is contributed to the shape and position of dorsal idiosomal setae, chaetotaxy of the palp, leg chaetotaxy and solenidiotaxy. Both species are compared with their nearest congeners.

Oxygen Isotopic Ratios of Quartz from Wind‐Erosive Soils of Southwestern United States in Relation to Aerosol Dust

AbstractThe oxygen isotopic ratios (expressed as parts per thousand relative to mean ocean water, SMOW, δ18O) of the quartz from 13 soils undergoing much wind erosion during the study period of 1972–1975 in four southwestern states and from comparison areas were determined. The δ18O for quartz from eight Texas (TX) and Arizona (AZ) soils range from 13.0 to 15.9 ‰. The quartz of the sands and silts coarser than 20 µm from three of the soils had δ18O values ranging from 13.1 to 15.1 ‰, characteristic of an ultimate igneousmetamorphic origin. The δ18O values increase greatly with decreasing particle size of quartz from three soils ranging from loamy fine sand to loam to clay in texture. The δ18O of the 1–10 µm quartz fraction (aerosol size) ranged from 19.2 to 20.2 ‰ (19.55 ± 0.28 ‰; ± sigma) for the thirteen soils most affected by dust storms. The higher values are suggestive of an admixture of some quartz crystallized at low temperature to quartz of high temperature origin. These results are not significantly different from the mean of 20.73 ± 1.82 ‰ found earlier for the same fraction from 30 shales of the midcontinental United States, but are 4 to 7 ‰ higher than those reported for New Zealand, Australian, and southern African soils. The 1–10 µm quartz from the comparison areas of TX and AZ soils in Fluvial materials further southeast and southwest has a δ18O mean of 17.7 ± 0.6 ‰, reflecting the influx of a slightly higher proportion of quartz from igneous sources than in the soils most affected by wind erosion. The oxygen isotopic ratios of 1–10 µm quartz from three Hawaiian soils and two sediments from Lake Waiau occurring at 3,970 m altitude on the Mauna Kea summit on the Island of Hawaii give a δ18O mean of 18.3 ± 0.2 ‰. Quartz of the above soils, including sandy, loamy, and clayey soils of southwestern United States, in addition to shales reported earlier, are source reservoirs of quartz having intermediate oxygen isotopic ratios characteristic of mixtures of two sources of quartz, one crystallized at high (e.g., δ18O = 11 ‰) and one at low temperature (e.g., δ18O = 30 ‰).

Moisture redistribution in Southern African soils : Conference. Session four de Bruyn, CM 10F, 1T, 12R Proc. Eighth Int. Conf. on Soil Mech. Found. Engng. Moscow, 1973, V2.2, P37–44

Moisture redistribution in Southern African soils : Conference. Session four de Bruyn, CM 10F, 1T, 12R Proc. Eighth Int. Conf. on Soil Mech. Found. Engng. Moscow, 1973, V2.2, P37–44