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Pearl millet is an important crop for food security in Asia and Africa's arid and semi-arid regions. It is widely grown as a staple cereal grain for human consumption and livestock fodder. Mechanistic crop growth and water balance models are useful to forecast crop production and water use. However, very few studies have been devoted to the development of the model parameters needed for such simulations for pearl millet. The objectives of the study were to determine crop-specific model parameters for each of three pearl millet varieties (landrace, hybrid, and improved), as well as to calibrate and validate the Soil Water Balance (SWB) model for predicting pearl millet production and water use based on weather data. The SWB was chosen because it is widely used in southern Africa; however, the developed parameters should benefit other models as well. The presented crop-specific parameter values were derived from field observations and literature. Varieties with different phenology, maturity dates and tillering habits were grown under well-watered and well-fertilised conditions for calibration purposes. The calibrated model was used to predict biomass production, grain yield and crop water use. The hybrid's water use efficiency was higher than that of the landrace and improved variety.

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BACKGROUND: There are concerns that fertilization using sludge in semi-arid areas, where water is limiting, will compound the effect of drought, resulting in the decline of yield from potential salt accumulation. This study investigated impacts of annual sludge application at 0, 4, 8 and 16 Mg ha-1 on weeping lovegrass hay yield, crude protein (CP) content, rainfall use efficiency (RUE), nitrogen use efficiency (NUE) and trace metal uptake over eight consecutive years. RESULTS: Both hay yield and RUE increased by 5-53% as the sludge rate increased. Hay yield was highest (13.3 Mg ha-1 ) during the wet season and RUE (27.1 kg mm-1 ) during the dry season. RUE was highest at sludge rates of 16 Mg ha-1 and NUE at 4 Mg ha-1 . Similarly, municipal sludge application increased CP content as well as crop Cr and Zn uptake from the 16 Mg ha-1 treatment. CONCLUSION: Results from this study indicated that eight consecutive years of treated municipal sludge application increased weeping lovegrass hay yield, CP content and RUE. Similarly, trace metal uptake by crop did not differ between the zero control and the 16 Mg ha-1 treatment, except for Zn and Cr, which showed a slight increment. Nonetheless, all trace metals remained well below the maximum tolerable dietary concentrations for domestic animals. © 2017 Society of Chemical Industry.

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Future water scarcities in the face of an increasing population, climate change and the unsustainable use of aquifers will present major challenges to global food production. The ability of water footprints (WFs) to inform water resource management at catchment-scale was investigated on the Steenkoppies Aquifer, South Africa. Yields based on cropping areas were multiplied with season-specific WFs for each crop to determine blue and green water consumption by agriculture. Precipitation and evapotranspiration of natural vegetation and other uses of blue water were included with the agricultural WFs to compare water availability and consumption in a catchment sustainability assessment. This information was used to derive a water balance and develop a catchment WF framework that gave important insights into the hydrology of the aquifer through a simplified method. This method, which requires the monitoring of only a few key variables, including rainfall, agricultural production, WFs of natural vegetation and other blue water flows, can be applied to inform the sustainability of catchment scale water use (as opposed to more complex hydrological studies). Results indicate that current irrigation on the Steenkoppies Aquifer is unsustainable. This is confirmed by declining groundwater levels, and suggests that there should be no further expansion of irrigated agriculture on the Steenkoppies Aquifer. Discrepancies between in- and outflows of water in the catchment indicated that further development of the WF approach is required to improve understanding of the geohydrology of the aquifer and to set and meet sustainability targets for the aquifer. It is envisaged that this 'working' framework can be applied to other water-stressed aquifers around the world.

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Sludge application to agricultural lands is often limited mainly because of concerns about metal accumulation in soils and uptake by crops. The objective of the study was to test the following hypotheses: (i) in the short to medium term (5-10 yr), the application of good-quality sludge according to crop N requirements will not lead to significant accumulation of water-soluble metal fractions in soil, (ii) mobility and uptake of metals is higher under irrigated than dryland systems, and (iii) metal concentrations in plant tissue could reach phytotoxic levels before the soil reaches environmental threshold levels. Field plots were arranged in a randomized complete block design comprising four replications of three treatments (0, 8, and 16 Mg ha yr anaerobically digested municipal sludge) planted to dryland maize and irrigated maize-oat rotation. Soil and plant samples were collected after 7 yr of treatment application for selected metal analyses. A large fraction of the Zn, Ni, and Pb in the soil profile was ethylenediaminetetraacetic acid extractable (46-79%). Saturated paste-extractable fractions of Cd and Pb were <1 mg kg. Plant uptake of Cd, Pb, and Ni under irrigation was double that for dryland systems. Concentrations of the metals considered in plant tissue of both cropping systems remained well below phytotoxic levels, except for Zn under dryland maize, which received 16 Mg sludge ha yr. Metal concentrations in the soil remained far below total maximum threshold levels. Therefore, hypotheses 1 and 3 were accepted for the metals considered, and hypothesis 2 was rejected for Zn.

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Rehabilitating coal gasification ash dumps by amendment with waste-activated sludge has been shown to improve the physical and chemical properties of ash and to facilitate the establishment of vegetation. However, mineralization of organic N from sludge in such an alkaline and saline medium and the effect that ash weathering has on the process are poorly understood and need to be ascertained to make decisions regarding the suitability of this rehabilitation option. This study investigated the rate and pattern of N mineralization from sludge in a coal gasification ash medium to determine the prevalent inorganic N form in the system and assess the effect of ash weathering on N mineralization. An incubation experiment was performed in which fresh ash, weathered ash, and soil were amended with the equivalent of 90 Mg ha sludge, and N mineralization was evaluated over 63 d. More N (24%) was mineralized in fresh ash than in weathered ash and soil, both of which mineralized 15% of the initial organic N in sludge. More nitrification occurred in soil, and most of the N mineralized in ash was in the form of ammonium, indicating an inhibition of nitrifying organisms in the ash medium and suggesting that, at least initially, plants used for rehabilitation of coal gasification ash dumps will take up N mostly as ammonium.

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The nutrient content of sludge produced by municipal water treatment works often far exceeds the requirements of nearby crops. Transporting sludge further afield is not always economically viable. This study reports on the potential to export large volumes of anaerobically digested municipal sewage sludge through turfgrass sod production. Hypotheses examined are that sludge loading rates far above recommendations based on crop nutrient removal (i) are possible without reducing turf growth and quality, (ii) do not cause an accumulation of N and P below the active root zone, (iii) can minimize soil loss through sod harvesting, and (iv) do not cause unacceptably high nitrate and salt leaching. An 8 Mg ha(-1) sludge control (the recommended limit) was compared with sludge rates of 0, 33, 67, and 100 Mg ha(-1) on a loamy, kaolinitic, mesic, Typic Eutrustox soil near Johannesburg, South Africa. Sludge application rates up to 67 Mg ha(-1) significantly improved turfgrass establishment rate and color. The ability of sods to remain intact during handling and transport improved as the sludge application rate increased to 33 Mg ha(-1) but deteriorated at higher rates. A sludge application rate of 100 Mg ha(-1) was needed to eliminate soil loss, but this rate was associated with unacceptably high N leaching losses. All our hypotheses were accepted for application rates not exceeding 33 Mg ha(-1) on the proviso that some soil loss was acceptable and that the leaching fraction was carefully managed during the first 2 mo after sludge application.

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