Modelling maize grain yield and nitrate leaching from sludge-amended soils across agro-ecological zones: A case study from South Africa

When applying municipal sludge according to crop N requirements, the primary aim should be optimizing sludge application rates in order to maximize crop yield and minimize environmental impacts through nitrate leaching. Nitrate leaching and subsequent groundwater contamination is potentially one of the most important factors limiting the long-term viability of sludge application to agricultural soils. This study assessed maize grain yield and potential nitrate leaching from sludge-amended soils, using the SWB-Sci model, based on crop nitrogen requirements and inorganic fertilizer. The following hypotheses were tested using the SWB-Sci model and 20 years of measured weather data for 4 of the 6 South African agro-ecological zones. Under dryland maize cropping, grain yield and nitrate leaching from sludge-amended soils compared to inorganic fertilizer: (1) will remain the same across agro-ecological zones and sites, (2) will not vary across seasons at a specific site, and (3) will not vary across soil textures. Model simulations showed that annual maize grain yield and nitrate leaching varied significantly (P > 0.05) across the four agro-ecological zones, both for sludge-amended and inorganic fertilizer amended soils. The annual maize grain yield and nitrate leaching from sludge-amended soils were 12.6 t∙ha−1 and 32.7 kgNO3 -N∙ha−1 compared to 10.2 t∙ha−1 and 43.2 kgNO3 -N∙ha−1 for inorganic fertilizer in the super-humid zone. Similarly, maize grain yield and nitrate leaching varied significantly across seasons and soil textures for both sludge and inorganic fertilizer amended soils. However, nitrate losses were lower from sludge-amended soils (2.3–8.2%) compared to inorganic fertilizer (11.1–26.7%) across all zones in South Africa. Therefore, sludge applied according to crop N requirements has a lower environmental impact from nitrate leaching than commercial inorganic fertilizer. Further validation of these findings is recommended, using field studies, and monitoring potential P accumulation for soils that received sludge according to crop N requirements.Keywords: sewage sludge, inorganic fertilizer, nitrate leaching, maize yield, agro-ecological zones, SWB-Sc

Model parameters of four important vegetable crops for improved water use and yield estimation

High-value vegetable crops are typically grown under irrigation to reduce production risk. For water resource planning it is essential to be able to accurately estimate water use of irrigated crops under a wide range of climatic conditions. Crop water use models provide a means to make water use and yield estimates, but need crop- and even cultivar-specific parameters. There is generally a lack of crop-specific model parameters for some important commercially grown vegetable crops, especially parameters determined over both summer and winter seasons. The experimental site used in this study was on the Steenkoppies Aquifer, a catchment under stress and an important vegetable production area in South Africa. Crop-specific growth parameters and water use for 4 selected high-value vegetable crops (beetroot, cabbage, carrots and broccoli) were measured over multiple seasons (two summers and one winter). These were used to parameterise the Soil Water Balance (SWB) generic crop growth model for both summer and winter seasons. In seasons where the same cultivar was planted, a single set of model parameters could be used to successfully simulate crop growth and water use. Results show that the amount of irrigation water required is dependent on season and rainfall, with broccoli having the lowest (1.8–2.7 kg m−3) and beetroot the highest (12.2–23.4 kg m−3) water productivity (WPFM), defined as fresh mass of marketable product per unit water consumed. The root crops had a greater harvest index (HIDM) than cabbage and broccoli. The parameters obtained expand the current database of SWB crop growth parameters for vegetables and can be used in a wide range of mechanistic simulation models to improve water management at field and catchment levels.Keywords: SWB model, Steenkoppies Aquifer, carrot, broccoli, beetroot, cabbag

Suitability indicators to assess specific site, risk-based irrigation water quality

In the first of this two-paper series, we introduced readers to a newly developed electronic Decision Support System (DSS) that helps the user to assess the fitness for use of irrigation water (du Plessis et. al., 2019). In this paper, we provide an overview of the suitability indicators used by the DSS to assess the fitness for use of water for irrigation.http://www.sabi.co.za/magazine.htmlam2019Town and Regional Plannin

Help to assess the fitness for use of irrigation water at a specific site using a risk-based approach

Water quality guidelines have two main applications. Firstly, and more commonly, water quality guidelines are used to assess the fitness for use (FFU) of a given water for a specific purpose. Secondly, water quality guidelines are used to determine the desired composition of water that would pose minimal risk when used for a specific purpose, the so-called Water Quality Requirements (WQR). In this article we will focus on the first of these applications and specifically introduce a newly developed electronic Decision Support System (DSS) that guides the user to assess the fitness for use of a potential irrigation water.http://www.sabi.co.za/magazine.htmlam2019Plant Production and Soil Scienc

Modelling maize grain yield and nitrate leaching from sludge-amended soils across agro-ecological zones : a case study from South Africa

When applying municipal sludge according to crop N requirements, the primary aim should be optimizing sludge application rates in order to maximize crop yield and minimize environmental impacts through nitrate leaching. Nitrate leaching and subsequent groundwater contamination is potentially one of the most important factors limiting the long-term viability of sludge application to agricultural soils. This study assessed maize grain yield and potential nitrate leaching from sludge-amended soils, using the SWB-Sci model, based on crop nitrogen requirements and inorganic fertilizer. The following hypotheses were tested using the SWB-Sci model and 20 years of measured weather data for 4 of the 6 South African agro-ecological zones. Under dryland maize cropping, grain yield and nitrate leaching from sludge-amended soils compared to inorganic fertilizer: (1) will remain the same across agro-ecological zones and sites, (2) will not vary across seasons at a specific site, and (3) will not vary across soil textures. Model simulations showed that annual maize grain yield and nitrate leaching varied significantly (P > 0.05) across the four agro-ecological zones, both for sludge-amended and inorganic fertilizer amended soils. The annual maize grain yield and nitrate leaching from sludge-amended soils were 12.6 t∙ha−1 and 32.7 kgNO3 -N∙ha−1 compared to 10.2 t∙ha−1 and 43.2 kgNO3 -N∙ha−1 for inorganic fertilizer in the super-humid zone. Similarly, maize grain yield and nitrate leaching varied significantly across seasons and soil textures for both sludge and inorganic fertilizer amended soils. However, nitrate losses were lower from sludge-amended soils (2.3–8.2%) compared to inorganic fertilizer (11.1–26.7%) across all zones in South Africa. Therefore, sludge applied according to crop N requirements has a lower environmental impact from nitrate leaching than commercial inorganic fertilizer. Further validation of these findings is recommended, using field studies, and monitoring potential P accumulation for soils that received sludge according to crop N requirements.Water Research Commission of South Africa (WRC) and East Rand Water Care Works (ERWAT).https://www.ajol.info/index.php/wsapm2020Plant Production and Soil Scienc

Can productivity and post-pruning growth of Jatropha curcas in silvopastoral systems be regulated by manipulating tree spacing/arrangement without changing tree density?

The potential of tree spacing/arrangement to alleviate effects of interspecific competition of hedgerow intercropping systems on productivity and response to pruning of Jatropha curcas (Jatropha) was investigated using a Jatropha e Pennisetum clandestinum (kikuyu) silvopastoral system at Ukulinga (KwaZulu Natal, South Africa). Treatments were differentiated by presence/absence of kikuyu and arrangement/spacing of Jatropha trees (one, two and three tree hedgerows on either side of the alley) and had the same tree density (1110 ha 1). When high water availability and kikuyu dormancy coincided, tree growth rates did not significantly differ across treatments. Trees of the treatment without interspecific competition (tree-only) were the tallest. Even when rainfall was high, post-pruning tree height growth rate was affected by belowground (BG) interspecific competition and tree spacing/arrangement. Treatment with a single tree hedgerow between alleys had the most frequent maximum stem growth rate and was the most efficient during limited water availability. Its trees showed slow initial response to pruning due to a high tree-grass interface, followed by compensatory growth when competition for water with grass was low. Generally, length of tree-grass interfaces affected yield inversely especially as trees matured toward their maximum-yield age (4e5 years). BG competition reduced tree yield more than tree biomass, while tree spacing/arrangement did not affect tree harvest index. Manipulation of tree arrangement/spacing without changing tree density had no consistent effects on tree productivity.Water Research Commission (WRC) and National Research Foundation (NRF) of South Africa for funding the project (K5/1480/1/12).http://www.elsevier.com/locate/biombioe2016-03-31hb201

Nitrogen mineralization from sludge in an alkaline, saline coal gasification ash environment

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−1 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.Th is study was supported by the Water Research Commission and Sasol.https://www.agronomy.orgpublications/jeqam2017Plant Production and Soil Scienc

Phenology and reproductive biology of Acacia karroo Hayne (Leguminosae : Mimosoideae)

The architectural development of Acacia karroo conforms to Troll’s model. Growth of the branches is modular and sympodial with heteroblastic leaves on all long shoots of the tree, including the seedling. Axillary buds tend to proliferate especially on flowering shoots where they form fascicles consisting of up to 10 inflorescences arranged in two parallel serial rows per leaf axil. Most axillary buds are sylleptic and basal buds which give rise to short shoots, each producing two to five cataphylls each season, but no flowers. Inflorescences are only produced on long shoots (modules) of the current season. After flowering the terminal part of the module aborts, trees are usually andromonoecious with capitate inflorescences containing 40 to 100 flowers each, with some male and some hermaphrodite. Some trees produce only male flowers. Anthesis in the same inflorescence, the same tree as well as amongst trees of the same community are synchronised and occur at intermittent intervals, each lasting three or more days at a time. Flowers are protogynous and pollen is produced in polyads, each consisting of 16 pollen grains. Ovaries contain 10 to 14 ovules each. The concave stigma has space for only one polyad which can fertilise all ovules in the ovary after a single pollination event. Fruit set is low with 0 to 10 fruits (pods) per inflorescence.http://www.scirp.org/journal/ajpshb201

Crop model parameterisation of three important pearl millet varieties for improved water use and yield estimation

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 cropspecific 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 varietyhttps://www.mdpi.com/journal/plantsdm2022Plant Production and Soil Scienc

Forage yield and quality response of annual ryegrass (Lolium multiflorum) to different water and nitrogen levels

Water and nitrogen shortages in annual ryegrass (Lolium multiflorum) production can be improved by using proper irrigation and nitrogen scheduling methods. The aim of this research was to analyse the effect of water and nitrogen application on dry matter production and forage quality of annual ryegrass. Treatments consisted of three irrigation levels, scheduling application to field capacity once every two weeks (W1), once a week (W2) or twice a week (W3), and three nitrogen (N) levels, top-dressing after each harvest at a rate of 0 (N1), 30 (N2) or 60 (N3) kg N h−1 cycle−1. Yield increased as a function of the amount of water and nitrogen fertiliser applied. The dry matter (DM) content, in vitro organic matter digestibility (IVOMD), crude protein (CP) and metabolisable energy (ME) were higher in the treatments being irrigated once every two weeks. These results conclude that higher irrigation coupled with high N application significantly improved the dry matter yield, while water stress, to some extent, did improve the quality of the pasture by increasing the DM content, IVOMD, CP and ME values.Water Research Commission (Project 1650 ‘Guidelines for irrigation management in pasture production’).http://www.tandfonline.com/loi/tarf202016-04-30hb201