Modelling biomass accumulation and soil organic carbon under urban vegetable production systems in Tamale, Northern Ghana

To develop sustainable urban and peri-urban agriculture (UPA) systems with high water and
nutrient use efficiency and to promote food security on nutrient poor soils of West Africa
under intensive rural-urban transformation and climate change, the Denitrification-
Decomposition (DNDC) model can help to understand carbon (C) and nitrogen (N) turnover
resulting from the complex interaction between soil, climate, management practices, and
plant growth in vegetable production systems. To achieve those purposes, this study
modelled biomass accumulation of the locally important vegetables amaranth (Amaranthus
cruentus), lettuce (Lactuca sativa L.), jute mallow (Corchorus olitorius L.), and roselle
(Hibiscus sabdariffa L.), and soil organic carbon (SOC) dynamics in response to a different
level of N-fertilization, clean and waste water irrigation quantities, and biochar addition
under current and future climate change scenarios (RCP 2.6, RCP 4.5, and RCP 8.5) in
Tamale, Northern Ghana. Crop modelling was preceded by a series of the DNDC model
evaluation processes, that is calibration, validation, and analysis of uncertainty and its
contributing factors, using field data of soil, climate, and crop growth from experimental
plots in Tamale.
The studies described in Chapter 2 and Chapter 3 of this thesis confirm that the DNDC model
performed well in simulating biomass accumulation of all vegetables and SOC under
different input intensities with acceptable tolerance (α 5%) as reflected by the root mean
square error (RMSE), relative error (E), and the correlation (r) values. However, DNDC
modelling was limited to conditions without pest and disease incidence and was unable to
simulate the impacts of biochar addition to SOC. For up-scaling the DNDC model in
simulating vegetable biomass accumulation and the effects of SOC in West Africa, soil (pH
and SOC) parameters need to be parameterized carefully due to spatial-temporal variation
of these parameters and their contributing to the uncertainty of model outputs. Based on the simulation using the DNDC model under different management practices and projected
climate change scenarios (baseline and RCPs), we propose transformative management
practices combining N-fertilization and high nutrient loaded-waste water irrigation to
stabilize marketable yield of all studied leafy vegetables without detrimental effects on soil
fertility. Um die nachhaltige Wasser- und Nährstoffnutzungseffizienz in stadtnahen Gemüseanbausystemen Westafrikas zu erhöhen und die Ernährungssicherheit in nährstoffarmen Böden unter intensiver Land-Stadt-Transformation und Klimawandel zu fördern, kann das DNDC-Modell beim Verständnis der Kohlenstoff ©- und Stickstoff (N)-Umsätze helfen. Diese spiegeln die komplexen Wechselwirkung zwischen Boden, Klima, Bewirtschaftungspraktiken und Pflanzenwachstum wideren. Um diese Ziele zu erreichen, modellierte die vorliegende Studie die Biomasseakkumulation der lokal wichtigen Gemüsearten Amaranth (Amaranthus cruentus), Salat (Lactuca sativa L.), Malve (Corchorus olitorius L.) und Roselle (Hibiscus sabdariffa L.). sowie die SOC-Dynamik als Reaktion auf ein unterschiedliches Niveau der N-Düngung, der Bewässerungsmengen mit Trink und Abwasser sowie der Zugabe von Pflanzenkohle unter aktuellen und zukünftigen Klimawandelszenarien (RCP 2.6, RCP 4.5 und RCP 8.5). Vor der Modellierung erfolgten umfangreiche Bewertungsprozesse des DNDC-Modells, insbesondere seine Kalibrierung, Validierung und eine Analyse der Unsicherheit in Abhängigkeit von Felddaten zu Boden, Klima und Pflanzenwachstum aus Versuchsflächen in Tamale.