PhD Thesis
Nitrogen and water management practices are key components in Crop production. Plants cannot grow in soil without water, so too when soil is saturated with water. Plants lacking N show stunted growth and yellowish leaves. Too much N can have negative environmental impacts such as contamination of water, pollution, and eutrophication. Leaching is the main vehicle through which applied nitrogen can contaminate groundwater. Identifying the most economic application rate of N fertilizer is most important in high N demanding crops such as maize. Understanding of N movement through soil profile is also essential for more N efficient and minimizing N leaching. The aims of this study were to evaluate the agronomic response of maize to different water and N application regimes; to determine the lateral and vertical movement of nitrogen under different irrigation regimes; and to model the distribution pattern of nitrogen in the maize root zone. The research study was conducted in two irrigation growing seasons from 1st June to 8th September 2012, and from 10th September to 15th December, 2012 at Nkango Irrigation Scheme in Kasungu, Malawi. The factors under study were water and nitrogen with four levels each. A V-notch flume was used to measure volume of applied water to the plots.
Triscan Sensor (EnviroScan, Sentek Pty Ltd, Stepney, Australia), which has ability to monitor the direction and movement of nitrogen in the soil at instant time of inserting monitoring probe in the soil, was used to measure total nitrogen concentration at lateral distances. The measurement of the sensor is in Volumetric Ion Concentration (VIC), but using standazation equation the concentration of total nitrogen on each point was calculated. The lateral distances at which measurements were taken were at point of application (represented by 0 cm), at 5 cm away from the plant (represented by -5 cm), at 5 cm towards the plant, 10 cm towards the plant (this point was maize planting station), and 15 cm (this point was 5 cm after planting station in the direction opposite from where N was applied). The lateral distances were taken based on spreading and elongation pattern of lateral roots of maize plants. The lateral readings of nitrogen were respecively taken at five soil depths of 20, 40, 60, 80, and 100 cm. The R version 3.2.2, open source statistical software (R Core Team, 2015) was used to run ANOVA statistical analysis on yield data, separate treatment effect means and plot various graphical plots such as box and whisker and interaction effects.
The following conclusions were drawn from the study:
The study has concluded that statistically interactive effect of 60% of FWRR and 92 N Kg/ha gave optimum yields compared to other combination of treatments. The interactive effect of nitrogen and water on maize yield has indicated that it is only a combination of 100% FWRR: 92 Kgs N/Ha – 60% FWRR: 92 Kgs N/Ha that has no significant difference on maize yield (p-values <0.1). The irrigated smallholder farmers can therefore be advised to apply 60%FWRR to their maize fields to save water.
The study has identified that vertical movement of nitrogen is influenced by water flux, and the direction of flow is greatly influenced by absorption rate of plants roots due to gradient created by absorption. When supply of nitrogen is low due to high absorption of plants roots especially during the period when plants require large quantities of nitrogen, the lateral movement of nitrogen towards plant roots is greatly influenced by pulling effect by plant roots, caused by negative gradient due to water uptake known as diffusion. The study has also shown that the factors that influence movement patterns, direction, and distribution of nitrogen concentration are: evaporation of water from the soil surfaces, pulling effects by plant roots, deep percolation through gravitational force, and ability of plant roots to create environment that is conducive to diffusion of nitrogen.
The study has inferred that the soil moisture redistribution in the root zone is directly related to the amount of applied irrigation water, and spatial distribution of soil moisture content was primarily influenced by roots water uptake and evaporation.
Review of N leaching simulation models has indicated that models that use cascading soil water balance approach in simulating water and solute transport through soil profile are much better compared with models that uses Richards’ and Convection-Dispersion equations. Richards’ (and Darcy’s) and Convection-Dispersion equations are suitable to model unsaturated flow in laboratory-scale soil columns with limited heterogeneity, but have limited capability to simulate water flow in field soils which have high soil variability.
The study has shown that N leaching can be delayed, which consequently means it can easily be managed. The results of this paper has indicated that N leaching is directly related to water, higher amount of applied water result in high N leaching and less amount of applied water result into less or zero N leaching. In order to minimize N leaching it is of paramount important to squarely manage applied water. Applied water in the soil should not exceed field capacity of the soil and in such way leaching of nitrogen will be minimized.
The study has concluded that treatments that received high amount of inorganic N fertilizer lost more nitrogen through N leaching because plant roots only absorb nitrogen it requires leaving excess N to be leached by water below the active rooting zone. The study has also concluded that with the use EU-Rotate N model, it is possible to minimize N leaching by reducing amount of applied water during the time when leaf area index is 1 or nearly there.
The following recommendations were concluded from this study:
The study was conducted in sandy loam soils. Further study needs to be done in different types of soil to establish the maize responses to different levels of water and nitrogen for different types of soils.
While it was not economicaly viable to apply 125% of TNPRA in the study, technically the treatment gave very good insight of behaviour of nitrogen in the soil when its content is ‘high’. It is therefore recommended that further study needs to be done in which two or more treatments with high nitrogen application levels than TNPRA will be tested so as to know behaviour of nitrogen in soil when its application content is high.
Further study needs to be done on the pulling effect of plant roots. Maize at different stages has different pulling effects. The study need to unearth whether the pulling effect is also influenced by soil types, soil moisture contents, availability of nitrogen in the soil etc. and to what extent does pulling effect affect the movement of solutes in the soil.
N leaching is simply defined as N which is below active root zone of a crop. This means that for shallow-rooted crops, N leaching can be below few depth of soil while deep-rooted crops, N leaching can be below far deeper. In this case, N leaching can be managed by rotating shallow- and deep-rooted crops. Intercropping of shallow- and deep-rooted crops can also manage N leaching. This is because deep-rooted crops can be efficiently absorbing nitrogen which has ‘leached’ from shallow-rooted crops.
Alliance for Green Revolution in Africa (AGRA