D. R. Scotter

Losses of nitrogen fertiliser under oil palm in Papua New Guinea: 1. Water balance, and nitrogen in soil solution and runoff

Nitrogen (N) fertiliser is an important and expensive input to oil palm in Papua New Guinea. Of about 3000mm/year of rainfall, about 1300mm is lost as evaporation. This leaves an excess of >1000mm/year lost as surface runoff and/or deep drainage, and with it the potential for N loss. Approximately 11% of rainfall reached the ground as stem flow. Throughfall was generally lowest near the trunk and highest where canopies overlapped, but random spatial variability was large. The difference between the measured rainfall and stem flow plus throughfall was 6%, indicating relatively little interception. Surface runoff from the volcanic ash soils was 6% of rainfall at one site, but only 1.4% at the other. Less than 2% of the applied N was lost in the surface runoff after an ammonium chloride application. Calculations of N leaching losses made using suction cup data and the water balance indicated that significant losses occur, but the estimates were not reliable due to the huge spatial variability in the suction cup and throughfall data. Therefore, another technique is needed to study N leaching in oil palm plantations.

Solute movement during intermittent water flow in a field soil and some implications for irrigation and fertilizer application

Abstract The movement of a bromide tracer in response to intermittent water flow was investigated in the field. Two experiments were conducted. The first involved application of a 5 mm pulse of dilute potassium bromide solution to pasture plots of contrasting initial water content. All plots then received a further application of 50 mm of water. Twenty-four hours later core samples of soil were collected and the distribution of water and bromide measured. The final bromide distributions were found to be dependent on the initial water content of the soil. Bromide applied to initially dry soil was very resistant to movement by subsequent leaching water. The second experiment also involved the application of a 5 mm pulse of potassium bromide solution followed by some leaching water. The time scale was however much greater. There was a 12 day period between solute application and final sampling, with a leaching treatment applied at different times. Coupling a mobile-immobile variant of the convection-dispersion model with a description of the transient water flow and root water extraction provided a mechanistic model. This model could successfully describe the main features of the solute movement under the four different regimes of water application. The assumed depth of water extraction by roots strongly affected the predicted final solute distribution. Some practical implications for the scheduling of fertiliser applications and irrigation events are discussed.

Leaching of copper from contaminated soil following the application of EDTA. I. Repacked soil experiments and a model

The EDTA-enhanced remediation of copper contaminated sandy-loam soil of volcanic origin was investigated. The soil, from an orchard, was contaminated with about 250 mg/kg of copper due to the extensive use of copper sprays. Copper-contaminated soil was packed into 100-mm-long columns, and solutions of Na2H2EDTA with CaCl2, raised to a pH of 6.2, were applied at a flow rate of 24 mm/h. Application of an excess of 0.01 M EDTA leached about half the acid-extractable copper from the soil; most of it coming out in the first 3 liquid-filled pore volumes (PV). Also a 0.5 PV pulse of 0.001 M EDTA was applied to similar soil columns and then either leached immediately with 0.005 M CaCl2, or left for periods of up to 1 month before leaching. With immediate leaching, 70% of the EDTA applied was complexed with copper in the leachate, but after a months delay only 24% was complexed with copper in the leachate, the rest being complexed with iron. There was no evidence of EDTA retardation or adsorption. The experimental results were simulated using the convection–dispersion equation, incorporating a source/sink term. This described the competing time-dependent reactions of copper and iron with EDTA, and the reversion of CuEDTA2– to adsorbed Cu2+ and Fe(III)EDTA– in solution. Reasonable simulations were achieved, mostly within errors of observation.

Leaching of copper from contaminated soil following the application of EDTA. II. Intact core experiments and model testing

In Part I, we proposed a model for EDTA-facilitated transport of copper, and we tested the robustness of this scheme by reference to experiments with various initial and boundary conditions on a repacked soil. Now we examine a quite different repacked soil, and consider intact cores of the same soil as in Part I. Continuous leaching of an intact core of a contaminated orchard soil of volcanic origin under unsaturated conditions with excess 0.01 M EDTA (510 mm) reduced the copper concentration from 240 down to 80 µg/g in the top 25 mm. When a 480 µmol pulse of 0.001 M EDTA was applied to similar intact cores, 126 µmol of copper came out with immediate leaching, but only 18% of that was leached out from a core that was left for 1 month before leaching. The month’s delay caused an increase in the amount of iron leached from 116 to 286 µmol. An experiment in which daily pulses CuEDTA 2– were applied to a non-contaminated soil for 3 months indicated that these changes were due to CuEDTA 2– slowly changing to Fe(III)EDTA – , with the re-adsorption of the Cu 2+ released. Application of our model from Part I indicated that different rate constants are required for the chemical reactions for different soils, and also for the same volcanic soil when the pH is lower. But by changing just the dispersivity from 3 mm to 23 mm, the model with the same chemical parameters was able to simulate both the repacked columns and the intact core experiments using the volcanic soil. SRu- natit I Taku et

In situ ion exchange resin membrane (IEM) technique to measure soil mineral nitrogen dynamics in grazed pastures

This paper explored the potential of application of in situ ion exchange resin membrane (IEM) technique for assessing soil nitrogen (N) availability and spatial distribution in New Zealand grazed pastures. Field and incubation experiments conducted to test the technique proved IEM technique to be a useful approach to monitoring the continuous changes in soil mineral N in pasture soils. The field testing showed that the IEM technique reflects both differences in pool size and mineral N flux, while 2-M KCl extraction reflects only pool size at the sampling. Testing the effects of residence time, temperature, soil inorganic N content, and soil water content through diffusion modeling offers further support for using IEM to explore the complex dynamics of nitrogen availability in pasture soils.

A critical assessment of the role of measured hydraulic properties in the simulation of absorption, infiltration and redistribution of soil water

Abstract Free-water absorption and infiltration were measured in a repacked fine sand. Redistribution following constant-flux infiltration was also measured. These processes were then computer-simulated using independently measured soil water retentivity and conductivity data. Neither free-water absorption, nor ponded infiltration could be accurately similated, as they depended so critically on the soil water retentivity near saturation, where errors of observation prohibit accurate measurement. For simulating redistribution, useful answers were obtained even when hysteresis was ignored. But redistribution was grossly over-predicted when the Campell-Brooks and Corey method was used for inferring the unsaturated hydraulic conductivity from saturated conductivity and retentivity data. Campbell has produced a simple yet elegant, computer simulation package for soil water movement. But to use it successfully required alternative ways of evaluating the hydraulic parameters. Ponded infiltration was successfully simulated only when the sorptivity was used as an independent input. To simulate redistribution required accurate determination of the unsaturated hydraulic conductivity. Software such as Campbells is now readily available, and likely to come into increasing use. For it to be used effectively more effort and physical insight needs to be directed towards appropriate characterization of the critical hydraulic properties. This physical emphasis will lessen the likelihood of such useful software generating erroneous and misleading results.

Losses of nitrogen fertiliser under oil palm in Papua New Guinea: 2. Nitrogen transformations and leaching, and a residence time model.

Nitrogen fertiliser is an expensive input to oil palm in Papua New Guinea, and prone to leaching due to the about 3000mm/year of rainfall. Transfer function theory is used to describe this leaching, and to devise ways of reducing it. Four variants of a leaching experiment were conducted at 2 sites to parameterise and check the theory. The experiment involved the application of ammonium chloride to an area of 25m 2 , and then from 6 days to 5 months later taking soil samples at 200-mm intervals down to 2m depth and analysing them for chloride, ammonium, and nitrate. Background concentrations were obtained by contemporaneous sampling nearby. In one variant of the experiment 353mm of rain in 6 days moved nearly half the applied nitrogen to below 400mm depth. Nitrification was rapid, with ammonium half-lives ranging from 2 to 16 days once the soil was wet. The theory is used to demonstrate how the fertiliser residence time in the root-zone can be increased by applying it in certain months and about 2m from the trunk where there is less throughfall.

Effects of irrigation frequency on ryegrass and white clover growth. 2. Improving irrigation efficiency

Growing concerns about limited water inputs for, and deleterious environmental outputs from, irrigation of pasture make the identification of more efficient scheduling strategies imperative. A climate-driven pasture production model is used to simulate the daily soil water deficit in the topsoil and subsoil, and so pasture production under a range of irrigation strategies. Soil water content and pasture production data from a companion paper, describing a Manawatu experiment where irrigation was applied to plots at trigger deficits of 20, 40, and 60 mm, were used to evaluate model parameters. The model was then used with 30 years of meteorological data to simulate a range of irrigation strategies at Palmerston North and Winchmore. Applying 20 mm of irrigation when a 20-mm trigger deficit is reached as opposed to 60 mm of irrigation at a 60-mm deficit increased the simulated average annual pasture response to irrigation by ~80%, but this increase was at the cost of ~40% more irrigation water. A suggested alternative strategy is to apply 20 mm of irrigation whenever a 60-mm trigger deficit is reached, which in summer will be about every 5 days if no rain falls. Keeping the topsoil moist most of the time, while leaving room for rainfall in the subsoil, will increase water use efficiency (the production response per mm of irrigation) by substantially reducing the irrigation requirement and drainage excess, even though it does not maximise production.

Effects of irrigation frequency on ryegrass and white clover growth. 1. Experimental results

The rising demand for the finite amount of water available for pasture irrigation in New Zealand means increasing interest in irrigation efficiency. The effects of frequency of sprinkler irrigation of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) were investigated near Palmerston North on Manawatu fine sandy loam (Dystric Fluventric Eutrochrept), using replicated plots of 1.5 m2. As values range from 12 to 90 mm for the reported soil water deficit at which stress first reduces pasture growth, with the most common values between 20 and 60 mm, deficits of 20 mm (involving irrigation about every 5 days if no rain fell), 40 mm, and 60 mm were chosen to trigger irrigation. There were replicated plots with and without superphosphate treatments for each irrigation and species treatment, as an interaction between fertility and the optimal trigger deficit was considered possible, and such interactions have rarely been investigated. There were also four non-irrigated plots. Yield was measured and soil water content monitored. Irrigating at a soil water deficit of 20 mm (meaning readily available water was always present in the top 150 mm of soil) rather than 60 mm resulted in significantly more ryegrass and white clover production over the irrigation season. This was particularly so when superphosphate was applied to the ryegrass.

Chapter 26 Contaminants in the rootzone: Bioavailability, uptake and transport, and their implications for remediation

Publisher Summary This chapter focuses on the mechanisms that control the bioavailability, transport, and plant uptake of both metals and inorganic contaminants in soil. Their implications for phytoremediation of contaminated sites using poplars and willows are discussed. The chapter presents the results from screening experiments that have identified a huge range in the ability of willow clones to extract cadmium from contaminated soil. Beyond these pot experiments with willow clones, the emphasis is on the entire soil–plant–atmosphere continuum so that the dynamics and ultimate fate of water and contaminants in the rootzone can be detailed. A field project and large-scale lysimeter experiments are described in the chapter, exploring the ability of poplars to both dewater and remove the boron from contaminated sawdust piles. Experiments and analyses that have determined the fate of Cu, which is mobilized from soil by ethylenediamine tetra-acetic acid (EDTA) in an effort to enhance phytoextraction, are presented in the chapter. The power of quantitative modeling for providing predictions of the fate of rootzone water and contaminants is also demonstrated in the chapter.