Part II - IN-HOUSE RESEARCH
NSCP Research - Project G

Soil Macropore Development as a Mechanism for
Root Distribution and Solute Transport

Purpose

Conventional wisdom has held that an advantage of using deep rooted crops, either as a main crop or soil improving crop, has been the development of deep root channels which improve drainage and offer new root channels for the growth of following crops. Soil drying creates extensive cracking in soils such as Brookston clay. It is not well understood how these factors may be related.

Objectives

  1. To characterize macropore development under the influence of crops with different root development capabilities.

  2. To relate macropore development to development of the subsequent crop and to solute transport.

  3. To suggest or develop soil management

  4. systems which will curtail excessive amounts of adverse transport while retaining advantages of enhanced root development.

Scope

This statement of work offers large scope for innovative approaches to the problem. The requirements may be met by practical field observation, by innovative technical evaluation where suitable existing facilities are available, by more theoretical modelling approaches, or by any combination of these. The technical approach to the problem should be well defined with clear definition of the chosen approach. The foreseeable difficulties which put a successful conclusion at risk should be recognized and potential solutions suggested.

Successful Bidder

London Research Centre; Dr. B. T. Bowman - $80,000.00 - Rainfall Simulator - Grid Lysimeter System for Preferential Solute Transport Studies Using Large, Intact Soil Blocks.

  View/Download Report   [101 KB pdf]

This research has been published in:

Bowman, B.T.; Brunke, R.R.; Reynolds, W.D.; Wall, G.J. 1994. Rainfall Simulator Grid Lysimeter System for Solute Transport Studies Using Large, Intact Soil Blocks. Journal of Environmental Quality 23:815 - 822

 


 

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Rainfall Simulator - Grid Lysimeter System for Preferential Solute Transport Studies Using Large, Intact Soil Blocks

Dr. Bruce T. Bowman
London Research Centre
Agriculture Canada

July 1993

STUDY TEAM

Dr. Bruce T. Bowman, L.R.C.
Mr. Richard Brunke, P.Eng., L.R.C.
Dr. W.D. Reynolds, C.L.B.R.R., Ottawa
Mrs. LouAnn Verellen, L.R.C.
Mr. William Bennett, L.R.C.

EXECUTIVE SUMMARY

During recent years there has been increasing interest in preferential water flow through soils and the resulting potential for rapid transport of pesticides, nutrients and other solutes to tile drains and groundwater. A technique has been developed to study in detail and under controlled conditions, the preferential flow of water and solutes in large, intact soil blocks, isolated at field sites and transported to the laboratory. The soil blocks, 46-cm (18") on each side (145 kg, 320 lb) were carefully cut with a flat shovel, then encased on the vertical sides with a polyurethane foam shell (inside a plywood box) to stabilize it during transport and later experimentation. The blocks were cut at least 46 cm deep to ensure that the A/B horizon interlayer was included. Unstable, preferential water flow (and solute movement) often occurs across dissimilar interlayer boundaries in soil profiles. A boiler plate was jacked under the block to isolate it, then the entire assembly was carefully lifted onto a truck and transported back to the laboratory.

In the laboratory the block assembly was turned on its side, the base plate was removed, the base of the soil block was cleaned and an aluminum plate (60 x 60 x 2 cm) grid solution collector (containing a 10 x 10 grid of shallow, 2.38-cm square collector funnels) was sealed to the base of the polyurethane foam shell. The assembly was turned upright onto a portable dolly, then wheeled under a precision rainfall simulator, capable of uniformly delivering water to the soil surface over a 5 to 80 mm hr-1 range. A grid of collector tubes below the collector plate was used to determine flow patterns of water and applied tracers. Much of the experimental data collected in this study were used to test the various components of the apparatus, as well as to characterize tracer movement through the soil blocks. Volumetric moisture content was continuously monitored at four depths in the soil block (2.5, 25, 33, 40 cm) using horizontally-inserted side-by-side pairs of Time Domain Reflectometry (TDR) probes. The probes at 25 and 33 cm were on opposite sides of the A/B horizon boundary. A tensiometer was horizontally inserted beside each TDR probe-set to monitor, in real-time, the matric potential (soil water tension).

Initial tracer tests were conducted using the bromide ion (Br-) as a conservative, non-reactive tracer, applied to the surface of an Embro silt loam soil block that had been in alfalfa for several years. Alfalfa roots, as well as numerous earthworm channels, had penetrated the full depth of the soil block. Under steady-state rainfall inputs and independent of the input rate (from 5.6 to 19.2 mm hr-1), initial traces of Br- ion were quickly detected in the outflow from the block within 0.5 hr after the pulse had been applied to the soil surface. At saturation input rates (19.2 mm hr-1), the peak concentration of the Br- pulse in the effluent occurred 1.25 hr after initial Br- introduction after collecting 5 L (< 1/7 pore vol.). At 5.6 mm hr-1 input, the Br- peak was delayed until 16 hr, after collecting 11.3 L (< 1/3 pore vol.). Approximately 85 % of the water in the soil block was "bypassed" by the Br- tracer. The distribution of water flow in the solution collector confirmed that extensive preferential flow of water and solute occurred in these tracer experiments.

This grid lysimeter apparatus will be used to characterize preferential water and solute movement in soil and is part of a larger collaborative effort which is investigating the same phenomena at larger scales at the same field sites where the soil blocks were obtained.

Concluding Remarks

Elucidation of the mechanisms controlling the preferential movement of water and solutes through soil require extensive and careful experimentation on intact soil samples that are large enough to be representative of true field conditions. Detailed and accurate measurements through space and time of the water and solute storage and transmission properties of the soil must be both known and controllable. The grid lysimeter system and the sample collection, containment and storage techniques described in this report are an attempt to satisfy these requirements.

Future Directions

This study has focused on the careful validation of the various components of the grid lysimeter system to accurately characterize preferential water and solute transport in intact soil blocks. We are quite confident that the current setup could be scaled up to accommodate considerably deeper intact soil profiles (perhaps up to 1 m) with modest strengthening of the support structures. This would permit realistic ground truthing of various solute transport models to tile drain depths, and to further investigate the behaviour of solute transport across heterogeneous soil interlayer boundaries. The grid lysimeter system is also well suited to examine stop-and-go (intermittent) flow phenomena in soils, which may well prove to be more relevant than much of the steady state flow experiments which have been traditionally used to characterize solute behaviour in soil profiles.

Published Paper
Bowman, B.T.; Brunke, R.R.; Reynolds, W.D.; Wall, G.J. 1994. Rainfall Simulator Grid Lysimeter System for Solute Transport Studies Using Large, Intact Soil Blocks. Journal of Environmental Quality 23:815 - 822

A grid lysimeter system and sample collection, containment, and storage techniques were developed for detailed laboratory studies of water and solute movement through intact soil blocks. This was done because existing designs and techniques bad important deficiencies and were limited in their range of capabilities. Intact 46-cm soil cubes were isolated, then contained within a polyurethane foam shell, which formed a stable, intimate soil bond, was impermeable to water and strong enough to support a large soil block, while sufficiently elastic to accommodate soil shrink-swell with changing water content without rupturing. The soil blocks were instrumented with solution delivery, collection, and monitoring systems. A dripper-based simulator delivered steady rainfall ranging from 4.8 to 30.0 mm h-1. The solution collection system was a 10 by 10 grid of cells (3.8 by 3.8 by 1.3 cm deep) milled into an aluminum block, which individually drained into collection tubes housed within a vacuum chamber. The collection grid permitted characterization of spatial and temporal water and solute movement through the block. The solution monitoring system consisted of side-by-side tensiometer pairs and time domain reflectometry (TDR) probes inserted horizontally through the foam shell at four depths in the block. As a partial test of the system, a bromide (Br-) tracer breakthough curve (saturated flow) was generated at a simulated rainfall rate of 19.2 mm h-1. Flow data indicated that 85% of the water in the block was bypassed by the Br-, and that >99% of the water flow passed through only 26% of the basal area of the block. The water flow pattern in the solution collector exhibited no evidence of preferential flow along the interface between the soil and the outer polyurethane shell. It was concluded that the rainfall simulator-grid lysimeter system was operating effectively.

 

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Created: 09-21-1996
Last revised: Sunday, May 08, 2011 07:57:44 AM