Rhizon and Solu Systems for Soil pore Water Sampling

Soil Water Samplers. Inert samplers can extract small or larger volumes of (pore) water from soils and sediments in a simple, non-destructive and repeatable manner. Different soil water samplers, developed by i) Rhizosphere Research Products, and ii) Sentek can be used to sample water from both unsaturated and saturated soils, and the extract can be used to measure dissolved compounds such as salt / nutrients, dissolved organic carbon, plant exudates incl. enzymes, or contaminants. 

For MicroRhizons, any standard syringe with a "tip" and a wooden retainer can be used for sampling. The MicroRhizons and the syringe tip are connected by a "silicone connector" - i.e. a small piece of silicone tubing.

Table 1 MicroRhizons & accessories for soil pore water sampling in RhizonBox back panels (RRP19.21.81), or with silicone sleeve (RRP19.21.83) for entry from the side of Rhizonboxes (4.5 mm holes).

 Rhizon type are available in varieties (see Tables 2, 3 and 4 for details / product numbers):

• with a 5 cm or 10 cm porous part (9 cm for MacroRhizons)
• with a male luer lock - suitable for creating a vacuum with needles + vacuum tubes, or with a female luer lock - suitable for creating a vacuum with syringes 
• with 0.15 µm or 0.60 µm pore size.
• with (# + S) and without silicone sleeve.

Table 2. Rhizon types CSS and Flex for soil pore water sampling in RhizonBoxes and Rhizobox, and pot studies. Rhizon CSS (Rhizonbox, from side, immediately) / RhizonFlex (Rhizobox, from top during set-up) samplers are particular suitable for use with Rhizo(n)boxes. See Table 5 for Rhizon Sampler Accessories.

Table 3. Rhizon types SMS and MOM for soil pore water sampling in pots, columns etc. (disturbed soil). See Table 5 for Rhizon sampler accessories.

Table 4. MacroRhizon types for field soil pore water sampling. Outer diameter of membrane 4.5 mm. See below for notes on the installation of MacroRhizons.

What accessories do you need for sampling? This depends on the type of Rhizon ... read more on Ways how to do sampling with Rhizons below.

Table 5. Rhizon Accessories for soil pore water sampling with the different Rhizon samplers. Essentials for Rhizonbox studies marked in green. (Sampling method: male luer connectors M = needle & vial, OR female luer connectors F = syringe + retainer sampling!).

The microfiltration membrane of (Micro-)Rhizon samplers is made of a hydrophilic, inert polymer with no ion exchange properties. Once wet, the membrane is only permeable for liquids, not air. Rhizons have very small diameters and low dead volumes and are easy to handle; once installed for one experiment, they can be used many times.

• Microfiltration membrane with nominal pore sizes of either 0.12 - 0.18 µm (mean pore size 0.15 µm) OR a mean pore size of 0.6 µm
• Hydrophilic membrane composed of a blend of polyvinylpyrrolidine and polyethersulfone
• Structure is asymmetric/microporous
• High performance and a very good anti-fouling behaviour
• Suitable for filtration outside-in and inside-out
• pH range: 2-12
• Temperature range: 1°C to 90°C
• Pressure range: -300 to +300 kPa
• Very high resistance against acids, bases, and oil. High resistance to aliphatic alcohols and aliphatic hydrocarbons
• Not suitable for organic esters, ketones, ethers, halogenated hydrocarbons, aromatic hydrocarbons and polar organic solvents

Store in a dry, normally ventilated place, away from sources of heat, ignition and direct sunlight. Store between 0 and 50°C (32 - 122F). The membranes should not be exposed to freezing.

For sampling rhizosphere pore water, you need a (Micro-)Rhizon sampler and accessories to create a (partial) vacuum (see images below).

1. MicroRhizon samplers need a syringe with tip and a silicone connector
2. Rhizon samplers with a female luer lock need a syringe + retainer (OR a male-male luer connector and a vacuum tube)
3. Rhizon samplers with a male luer lock need a needle and a vacuum tube (evacuated vial), (OR a female-female luer connector and a syringe)

What is the difference? The advantage of vacuum vials is that the sample does not come into contact with air. The advantage of needles/syringes is that the vacuum can be easily restored by pulling back the plunger. However, male-to-male and female-to-female luer connectors are available to easily "correct" your initial choice of sampler luers later. If you use syringes to create a vacuum, wooden retainers can be used to block the plunger -which keeps the vacuum. If you use vacuum vials, you can buy them pre-evacuated or prepare them cost-effectively using the Vacuum Vial Evacuator EV-101. Simply connect a needle to the male Luer lock on Rhizon samplers (or use a female-to-female connector) and insert it through the septa of the vacuum tube.

If you cannot extract soil water, you may have forgotten to remove the (blue or red) protective cap from the Rhizon before use. Please check and try again. If the protective cap is already off you may have to wait a little longer. The porous membranes of any (Micro-)Rhizon samplers must be completely wet before you can apply vacuum.

If you are interested in testing the response of specific roots or rhizosphere compartments to changes in the soil environment (e.g. salts, heavy metals or other pollutants, dissolved carbon, etc.), you may consider using (micro)rhizons to inject a solution into the soil (next to a specific root or soil region) instead - or use RhizonIrrigators for setting specific irrigation treatments by passive suction or active outflow.

Table 6. SoluSamplers for Soil Water Sampling and Accessories. SoluSamplers are most suitable for field applications and large sample volumes.

Installation:

1. Select Solu Sampler type: high or low flow according to texture/soil moisture.
2. Core / drill a hole (c. 40 mm diameter), and to required depth. Keep soil for back-filling.
3. Wet SoluSamplers. Attach to installation tool for greater depth. 
4. Insert SoluSampler with tube protruding from the surface (keep stopcock clean).
5. Pepare a slurry (sieved, sampled soil & water). Preferably use soil from the installation depths soil profile to mix the slurrry.
6. Pour the slurry into the hole to ensure good soil contact, up to 1-2 cm above the SoluSampler. Add some soil, gently tap. Add a 1:1 mixture of sand and betonite to seal the hole and act as a hydraulic barrier.
7. Fill in the remaining soil to the surface. Allow the Solu Sampler to settle / wet.

Generally, it takes about two weeks after installation for the SoluSampler to provide a representative soil water sample. During this two week period, discard two or more extracted samples to avoid unrepresentative samples.

Sampling:

1. Vacuum is created using a large syringe (60 ml). Connect the syringe to the stopcock. Create vacuum (potentially using several cycles), close stopcock. 
2. Vacuum is maintained for at least 4 hours. During this time: Soil water is drawn through the ceramic cup and stored in the unit.
3. After this time, water can be extracted with a (clean) syringe.
4. After drawing out the soil water, the Sentek SoluSAMPLER should not remain under vacuum conditions until the next time it is used. Consider the long-term effects of bacterial/fungal growth on nutrient analysis; chemical sterilization may be required for extended SoluSampler incubation times but consider effects on soil microbiome etc. 

Analysis of the collected soil water can be carried out by:

• Portable Electrical Conductivity (EC) meter (in the field)
• Nitrate strips (in the field)
• A wide range of laboratory analysis

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