Ion Exchange Resin Strips to Estimate Nitrogen Availability

Active

In use from 2009-07-01 to 2009-11-30

Abstract

Sampling frequency: Monthly basis from August 2008.

Where used, ion resin strips are placed in the field for 1 month starting from March to November. Three pairs of anion and cation strips (2.5 cm x 10cm) are placed in each plot (1 pair at each of 3 stations). After one month, strips are collected by plot and both anion and cation strips (6 strips total) are put in the same 250 mL vial for that plot. After bringing the vials into the laboratory, percentage area remaining on each strip is measured using a small template (15 cm x 20 cm). Strips are then cleaned with deionized water to remove visible soil and then placed in plastic tubs with 2.0 M KCl (35 mL/strip) and shaken at 40 rpm for 1 hour. The extract (5ml) is stored in 7 mL polyethylene scintillation vials and frozen until analysis. Extracted strips can be regenerated with 0.5 M HCl and 0.5 M NaHCO3 prior to next use.

Protocol

Overview

Ion exchange resins can be used to measure relative amounts of plant-available nutrient ions in soils and the rates at which they are released from soil organic matter. Typical ion exchangers are ion exchange resins (functionalized porous or gel polymer), zeolites, montmorillonite, clay, and soil humus. They are either negatively charged exchangers that capture positively charged ions (cations) or anion exchangers that capture negatively charged ions (anions). There are also amphoteric exchangers that are able to exchange both cations and anions simultaneously. Ion exchange resins are widely used for assessing soil nitrogen availability and its spatial distribution. Anion-exchange membranes sorb nitrate ions (NO3−) and cation-exchange membranes sorb ammonium ions (NH4+). Direct placement of ion-exchange resin strips in soils has proven to be a simple and effective method for assessing relative ammonium and nitrate-release rates by mineralization among different soils. Acting as an ion sink over a certain period in a manner similar to plant roots, in situ extraction can overcome the disadvantages of extractions that are inherently static and do not account for the kinetics of nutrient release and transport.

Materials

  • Anion and Cation exchange membranes (18”x 40”) (General Electricals;Watertown, MA)
  • Template (100 cm x 45 cm)
  • Specimen collection cups with a lid (250 mL)
  • Scintillation vials (7 mL)
  • Plastic tub with a lid (600 mL or 2000 mL)
  • pH paper
  • Needle nose pliers
  • Trowel
  • Mallet
  • Putty knives
  • Brush
  • Clean paper towel
  • Gloves
  • Ziploc bag (2 gallon)

Procedure

  1. Cut ion sheets into 2.5 × 10 cm strips using the large plastic template. Once cut, punch a star shaped hole on one end of the cation strips and a rectangle shaped hole in one end of the anion strips to distinguish between the two types of resin.
  2. Regenerate ion strips to remove existing nutrient ions. This step must be performed prior to every use whether new or reused. Place the cation and anion strips into separate and labeled plastic tubs with a lid. Fill the tubs to fully cover strips with 0.5 M HCl (hydrochloric acid). Place the tubs on a shaker table and shake at 40 rpm for at least one hour.
  3. Pour off HCl into a beaker in the sink and neutralize with 0.5 M NaHCO3 (sodium bicarbonate), taking care to not add so much that it bubbles over the edge of the beaker. Use pH paper to ensure that the acid has been neutralized before washing the solution down the sink.
  4. Soak the strips in 0.5 M NaHCO3 in the same plastic tubs and shake them at 40 rpm for at least five hours. Change the solution once every hour during this process and discard the used solution every time.
  5. Rinse the strips with deionized H2O and place anion and cation strips into separate, labeled clean Ziploc bags. Add some deionized H2O into the bags to keep the strips moist until placed in soil.
  6. For placing the strips into soil at designated places, create two vertical slits each about 10 cm deep in the soil using a putty knife or trowel. One slit is for a cation strip and the other for an anion strip. In row crops, place one pair in the row and another pair between the rows. If desired, a third pair can be inserted midway between the first two pairs. Carefully remove a strip from the bag without introducing any soil particles into the bag (gently push strip up from the outside of the bag before grabbing). Place the strip into the soil so that 5 mm of the hole-punched end is above the soil surface. Firmly close the slit by hand to create contact between the soil and the strip. Mark the sample site with a flag. To avoid the rodent damage, the site should be covered with wire mesh cage (12×12×1.5cm).
  7. After one month, carefully pull the strip from the soil using fingers or needle nose pliers (pull the strip directly vertical to prevent cracking). Gently brush the strip with a wire or plastic brush to remove as much soil as possible. Place all anion and cation strips from one plot into the same 237 ml specimen cup, cap it, and bring back to the lab in a plastic tub. There will be a little soil left on the strips which can be removed later in the laboratory.
  8. In the laboratory, measure the percentage of the original area that remains on each strip (some area may be missing due to rodent or other damage). Templates with little squares (each square represents 2.5% of the total area) can be easily made. Keep them in Ziploc bags so that they can be washed. This estimate goes in the “% remaining” column in the excel spreadsheet.
  9. Add a squirt of deionized water to each cup, recap, and shake by hand. Open the cup again, pour out the water, and shake it vigorously until most of the water is out. Inspect for remaining debris and repeat process if there is obvious soil left in cup. This process should remove nearly all of the remaining soil.
  10. Add 35 ml of 2.0 M KCl per strip to each cup containing all strips from a plot (e.g. 210 mL for 6 strips). Make sure cap is tight and place plastic tub containing cups on the shaker. Shake for one hour at 40 rpm. After shaking, filter the extract with a syringe filter using a Type A/E, 1um pore size, glass fiber filter. Rinse a labeled scintillation vial with extract and pour enough to fill up to the shoulder to allow expansion of the solution in the freezer. Place labeled scintillation vials into the freezer for future analysis. Remove strips from the cups, separate them into cation and anion tubs, and prepare for regeneration.

Spreadsheet calculations

Enter into appropriate spreadsheet columns: date, plot deployment (days), % area remaining, µg N per mL. The spreadsheet will calculate N adsorbed by the formula:

[(conc in µg N per mL) × 70 mL of KCl)] / (50.8 cm2 area of the strip × %strip remaining × number of days in the ground)

Recipes for solutions for strip maintenance and extraction

0.5 M HCl for recharging strips
Put 3L of deionized water in a 5L container labeled “HCl for strip recharging.” In the fume hood (turned on) add 215 mL of concentrated HCl using a graduated cylinder. Cover and shake the container. Add enough deionized water to the container to bring it up to the 5 L mark (taped on the side) and shake one final time.

0.5 M Na HCO3 for recharging strips
Put 15L of deionized water in a 25 L carboy labeled “Sodium bicarbonate for strip recharging.” Weigh out 1050 g of sodium and pour into container with deionized water. Shake until it dissolves, adding deionized water, if needed, to aid in dissolution. Once all the bicarbonate is dissolved, add enough deionized water to bring the level in the container up to the 25 L mark taped on the side of the container and shake one final time.

2.0 M KCl for extracting strips
Put 5 L of deionized water into a 10 L carboy labeled “KCl for strip extraction.” Weigh out 1491 g of potassium chloride into big plastic beaker labeled as KCl. Using funnel, transfer KCl into the carboy and shake until all the KCl is dissolved. Add enough deionized water to the container to bring it up to the 10 L mark taped on the side and shake one final time.

References

Qian, P., and J. J. Schoenau. 1995. Assessing nitrogen mineralization from organic matter using anion exchange membranes. Fertilizer Research 40:143–148.

Qian, P., and J. J. Schoenau. 1996. Ion exchange resin membrane (IERM): A new approach for in situ measurement of nutrient availability in soil. Plant Nutrition and Fertilizer Sciences 2: 322–330.

Qian, P., and J. J. Schoenau. 2002. Practical applications of ion exchange resins in agricultural and environmental soil research. Canadian Journal of Soil Science 82: 9–21.

Qian, P., and J. J. Schoenau. 2005. Use of Ion-Exchange Membrane to Assess Nitrogen-Supply Power of Soils. Journal of Plant Nutrition 28:2193 -2200

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