To monitor nitrogen availability over the growing season, place ion exchange resin strips in the soil for 1 month for each month from March to November. Bury one pair of anion and cation strips (2.5 cm x 10 cm) at each sampling station in each plot (e.g., for the BCSE, 1 pair at each of 3 stations). After one month, collect strips by plot, putting both anion and cation strips (e.g., for the BCSE, 6 strips total) in the same 250 mL vial for that plot. Return vials to the laboratory and measure the percent area remaining on each strip using a grid template. Rinse strips with deionized water to remove visible soil, place in plastic cups, add 35 ml of 2.0 M KCl per strip and shake at 40 rpm for 1 hour. Remove 5 ml of the extract, store in 7 mL polyethylene scintillation vials, and freeze until analysis. Extracted strips can be re-used by regenerating with 0.5 M HCl and 0.5 M NaHCO3 prior to next use.

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)
• 0.5M HCl, 0.5M NaHCO3 and 2.0M KC
• 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 a 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) and securely attach lid. 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. To place the strips into soil, use a putty knife or trowel to create two vertical slits each about 10 cm deep in the soil at designated sampling sites in the plot. 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 remove the strip from the soil using your fingers or needle nose pliers to 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 the cup and place in a plastic tub for transport back to the lab. A little soil may be left on the strips which can be removed later in the lab.
8. In the lab, measure the percentage of the original area that remains on each strip; some area may be missing due to rodent or other damage. Grid templates with each square representing 2.5% of the total area can be made by laminating grid paper. Keep the templates in Ziploc bags so they can be washed. Record “% strip area remaining” for each strip.
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 the cap is tight and place cups in a plastic tub and secure tub to shaker. Shake for one hour at 40 rpm. After shaking, filter the extract with a syringe fitted with a Type A/E, 1um pore size, glass fiber filter. Rinse a labeled scintillation vial with extract, then fill vial with extract up to the shoulder to allow room for expansion 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.

Calculations

Analyze samples for ammonium and nitrate (Soil Inorganic N). Create a spreadsheet with date installed, date removed, site/replicate/station, number of strips in sample, total strip area, % strip area remaining, ammonium concentration in µg N per mL, nitrate concentration in µg N per mL, and number of days in ground. For both ammonium and nitrate, calculate micrograms N adsorbed per centimeter squared per day using:

µg N cm-2 day-1 = [(conc in µg N per mL) × mL of KCl added)] / (total area of the strips × % strip area 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 bicarbonate 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