Gary Clark, Xiaojuan Wang
Department of Agricultural Sciences/The Agribioscience Centre, La Trobe University
This protocol outlines the use of resin membranes for the measurement of soil nutrients in the field.
The bioavailability of soil nutrients is a function of the soil chemical, physical and the mineralogical properties which influence the ion-exchange reactions in the soil (Sherrod et al., 2002). Conventional methods of soil nutrient availability involve the chemical extraction of soil sampled from a field site, and subsequently analysed in the laboratory. However, this method is unable to elucidate the dynamics of labile nutrients. This especially concerns nutrients, such as N and P, which are dependent on soil microbial processes, and thus influenced by soil temperature and moisture. The use of ion-exchange resins allows for the extraction of nutrients in situ in a number of ecological studies (Qian et al., 1992; Qian and Schoenau, 2005; Hovenden et al., 2008).
Ion-exchange membranes are most useful as a measure of soil nutrients over a defined time period, rather than as a measure of the absolute plant available nutrient that is most often used to aid crop production (Sherrod et al., 2003). Furthermore, the method of extraction of nutrients, chemical as opposed to resin/counter ion, can lead to chemical modification of the soil mineral matrix, thus resulting in distinctly different levels of extracted nutrient (Sherrod et al., 2002).
This method is currently being used for the measurement of soil nutrients for a study of nutrient dynamics in the alpine zone of Victoria.
Resin membranes – cation and anion exchangeable, HCl, NaCl, NaHCO3
Resin membranes are prepared with a suitable counter-ion of the same charge, but importantly with less ability to adsorb to the exchange surface than the nutrient ions of interest. This counter-ion has the ability to exchange with the nutrient ions in solution. Nutrient ions are then removed by immersion of the strip in an excess of a salt solution. The concentration of nutrients in solution can then be measured with ICP, AAS or FIA.
Importantly, as P is strongly sorbed to colloid, and other exchangeable surfaces, the choice of anion counter-ion was quite important. Measurement of rates of sorption with nutrient solutions demonstrated that bicarbonate, HCO3 , in comparison to chloride, was the most suitable counter-ion for the measurement of P (Figure 1). In comparison, HCl, was chosen for the preparation of the cation counter-ion as H+ will easily exchange with all of the nutrient cations.
Figure 1. Absorption of P by the AEM saturated with counter ions as HCO3– and Cl–.
Preparation of resin membrane strips
1. The cation exchange membrane (CEM) and anion exchange membrane (AEM) are prepared by cutting a sheet of membrane (Membranes International) into 2 x 12 cm strips.
2. All strips are washed with distilled water to remove impurities.
3. The CEM was saturated with H+ by shaking CEM strips overnight in 2 M HCl.
4. The AEM were saturated with HCO3– by shaking the AEM strips overnight in 1 M NaHCO3 .
5. The membranes are rinsed free of excess HCl and NaHCO3 with deionized water.
6. For the CEM, a further 12 h equilibration with distilled water is required. A frequent change of the distilled water during the 12 h is necessary to remove the last trace of acid.
7. Prepared strips are kept moist in sealed plastic bags prior to installation in the field. Installation in the field
Installation in the field
1. At each 1-m2 plot, 5-paired CEM and AEM (10 x 2 cm) are inserted vertically into, as practicably possible, the non-vegetated area of surface soil to 10 cm in depth (Figure 2).
2. Strips are placed vertically into a slice opened gently with a spatula. The slit is then closed by firmly pressing both sides together to ensure uniform membrane-soil contact.
3. For ease of use, the membrane strips can be cut longer than 10 cm. This allows for additional membrane to be above the soil surface, and a small overlap of membrane on the bottom of the spatula greatly assists the insertion and retrieval of the membrane strips.
Figure 2. Membrane installation in field
1. Strips placed into pre-labelled zip-lock bags. New membrane strips are inserted in the same position or at different randomized locations within the plots. (This needs careful thought as placement in the same location could lead to local depletion of soil nutrients).
2. All strips should be refrigerated until further processing in the laboratory.
Desorption of nutrients
1. Membranes are carefully rinsed with DIW until all traces of soil are removed.
2. Desorption of nutrients is achieved by shaking the 5 CEM strips from one plot in 250 mL of 0.5M HCl for 2 hours. This solution can be used to measure cations of interest with ICP-OES (or AAS). To measure NH4+ by FIA requires a small addition of NaOH to the extracted solution to increase the pH above 3. This ensures that the colorimetric reaction for the ammonium is able to proceed in an alkaline solution.
3. Desorption of anions, such as NO3– and H2PO4– etc, is achieved by shaking all 5 AEM strips in 1 M NaCl for 2 hours. The solution is then used to measure nitrate using FIA, and phosphate using the malachite green method (Diatloff and Rengel, 2001). Results All concentrations of nutrients should be quoted as mg, or g cm-2 (of membrane)
Other useful measurements at the time of installation/retrieval
1. Soil moisture – Theta probe® or similar
2. Soil temperature
Links to resources and suppliers
Diatloff E, Rengel Z (2001) Compilation of simple spectrophotometric techniques for the determination of elements in nutrient solution. Journal of Plant Nutrition 24, 75-86.
Hovenden MJ, Newton PCD, Carran RA, Theobold P, Wills KE, Vander Schoor JK, Williams AL and Osanai Y (2008) Warming prevents the elevated CO2-induced reduction in available soil nitrogen in a temperate, perennial grassland. Global Change Biology 14, 1018-1024.
Qian P, Schoenau JJ (2005) Use of ion-exchange membranes to assess nitrogen-supply power of soils. Journal of Plant Nutrion 28, 2193-2200.
Qian P, Schoenau JJ, Huang WZ (1992) Use of ion exchane membranes in routine soil testing. Communications in Soil Science and Plant Analysis 23, 1791-1804.
Sherrod SK, Belnap J, Miller ME (2002) Comparison of methods for nutrient measurement in calcareous soils: ion-exchange resin bag, capsule, membrane, and chemical extractions. Soil Science 167, 666-679.
Sherrod SK, Belnap J, Miller ME (2003) Comparison of ion-exchange counterions in the nutrient measurements of calcareous soils: implications for correlative studies of plant-soil relationships. Communications in Soil Science and Plant Analysis 34, 1981-2001.