soil extraction, ion exchange resin, and ion exchange membrane

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Published January, 2005

of nutrient per unit weight of soil. Correlations between IER measurements of soil nutrient availability and plant nutrient uptake have produced mixed results, with some studies showing good correlations (e.g., Binkley, 1984; Binkley et al., 1986; Lajtha, 1988; Qian and Schoenau, 1995) and some showing little or no correlation (e.g., Giblin et al., 1994; Walley et al., 2002). Nutrient availability measurements with IERs have been shown to be sensitive to soil water status and temperature and to competition from both microbial and root competition for nutrients (Schaf and Skogley, 1982; Binkley, 1984; Huang and Schoenau, 1997; Hangs et al., 2004). The use of resins for measuring soil nutrient availability in plots of limited size is especially attractive because of their nondestructive nature (e.g., Johnson et al., 2001). We have such a need in a study of the effects of climate variability on ecosystem processes in tallgrass prairie. This study involves temperature treatments of intact soil monoliths that were excavated and transported at great expense and with great care from a field site near Washington, Oklahoma, to the Great Basin Environmental Research Laboratory at the Desert Research Institute in Reno, Nevada. Measurements of soil N availability are a vital part of this study, yet disturbance must be kept to a minimum; thus, we used two commercially available resins—the Unibest resin capsules (Yang and Skogley 1992; Dobermann et al., 1994) and the PRS. To evaluate the effectiveness of these IERs in measuring soil N availability and its responses to warming in the study soil, we conducted a laboratory incubation in which these devices were placed in with soils incubated and extracted with one of the standard incubation procedures (Johnson et al., 1980).

SOIL EXTRACTION, ION EXCHANGE RESIN, AND ION EXCHANGE MEMBRANE MEASURES OF SOIL MINERAL NITROGEN DURING INCUBATION OF A TALLGRASS PRAIRIE SOIL Dale W. Johnson,* P. S. J. Verburg, and J. A. Arnone Abstract Two commercially available ion exchange resin (IER) devices— Unibest resin capsules (Unibest, Inc., Bozeman, MT) and Plant Root Simulator (PRS) probe–ion exchange membranes (Western Ag Innovations, Inc., Saskatoon, Canada)—for measuring soil nutrient availability were compared to traditional soil NHⴙ4 and NO⫺ 3 measurements during incubation of an Oklahoma tallgrass prairie soil at two temperatures (16ⴗ and 25ⴗC) and two moisture contents (15 and 25% by weight). Nitrate dominated the soil mineral N pool in soils and in both IER devices. Soil extractable and resin capsule mineral N showed significant responses to both temperature and moisture whereas PRS probe mineral N showed responses to moisture only. Both devices were more sensitive to moisture than soil mineral N was. Neither device related well to N mineralization or the patterns of extractable mineral N over time. Possible reasons for the differences include the integration of soil mineral N over time in the IERs as opposed to snapshots in time for soil mineral N, spatial variation within the incubated soils, and the importance of solution contact with IERs.

I

on exchange resin–based techniques are becoming more popular for measuring soil nutrient availability (Binkley and Matson, 1983; Binkley, 1984; Binkley et al., 1986; Lajtha, 1988; Dobermann et al., 1994; Giblin et al., 1994; Qian and Schoenau, 1995; Schnabel, 1995; Huang and Schoenau, 1996, 1997; Walley et al., 2002; Hangs et al., 2004) and leaching fluxes (Schnabel, 1983; Crabtree and Kirkby, 1985; Kjønaas, 1999; Susfalk and Johnson, 2002) in both field and laboratory settings. In addition to cost-effectiveness and generally simpler chemical workup, IER-based field techniques have advantages over traditional soil sampling in that they cause minimal disturbance and can allow remeasurement of specific points in the soil over time. Comparisons between field measurements of IERs and traditional soil measurements are difficult, however, because values from IER exposure cannot be related to specific amount of soil; thus, values for IER measurements tend to be expressed as weights or moles of nutrient per unit weight or surface area of resin rather than in weights or moles

Materials and Methods

D.W. Johnson, Natural Resources and Environmental Science, Univ. of Nevada, Reno, Reno, NV 89557; P.S.J. Verburg and J.A. Arnone, Earth and Ecosystem Science, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512. Received 13 May 2004. *Corresponding author ([email protected]).

The soil used in this study was the Pulaski series, Typic Ustifluvents derived from alluvium. A bulk sample of A horizon material (0–25 cm) was taken from a newly established field research site near Washington, Oklahoma, for the purposes of this and other laboratory studies. The soil was bulked in the field and homogenized before being used. Vegetation at the site consisted of tallgrass prairie, including Panicum virgatum L., Schizachyrium scoparium (Michx.) Nash, Andropogon gerardii Vitman, Sorghastrum nutans (L.) Nash, Ambrosia psilostachya DC., Xanthocephalum texanum (DC.), Bromus japanicus Thunb., and Eragrostis spp. In this laboratory experiment, 900 g of the sieved (2 mm), previously homogenized soil was placed into each of 16 Mason jars. Moisture in 8 of the jars was adjusted to 15% by weight and that in another 8 was adjusted to 25% by weight. These moisture contents were chosen because they represent the maximum and minimum values observed during a greenhouse study with intact monoliths using this soil in the Great Basin Environmental Research Laboratory (P. Verburg, unpublished data, 2003). Within each jar, one resin capsule and one pair of ion exchange stakes were buried. The resin capsule was a commercially available mixed-bed cation–anion ex-

Published in Soil Sci. Soc. Am. J. 69:260–265 (2005). © Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: IER, ion exchange resin; PRS, Plant Root Stimulator; SWAFL, Soil, Water, and Forage Analytical Laboratory.

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Fig. 1. (a) Soil extractable, (b) net soil change, (c) Plant Root Stimulator (PRS) probe, and (d) resin capsule mineral N (NHⴙ4 ⫹ NO⫺ 3 ) during the incubation. The net change in mineral N (b) represents N mineralization. Standard errors are given.

change resin capsule, Unibest PST-1 (Dobermann et al., 1994; Yang and Skogley 1992). The counter ions on the resins were H⫹ and OH⫺. The resin membrane stakes were PRS probes, which consist of anion or cation exchange membranes imbedded in plastic stakes (Western Ag Innovations, Inc., Saskatoon, Canada). The counter ions on the resin membranes are Na⫹ and HCO⫺ 3 . Four jars from each moisture level were then incubated at two temperatures, 16 and 25⬚C (representing a range of conditions expected during the growing season in the field). The lower temperature (16⬚C) incubator was a Galaxy regular refrigerator (Sears Roebuck Inc., Hoffman Estates, IL) with temperature control using a Campbell CR10 datalogger (Campbell Scientific Inc., Logan, UT) triggering the cooling unit. The higher temperature (25⬚C) incubator was a Fisher Scientific Isotemp incubator (Fisher Scientific, Pittsburgh, PA) with an Athena series 16 temperature controller (Athena Controls, Inc., Plymouth Meeting, PA. In both cases temperature never deviated more than 0.1⬚C from the setpoints.

At the beginning of the experiment and at days 28, 56, and 84, soils were removed from the jars, homogenized again, and subsampled. At each sampling, both the resin stakes and capsules were also removed and replaced with new ones when the soil was placed into the Mason jars for the next incubation period. The soil subsamples from each collection were ana⫺ lyzed for NH⫹ 4 and NO3 at the Soil, Water, and Forage Analytical Laboratory (SWAFL) at Oklahoma State University. At SWAFL, 10 g of dried, ground soil were shaken with 20 mL of 1 M KCl for 30 min. The extractant was filtered through a Fisher P4 qualitative filter (Fisher Scientific, Pittsburgh, PA) ⫺ and analyzed for NH⫹ 4 and NO3 on a Lachat 8000 flow-injection analyzer (Lachat Instruments, Milwaukee, WI) with Cetac xyz autosampler (Cetac Technologies, Portland, OR). The capsules and PRS probes were washed with distilled water to remove adhering soil after removal and then extracted for ⫺ NH⫹ 4 and NO3 also. The capsules were extracted with three sequential 20 mL solutions of 2 M KCl on a platform shaker for 20 min. each. The combined extractant solution (60 mL)


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Fig. 2. (a) Soil extractable, (b) net soil change, and (c) Plant Root Stimulator (PRS) probe, and (d) resin capsule NO⫺ 3 during the incubation. Standard errors are given.

was analyzed at SWAFL as described above. Values were expressed at ␮mol N cm⫺2 of resin area (which for two sides of the capsule is 22.8 cm2). The PRS probes were sent to Western Ag Innovations, Saskatoon, Canada for extraction. At Western Ag, the probes were extracted with 17.5 mL of 0.5 M HCl for one hour in a zip lock bag, and the extractant ⫺ was analyzed for NH⫹ 4 and NO3 using a Technicon autoanalyzer (Bran and Lubbe, Inc., Buffalo, NY). The values for both the probes were reported in units of 10 ␮mol N cm⫺2 of resin surface, and these were converted to ␮mol N cm⫺2 in this study for the purpose of comparing to the resin capsules.

Results The results of the mineral N (NH4⫹ ⫹ NO3⫺), NO3⫺, and NH4⫹ extractions in soils, PRS probes, and resin capsules are shown in Fig. 1 to 3 (respectively). In each case, the net change in soils is shown in panel B; in the case of mineral N, this represents N mineralization.

Statistical analyses are given in Table 1 and 2. Because of the large changes in N mineralization between the first incubation period (28 d) and the next two (56 and 84 d), statistical analysis were conducted separately on data from the first period as well as on the entire data set. Nearly all soil N mineralization occurred during the first 28 d incubation period (Fig. 1b), and nearly all (84–97%) of this mineral N consisted of NO3⫺ (Fig. 2a, 2b). Both temperature and moisture had significant effects on soil mineral N, soil mineralization, and soil NO3⫺ during both the first incubation period and during the incubation as a whole (Table 1). Time (incubation period) was significant for soil N mineralization but not for soil NO3⫺ or NH4⫹ (Table 1). The temperature ⫻ moisture interaction term was not significant for soil mineral N, N mineralization, NO3⫺, or NH4⫹ with the exception of NH4⫹ during the first incubation period.


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Fig. 3. (a) Soil extractable, (b) net soil change, (c) Plant Root Stimulator (PRS) probe, and (d) resin capsule NHⴙ4 during the incubation. Standard errors are given.

Moisture had significant, positive effects on PRS probe mineral N, NO3⫺, and NH4⫹ during the incubation as a whole, but effects on NH4⫹ were not significant during the first incubation period (Fig. 1–3, Table 1). Temperature had no significant effects on PRS probe mineral N or NO3⫺. Temperature had a significant negative effect on PRS probe NH4⫹ during the incubation as a whole but not during the first incubation period. The patterns of response in the PRS probes did not match those in the soils during the second and third incubation periods. The PRS probes showed an overall reduction during the second incubation period, as was the case for soil N mineralization, but the decline in the PRS probes was much less than for N mineralization and reversed somewhat during the third incubation period (Fig. 1). Both temperature and moisture had significant, positive effects on resin capsule mineral N and NO3⫺ during

both the first incubation period and during the incubation as a whole (Table 1). However, the patterns of response to temperature and moisture in the capsules during the second and third incubation periods (which were similar to those in the first incubation period) differed substantially from those in soils (Fig. 1 and 2). In contrast to the soil and PRS probes, the interaction term of moisture ⫻ temperature is significant for resin capsule mineral N and NO3⫺ in both the first incubation period and in the incubation as a whole, suggesting that the effects were more than additive. Also in contrast to the soils, the resin capsules showed increases in mineral N recovery in the second and third incubation periods whereas soil mineral N either remained constant or declined slightly and soil N mineralization declined substantially (Fig. 1). Resin capsule NH4⫹ was at trace levels (⬍0.0001 ␮mol cm⫺2 day⫺1) in all samples taken during the first incubation and nearly all samples taken during

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Table 1. ANOVA probabilities for analysis of the effects of temperature and moisture on soil extractable, resin capsule, and Plant Root Stimulator (PRS) probe NH4ⴙ ⫹ NO3⫺ during the incubation. Source

Temperature Moisture

First incubation period (28 d) 0.02 Soil NHⴙ4 ⫹ NO⫺ 3 Soil mineralization 0.02 Capsule NHⴙ4 ⫹ NO3⫺ ⬍0.01 0.58 PRS probe NH4ⴙ ⫹ NO3⫺ Soil NHⴙ4 0.77 Capsule NHⴙ4 1.00 PRS probe NH4ⴙ 0.25 0.02 Soil NO⫺ 3 Capsule NO⫺ ⬍0.01 3 PRS probe NO3⫺ 0.34 All data (repeated measures analysis) ⬍0.01 Soil NHⴙ4 ⫹ NO⫺ 3 Soil mineralization ⬍0.01 Capsule NHⴙ4 ⫹ NO3⫺ ⬍0.01 PRS probe NH4ⴙ ⫹ NO3⫺ 0.60 0.19 Soil NHⴙ4 Capsule NHⴙ4 0.34 PRS probe NH4ⴙ 0.05 ⬍0.01 Soil NO⫺ 3 Capsule NO⫺ ⬍0.01 3 PRS probe NO3⫺ 0.57

Temperature ⫻ moisture Time

⬍0.01 ⬍0.01 ⬍0.01 ⬍0.01 0.18 1.00 0.25 ⬍0.01 ⬍0.01 ⬍0.01

0.94 0.94 ⬍0.01 0.35 0.02 1.00 0.48 0.79 ⬍0.01 0.35

⬍0.01 0.10 ⬍0.01 ⬍0.01 0.54 0.34 0.05 ⬍0.01 ⬍0.01 ⬍0.01

0.48 0.98 ⬍0.01 0.11 0.88 0.34 0.10 0.49 0.04 0.11

0.49 ⬍0.01 ⬍0.01 0.01 0.49 0.34 ⬍0.01 0.62 ⬍0.01 0.01

the incubation as a whole, and there were no statistically significant effects of either temperature or moisture on resin capsule NH4⫹ (Fig. 3d and Table 1). Soil mineral N, N mineralization, and NO3⫺ were significantly correlated with both resin capsule and PRS probe mineral N and NO3⫺ during the first incubation period (Table 2). During the incubation as a whole, correlations between soil and resin capsule mineral N and NO3⫺ were much weaker and there were no significant correlations between soil N mineralization and PRS probe mineral N, NO3⫺, or NH4⫹. Soil NH4⫹ was weakly but significantly correlated with resin capsule mineral N but not with any other measured parameter. During the first incubation, PRS probe and resin capsule mineral N and NO3⫺ were significantly correlated, but during the incubation as a whole there were no significant correlations between PRS probe and resin capsule measurements aside from a weak and probably spurious correlation between PRS NH4⫹ and resin capsule NO3⫺ and mineral N (Table 2).

Discussion According to the statistical analyses, soil mineral N responses to temperature and moisture treatments correlated better with those of the resin capsules than with the PRS—specifically, soil and resin capsule mineral N showed statistically significant response to temperature and moisture whereas PRS probes showed significant response to moisture only (aside from the negative response of NH4⫹ to temperature). Neither the capsules nor the probes reflected the patterns observed in the soil mineral N or N mineralization at the end of the second and third incubation periods, however. The patterns shown by the capsules differed very substantially from those in soils with respect to treatment effects and temporal changes during the second and third incubation periods. The resin capsules showed increased N availability with time of incubation, whereas the soil incubation showed approximately stable mineral N pools and greatly reduced mineralization after the first incubation period. The PRS probes more closely approximated the temporal patterns in soil N mineralization, showing a reduction after the first incubation period. Thus, it cannot be said from the statistical analyses alone that the capsules performed better than the probes in this experiment. Both the resin capsules and the PRS probes were more sensitive to moisture than to temperature, and the magnitude of the responses in both the capsules and PRS probes were much greater than in the soil incubations. This was especially true for the capsules, which showed increases of up to 15-fold with treatment whereas the soil incubation responses were less than 100% in all cases. Although the capsules did show significant responses to temperature (up to 200% increase), they showed much greater responses to moisture (up to 1000% increase), whereas responses in the soil incubations were approximately equal (up to 60–70% increase in each). The greater sensitivity of both resin capsules and PRS probes to moisture rather than temperature suggests that soil solution is a major vector of contact between soil exchange sites and resin surfaces in both cases and suggests further that these techniques will not be especially accurate indices of soil mineral N in dry conditions. The comparisons among soil mineral N, mineralization, capsules, and probes may be confounded to some

Table 2. Correlation coefficients (r 2) for soil extractable, resin capsule, and Plant Root Stimulator (PRS) probe mineral N (NHⴙ4 ⫹ NO⫺ 3 ) during the incubation. 1st Incubation Prd. All Data Soil NH4ⴙ ⫹ NO3⫺ Soil mineralization Soil NH4ⴙ Soil NO3⫺ PRS NH4ⴙ ⫹ NO3⫺ PRS NH4ⴙ PRS NO3⫺ Capsule NO3⫺ Capsule NH4ⴙ Capsule NH4ⴙ ⫹ NO3⫺

Soil NH4ⴙ ⫹ NO3⫺ 0.09‡ 0.01 0.96§ 0.06† 0.08† 0.06† 0.20§ 0.04† 0.20§

† Regression significant at the 90% level ‡ Regression significant at the 95% level § Regression significant at the 99% level

Soil N min.

Soil NH4ⴙ

Soil NO3⫺

PRS NH4ⴙ ⫹ NO3⫺

PRS NH4ⴙ

PRS NO3⫺

Capsule NH4ⴙ ⫹ NO3⫺

Capsule NH4ⴙ

Capsule NO3⫺

1.00§

0.18‡ 0.18†

0.99§ 0.99§ 0.12

0.53 0.53§ 0.01 0.54§

0.07 0.07 0.02 0.07 0.06

0.53§ 0.53§ 0.01 0.54§ 1.00§ 0.06

0.60§ 0.60§ 0.15† 0.59§ 0.62§ 0.04 0.62§

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

0.60§ 0.60§ 0.15† 0.59§ 0.62§ 0.04 0.62§ 1.00§ 0.00

0.02 0.09‡ 0.02 0.01 0.01 0.14† 0.02 0.14†

0.01 0.01 0.02 0.01 0.02 0.02 0.14†

0.07† 0.08† 0.07† 0.19§ 0.05† 0.19§

0.08† 1.00§ 0.03 0.01 0.03

0.08† 0.09† 0.01 0.09†

0.03 0.01 0.03

0.05† 1.00§

0.05†

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degree by the scale at which these methods sample the soil. The capsules were exposed to a virtual point within the soil at approximately halfway through the artificial profile with the incubation jars (where they were buried). The probes were exposed to a much greater vertical segment of the profile (5.5 cm). The soil samples should have been representative of the bulk of the soil in the incubation because the samples were thoroughly mixed before subsampling on each occasion. Aside from the first incubation period, the resins did not represent soil N mineralization rates very well. This is as expected, because soil N mineralization rates were di⫹ rectly proportional to standing pools of NO⫺ 3 and NH4 at the end of the first incubation period due to the homogenization of the soil before incubation. After that time, soil standing pools of NO3⫺ and NH4⫹, which the resins are presumably most responsive to, had stabilized and were therefore greater than mineralization rates. The nature of this study does not allow us to comment on which of these methods represents truly “available” N in soils (i.e., as seen by plant roots) because no plants were present in the incubations. The greater sensitivity of both the capsules and probes to moisture is as one would expect in a plant root as well—high values of soil mineral N do not necessarily translate into greater available N to plants if the soil is too dry to facilitate fine root growth or transport of N to plant roots. Furthermore, both capsules and probes represent more of an integrated and perhaps cumulative measure of soil N availability over time whereas the soil mineral N measurements are snapshots at discrete intervals. For our purposes in the study described in the Introduction—to obtain non-destructive measurements of soil available N—we will use both methods, duly noting the differing sensitivities describe here, and in future papers we will directly compare plant N uptake with both IER measures of soil N availability. Finally, we note that long-standing concepts of what constitutes soil N “availability” and how it is measured are in the process of changing, with more emphasis on spatial and temporal variability on a small scale, among other things (Schimel and Bennett, 2004). Hopefully, this simple experiment and others like it can shed more light on the nature of such variability and on how well different methods for measuring soil N are able to characterize it. ACKNOWLEDGMENTS Research supported by the NSF Integrated Research— Challenges in Environmental Biology Project No. DEB-0078325 (Interannual Climate Variability and Ecosystem Processes: A Quantitative Assessment of Combining Modeling with Field

and Mesocosm Experiments) and by Nevada Agricultural Experiment Station (publication 52042940). Technical assistance from Matt Church is greatly appreciated.

REFERENCES Binkley, D. 1984. Ion exchange resin bags: Factors affecting estimates of nitrogen availability. Soil Sci. Soc. Am. J. 48:1181–1184. Binkley, D., J. Aber, J. Pastor, and K. Nadelhoffer. 1986. Nitrogen availability in some Wisconsin forests: Comparisons of resin bags and on-site incubations. Biol. Fertil. Soils 2:77–82. Binkley, D., and P. Matson. 1983. Ion exchange resin bag method for assessing available forest soil nitrogen. Soil Sci. Soc. Am. J. 47: 1050–1052. Crabtree, R.W., and M.J. Kirkby. 1985. Ion-exchange resin samplers for the in situ measurement of major cations in soil water solute flux. J. Hydrol. (Amsterdam) 80:325–335. Dobermann, A.H., A.H. Langner, H. Mutscher, J.E. Yang, E.O. Skogley, M.A. Adviento, and M.F. Pampolino. 1994. Nutrient adsorption kinetics of ion exchange resin capsules: A study with soils of international origin. Commun. Soil Sci. Plant Anal. 25:1329–1353. Giblin, A.E., J.A. Laundre, K.J. Nadelhoffer, and G.R. Shaver. 1994. Measuring nutrient availability in arctic soils using ion exchange resins: A field test. Soil Sci. Soc. Am. J. 58:1154–1162. Hangs, R.D., K.J. Greer, and C.A. Sulewski. 2004. The effect of interspecific competition on conifer seedling growth and nitrogen availability. Can. J. For. Res. 34:754–761. Huang, W.Z., and J.J. Schoenau. 1996. Forms, amounts and distribution of carbon, nitrogen, phosphorus, and sulfur in a boreal aspen forest soil. Can. J. Soil Sci. 76:373–385. Huang, W.Z., and J.J. Schoenau. 1997. Seasonal and spatial variations in soil nitrogen and phosphorus supply rates in a boreal aspen forest. Can. J. Soil Sci. 77:597–612. Johnson, D.W., N.T. Edwards, and D.E. Todd. 1980. Nitrogen mineralization, immobilization, and nitrification following urea fertilization of a forest soil under field and laboratory conditions. Soil Sci. Soc. Am. J. 44:610–616. Johnson, D.W., B.A. Hungate, P. Dijkstra, G. Hymus, and B. Drake. 2001. Effects of Elevated CO2 on Soils in a Florida scrub oak ecosystem. J. Environ. Qual. 30:501–507. Kjønaas, O.J. 1999. In situ efficiency of ion exchange resins in studies of nitrogen transformation. Soil Sci. Soc. Am. J. 63:399–409. Lajtha, K. 1988. The use of ion-exchange resin bags for measuring nutrient availability in an arid ecosystem. Plant Soil 105:105–111. Qian, P. and J.J. Schoenau, J.J. 1995. Assessing nitrogen mineralization from soil organic matter using anion exchange membranes. Fert. Res. 40:143–148 Schaf, B.E., and E.O. Skogley. 1982. Diffusion of potassium, calcium, and magnesium in Bozemann silt loam as influenced by temperature and moisture. Soil Sci. Soc. Am. J. 46:521–524. Schimel, J.P., and J. Bennett. 2004. Nitrogen mineralization: Challenges of a changing paradigm. Ecology 85:591–602. Schnabel, R.R. 1983. Measuring nitrogen leaching with ion exchange resin: A laboratory assessment. Soil Sci. Soc. Am. J. 47: 1041–1042. Schnabel, R.R. 1995. Nitrate and phosphate recovery from anion exchange resins. Commun. Soil Sci. Plant Anal. 26:531–540. Susfalk, R.B., and D.W. Johnson. 2002. Ion exchange resin based soil solution lysimeters and snowmelt collectors. Commun. Soil Sci. Plant Anal. 33:1261–1275. Walley, F., T. Yates, J.-W. van Groenigen, and C. van Kessel. 2002. Relationships between soil nitrogen availability indices, yield, and nitrogen accumulation in wheat. Soil Sci. Soc. Am. J. 66:1549–1561. Yang, J.E., and E.O. Skogley. 1992. Diffusion kinetics of multinutrient accumulation by mixed-bed ion-exchange resin. Soil Sci. Soc. Am. J. 56:408–414.