Agronomy Journal 94:864-869 (2002)
© 2002 American Society of Agronomy
NOTES & UNIQUE PHENOMENA
Refinements to an In-Situ Soil Core Technique for Measuring Net Nitrogen Mineralization in Moist, Fertilized Agricultural Soil
Kristofor R. Brye*,a,
John M. Normanb,
Erik V. Nordheimc,
S. Thompson Gowerd and
Larry G. Bundyb
a Dep. of Crop, Soil, and Environ. Sci., Univ. of Arkansas, 115 Plant Sci. Bldg., Fayetteville, AR 72701
b Dep. of Soil Sci., Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299
c Dep. of Stat. and Dep. of Forest Ecol. and Manage., Univ. of Wisconsin, 1630 Linden Dr., Madison, WI 53706-1598
d Dep. of Forest Ecol. and Manage., Univ. of Wisconsin, 1630 Linden Dr., Madison, WI 53706-1598
* Corresponding author (kbrye{at}uark.edu)
Received for publication September 24, 2001.
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ABSTRACT
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Diffusion of NO-3–N ions and nonuniform distribution of surface-applied N fertilizers contribute to large variations in field measurements of net N mineralization. An in situ soil core–ion exchange resin bag (ISC–IERB) field method has been used reliably to measure net N mineralization in intact soil cores but has not been widely tested in moist, though nonirrigated, N-fertilized agricultural soils. From 1996 through 2000, net N mineralization was measured in the top 20 cm using a refined version of the ISC/IERB technique for the first 1-mo period following planting and fertilization of N-fertilized and N-unfertilized, no-tillage and chisel-plowed corn (Zea mays L.) agroecosystems on Plano silt loam (fine-silty, mixed, superactive, mesic, typic argiudoll) in south-central Wisconsin. Progressive modifications were made to the ISC/IERB technique, which ultimately resulted in reduced sample variability. Following the refinements, a significant N fertilization effect was shown for the most variable period of the growing season where net N mineralization rates for N-fertilized corn treatments were significantly higher (p < 0.001) than for N-unfertilized corn treatments.
Abbreviations: CP, chisel plowed • ISC–IERB, in situ soil core/ion exchange resin bag • NT, no-tillage
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INTRODUCTION
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THE AMOUNT and potential availability of plant nutrients in soil, particularly N, are key components of high-yielding crop management. However, relatively few methods exist to quantify N availability to plants from mineralization (Subler et al., 1995). In fact, the potential differences in net N mineralization between N-fertilized and N-unfertilized agricultural soils have not been adequately assessed under field conditions. Ion exchange resin bags have been buried in the field to evaluate nutrient availability in forests (Binkley and Matson, 1983), grasslands (Gibson et al., 1985), and deserts (Lajtha, 1988). DiStefano and Gholz (1986) proposed an in situ field technique using incubated soil cores and ion exchange resin bags (ISC/IERB) to measure net N mineralization and capture the leaching of inorganic N in agricultural soils. Estimates of net N mineralization using this technique are positively correlated to the buried bag technique (Eno, 1960) and are an index of soil N availability (Binkley and Hart, 1989).
The ISC/IERB technique, which has been used successfully for net N mineralization in grasslands (Hook and Burke, 1995), forests (Binkley et al., 1986; Smethurst and Nambiar, 1989), and dryland agroecosystems (Kolberg et al., 1997, 1999) and recommended in the standard Methods of Soil Analysis (Hart et al., 1994), has considerable potential for use in agricultural soils (Subler et al., 1995). However, the use of the ISC/IERB technique to assess net N mineralization has not been widely tested in moist, N-fertilized agroecosystems (Hübner et al., 1991) that do not require irrigation.
Two potential limitations to accurately quantifying net N mineralization in moist, N-fertilized agroecosystems are the following: (i) lateral and upward diffusion of NO-3–N ions from outside the core under moist soil conditions and (ii) inherently high spatial variability of inorganic N in surface soils because of nonuniform distribution of fertilizer N. These potential limitations contribute to errors in quantifying the initial inorganic N content of the fertilized soil adjacent to the core before incubation. Consequently, the ISC/IERB technique may require methodological refinement before it is applicable in moist, N-fertilized agricultural soils. Kolberg et al. (1997) tested the ISC/IERB field technique in a dryland agroecosystem setting and determined that this method was reliable but that variability of the results was high. This necessitated increasing the number of samples to detect treatment differences at a desirable confidence level (Kolberg et al., 1997).
The objective of this exploratory work was to develop refinements to the ISC/IERB technique so as to minimize the potential for ion diffusion and reduce sample variability, without increased sampling, to assess potential differences in net N mineralization between N-fertilized and N-unfertilized agricultural soil. Brye (1999) demonstrated that the first 1-mo sampling interval immediately following planting and fertilization was the most variable month of the season for measuring net N mineralization. Therefore, we hypothesized that sample variability could be significantly reduced for a N-fertilized, chisel-plowed (CP) corn (Zea mays L.) agroecosystem using a refined ISC/IERB field technique because, compared with N-fertilized, no-tillage (NT) or N-unfertilized tillage treatments, this was the treatment in which net N mineralization estimates were most variable (Brye, 1999).
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MATERIALS AND METHODS
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Experimental Treatments
Between 1996 and 2000, net N mineralization was measured in situ in the top 20 cm of soil using the ISC/IERB technique, similar to that of DiStefano and Gholz (1986). Measurements were conducted during the first 1-mo period following planting and fertilization because this is typically the period of the growing season in which mineral N is most variable due to fertilization and temperature- and moisture-mediated changes in N mineralization. We incorporated several modifications to the ISC/IERB technique of DiStefano and Gholz (1986) and measured net N mineralization in N-fertilized and N-unfertilized NT and CP corn agroecosystems. The agroecosystems were located at the University of Wisconsin's Arlington Agricultural Research Station near Arlington, WI (43°17' N, 89°22' W). The treatments were arranged in a randomized complete block replicated four times and reside on Plano silt loam (fine-silty, mixed, superactive, mesic, typic argiudoll). A detailed description of the agroecosystems is found in Brye et al. (2000).
Chronology of Modifications to Methodology
The technique modifications dealt mainly with placement of the resin bag relative to the bottom of the soil core. Preliminary investigations (i.e., from data collected in 1995) suggested that lateral and upward diffusion of inorganic N occurs in moist, N-fertilized Wisconsin soils (Brye, 1999). Therefore, in 1996, a silica sand bag was placed below the resin bag outside the soil core (i.e., n = 8 cores per treatment) to act as a diffusion barrier (Fig. 1B)
. This diffusion-barrier approach has not previously been reported for studies using the ISC/IERB method. Upon analyzing the 1996 data, we realized that total isolation of the resin bag from the bulk soil was critical to avoid contamination of the resin bag by inorganic N not contained within the incubating soil column. Consequently, the resin and sand bags were placed entirely inside the enclosed soil column for measurements conducted in 1997 through 1999 (i.e., n = 8 cores per treatment in 1997 and 1998 and n = 4 cores per treatment in 1999) (Fig. 1C). During 1999, mineralization measurements were not conducted in N-unfertilized plots, but in the N-fertilized plots, we continued as before to (i) collect initial soil samples from outside the incubating core, assuming that the inorganic N status outside the core was the same as inside the soil core and (ii) install soil cores for incubation after field plot fertilization. In 2000, we realized that these last two procedures may also cause significant variability. Therefore, we extracted small initial samples from the core and fertilized the core with precise amounts of fertilizer. The final, refined procedure is described in the following section.
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Fig. 1. Schematic diagrams of modifications made to the incubated soil core–ion exchange resin bag technique of DiStefano and Gholz (1986) that were used in (A) 1995, (B) 1996, (C) 1997 and 1998, and (D) the minilysimeter method used in 2000. Diagrams not drawn to scale. (Dimensions are given in Materials and Methods.)
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The ISC/IERB technique provides an estimate of net N mineralization that may potentially be artificial because, in theory, the bottom of the column must become saturated before drainage from the core will commence. Therefore, the residence time of inorganic N in solution in the soil column above the resin bag could be longer than in undisturbed soil. Because the soil at the base of the column requires saturation before draining through the resin bag, inorganic N in solution could experience immobilization and/or denitrification before it would have otherwise leached from the soil column under more natural drainage conditions. Consequently, a new technique for measuring net N mineralization, consisting of a stainless-steel minilysimeter, was developed and implemented in 2000 for several reasons: (i) to maintain a natural drainage pattern in the incubating core that is more typical of the bulk soil, (ii) to reduce the potential for uncharacteristically high immobilization and/or denitrification rates, and (iii) to have an alternative technique that did not use the resin bags and completely isolated the incubating soil core from the bulk soil to serve as a test of the ISC/IERB field method.
Refined Net Nitrogen Mineralization Procedure
In 2000, soil cores were incubated in situ for about 1 mo from the day the corn crop was planted until roughly 2 wk after emergence (i.e., 3 May through 5 June). Two, 20-cm-long soil cores were incubated per experimental plot in polyvinyl chloride (PVC) tubes, 7.6 cm in diameter by 30 cm long. The soil cores were placed in the furrows without wheel tracks within each N-fertilized and N-unfertilized NT and CP plot (i.e., n = 8 cores per treatment) immediately after planting so that plant N uptake was not an issue. The PVC tubes were sharpened with a bevel to the outside at the base to minimize soil compaction in the core during installation. To minimize variability associated with a possible heterogeneous distribution of fertilizer N, the initial soil samples were collected from inside the PVC core. The bottom 2 cm of soil was removed from the soil core. A 1-cm-diam. cork borer was used to take four 8-cm-long samples from the PVC core; two from the top portion and two from the bottom portion (i.e., taken so holes from both ends did not intersect) and the subsequent holes were plugged with 8-cm-long sharpened plastic rods that were 1 cm in diameter. Final net N mineralization rates were adjusted appropriately for the 3% reduction in area caused by removing the initial soil cores.
A 15-g subsample of fresh soil from the composite cork-borer sample was immediately extracted in preweighed 125-mL wide-mouth bottles containing 100 mL of 2 M KCl for determination of initial inorganic NO-3–N and NH+4–N. Subsamples of initial soil were also placed in small tins for gravimetric soil moisture determinations. The preweighed bottles containing extracting soil were transported to the laboratory where they were postweighed, shaken for 1 h, and set out overnight for the soil suspension to settle. After 24 h, aliquots were collected by pipet from the supernatant solution for colorimetric analysis of inorganic N using a continuous-flow ion analyzer (Lachat, 1986; Lachat, 1987).
After the initial soil samples were collected from the PVC core and chemically extracted, a resin bag was placed into the excavated area at the base of the PVC core, and a bag of silica sand was placed below the resin bag. The resin bags consisted of 15 g of anion and 15 g of cation exchange resin beads (Anion Exchange Resin 4601-01 and Cation Exchange Resin 1927-01, J.T. Baker, Phillsburg, NJ) tied into a nylon stocking. Laboratory incubations of the resin beads alone showed that the resin beads, despite being N based from manufacturing, do not break down to release N within a 1-mo period (P. Barak, personal communication, 1996). A square piece of stiff wire mesh was forced into the base of the tube below the sand bag to keep the resin and sand bag inside the PVC tube. The resin and sand bags each occupied the full area of the tube (46 cm2) and were approximately 1 cm thick. The PVC tube containing the soil core was inserted back into its original hole and tapped back into its original position. The top of the PVC tube remained open to the atmosphere to allow natural moisture fluctuations to occur.
Fertilizer N was then added to the surface of each individual fertilized PVC core with 15 mL of distilled water containing 0.233 g of NH4NO3, equivalent to a rate of 189 kg N ha-1. The fertilized PVC cores were temporarily covered as the remaining plot area was fertilized by hand with a broadcast application (i.e., 189 kg N ha-1) of pelletized NH4NO3.
After incubating for approximately 1 mo, the PVC cores were collected from the field plots. The ion exchange resin bags were immediately extracted in 100 mL of 2 M KCl. For this study, we assumed 100% efficiency for the one-time batch extraction of the resin bags. The soil in the PVC tube was removed, homogenized by hand, and approximately 15 g of fresh soil was added to preweighed 125-mL wide-mouth bottles containing 100 mL of 2 M KCl. Subsamples of incubated soil were placed in small tins for gravimetric soil moisture determinations. Net N mineralization was calculated on a dry soil basis by adding the inorganic N extracted in the resin bag to the difference between post- and preincubation soil inorganic N levels. No plants or plant activity were involved in the system.
Minilysimeter Technique
A new, in-situ minilysimeter technique was also tested during the first 1-mo period following planting and fertilization in 2000 (i.e., 3 May through 5 June) and compared to net N mineralization results from the refined ISC/IERB technique. Cylindrical minilysimeters, 7.6 cm wide (i.e., 46 cm2) by 2.5 cm tall, were constructed out of stainless steel with a 0.2-µm porous stainless-steel plate surface similar to that used by Brye et al. (1999). Two PVC cores were outfitted with minilysimeters (Fig. 1D) and installed in each of two N-fertilized and N-unfertilized CP plots (i.e., n = 4 per treatment).
Roughly 1 cm of soil was excavated at the base of the PVC core for the placement of the lysimeter inside the base of the core. Initial soil samples were collected from soil inside the PVC core and their holes plugged with lucite rods at the surface and base of the PVC core as previously described. The PVC core was then inverted with a retainer holding the soil core in place from the top, a wet disc of glass fiber filter paper was placed on the surface of the minilysimeter, and a thin layer of soil slurry was spread out over the base of the soil core to ensure capillary contact between the base of the soil core and the porous plate surface of the minilysimeter (Brye et al., 1999). The minilysimeter, with a spring to keep it pressed against the soil, was put into a PVC cup with a top edge that was beveled to match the bottom sharpened edge of the PVC core (Fig. 1D). The seam between the PVC core and the PVC cup was taped together with electrical tape, and a hose clamp was tightened around the seam to ensure the PVC core was sealed. Plastic tubing was attached on one end to a short stainless-steel drain tube, which was welded to the minilysimeter and extended out from the base of the PVC cup, and on the other end to one of two bulk-head fittings on the cap of a 1-L plastic leachate-collection bottle (Fig. 1D). Vacuum tubing was attached to the other bulk-head fitting, which connected the leachate-collection bottle to a regulated vacuum system with 10 kPa of continuous suction (Brye et al., 1999).
The incubating soil cores containing minilysimeters were fertilized individually as described earlier with 15 mL of distilled water containing 0.233 g of NH4NO3, equivalent to a rate of 189 kg N ha-1. The fertilized PVC cores with minilysimeters were also temporarily covered as the remaining plot area was fertilized as previously described.
Leachate solutions were collected from the minilysimeters three times following rainfall events during the 1-mo measurement interval. Leachate volumes were measured, aliquots were filtered through glass fiber filter paper, and analyzed for inorganic N concentrations using a continuous-flow ion analyzer (Lachat, 1993a, 1993b). Final core sampling and analyses followed the procedure previously described for the refined ISC/IERB method. Net N mineralization was calculated on a dry soil basis by adding the difference between post- and preincubation soil inorganic N levels to the sum of inorganic N in the minilysimeter leachate solutions.
Statistical Analyses
The tillage and fertilizer rate combinations were considered as a single management practice; thus, four treatments were evaluated. Therefore, a one-way analysis of variance was performed, and least significant differences (LSDs; 
= 0.05 level) were calculated to determine net N mineralization differences among treatments and methods for the 2000 data (SAS Inst., 1990). Replicate soil cores per plot were summed to give the number of cores (n) per treatment per year as the data set used for statistical tests. Statistical tests for data normality and homogeneity of variance (i.e., Levene's test; Levene, 1960) were performed on net N mineralization data (Minitab, 1997). Coefficients of variation were also calculated.
The null hypothesis that the refined ISC/IERB field technique had the same sample variability as the unrefined ISC/IERB technique was tested using the bootstrap resampling technique (Efron and Tibshirani, 1993) against the alternative hypothesis that the refined technique reduced the sample variability (MathSoft, 1998). Separate bootstrap tests were performed to compare the 2000 data with each of the previous years' data, except for the N-unfertilized tillage treatments in 1999, for which measurements were not conducted. The test statistics used for testing the hypothesis were the ratios of the variance from the 2000 data (VAR00) to the variance of each previous year's data (i.e., 1996, 1997, 1998, and 1999) (VAR9X), i.e., VAR00/VAR9X. The 95% confidence limits for the variance ratio were calculated by ordering the ratios obtained from 1000 subsets (i.e., of the initial sample population) that were resampled with replacement from the original net N mineralization observations. Significant differences between the variances of the 2000 and previous years' data were determined when the variance ratio VAR00/VAR9X = 1 fell outside the 95% confidence limits. The bootstrap resampling technique has been used before to explore significant treatment differences in agriculturally related studies (Reich et al., 1993; Roche et al., 1997; Lui et al., 1999).
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RESULTS AND DISCUSSION
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The field technique comparison demonstrated that estimates of net N mineralization rates did not differ significantly (p > 0.10) between the refined ISC/IERB and the minilysimeter methods (Fig. 2)
. Preliminary results suggest that the potential for immobilization and/or denitrification of inorganic N in solution using the ISC/IERB technique was no greater than using the minilysimeter method, and therefore is not of significant concern.
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Fig. 2. Nitrogen mineralization rates and technique comparison conducted between 3 May and 5 June 2000 for the N-fertilized and N-unfertilized, chisel-plowed (CP), and no-tillage (NT) corn treatments. Asterisks denote significant differences (** = p < 0.05 and *** = p < 0.001) in N mineralization estimates from the same method [i.e., incubated soil core–ion exchange resin bag (ISC–IERB) or minilysimeter] between N-fertilized and N-unfertilized treatments. Least significant differences ( = 0.05 level) between methods were 12.5 and 20.3 kg N ha-1 mo-1 for the N-unfertilized (p = 0.114) and N-fertilized (p = 0.521) CP treatments, respectively.
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The refined ISC/IERB field method significantly reduced sample variability (p < 0.05) in the N-fertilized CP agroecosystem for three out of four comparisons of the 2000 data to data from previous years (i.e., when the variance ratio of 1 falls outside the 2.5 and 97.5 percentile columns in Table 1). The effect of slightly higher precipitation on the variability of net N mineralization estimates during the 2000 sampling period cannot be distinguished; it may have contributed to the observed differences. Results from Levene's tests for homogeneity of variance, though a conservative test, also supported the preliminary inference drawn that the refined ISC/IERB method reduced sample variability in the N-fertilized CP treatment (Table 1). However, preliminary results also showed that the refined ISC/IERB technique did not significantly reduce sample variability for the N-unfertilized or the N-fertilized NT treatments (Table 1).
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Table 1. Summary of net N mineralization rates measured by the previous (i.e., 1996–1999) and refined (i.e., 2000) in situ soil core–ion exchange resin bag (ISC–IERB) method for the N-unfertilized chisel-plowed (CPnf) and no-tillage (NTnf) agroecosystems and N-fertilized chisel-plowed (CPf) and no-tillage (NTf) agroecosystems, with results from bootstrap, normality, and homogeneity-of-variance (i.e., Levine's) statistical tests. Mean rates of net N mineralization are accompanied by their sample size (n), standard deviations (SD), and coefficients of variation (CV).
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The sample variances for the 1997 and 2000 N-fertilized CP treatment data were similar even though the mean was higher and the coefficient of variation was smaller for the 2000 data than the 1997 data (Table 1). This result was not surprising because both precipitation and daily air temperatures during the 1-mo sampling interval following planting and fertilization in 1997 were below 30-yr averages for the Arlington Agricultural Research Station (Owenby and Ezell, 1992) (Table 2), a condition conducive to uniformly low N-mineralization rates and, thus, unusually low variability. Mean daily 10-cm soil temperatures were also the lowest in 1997 compared with the other 4 yr (Table 2). Precipitation and air temperatures for the same time period were similar and above average in 1996, 1998, 1999, and 2000. Consequently, in years where environmental conditions deviate from near optimal for N mineralization (i.e., dry and cool vs. wet and warm), as in 1997, differences in net N mineralization among tillage treatments might be more difficult to measure accurately because subnormal environmental conditions could mask treatment differences.
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Table 2. Summary of precipitation, air, and 10-cm soil temperatures for the first 1-mo sampling interval of net N mineralization following planting and fertilization from 1996 through 2000.
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From 1995 through 1998, estimates of net N mineralization rates for the first 1-mo sampling period following planting and fertilization were frequently higher for N-fertilized than N-unfertilized tillage treatments, but sample variability was often too large to infer statistical significance for differences among fertilizer rate treatments (Brye, 1999). As a result of reduced sample variability using the refined ISC/IERB technique, net N mineralization rates for the first 1-mo period following planting and fertilization in 2000 were significantly higher (p < 0.001) in the N-fertilized than N-unfertilized CP and NT treatments (Fig. 2). These preliminary results support the fertilizer N priming effect theory of Bingeman et al. (1953), who suggested that N mineralization was different in fertilized and unfertilized agricultural soils (Westerman and Kurtz, 1973; Lovell and Hatch, 1998), and the findings of Kolberg et al. (1999), who showed higher rates of net N mineralization with increasing applied fertilizer N.
The results suggest that the refined ISC/IERB field method could reduce sample variability in a N-fertilized CP agroecosystem; this was very encouraging. We expected the N-fertilized CP agroecosystem to be the better field setting to test the refined method because the plow layer is mixed annually by tilling, which reduces spatial (i.e., lateral and vertical) variability in soil organic matter and inorganic N distributions. Sample variability for the N-unfertilized NT and CP and the N-fertilized NT treatments was not significantly reduced by use of the refined ISC/IERB field method even though sample variability was often smaller in 2000 than in previous years (Table 1). The NT setting possesses the confounding effect of increasing vertical stratification of chemical properties near the soil surface as the age of the NT practice increases, which could potentially influence N mineralization processes.
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SUMMARY
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Reliable estimates of net N mineralization for the growing season are needed to improve commercial fertilizer recommendations and minimize N leaching to ground water. Critical improvements to the ISC/IERB suggested that measurement variability could be reduced for net N mineralization estimates in N-fertilized CP soil and allowed us to show that net N mineralization was significantly higher in N-fertilized than N-unfertilized NT and CP soil. Results suggest that reliable, quantitative field estimates of net N mineralization can be obtained in moist, well-mixed, N-fertilized soils using the refined ISC/IERB method, but the extra effort required may not be necessary in drier, cooler environments and may not improve the statistical significance of results in NT soils. Further methodological evaluations within the same year and over the growing season will be required.
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ACKNOWLEDGMENTS
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We wish to thank the Wisconsin Fertilized Research Council for providing the resources for this work. We also wish to thank Dr. Wesley M. Jarrell for his insightful collaboration during manuscript preparation.
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NOTES
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Research was supported by the Wisconsin Fertilizer Research Council.
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REFERENCES
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- Bingeman, C.W., J.E. Varner, and W.P. Martin. 1953. The effect of the addition of organic materials on the decomposition of an organic soil. Soil Sci. Soc. Am. Proc. 17:34–38.
- 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 S.C. Hart. 1989. The components of nitrogen availability assessment in forest soils. Adv. Soil Sci. 10:57–112.
- Binkley, D., and P. Matson. 1983. Ion exchange resin bag method for assessing soil nitrogen availability. Soil Sci. Soc. Am. J. 47:1050–1052.[Abstract/Free Full Text]
- Brye, K.R. 1999. Carbon and nitrogen budget evaluation of natural and managed ecosystems. Ph.D. diss. Univ. of Wisconsin, Madison.
- Brye, K.R., J.M. Norman, L.G. Bundy, and S.T. Gower. 1999. An equilibrium tension lysimeter for measuring drainage through soil. Soil Sci. Soc. Am. J. 63:536–542.[Abstract/Free Full Text]
- Brye, K.R., J.M. Norman, L.G. Bundy, and S.T. Gower. 2000. Water budget evaluation of prairie and maize ecosystems. Soil Sci. Soc. Am. J. 64:715–724.[Abstract/Free Full Text]
- DiStefano, J.F., and J.L. Gholz. 1986. A proposed use of ion exchanges resin to measure nitrogen mineralization and nitrification in intact soil cores. Commun. Soil Sci. Plant Anal. 17:989–998.
- Efron, B., and R.J. Tibshirani. 1993. An introduction to the bootstrap. Chapman & Hall, San Francisco.
- Eno, C. 1960. Nitrate production in the field by incubating the soil in polyethylene bags. Soil Sci. Soc. Am. Proc. 24:277–279.
- Gibson, D., I. Colquhoun, and P. Greig-Smith. 1985. A new method for measuring nutrient supply rates in soils using ion-exchange resins. p. 73–79. In A.H. Fitter (ed.) Ecological interactions in soils: Plants, microbes, and animals. Blackwell Publ., Oxford.
- Hart, S.C., J.M. Stark, E.A. Davidson, and M.K. Firestone. 1994. Nitrogen mineralization, immobilization, and nitrification. p. 985–1018. In R.W. Weaver (ed.) Methods of soil analysis. Part 2. SSSA Book Ser. 5. SSSA, Madison, WI.
- Hook, P.B., and I.C. Burke. 1995. Evaluation of methods for estimating net nitrogen mineralization in a semiarid grassland. Soil Sci. Soc. Am. J. 59:831–837.[Abstract/Free Full Text]
- Hübner, C., G. Redl, and F. Wurst. 1991. In situ methodology for studying N-mineralization in soils using anion exchange resins. Soil Biol. Biochem. 23:701–702.
- Kolberg, R.L., B. Rouppet, D.G. Westfall, and G.A. Peterson. 1997. Evaluation of an in situ net soil nitrogen mineralization method in dryland agroecosystems. Soil Sci. Soc. Am. J. 61:504–508.[Abstract/Free Full Text]
- Kolberg, R.L., D.G. Westfall, and G.A. Peterson. 1999. Influence of cropping intensity and nitrogen fertilizer rates on in situ nitrogen mineralization. Soil Sci. Soc. Am. J. 63:129–134.[Abstract/Free Full Text]
- Lachat. 1986. Ammonium in 2 M KCl soil extracts. Quikchem Method no. 12-107-06-2-A. Lachat Instruments, Milwaukee, WI.
- Lachat. 1987. Nitrate and nitrite in 2 M KCl soil extracts. Quikchem Method no. 12-107-04-1-A. Lachat Instruments, Milwaukee, WI.
- Lachat Instruments. 1993a. Ammonia (phenolate) in potable and surface water. Quikchem Method no. 10-107-06-1-A. Lachat Instruments, Milwaukee, WI.
- Lachat Instruments. 1993b. Nitrate/nitrite, nitrite in surface water, wastewaster. Quikchem Method no. 10-107-04-1-A. Lachat Instruments, Milwaukee, WI.
- Lajtha, K. 1988. The use of ion-exchange resin bags for measuring nutrient availability in an arid ecosystem. Plant Soil 105:105–111.
- Levene, H. 1960. Contributions to probability and statistics. Stanford Univ. Press, Stanford, CA.
- Lovell, R.D., and D.J. Hatch. 1998. Stimulation of microbial activity following spring applications of nitrogen. Biol. Fertil. Soils 26:28–30.
- Lui, W., E.A. Guertal, and E. Van Santen. 1999. Population differentiation, spatial variation, and sampling of tall fescue under grazing. Agron. J. 91:801–806.[Abstract/Free Full Text]
- MathSoft. 1998. S-Plus user's guide. Version 4.5. Data Analysis Products Div., MathSoft, Seattle, WA.
- Minitab. 1997. Minitab user's guide. 12th ed. Volumes 1–2. Minitab, State College, PA.
- Owenby, J.R., and D.S. Ezell. 1992. Monthly station normals of temperature, precipitation, and heating and cooling degree days 1961–90. U.S. Dep. of Commerce, Natl. Climatic Data Cent., Ashville, NC.
- Reich, R.M., C.D. Bonham, and K.K. Remington. 1993. Double sampling revisited. J. Range Manage. 46:88–90.
- Roche, C.T., D.C. Thill, and B. Shafi. 1997. Estimation of base temperatures for seed germination in common crupina (Crupina vulgaris). Weed Sci. 45:529–533.
- SAS Institute. 1990. SAS/STAT user's guide. Version 6. 4th ed. Volumes 1–2. SAS Inst., Cary, NC.
- Smethurst, P.J., and E.K.S. Nambiar. 1989. An appraisal of the in situ soil-core technique for measuring nitrogen uptake by a young Pinus radiata plantation. Soil Biol. Biochem. 21:939–942.
- Subler, S., J.M. Blair, and C.A. Edwards. 1995. Using anion-exchange membranes to measure soil nitrate availability and net nitrification. Soil Biol. Biochem. 27:911–917.
- Westerman, R.L., and L.T. Kurtz. 1973. Priming effect of 15N-labeled fertilizers on soil nitrogen in field experiments. Soil Sci. Soc. Am. Proc. 37:725–727.
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ABSTRACT
INTRODUCTION
