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Nitrogen Management

Fate of Nitrogen-15 in a Long-Term Nitrogen Rate Study

I. Interactions with Soil Nitrogen

^a Northern Plains Agric. Res. Lab., 1500 N. Central Ave., Sidney, MT 59270
^b Dep. of Crop Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
^c Dep. of Nat. Resour. and Environ. Sci., 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801

^* Corresponding author (bstevens{at}sidney.ars.usda.gov)

Received for publication September 2, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
A better understanding of how N management practices affect transformations and movement of fertilizer N may lead to more efficient N management. The objectives of this work were to determine how long-term N fertilizer history in a continuous corn (Zea mays L.) production system affects (i) movement of fertilizer N through the soil profile and (ii) cycling of fertilizer N between available and nonavailable soil forms. Nitrogen-15-labeled ammonium nitrate (¹⁵NH₄¹⁵NO₃) was applied at 0, 67, 134, 201, or 268 kg N ha^–1 to subplots of long-term N rate plots. Twenty to 55% of labeled N was converted into either organic or clay-fixed forms during the first growing season, with the percentage decreasing with increasing N application rate. Significantly more N was released from nonavailable forms in plots where the historical N application rate had exceeded the long-term optimum (186 kg ha^–1) than in plots that received lower rates. Little fertilizer-derived N leached from the profile during the first growing season, but losses did occur during the off-season and subsequent growing season when N application rate was higher than the optimum. It was concluded that a history of excessive N application may decrease response of subsequent crops to fertilizer N due to greater release from nonavailable N forms, most likely as a result of increased mineralization of crop residues and recently formed soil organic N.

Abbreviations: KMI, Kjeldahl nitrogen minus inorganic (nitrogen) • TKN, total Kjeldahl nitrogen

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
APPLICATION OF SYNTHETIC N fertilizer is a convenient, economical, and effective way to meet the N demands of modern high-yielding crops such as hybrid corn. Yet despite obvious benefits, there is evidence that N fertilizer use leads to increased levels of NO₃ in surface and ground water resources (Spalding and Exner, 1993; Randall and Mulla, 2001). Because the risk of NO₃–N loss is greater with excessive N application rates than with rates near the agronomic optimum (Jolley and Pierre, 1977), careful N management can significantly reduce the environmental risks of N fertilizer use by reducing postharvest NO₃ levels (Karlen et al., 1996). However, N application rates are difficult to optimize, owing to the uncertainty inherent in predicting net mineralization of soil organic N, which may vary greatly according to management and climatic variables.

Traditional N recommendation approaches, which typically include credits for previous legume crops, manure, and/or residual soil NO₃, have been only moderately successful at predicting N fertilizer requirements. In an Illinois study, Brown et al. (1993) compared three methods of calculating N fertilizer requirements for corn—the proven yield method described in the Illinois Agronomy Handbook (Hoeft, 2000), the preplant nitrate test, and the presidedress nitrate test (PSNT) developed by Magdoff et al. (1984). All three approaches performed less than satisfactorily in many cases when compared with actual N response for 75 site-years. Of particular interest were 12 nonresponsive site-years where recommended N application rates greatly exceeded N requirements. Because residual NO₃–N was not exceptionally high at these sites, the lack of response was attributed to unexpectedly high mineralization of organic N during the growing season.

Recent work by Mulvaney and Khan (2001) has identified serious defects in steam distillation methods of fractioning hydrolyzable soil N and has led to simple diffusion techniques that eliminate these defects. When the latter techniques were applied to some of the soil samples collected by Brown et al. (1993), a considerably higher concentration of amino sugar N was obtained for 11 nonresponsive sites than for seven responsive sites, whereas no consistent difference was observed in their concentrations of total hydrolyzable NH₄–N or amino acid N (Mulvaney et al., 2001). Subsequent work has led to a simple soil test for estimating amino sugar N as a means to detect sites where corn does not respond to N fertilization (Khan et al., 2001).

While the work of Mulvaney et al. represents a significant stride toward understanding how N cycling affects response to fertilizer N, more information is needed regarding the effects of rate and time of N application and N source on the relative contributions of mineralized N and fertilizer N to plant N uptake. Numerous studies have been conducted to evaluate the effects of cropping sequence and tillage on N cycling, among the more recent being those by Kitchen et al. (1997), Karlen et al. (1998), Wienhold and Halvorson (1999), and Ottman and Pope (2000). Fewer studies have addressed the effects of previous fertilizer N application rate on N mineralization and subsequent crop response to N fertilization. Moreover, the findings are rather difficult to interpret as in some cases, a history of heavy N fertilization has promoted net mineralization (Shen et al., 1989; Motavalli et al., 1992; Kolberg et al., 1999) while in others, the opposite effect has been observed (Lovell and Hatch, 1998; Carpenter-Boggs et al., 2000).

The use of ¹⁵N-labeled fertilizer has obvious application in long-term studies to evaluate fertilizer effects on soil N availability. This approach has been employed in long-term N rate experiments with wheat (Triticum aestivum L.; Powlson et al., 1986) and barley (Hordeum vulgare L.; Glending et al., 1997) while ¹⁵N-tracer research with corn has been limited to plots with no documented history of long-term N application rate (Kitur et al., 1984; Sanchez and Blackmer, 1988; Reddy and Reddy, 1993). Our objectives were to investigate the effects of previous fertilizer N application rate on (i) the movement of fertilizer N through the soil profile and (ii) cycling of fertilizer N between available and nonavailable soil forms.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Location
The experiment was conducted on plots of a long-term N fertilizer rate study at the University of Illinois Northwest Research and Education Center located near Monmouth, IL. Urea had been applied annually to plots at rates of 0, 67, 134, 201, or 268 kg N ha^–1, such that any given plot received the same application rate in every year since the establishment of the continuous corn study in 1983. Soil within the plot area is classified as a moderately permeable Muscatine silt loam (fine-silty, mixed, mesic Aquic Hapludolls) formed in a deep upland loess deposit on nearly level topography. The soil is classified as somewhat poorly drained, but no tile drainage exists within the plot area.

Treatment Application and Cultural Practices
Experimental units of the original long-term N rate study were 6.1 by 18.3 m in size and consisted of eight corn rows spaced 75 cm apart. To facilitate the establishment of microplots for ¹⁵N application without eliminating the main-plot harvest area, each experimental unit was divided into two 3.05- by 18.3-m areas, one of which was used for collection of grain yield data (main harvest area) and the other for establishment of 2.3- by 3.05-m ¹⁵N microplots. By the end of the 3-yr study, each experimental unit contained three microplots, a new microplot being established in each year (Fig. 1) . Urea was applied to the main-plot harvest area and immediately incorporated by tillage. Hybrid corn (DK 623, Dekalb Genetics Corp., DeKalb, IL) was then planted using a four-row tractor-drawn planter at a rate of 75500 seeds ha^–1. Just after planting, ¹⁵NH₄¹⁵NO₃ was applied to the microplot and unlabeled NH₄NO₃ to the remainder of the 3.05- by 18.3-m area that contained the microplots. To achieve a uniform application of labeled N, 2 L of ¹⁵NH₄¹⁵NO₃ solution was applied to microplot areas using a CO₂–pressurized spray boom, which spanned the entire width of the microplot. The boom was manually moved at a speed sufficient to travel the length of the plot 6 to 10 times before the solution was completely drained. A 3.1 atom% ¹⁵NH₄¹⁵NO₃ solution was prepared for each experimental unit by dissolving an enriched stock reagent (10 atom% ¹⁵N, Icon Services, Summit, NJ) and unlabeled reagent grade NH₄NO₃ (0.3663 atom% ¹⁵N) in 2 L of distilled water in amounts that resulted in an N application rate equal to that of the main-plot area.

View larger version (24K): [in this window] [in a new window]   Fig. 1. Location of 15N microplots and harvest area within main plots of the long-term N rate study at Monmouth, IL.

Fall tillage consisted of one chisel plow operation after all harvest and sampling operations were completed. Care was taken during the tillage operation to prevent excessive movement of soil and crop residues within the plot area.

Sample Collection
Soil samples for determination of initial chemical and physical characteristics were collected in the spring before application of N fertilizer. Following application of labeled N, soil samples were collected from a 1.5- by 1.5-m area within the microplots in the fall soon after crop harvest. Microplots were also sampled in the spring and fall of the second and third growing seasons following ¹⁵N application until the termination of the study in the fall of 1996. Soil cores were extracted from a depth of 120 cm and divided into five depth increments (0 to 15, 15 to 30, 30 to 60, 60 to 90, and 90 to 120 cm). Three cores were extracted from each microplot at each sampling time using either a 1.9-cm-diam. hand probe or a Giddings hydraulic soil probe equipped with a 3.5-cm-diam. sampling tube. Samples were maintained at field moisture and frozen until analysis.

Sample Analysis
Initial levels of total C and total N were determined on air-dried soil samples using a LECO CNS 2000 combustion analyzer (LECO Corp., St. Joseph, MI). Soil concentrations of available P and K were determined using the Bray P1 (Kuo, 1996) and the NH₄C₂H₃O₂ (Helmke and Sparks, 1996) methods, respectively. Soil pH was determined from a 2:1 mixture of deionized water and soil, using a pH meter equipped with a glass combination pH electrode.

Field-moist soil samples collected before ¹⁵N application were thawed and then wet-sieved to pass through a 2-mm screen. Ammonium N and NO₃–N concentrations were determined by extracting a portion of the sample with 2 M KCl using a 1:10 mass ratio of soil to KCl solution (Mulvaney, 1996). A 15-mL aliquot of the KCl extract was analyzed for NH₄–N and (NO₃ + NO₂)-N by steam distillation (Mulvaney, 1996). Labeled soil samples were prepared and extracted in the same way as were nonlabeled samples, with additional precautions to prevent cross-contamination between samples. To collect an adequate amount of N (50 µg N) to allow reliable N isotope analyses, a 50-mL aliquot of each ¹⁵N-labeled KCl extract was analyzed for total mineral N (NH₄ + NO₃ + NO₂) using mason-jar diffusion techniques (Mulvaney et al., 1997). Total N in ¹⁵N-labeled soil samples was determined by digesting approximately 1 g of field-moist soil using the reduced Fe-permanganate modification of the semimicro Kjeldahl procedure described by Bremner (1996). After the digests had cooled, each was diluted to a volume of 25 mL with deionized water and mixed. A 10-mL aliquot of each diluted digest was then analyzed for total Kjeldahl N (TKN) using mason-jar diffusion techniques (Stevens et al., 2000). The NH₃ liberated by diffusion of soil extracts or TKN digests was quantified using an automated titrator (Model 719 S Titrino, Metrohm, Herisau, Switzerland) equipped with a microelectrode (Model MI-411 S, Microelectrodes, Inc., Bedford, NH). Finally, 50 to 200 µg of N from each sample was transferred to a plastic microplate, dried, and analyzed for ¹⁵N using a mass spectrometer equipped with an automated Rittenberg apparatus (Mulvaney, 1993). Nonmineral N, which includes both organic N and nonexchangeable NH₄–N, was calculated by subtracting mineral (inorganic) N from TKN. The resulting fraction is referred to hereafter as KMI (Kjeldahl minus inorganic) N as per Sanchez and Blackmer (1988).

Experimental Design and Statistical Analysis
The experiment was conducted as a randomized complete block design with three replications. Statistical analysis was accomplished using the SAS GLM procedure (SAS Inst., 1998). Data collected from multiple soil depths within a given experimental unit were considered repeated measures and were analyzed as a split-plot arrangement of a randomized complete block design with N application rate as the main plot and depth as the split plot (Gomez and Gomez, 1984). Differences among treatment means were separated using the Fisher's least significant difference procedure.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Conditions Before ¹⁵N Application
The amount of total mineral N (NH₄ + NO₃ + NO₂) in the surface 30 cm of the soil profile before application of labeled N did not vary significantly with N fertilizer application rate within any of the 3 yr (Fig. 2) . In 1995 and 1996, significantly more mineral N was detected in the lower profile (60 to 120 cm) of plots receiving 201 or 268 kg N ha^–1 than at the same depth in plots receiving the lower application rates. No such difference was observed in 1994, presumably because unusually wet weather in 1993 (Fig. 3) had flushed residual NO₃–N from the profile. The relatively high NO₃–N concentrations of 7 to 13 mg kg^–1 at the 90- to 120-cm depth in 1995 and 1996 indicate there was some downward movement of NO₃–N, but total precipitation was inadequate to remove excess residual N from plots receiving the higher amounts of N fertilizer. Long-term N fertilizer treatment had no significant effect on total N content (Table 1).

View larger version (23K): [in this window] [in a new window]   Fig. 2. Distribution of total mineral N (NH4 + NO3 + NO2) by soil depth before spring N application. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (43K): [in this window] [in a new window]   Fig. 3. (a) Monthly average precipitation (crosshatched bars) and (b) monthly average air temperature (solid bars) recorded at the University of Illinois Northwest Research Center during the year preceding (1993) and 3 yr during which the experiment was conducted (1994–1996). The solid lines show the 100-yr average (a) precipitation and (b) air temperature for Monmouth, IL

First Growing Season after ¹⁵N Application
In each of the 3 yr, little fertilizer-derived mineral N remained in the 120-cm profile following harvest when the long-term N fertilizer application rate was either 67 or 134 kg N ha^–1 (Table 2). Significantly more labeled mineral N was recovered from plots receiving the two highest N treatments, with 11.4 to 15.3 kg N ha^–1 recovered from plots receiving 201 kg N ha^–1 and 23.2 to 43.9 kg N ha^–1 recovered from those receiving 268 kg N ha^–1. This same trend is evident in total mineral N data. With the exception of 1996, total mineral N content did not differ significantly among plots receiving 0, 67, or 134 kg N ha^–1 (Table 2). However, more residual mineral N was detected in plots receiving 201 or 268 kg N ha^–1 than in those receiving a lower N rate.

A more likely explanation is that plots with a history of high N fertilizer application accumulated a larger pool of easily released KMI-N than plots with a history of suboptimal N fertilization. Both organic-matter (Shen et al., 1989) and clay-fixed N pools (Mengel and Scherer, 1981; Norman and Gilmour, 1987) have been shown to release significant amounts of N to the plant-available pool during the growing season. While our data do not allow the separation of residual mineral N into organic-matter- or clay-derived fractions, other researchers have shown evidence that the amount of N fertilizer applied in previous years significantly affects net mineralization of organic N compounds in subsequent growing seasons (Motavalli et al., 1992; Shen et al., 1989). Conversely, Li et al. (1990) observed that release of clay-fixed NH₄–N was the same whether a prior urea application was 0 or 70 kg N ha^–1. Thus, it is likely that variation in the release of soil-derived N in our study is primarily the result of differences in organic N mineralization.

It is evident from the postharvest distribution of mineral N within the 120-cm soil profile that downward movement did occur (Fig. 4) . No labeled mineral N was detected at the 90- to 120-cm depth during the exceptionally favorable growing season of 1994, but trace amounts were detected in 1995 and 1996 with N fertilizer application rates of 201 or 268 kg N ha^–1. Regardless of N rate, trace amounts of fertilizer-derived KMI-N were also detected at profile depths of 60 to 90 cm and 90 to 120 cm, but the majority of the fertilizer-derived KMI-N was recovered from the top 15 cm of plots receiving 67 or 134 kg N ha^–1 and from the top 30 cm of plots receiving 201 or 268 kg N ha^–1 (data not shown). This suggests that, although some movement of fertilizer-derived N deep into the root zone may have occurred, it is unlikely that significant amounts of labeled N were lost from the rooting zone through leaching during the first growing season. Conversely, with the exception of 1994 when growing conditions were exceptional, significant amounts of total mineral N were detected throughout the profile of the plots receiving 201 and 268 kg N ha^–1 (Fig. 5) , indicating that significant amounts of soil-derived NO₃–N, which likely includes both native soil N and N from previous fertilizer applications, were leached from the rooting zone.

View larger version (19K): [in this window] [in a new window]   Fig. 4. Distribution of fertilizer-derived mineral N (NH4 + NO3 + NO2) by soil depth following crop harvest. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (23K): [in this window] [in a new window]   Fig. 5. Distribution of total mineral N (NH4 + NO3 + NO2) by soil depth following crop harvest. Each data point represents a mean mineral N concentration for the corresponding soil depth increment (0 to 15, 15 to 30, 30 to 60, 60 to 90, or 90 to 120 cm). Error bars show the LSD (P < 0.05) for mean comparisons within a depth increment. NS indicates differences were not significant.

View larger version (35K): [in this window] [in a new window]   Fig. 6. Recovery of labeled fertilizer N from mineral and Kjeldahl N minus inorganic (KMI) N fractions of a 1.2-m soil profile following a 1994 application of 15NH415NO3. Lowercase and uppercase letters are for comparing (P < 0.05) KMI-N and mineral N means, respectively, within a sampling event. Least significant differences (P < 0.05) for comparing mineral N and KMI-N means within a N treatment and across sampling dates are 4 and 13 kg ha–1, respectively. Parenthetical values are standard deviations for their corresponding means.

View larger version (35K): [in this window] [in a new window]   Fig. 7. Recovery of labeled fertilizer N from mineral and Kjeldahl N minus inorganic (KMI) N fractions of a 1.2-m soil profile following a 1995 application of 15NH415NO3. Lowercase and uppercase letters are for comparing (P < 0.05) KMI-N and mineral N means, respectively, within a sampling event. Least significant differences (P < 0.05) for comparing mineral N and KMI-N means within a N treatment and across sampling dates are 6 and 14 kg ha–1, respectively. Parenthetical values are standard deviations for their corresponding means.

Third Growing Season after ¹⁵N Application
In the spring 2 yr following the application of labeled fertilizer N, only trace amounts of labeled N were present in the mineral form, and no significant differences occurred among the four treatment levels (Fig. 6c). These data suggest that residual fertilizer-derived mineral N present in the profile the previous fall (Fig. 6b) had either been flushed from the soil profile by off-season precipitation or immobilized into organic or clay-fixed forms. There were no significant changes in the amount of labeled KMI-N from the fall of the second year to the spring of the third year, except for with the 268 kg ha^–1 application rate, which decreased from 74 to 49 kg N ha^–1 (Fig. 6b and 6c), presumably owing to mineralization and subsequent loss between the two sampling dates. These differences were not as great as those observed in the 1994–1995 off-season (Fig. 6a and 6b) when climatic conditions were more favorable for N mineralization and loss (Fig. 3). Similar to observations in the previous fall, plots receiving 268 kg N ha^–1 contained the most labeled KMI-N in the spring of the third year, followed by those receiving 201 kg N ha^–1, while the plots receiving 67 or 134 kg N ha^–1 contained the least. While differences among treatments were less clear in the fall of the third yr, the trend in labeled KMI-N content was similar to that observed in the preceding spring (Fig. 6c). This is not surprising given the low concentration of labeled N in the previous year's crop residues. The amount of labeled organic N mineralized was likely less than in the previous growing season because of the stabilization of organic N with time (Shen et al., 1989).

The pattern in labeled mineral N contents across N application rates was similar in the fall of the third year (Fig. 6c) as in the fall of second year (Fig. 6b) though concentrations were less. There was still significantly more labeled mineral N present with the 268 and 201 kg N ha^–1 application rates (3.4 and 2.4 kg ha^–1, respectively) than with the 67 or 134 kg ha^–1 application rates (<1.0 kg ha^–1). Thus, even in the third growing season after the application of labeled fertilizer N, a higher percentage of labeled N was being mineralized with the two highest long-term N fertilizer application rates than with the two lowest rates.

	SUMMARY AND CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 
Movement of mineral N to the lower depths of the soil profile was significantly greater in cases involving excessive application of fertilizer N. Very little of the fertilizer-derived N moved to the 120-cm depth during the first growing season, but by the spring of the second growing season, there were indications that fertilizer N was being leached from the profile, especially when N applications exceeded plant uptake.

Results also provide evidence that overapplication of fertilizer N on a consistent, long-term basis may lead to an accumulation of readily mineralizable soil organic N compounds in continuous corn production systems. After one growing season, profile concentrations of total mineral N were consistently greater with the two highest treatment levels than in the profiles of the lower treatments. As expected, the amount of fertilizer-derived mineral N in the profile also increased significantly with increasing N application rate, but more than twice as much soil-derived mineral N was present in the profile when the N application rate was 201 or 268 kg ha^–1 than when it was 134 kg ha^–1 or less. In the fall of both the second and third growing seasons, very little labeled mineral N was detected in the soil profile when the N application rate was 67 or 135 kg ha^–1, but significant amounts were still present when the N application rate was 201 or 268 kg ha^–1. By the third growing season, these differences had become quite small, indicating that the labeled portion of the organic N pool had begun to stabilize and become more resistant to mineralization. These data suggest that mineralization of residual organic N is promoted when N fertilizer application rates consistently exceed the optimum N application rate on a long-term basis.

These findings emphasize the environmental risk associated with excessive N fertilization and underscore the need for better predictions of N mineralization. In light of the recent work by Mulvaney et al. (2001) and Khan et al. (2001) that potentially mineralizable N may be estimated by a simple soil test for amino sugar N, further research is warranted to determine whether long-term N applications promote the accumulation of this N fraction and thereby reduce crop responsiveness to subsequent N fertilizer applications.

	ACKNOWLEDGMENTS

 
Financial support was provided in part by The Fertilizer Research and Education Committee (FREC) and the Illinois Groundwater Consortium. We gratefully acknowledge Dr. Eric Adee and staff of the University of Illinois Northwest Research and Education Center for field plot maintenance and Jeff Warren and Lisa Gonzini for assistance in collecting and analyzing samples.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION SUMMARY AND CONCLUSIONS REFERENCES

 

• Bremner, J.M. 1996. Nitrogen—total. p. 1085–1121. In D.L. Sparks (ed.) Methods of soil analysis: Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Brown, H.M., R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. p. 43–49. In R.G. Hoeft (ed.) Proc. Illinois Fert. Conf., Peoria, IL. 25–27 Jan. 1993. Coop. Ext. Serv., Univ. of Illinois, Urbana-Champaign.
• Carpenter-Boggs, L., J.L. Pikul, M.F. Vigil, and W.E. Riedell. 2000. Soil nitrogen mineralization influenced by crop rotation and nitrogen fertilization. Soil Sci. Soc. Am. J. 64:2038–2045.[Abstract/Free Full Text]
• Glending, M.J., P.R. Poulton, D.S. Powlson, and D.S. Jenkinson. 1997. Fate of ¹⁵N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status. Plant Soil 195:83–98.
• Gomez, K.A., and A.A. Gomez. 1984. Statistical procedures for agricultural research. 2nd ed. John Wiley & Sons, New York.
• Helmke, P.A., and D.L. Sparks. 1996. Lithium, sodium, potassium, rubidium, and cesium. p. 551–574. In D.L. Sparks (ed.) Methods of soil analysis: Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Hoeft, R.G. 2000. Soil testing and fertility. p. 84–124. In J. Atkinson (ed.) Illinois agronomy handbook 2001–2002. Univ. of Illinois, Urbana-Champaign.
• Jenkinson, D.S., R.H. Fox, and J.H. Rayner. 1985. Interactions between fertilizer nitrogen and soil nitrogen—the so-called ‘priming’ effect. J. Soil Sci. 36:425–444.[CrossRef]
• Jolley, V.D., and W.H. Pierre. 1977. Profile accumulation of fertilizer-derived nitrate and total nitrogen recovery in two long-term nitrogen-rate experiments with corn. Soil Sci. Soc. Am. J. 41:373–378.[Abstract/Free Full Text]
• Karlen, D.L., P.G. Hunt, and T.A. Matheny. 1996. Fertilizer ¹⁵nitrogen recovery by corn, wheat, and cotton grown with and without pre-plant tillage on Norfolk loamy sand. Crop Sci. 36:975–981.[Abstract/Free Full Text]
• Karlen, D.L., L.A. Kramer, and S.D. Logsdon. 1998. Field-scale nitrogen balance associated with long-term continuous corn production. Agron. J. 90:644–650.[Abstract/Free Full Text]
• Khan, S.A., R.L. Mulvaney, and R.G. Hoeft. 2001. A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Sci. Soc. Am. J. 65:1751–1760.[Abstract/Free Full Text]
• Kitchen, N.R., P.E. Blanchard, D.F. Hughes, and R.N. Lerch. 1997. Impact of historical and current farming systems on groundwater nitrate in northern Missouri. J. Soil Water Conserv. 52:272–277.
• Kitur, B.K., M.S. Smith, R.L. Blevins, and W.W. Frye. 1984. Fate of ¹⁵N depleted ammonium nitrate applied to no-tillage and conventional tillage corn. Agron. J. 76:240–242.
• 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]
• Kuo, S. 1996. Phosphorus. p. 869–919. In D.L. Sparks (ed.) Methods of soil analysis: Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Li, C., X. Fan, and K. Mengel. 1990. Turnover of interlayer ammonium in loess-derived soil grown with winter wheat in the Shaanxi Province of China. Biol. Fertil. Soils 9:211–214.
• Lovell, R.D., and D.J. Hatch. 1998. Stimulation of microbial activity following spring applications of nitrogen. Biol. Fertil. Soils 26:28–30.
• Magdoff, F.R., D. Ross, and J. Amadon. 1984. A soil test for nitrogen availability to corn. Soil Sci. Soc. Am. J. 48:1301–1304.[Abstract/Free Full Text]
• Mengel, K., and H.W. Scherer. 1981. Release of nonexchangeable (fixed) soil ammonium under field conditions during the growing season. Soil Sci. 131:226–232.
• Motavalli, P.P., L.G. Bundy, W.W. Andraski, and A.E. Peterson. 1992. Residual effects of long-term nitrogen fertilization on nitrogen availability to corn. J. Prod. Agric. 5:363–368.
• Mulvaney, R.L. 1993. Mass spectrometry. p. 11–57. In R. Knowles and T.H. Blackburn (ed.) Nitrogen isotope techniques. Academic Press, San Diego, CA.
• Mulvaney, R.L. 1996. Nitrogen—inorganic forms. p. 1123–1184. In D.L. Sparks (ed.) Methods of soil analysis: Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Mulvaney, R.L., and S.A. Khan. 2001. Diffusion methods to determine different forms of nitrogen in soil hydrolysates. Soil Sci. Soc. Am. J. 65:1284–1292.[Abstract/Free Full Text]
• Mulvaney, R.L., S.A. Khan, R.G. Hoeft, and H.M. Brown. 2001. A soil organic nitrogen fraction that reduces the need for nitrogen fertilization. Soil Sci. Soc. Am. J. 65:1164–1172.[Abstract/Free Full Text]
• Mulvaney, R.L., S.A. Khan, W.B. Stevens, and C.S. Mulvaney. 1997. Improved diffusion methods for determination of inorganic nitrogen in soil extracts and water. Biol. Fertil. Soils 24:413–420.
• Norman, R.J., and J.T. Gilmour. 1987. Utilization of anhydrous ammonia fixed by clay minerals and soil organic matter. Soil Sci. Soc. Am. J. 51:959–962.[Abstract/Free Full Text]
• Ottman, M.J., and N.V. Pope. 2000. Nitrogen fertilizer movement in the soil as influenced by nitrogen rate and timing in irrigated wheat. Soil Sci. Soc. Am. J. 64:1883–1892.[Abstract/Free Full Text]
• Powlson, D.S., B. Pruden, A.E. Johnston, and D.S. Jenkinson. 1986. The nitrogen cycle in the Broadbalk wheat experiment—recovery and losses of ¹⁵N-labeled fertilizer applied in spring and inputs of nitrogen from the atmosphere. J. Agric. Sci. 107:591–609.
• Randall, G.W., and D.J. Mulla. 2001. Nitrate nitrogen in surface waters as influenced by climatic conditions and agricultural practices. J. Environ. Qual. 30:337–344.[Abstract/Free Full Text]
• Reddy, G.B., and K.R. Reddy. 1993. Fate of nitrogen-15 enriched ammonium nitrate applied to corn. Soil Sci. Soc. Am. J. 57:111–115.[Abstract/Free Full Text]
• Sanchez, C.A., and A.M. Blackmer. 1988. Recovery of anhydrous ammonia-derived nitrogen 15 during three years of corn production in Iowa. Agron. J. 80:102–108.[Abstract/Free Full Text]
• SAS Institute. 1998. SAS user's guide: Statistics. 5th ed. SAS Inst., Inc., Cary, NC.
• Shen, S.M., P.B.S. Hart, D.S. Powlson, and D.S. Jenkinson. 1989. The nitrogen cycle in the Broadbalk wheat experiment: ¹⁵N-labelled fertilizer residues in the soil and in the soil microbial biomass. Soil Biol. Biochem. 21:529–533.[CrossRef]
• Spalding, R.F., and M.E. Exner. 1993. Occurrence of nitrate in groundwater: A review. J. Environ. Qual. 22:392–402.[Abstract/Free Full Text]
• Stevens, W.B., R.G. Hoeft, and R.L. Mulvaney. 1997. Effect of N fertilization on accumulation and release of readily-mineralizable organic N. p. 65–78. In R.G. Hoeft (ed.) Proc. Illinois Fert. Conf., Peoria, IL. 27–29 Jan. 1997. Coop. Ext. Serv., Univ. of Illinois, Urbana-Champaign.
• Stevens, W.B., R.L. Mulvaney, S.A. Khan, and R.G. Hoeft. 2000. Improved diffusion methods for nitrogen and ¹⁵nitrogen analysis of Kjeldahl digests. J. AOAC Int. 83:1039–1046.[ISI][Medline]
• Wienhold, B.J., and A.D. Halvorson. 1999. Nitrogen mineralization responses to cropping, tillage, and nitrogen rate in the northern Great Plains. Soil Sci. Soc. Am. J. 63:192–196.[Abstract/Free Full Text]

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