SSSAJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.
Soil Science Society of America Journal 64:1883-1892 (2000)
© 2000 Soil Science Society of America

DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Nitrogen Fertilizer Movement in the Soil as Influenced by Nitrogen Rate and Timing in Irrigated Wheat

Michael J. Ottmana and Nancey V. Popeb

a Plant Sciences Dep., Univ. of Arizona, Tucson, AZ 85721 USA
b 951 East 200 North, Provo, UT 84602 USA

mottman{at}ag.arizona.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Nitrogen fertilizer is a potential contaminant of groundwater supplies. The purpose of this study was to determine the influence of recommended N fertilizer rate and timing on N movement in the soil during the growing season. Durum wheat [Triticum turgidum L. subsp. durum (Desf.) Husn.] was grown at Maricopa, AZ, during the 1991 and 1992 growing seasons. A N rate study was conducted at two sites on a sandy loam soil [coarse loamy, mixed (calcareous), hyperthermic, Typic Natrargid (reclaimed)] and clay loam soil [fine loamy, mixed (calcareous), hyperthermic, Typic Torrifluvent] using 15N-labeled (NH4)2SO4 and Br- tracer. Three N rates that ranged from 5.4 to 10.1 g N m-2 for the less than recommended rate, 18.5 to 22.5 g N m-2 for the recommended rate and 28.0 to 37.8 g N m-2 for the greater than recommended rate were applied in split applications. The experimental design was a randomized complete block with six replications and three N rates. A N timing study was conducted on the sandy loam soil at the recommended N rate where 15N and Br- were applied at only one of the application times and nonlabeled N fertilizer was applied at the other times. The experimental design for the N timing study was a randomized complete block with six replications and four (1992) or five (1991) application times. Surface flood irrigation was applied in excess of soil water depletion (top 1.5 m), varying with year, soil type, and N rate. After harvest, the soil was sampled to a depth of 2.4 m and analyzed for 15N and Br-. Nitrogen rate had no influence on 15N fertilizer or Br- movement in the soil. Nitrogen rate increased the N content of the surface soil, but most of this N was not in NO3 form. In most cases, the median depth of movement of recovered 15N for all N rates was 0.23 m compared to 1.13 m for Br-. Timing of applications did not influence N fertilizer movement. Bromide overestimated the depth of 15N movement recovered in the soil possibly due to plant uptake and immobilization of N in the surface soil. We found that for irrigated wheat in Arizona, most of the N fertilizer recovered in the top 2.4 m of soil was in the surface soil, regardless of N fertilizer practices.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
NITRATE IN GROUNDWATER can originate from many sources, but N fertilizer and mineralization of soil organic N are regarded as primary components in NO3 contamination of groundwater supplies (Keeney, 1989). Nitrate leaching in irrigated agriculture has been documented for many crops (Pratt, 1984; Ritter, 1989) and for wheat in particular (Theocharopoulos, 1993). Nitrate leaching in irrigated agriculture is assumed to be an inevitable result of the relatively high N fertilizer rates applied and the need to periodically leach salts. Irrigated agriculture presently produces 33% of the world's food supply and is estimated to produce 50% by the year 2025 due to expansion of irrigation in developing countries (Pereira et al., 1996).

Nitrate movement in the soil and potential for leaching can be affected by many factors, such as crop N uptake dynamics, N fertilizer management, rainfall, irrigation management, soil texture, and N transformations in the soil. However, nitrogen rate and timing are two factors influencing the potential for N leaching that can be controlled by the grower. Conceptually, it seems that greater than recommended N fertilizer rates would lead to increased NO3 leaching potential and that low N fertilizer rates would result in the least potential for NO3 leaching, but this is not always the case. Nitrogen fertilizer rate has been shown to increase postharvest NO3 content of the soil in several studies (Chaney, 1990; Alcoz et al., 1993; Ayoub et al., 1995; Pilbeam et al., 1997), but the postharvest NO3 seems to be confined primarily to the plow layer. The increase in postharvest soil NO3 is greatest when the N fertilizer application rate exceeds that needed for optimum yields. In the study of Campbell et al. (1993), however, more NO3 leaching occurred in the low than the optimum N rate since more water leached in the low N treatment due to lower evapotranspiration. In other studies, N rates above optimum had little or no effect on soil inorganic N at harvest (MacDonald et al., 1989; Raun and Johnson, 1995; Porter et al., 1996) since greater than recommended N fertilization resulted in increased crop uptake and plant N volatilization, and incorporation of the residue and immobilization kept the N in organic form in the surface soil. Similar results have been obtained with several 15N studies with wheat, where most of the fertilizer remaining in the soil after one season was in the top 25 cm of soil in organic form (Powlson et al., 1986; Smith and Whitfield, 1990). Raun and Johnson (1995) found that N rates in excess of that required for maximum yield did not affect soil inorganic N until the soil–plant buffer for soil inorganic N was exceeded, at which point soil inorganic N would increase.

Best management practices for N fertilization usually contain a generic statement that N fertilizer be applied in split applications or timed according to crop needs. Nitrogen uptake efficiency has been shown to be greatest with split N applications to wheat (Alcoz et al., 1993; Sowers et al., 1994; Ayoub et al., 1995), and split applications presumably lower the potential for NO3 leaching. However, in other studies, timing of N application had no effect on residual N amount or distribution in the soil (Bronson et al., 1991; Boman et al., 1995).

Legislation has been enacted in Arizona, as in other states, to protect groundwater quality. All farmers in Arizona must follow Best Management Practices that include using soil and plant tissue testing as a guide to N fertilization. The purpose of this research was (i) to determine the potential for N leaching in irrigated wheat if fertilizer is applied according to soil and plant tissue testing guidelines, and if less than recommended or greater than recommended rates are applied, and (ii) to determine which N applications during the growing season are most susceptible to NO3 leaching.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Nitrogen rate and timing experiments were conducted at the University of Arizona's Maricopa Agricultural Center during the 1991 and 1992 growing seasons. The N rate experiment was conducted on two soils each year: (i) Casa Grande sandy loam [coarse loamy, mixed (calcareous), hyperthermic, Typic Natrargid (reclaimed)] and (ii) Trix clay loam [fine loamy, mixed (calcareous), hyperthermic, Typic Torrifluvent]. The N timing experiment was conducted on the Casa Grande sandy loam.

Sudangrass [Sorghum bicolor (L.) Moench subsp. drummondii (Nees ex Steud.) de Wet & Harlan] was grown each year prior to the establishment of the wheat crop to provide nutrient uniformity within the soil, especially with respect to N. The durum wheat cultivar Aldura was drilled in 15-cm rows at a rate of 15 g seed m-2 on 28 Nov. 1990 and on 22 Nov. 1991. Triple super phosphate (0-45-0) was applied at planting each year at a rate of 1.8 g P m-2, a relatively high rate and adequate for soils low in P. The soil was not tested for P before planting, but after harvest in 1992, both soil types contained 3 mg CO2-extractable P kg-1, which is considered a high soil P level. Weather data were provided by an automated weather station {approx}0.5 km from the sandy loam site and 1.3 km from the clay loam site. Monthly mean maximum and minimum temperature and precipitation for the two growing seasons are compared to the long-term mean in Table 1 .


View this table:
[in this window]
[in a new window]
  		Table 1 Mean monthly maximum temperature, minimum temperature, and precipitation at Maricopa for the 1990–1991 and 1991–1992 growing seasons compared with the 38-yr mean from 1961 to 1998

 
For the N rate experiment, N fertilizer was applied at less than recommended, recommended, and greater than recommended rates in split applications at planting, 5 to 6 leaf, once or twice between jointing and boot, and near anthesis (Tables 2 and 3) . The less than recommended and greater than recommended N rates averaged 36 and 154% of the recommended rate, respectively, but varied somewhat depending on year and soil type. The recommended N fertilizer rate was based on preplant soil NO3 for the preplant N applications and in-season lower stem NO3 samples for the in-season N applications (Doerge et al., 1991). The preplant soil sample was a composite of 40 cores (1.9-cm diam.) for each soil type to the recommended sampling depth of 15 cm (Doerge et al., 1991). The NO3 content in the soil profile was not characterized at planting since 15N was used as an indicator of fertilizer N movement in this study, and not differences between initial and final soil NO3 content. The soil was air-dried and ground to pass through a 1-mm sieve. Nitrate was extracted from the soil with deionized water and analyzed using a NO3 electrode (Keeney and Nelson, 1982). Preplant soil NO3 concentration was 2.8 mg NO3–N kg-1 for the sandy loam soil in 1991, 3.0 mg NO3–N kg-1 for the clay loam soil in 1991, 1.0 mg NO3–N kg-1 for the sandy loam soil in 1992, and 3.5 mg NO3–N kg-1 for the clay loam soil in 1992. The lower stem samples were obtained from the 1- to 2-cm portion of the plant between the crown and the soil surface at the 3- to 4-leaf stage and from the lower 5 cm of the aboveground stem at all other stages. The lower portion of the wheat stem was sampled before applications for each N rate, oven-dried at 65°C, ground to pass through a 550-µm mesh screen, extracted with a solution containing Al2(SO4)3, and analyzed for NO3–N content using a NO3 electrode (Milham et al., 1970). The in-season lower stem NO3 values are presented in Table 4 .


View this table:
[in this window]
[in a new window]
  		Table 2 Irrigation and 15N fertilizer application schedule for 1991

 

View this table:
[in this window]
[in a new window]
  		Table 3 Irrigation and 15N fertilizer application schedule for 1992

 

View this table:
[in this window]
[in a new window]
  		Table 4 Lower stem NO3 concentration as affected by N rate

 
The experimental design for the N rate test was a randomized complete block consisting of three N rates (less than recommended, recommended, and greater than recommended) and six replications. The main plots were 3.7 by 15.2 m in size. Microplots (1 by 1 m) within main plots received labeled N fertilizer (15N) as a tracer of N fertilizer movement. Labeled N fertilizer in the form of (NH4)2SO4 with a 5% enrichment of 15N was applied to the microplots at the same rates and times as the main plots. The microplots also received a one-time application of KBr at planting at a rate of 11.3 g Br- m-2. Bromide was applied to indicate the worst case scenario for NO3 movement and could be used as such since the native Br- content of soil and water is low, Br- is not transformed biologically or chemically, and Br- is not sorbed by the soil (Bowman, 1984). Labeled N and KBr were applied to the microplots as a solution with an automatic dispenser. At planting, 1 mL was dispensed in 5 by 5 cm squares in the 1 by 1 m microplots. At fertilizer applications after planting, 1 mL was dispensed between rows in 5 by 15 cm squares, with 15 cm representing the distance between rows. Unlabeled (NH4)2SO4 fertilizer (21-0-0) was broadcast by hand on the main plots at each fertilizer application date. The microplots were covered with plastic sheets whenever the unlabeled fertilizer was applied to the main plots.

The effect of N timing on N movement in the soil was studied by applying labeled N at only one of the four to five application times and applying unlabeled N fertilizer at the other times. This was accomplished by establishing additional microplots within the N rate study on the sandy loam soil. At each of the fertilizer application times (see Tables 2 and 3), a microplot was established in the main plots receiving recommended N. These microplots received the recommended rate of 15N-enriched fertilizer along with 11.3 g Br- m-2 as KBr at a single application time only, and received unlabeled fertilizer and no KBr at all other application times. The experimental design was a randomized complete block with five application times in 1991, four application times in 1992, and six replications in both seasons.

Soil water levels were monitored using a neutron probe (Model 503 DR, CPN Corp, Martinez, CA) to a depth of 1.5 m. A depth of 1.5 m was chosen since Erie et al. (1965) found this was the depth where 95% of the soil water depletion occurred for wheat in their study of consumptive water use patterns for crops in Arizona. Irrigation water was applied as dictated by the N fertilizer application schedule or when {approx}50 to 60% of the plant-available water was depleted in the top 1.5 m of soil. Irrigation water was applied using the surface flood method where water was applied to a rectangular, flat basin surrounded by earthen dikes. Irrigation water was introduced at one end of a rectangular basin and was shut off when the water had advanced the length of the field. The corresponding irrigation amounts are presented in Tables 2 and 3. The clay loam soil received more irrigation water than the sandy loam soil since, for the clay loam soil, the field length was longer (increasing irrigation time) and the initial infiltration rate was faster due to soil cracks (decreasing water advance rate). Volume of irrigation water applied was measured with a weir for the clay loam soil and with a meter for the sandy loam soil. The amount of irrigation water applied was the minimum that the surface flood irrigation system would allow, but this amount of water was often in excess of the soil water deficit. Irrigation water applied in excess of the soil water deficit in the top 1.5 m of soil was calculated.

The crop was harvested on 4 June in 1991 and 19 May in 1992. Entire plants were removed from 0.6 by 0.6 m areas within each microplot. A plant sample was also removed outside the plot area for background 15N calculation. The plants were dried at {approx}60°C, weighed, the number of heads was counted, and grain was threshed from the heads. Harvest index was calculated by dividing grain yield by total yield. Kernel weight was determined for 500 kernels. The number of kernels per spike was determined arithmetically from grain yield, weight per kernel, and spikes per unit area. Hard vitreous amber count was estimated from 100 kernels.

Soil was sampled after harvest from a 5-cm-diam. auger hole in the center of each microplot and in two areas outside the plots where the wheat was not fertilized for background 15N determination. The soil was sampled in 0.3-m increments to a depth of 2.4 m, well below our estimated active rooting zone of 1.5 m. Soil samples were air-dried and ground to pass a 1-mm screen. Total N, N isotope ratio, NO3, and Br- were determined in the soil samples and calculated on a volumetric basis using bulk density values from Post et al. (1988).

Nitrate and Br- in the soil at harvest were extracted with deionized water and analyzed in 1991 with an ion chromatograph (Dionex 2200I, Dionex Corp., Sunnyvale, CA) and with a continuous flow colorimetric autoanalyzer (Alpkem Model RFA2, OI Analytical, College Station, TX) in 1992. Total N in the soil and plant was determined by permanganate-reduced Fe modification of the Kjeldahl procedure (Bremner and Mulvaney, 1982). Nitrogen isotope analysis was performed by the 15N analysis service at the University of Illinois using the Rittenberg procedure (Mulvaney and Liu, 1991). Fertilizer-derived 15N in the soil and plant was calculated using the procedure of Hauck and Bremner (1976).


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Yield and Yield Components
We were not able to detect differences in grain yield between the recommended and greater than recommended N rates, except for the clay loam soil in 1992 where the highest grain yield was obtained at the greater than recommended N rate (Table 5) . Similarly, the highest grain yield was obtained at the greater than recommended N rate when averaged across years and soil types. However, the economic optimum occurred at the recommended N rate when averaged across years and soil types since the value of the increased grain yield obtained at the greater than recommended N rate was less than the cost of the additional fertilizer.


View this table:
[in this window]
[in a new window]
  		Table 5 Grain yield and yield components as affected by N rate

 
Greater than recommended N had a negligible effect on grain yield, but increased straw yield and total plant yield. The recommended N rate had the highest harvest index, or proportion of grain to total plant yield. Recommended and greater than recommended N rates had larger kernels and more spikes per unit area than the less than recommended rate averaged across years and soil types. The recommended rate had more kernels per spike than the less than recommended rate averaged across years and soil types. Increased N rate increased the amount of hard vitreous amber kernels, an indicator of grain quality. A hard vitreous amber count of 90% or greater is desirable in commercial durum trade, and this was achieved with the recommended and greater than recommended N rates in all cases except for the recommended N rate on the clay loam soil in 1992. The response of yield and yield components to N fertilizer rate is similar to that achieved at this location in the past (Knowles et al., 1991).

Soil Water Depletion
Nitrogen rate affected soil water depletion (Tables 6 and 7) . Less than recommended N resulted in less soil water depletion than recommended and greater than recommended N in most cases. Differences in soil water depletion between the recommended and greater than recommended N rates were not detected, except on the clay loam soil in 1992. Nitrogen rate affected soil water depletion through its influence on the amount and duration of crop growth. Total yield was increased by N rate as mentioned previously. Growth in the less than recommended N rate was stymied to the extent that full ground cover was not achieved, and soil water depletion was consequently affected. Less than recommended N hastened the amount of time to reach anthesis compared with the recommended and greater than recommended N rates, and leaves of these N deficient plants senesced prematurely. Soil water depletion was influenced by N rate later in the season and not at earlier stages, indicating that the influence of N on maintaining green leaf area was the major reason for differences in soil water depletion.


View this table:
[in this window]
[in a new window]
  		Table 6 Soil water depletion in the top 1.5 m of soil as affected by N rate in 1991

 

View this table:
[in this window]
[in a new window]
  		Table 7 Soil water depletion in the top 1.5 m of soil as affected by N rate in 1992

 
The consequence of increased soil water depletion for the recommended and greater than recommended N rates is that these treatments may have a lower leaching volume since the same amount of irrigation water was applied to all treatments. The recommended and greater than recommended N rates had less irrigation water applied in excess of soil water depletion than the less than recommended N rate in the 1992 growing season, but we were not able to detect a difference among N treatments in 1991 (Tables 8 and 9) .


View this table:
[in this window]
[in a new window]
  		Table 8 Irrigation water applied in excess of soil water deficit in the top 1.5 m of soil as affected by N rate in 1991

 

View this table:
[in this window]
[in a new window]
  		Table 9 Irrigation water applied in excess of soil water deficit in the top 1.5 m of soil as affected by N rate in 1992

 
Bromide Movement in the Soil
Nitrogen fertilizer had no influence on Br- distribution in the soil when averaged across depths, but some differences were detected at individual depths (Fig. 1) . These differences may be explained in many cases by the fact that N rate increased soil moisture depletion and decreased excess irrigation water applied. Despite the differences in Br- concentration detected at certain depths, the Br- curves had similar shapes and location of peaks in the soil regardless of N rate. Nitrogen fertilizer rate had no influence on the median depth of recovered Br- in the soil (data not presented), which averaged 1.13 m across N rates, except for the sandy loam soil in 1992 where most of the Br- appeared to leach past 2.4 m. Differences in depth of Br- movement might be expected due to variation in excess irrigation water applied. However, variation among N rates in the amount of excess water applied at any irrigation was small. Water and solutes may have moved up in the soil profile toward the soil surface as the soil dried between irrigation cycles and masked any small differences in depth of Br- movement as a result of differences in excess water applied. Among soil types, though, variation in the amount of excess water applied was great. Despite the fact that more excess water was applied to the clay loam soil in both years than the sandy loam soil in 1992, Br- movement was much less for the clay loam soil possibly due to water bypassing Br- through preferential flow or greater tortuosity of solute diffusion in the finer textured soil. Tortuosity accounts for the path length and cross-sectional area of solute diffusion as a result of the size of solids and air spaces (Jury and Nielsen, 1989).



View larger version (41K):
[in this window]
[in a new window]
  		Fig. 1 Nitrogen fertilizer rate effect on soil Br- concentration as a function of depth for a clay loam and sandy loam soil in 1991 and 1992. Potassium bromide was applied at a rate of 11.3 g Br- m-2 at planting. The error bars represent the least significant difference at P = 0.05 (LSD0.05). The LSD is not significant if the error bar is followed by "ns" and significant if the error bar is followed by "*"

 
Soil Br- concentration at harvest was affected by timing of Br- application in both years (Fig. 2) . In 1991, we recovered an average of 50% of the Br- applied at the planting and 5-leaf stages and 74% of the Br- applied at the 2-node and boot stages. In 1992 at the first two application times, a Br- peak was not discernable and we recovered only 17% of the Br- applied. However, we recovered 58% of the Br- applied at the 3-node stage where the Br- peak occurred at {approx}2 m, and we recovered 80% of the Br- applied at anthesis where the Br- peak occurred at slightly less than 1 m. In comparing years, more excess water was applied to the sandy loam soil in 1992 and may have been responsible for greater leaching of Br- that year. Our results are similar to other studies that have also shown that leaching potential is the highest for early season applications of solutes (Addiscott and Darby, 1991; Silvertooth et al., 1992).



View larger version (28K):
[in this window]
[in a new window]
  		Fig. 2 Bromide application timing effect on soil Br- concentration as a function of depth in 1991 and 1992 on a sandy loam soil. Potassium bromide was applied at a rate of 11.3 g Br- m-2 each application time. The error bars represent the least significant difference at P = 0.05 (LSD0.05). The LSD is not significant if the error bar is followed by "ns" and significant if the error bar is followed by "*"

 
Bromide indicates the maximum depth of movement of solutes, but the actual depth of movement of NO3 may be less than indicated by Br- due to plant uptake and immobilization of NO3. The depth of water movement is not necessarily identical to the depth of Br- movement unless the soil is structureless, since diffusion and hydrodynamic dispersion prevent solutes from moving strictly by mass flow in the water (Jury and Nielson, 1989). The concentration profile of solutes that have been subject to leaching usually contains a single peak that can be described as asymmetric or lognormal due to the tail that occurs deeper in the soil (Wild and Babiker, 1976; Rice et al., 1986; Jaynes et al., 1988). The Br- curves in our study also contained single peaks that are asymmetric or lognormal, at least to the sampling depth of 2.4 m. Recovery of applied Br- in our study was 48, 66, 80, and 11% for the clay loam in 1991, the sandy loam in 1991, the clay loam in 1992, and the sandy loam in 1992, respectively. Thus, about one-half of the applied Br- in our study is not accounted for and could have been lost to leaching. Alternatively, Br- may be accounted for in the plant since plant uptake of applied Br- was 2% for wheat in a laboratory column study (Gish and Jury, 1982), 30% for a grass pasture (Owens et al., 1985), and 53% for potato (Solanum tuberosum L.) in a field study (Kung, 1990).

Nitrogen-15 Movement
Most of the 15N fertilizer recovered in the soil was in the top 1 m and the depth of movement of 15N was not influenced by N rate (Fig. 3) or timing (Fig. 4) . The median depth of recovered 15N fertilizer was actually 0.23 m on average, much less than the depth of movement indicated by Br- data. Nitrogen fertilizer movement in the soil was not evident even for the sandy loam in 1992, where appreciable Br- movement occurred. In contrast to recovered Br-, most of the recovered 15N fertilizer was in the surface soil.



View larger version (35K):
[in this window]
[in a new window]
  		Fig. 3 Nitrogen fertilizer rate effect on amount of 15N recovered in the soil as a function of depth for a clay loam and sandy loam soil in 1991 and 1992. The error bars represent the least significant difference at P = 0.05 (LSD0.05). The LSD is not significant if the error bar is followed by "ns" and significant if the error bar is followed by "*"

 


View larger version (29K):
[in this window]
[in a new window]
  		Fig. 4 Nitrogen fertilizer application timing effect on amount of 15N recovered in the soil as a function of depth in 1991 and 1992 on a sandy loam soil. The error bars represent the least significant difference at P = 0.05 (LSD0.05). The LSD is not significant if the error bar is followed by "ns" and significant if the error bar is followed by "*"

 
Nitrogen fertilizer rate and timing did not affect depth of N movement in the soil, but did affect the amount of fertilizer remaining in the soil after the season. This residual fertilizer could possibly be subject to leaching in future seasons. Greater N fertilizer rates increased the amount of residual N in the surface soil to a depth of {approx}1 m (Fig. 3). Greater than recommended N led to a slight increase in residual N below 1.5 m for the sandy loam soil in 1992. Nitrogen fertilizer rate has increased postharvest N content of soils in other studies (Ayoub et al., 1995; Pilbeam et al., 1997). In our study, the 15N application at planting time contributed about three times more residual N at harvest than any other application (Fig. 4). The microbes may have immobilized the N fertilizer applied at planting and contributed to increased residual N at this application time, whereas plant uptake in post-plant applications may have reduced soil residual N.

Recovery of 15N in the soil and plant ranged from 53 to 78% (Table 10) . As N rate increased, percentage recovery decreased in the soil, remained constant in the grain, increased in the straw, and decreased in the sum of the soil, grain, and straw. Powlson et al. (1986) also found decreasing percentage recovery as fertilizer rates increase. The N not accounted for in our study could have been contained in the crowns, which were not sampled, or leached, denitrified, or volatilized as NH3 from the soil or plant. Freney et al. (1983) reported that high pH (>7) and free CaCO3, characteristic of soils in this experiment, enhance volatilization of surface-applied NH4. Excess N increased gaseous N loss from wheat and ranged from 8 to 59% of the N in the plant at anthesis in the study of Kanampiu et al. (1997).


View this table:
[in this window]
[in a new window]
  		Table 10 Labeled N fertilizer percentage recovery in the soil, grain, and straw at harvest as influenced by N rate

 
We were not able to detect differences in percentage recovery of 15N in the soil due to N, but were able to detect differences in the grain and straw (Table 11) . Later applications increased percentage recovery in the grain except for the watery kernel stage in 1991, which may have been too late for maximum utilization by the plant. A greater amount of the fertilizer applied early was recovered in the straw. Recovery in the plant (grain + straw) was greatest for later applications except for the watery kernel stage in 1991. Total percentage recovery (soil + grain + straw) was not consistent based on timing. Results from 1992 are similar to Wuest and Cassman (1992a, 1992b) who demonstrated a greater percentage recovery in the plant of fertilizer applications at anthesis and that those applications are more efficiently allocated to grain.


View this table:
[in this window]
[in a new window]
  		Table 11 Labeled N fertilizer percentage recovery in the soil, grain, and straw at harvest as influenced by timing of N application for the recommended N rate on a sandy loam soil

 
The labeled N fertilizer in our study was applied in the form of (NH4)2SO4 and, due to the fact that most of the recovered 15N was in the surface soil, the fertilizer may have been immobilized in the soil organic fraction or fixed as NH4 in the clay lattices. Alternatively, the plant may have taken the fertilizer up as NH4, or as NO3 once nitrified but before it could be leached beyond the root zone by the next irrigation. MacDonald et al. (1989) reported that most of the fertilizer remaining in the soil was immobilized in his study and <1.3% of the N from fertilizer ended up in the form of residual soil NO3. We did not determine residual soil NO3 originating from 15N fertilizer in our study, but the residual soil NO3 from all sources averaged 0.8 mg NO3–N kg-1 in our sampling depth of 2.4 m, or a mean of 0.37 g NO3–N m-2 for each 0.3-m soil increment. Except for the surface 0.3 m, most or all of the recovered 15N fertilizer could have been in the NO3 form. In the surface 0.3 m where most of the 15N fertilizer was recovered in the soil, residual soil NO3 from all sources was {approx}24% of the recovered 15N fertilizer, so most of the 15N in the surface soil was not in the NO3 form. Immobilization may conserve N from leaching temporarily, but eventually immobilized N could be mineralized, nitrified, and subject to leaching.

Determining the long-term fate of the fertilizer was beyond the scope of this study. However, we showed that N rate and timing did not affect depth of movement 15N recovered at the end of a single season for irrigated wheat. Several studies with rainfed wheat have similarly shown that N rate (Powlson et al., 1986; Smith and Whitfield, 1990) and timing (Bronson et al., 1991; Boman et al., 1995) do not affect residual N distribution in the soil. The soil–plant system seems to be buffered against or resists accumulation of inorganic N in the soil profile, even when greater than recommended N rates are applied (Raun and Johnson, 1995; Porter et al., 1996). Excess N can be taken up by the plant, volatilized, or incorporated into the soil organic fraction as residue at the end of the season. MacDonald et al. (1989) showed that the NO3 at risk for leaching in the winter for wheat comes from mineralization of organic N, not from unused fertilizer applied the previous spring. Nitrate leaching is often controlled predominantly by factors other than N rate and timing, such as growing season conditions and crop (Sieling et al., 1997). Nevertheless, long-term application of high N rates to wheat has been reported to increase NO3 levels in the subsoil for rainfed wheat (Westerman et al., 1994). In summary, soil NO3 concentration at the time of leaching events may be a more direct indicator of leaching potential than in-season fertilizer practices for irrigated wheat in Arizona.Theocharopoulos Karayianni Gatzogiani Afentaki Aggelides 1993; Wuest Cassman 1992; Wuest Cassman 1992


   ACKNOWLEDGMENTS
 
The financial support given by the Arizona Department of Environmental Quality for this project and the technical assistance of David Parsons and Mark Rogers is greatly appreciated.

Received for publication July 2, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




This article has been cited by other articles:


Home page
 Agron. J.Home page
J. D. Booker, K. F. Bronson, C. L. Trostle, J. W. Keeling, and A. Malapati
Nitrogen and Phosphorus Fertilizer and Residual Response in Cotton-Sorghum and Cotton-Cotton Sequences
Agron. J., April 4, 2007; 99(3): 607 - 613.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
R.-F. Zhao, X.-P. Chen, F.-S. Zhang, H. Zhang, J. Schroder, and V. Romheld
Fertilization and Nitrogen Balance in a Wheat-Maize Rotation System in North China
Agron. J., June 5, 2006; 98(4): 938 - 945.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
W. B. Stevens, R. G. Hoeft, and R. L. Mulvaney
Fate of Nitrogen-15 in a Long-Term Nitrogen Rate Study: I. Interactions with Soil Nitrogen
Agron. J., June 17, 2005; 97(4): 1037 - 1045.
[Abstract] [Full Text] [PDF]

Home page
 J. Environ. Qual.Home page
D. E. Clay, Z. Zheng, Z. Liu, S. A. Clay, and T. P. Trooien
Bromide and Nitrate Movement through Undisturbed Soil Columns
J. Environ. Qual., January 1, 2004; 33(1): 338 - 342.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (6)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Ottman, M. J.
Right arrow Articles by Pope, N. V.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome