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


     

Published online 7 February 2006
Published in Agron J 98:255-264 (2006)
DOI: 10.2134/agronj2005.0028
© 2006 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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 (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Agricola
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Related Collections
Right arrow Nitrogen
Right arrow Plant Nutrition
Right arrow Tillage

Production Papers

Spatial Variability of In-Season Nitrogen Uptake by Corn Across a Variable Landscape as Affected by Management

R. S. Dharmakeerthia, B. D. Kayb,* and E. G. Beauchampb

a Dep. of Soils and Plant Nutrition, Rubber Research Inst. of Sri Lanka, Dartonfield, Agalawatta, Sri Lanka
b Dep. of Land Resource Science, Univ. of Guelph, Guelph, ON, N1G 2W1, Canada

* Corresponding author (bkay{at}lrs.uoguelph.ca)

Received for publication January 28, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
An understanding of the spatial and temporal variability of N uptake at a landscape scale is required to implement site-specific N management. We determined the spatial variation of in-season N uptake and N nutritional status of corn (Zea mays L.) in a variable landscape in southern Ontario from 1997 to 2001 under three management conditions involving corn under no-tillage or conventional tillage using barley (Hordeum vulgare L.) or barley under-seeded with red clover (Trifolium pratense L.) as the preceding crop and with or without fertilizer N. The aerial dry matter content (DM) of corn and N concentrations in the DM (Ni) were determined at 2-wk intervals. Organic C (OC) content (0–0.3 m) was used as the landscape-based variable to account for the spatial variability of N uptake. Fertilizer N addition, legume incorporation, and tillage had significant positive effects on N uptake, but the magnitude of these effects varied within and among growing seasons. Nitrogen uptake increased with OC content in a quadratic relationship, reaching a maximum at OC content of about 26 g kg–1. The N nutritional status in the DM, estimated using previously established critical dilution curves, also increased with OC content. However, the nutritional status decreased as the growing season progressed under all management treatments; this decrease was largest at the smallest OC contents. These trends suggest that measurements of plant N early in the season cannot be used in site-specific N management to accurately identify areas of adequate or excess N later in the growing season.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
NITROGEN UPTAKE by plants is controlled by their demand and by N supply. Because spatial variation in soil properties is a characteristic of most landscapes used for corn production in southern Ontario, the demand for N by plants and the supply of N from soils is expected to vary across the landscape. The supply of N can be altered by management practices, such as introduction of a legume into the cropping system, tillage, or adding N fertilizers. Legume incorporation can provide almost all plant N requirements (Stute and Posner, 1995; Vyn et al., 1999), thus resulting in greater profitability (Tiffin and Hesterman, 1998) and less risk to the environment (Stute and Posner, 1995). Nitrogen availability may also vary with tillage practices (Eriksen and Jensen, 2001; Dharmakeerthi et al., 2004). Although several studies have dealt with spatial variability of N availability (Jowkin and Schoenau, 1998; Dharmakeerthi et al., 2005) or fertilizer N requirement (Fiez et al., 1994; Ferguson et al., 2002; Schmidt et al., 2002; Scharf et al., 2005), only a few studies have considered the spatial variability of plant N demand, the ability of incorporated legumes to supply plant N requirements, or the effect of tillage on N uptake across a variable landscape.

Measurement of N uptake by plants (i.e., tissue concentration multiplied by biomass weight) does not necessarily indicate the N requirement of the plant. Several studies have shown that the N concentration in shoots can be greater than the minimum plant requirements for maximum growth. The excess N does not provide any additional benefit for the shoot growth but accumulates as reduced N compounds or nitrate in the stem. Studies conducted by Plenet and Lemaire (1999) on 11 field experiments with irrigated corn showed that the N concentration in shoot DM could be up to 65% higher than the minimum requirements for maximum growth. Tiffin and Hesterman (1998) have also observed that corn continued to accumulate N in the aboveground biomass as the addition of fertilizer N increased from 0 to 200 kg ha–1 in red clover–incorporated plots.

The ability of the plant to store more N than is required at early phases of development could be an advantage under conditions of diminishing N availability as the growing season progresses. As N supply decreases, N uptake, translocation, and remobilization are affected, especially during the grain-filling stage, because these processes are influenced by the source/sink ratio (Tollenaar, 1991). Delayed remobilization of N from leaves during the grain filling stage maintains the photosynthetic capacity for longer by decreasing the rate of leaf senescence (Rajcan and Tollenaar, 1999; Borrell et al., 2001) and therefore maintaining a high source/sink ratio. It is therefore important to relate the dynamics of N uptake to soil, plant, or landscape attributes and to identify parts of the landscape where plant N uptake is deficient, sufficient, and excessive at different growth stages.

Identification of an indicator of the N status of the soil–plant system is particularly important for site-specific N management. Many plant indicators have been proposed as diagnostic tools, but few of them have been effective in identifying cases of N excess. Recently, Plenet and Lemaire (1999) proposed a critical dilution curve for corn that describes the relationship between the DM accumulation and the critical N concentration in the DM. The critical N concentration was defined as the minimum N concentration in shoots required to produce the maximum DM at a given time under nonlimiting conditions. This critical N concentration decreases with time as plants accumulate more DM. The critical dilution curve seems to be a reliable tool for diagnosing the N nutrition status of a crop, using only the DM and N concentration in the crop (Lemaire and Gastal, 1997). Nitrogen is not a limiting factor for crop growth rate if the actual N concentration exceeds the critical concentration. Conversely, if the actual N concentration is lower than the critical N concentration, then the above-ground DM accumulation is, or has been, limited by N availability in the soil.

An understanding of the dynamics of N availability in soil and plant N uptake through the growing season and their variability across the landscape is required to implement site-specific N management. In a previous paper we reported that the spatial variability of N availability across a landscape under corn production in southern Ontario was governed mainly by the spatial variation in OC content and that the effects of management practices, such as tillage and legume incorporation on N availability, did not vary across the landscape (Dharmakeerthi et al., 2005). This paper focuses on measurements of N uptake on the same site. The objectives of this phase of the study were to determine (i) the extent and causes of spatial variability of in-season N uptake by corn in cropping systems that included a legume (red clover cover crop) in a variable landscape and (ii) the degree to which minimum plant N requirements have been met at different parts of the landscape within and between growing seasons. These objectives were achieved in a field experiment conducted over four growing seasons.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Design
Details on the experimental design, treatments, sampling, and analysis have been described in a previous paper that focused on soil N availability (Dharmakeerthi et al., 2005). In this section we provide the relevant details for the N uptake study. The experiment was conducted on a variable landscape at the Elora Research Station (43° 39' N lat; 80° 35' W long), which is about 23 km northwest of Guelph, Ontario and extended from 1997 to 2001 (inclusive). Plots (4.5 by 8 m) were established at five positions across the landscape that were visually related to slope and referred to as the summit, shoulder, backslope, footslope, and toeslope positions. The soil was a Typic Hapludalf with a silt loam soil texture in the A horizon. The clay content was relatively uniform across the site (85–187 g kg–1), whereas the OC content exhibited much larger variation (7.7–31.2 g kg–1). The pH (0.01 M CaCl2) was neutral to slightly alkaline. The elevation of the landscape dropped by about 10 m along a 160-m traverse from the summit to the toeslope position.

Three management systems were used in this 4-yr experiment: (i) barley followed by corn under no-tillage (Barley-NT), (ii) barley followed by corn under a spring plowing and secondary tillage operation (Barley-CT), and (iii) barley under-seeded with red clover that was plowed down in the following spring and plots planted to corn (Barley + RC-CT). In all seasons, barley was grown as the preceding crop after spring cultivation. Corn varieties were Pioneer 3902 in 1997 and Northrup King N17-C4 in the other four seasons. At the time of planting, 200 kg 0–20–20 N–P–K ha–1 was banded beside the corn seed. Corn was planted down the slope, and the experimental plots were established at the five positions in the landscape. Each experimental plot had six rows, of which the four center rows were used for plant sampling, and the remaining two rows were used as border rows. Planting density was about 60,000 plants ha–1 with a row spacing of 0.75 m. Corn received 0 or 140 kg N ha–1 in the form of NH4NO3 around the V2 stage.

There were three management systems (main plot), five landscape positions (subplot), and two fertilizer N levels (sub-subplot). The experimental design was a modified split-split plot design with three replicates. The experiment was conducted in adjacent areas on the same landscape with year 1 of the rotation in alternate areas. Thus, measurements in the 1997, 1999, and 2001 seasons were conducted in one area, whereas measurements in the 1998 and 2000 seasons were conducted in the other. The locations of cropping treatments within a replication were re-randomized at the beginning of each cycle of the rotation. Very wet soil conditions delayed the planting in the 2000 growing season until the second week of June, which resulted poor crop growth and yields. Therefore, N uptake data for the year 2000 were excluded from analyses.

Measurements
Soil samples were collected from 0 to 0.3 m depth for the determination of soil mineral N content (Keeney and Nelson, 1982) and gravimetric water content (WC). Sampling was conducted at 2-wk intervals throughout each growing season from early May to late September or early October. Soils collected on the second sampling (around late May) were used for the determination of OC using a Leco Carbon Analyzer (LECO SC444; Leco Corporation, St. Joseph, MI) and particle size distribution (Gee and Bauder, 1986). Dry bulk density of each plot was also estimated in early October when the corn was at post-physiologic maturity. From the soil measurements, only OC data are used in the subsequent analyses in this paper.

Corn plants were harvested (tops only) at 2-wk intervals, beginning about 4 wk after planting at the time of soil sampling. An area equivalent to 1 by 2 m rows was sampled in the first sampling, and thereafter a 1 by 1 m row area was harvested for determination of DM and N concentration. In 1999 and 2001, plants in the 0N and +N treatments were sampled, whereas in 1997 and 1998 only the 0N treatments were sampled. The sampling dates in each year and the crop heat units (CHU) accumulated (Brown, 1978) from planting to each sampling date are given in Table 1.


View this table:
[in this window]
[in a new window]
  		Table 1. Crop heat units (CHU) accumulated from planting to different plant sampling dates in the 1997, 1998, 1999, and 2001 growing seasons.

 
Weather data (daily rainfall [mm] and minimum, maximum, and average daily temperature) were obtained from the weather station situated on the Elora Research Station, about 1 km away from the experimental plots.

Diagnostic Tool for Nitrogen Nutrition Status of the Plant
The N nutrition status in each plot was diagnosed by referring the N concentration of the above-ground plant parts and their DM to the critical dilution curve proposed by Plenet and Lemaire (1999) for corn. We assumed that the model proposed by the Plenet and Lemaire (1999) was applicable to our ecophysiologic environment. The model was:

Formula

Formula
where DM is the DM (Mg ha–1) and Nc is the critical N concentration (%) in the above-ground plant parts. The critical N concentration has been defined as the minimum N concentration in shoots required to produce the maximum DM at a given time under nonlimiting conditions. This model is applicable to corn crop development between plant emergence and silking + 25 d.

Lemaire and Gastal (1997) used the ratio between the observed N concentration at a given time (Ni) and the Nc for DM observed at that time as a diagnostic tool of N nutrition status of the plant. Values of Ni/Nc < 1 were considered to represent N deficiency, whereas values of Ni/Nc > 1 represented conditions in which the plants were not limited by N and the soil N availability allowed maximal growth (Devienne-Barret et al., 2000). This interpretation should be valid when there are no limitations other than N for plant growth under a given set of climatic conditions. Because there was evidence that plant growth on some parts of the landscape was affected by the lack of available water, we used the Ni/Nc ratio in our study primarily to identify the situations of adequacy or excess N during the growing season (i.e., Ni/Nc ≥ 1).

Statistical Analysis
Analysis of variance was conducted using the PROC GLM program of the SAS software package (SAS Institute, 1996), and multiple regression analyses were conducted using the stepwise regression procedure in the S-PLUS software package (Insightful Corporation, 2002). Before the regression analysis, outliers and influential data points were removed (Dharmakeerthi et al., 2005). During the regression analysis, the higher-order interactions that were not significant at P < 0.05 were removed from the list of independent variables and the stepwise procedure rerun. Moreover, when an interaction effect was significant, its main effects were retained in the final model even if they were not statistically significant.

Soil properties do not vary consistently with landscape position; therefore, landscape position may not always be a good variable to account for the spatial variability of biological processes, although we used landscape position as a source of variance in the ANOVA. However, the OC content, as a measure of organic matter content, can reflect the spatial variation in soil structure (Kay et al., 1997) and water content (da Silva et al., 2001). Both of these soil properties can influence biological processes. In addition, OC contents are generally constant over one or more growing seasons when cropping systems remain constant. For these reasons and because of the observation that the variation in OC was much greater than the variation in texture on the site, OC content was used as the landscape-based variable in the regression analyses to account for the spatial variability of N uptake. The effects of tillage, legumes, and fertilizer N on plant N uptake were regressed as categorical variables using 0 and 1 for the "without" and "with" treatments, respectively.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather
The weather of the four growing seasons varied considerably in terms of rainfall distribution and CHU accumulation. The 1998 season experienced the lowest rainfall and accumulated the greatest CHU after planting. Although the rainfall distribution was near average in the 1997 season, the CHU accumulation was the lowest among the four seasons. The 1999 season received comparatively more rainfall during the growing season and accumulated nearly as many CHU as the 1998 season (Fig. 1 ). Generally, the rainfall received during July and early August was less than in other months, and the 1998 and 2001 seasons received the least rainfall during this period. Distribution of water-filled pore space during the growing period also indicated that the soil was driest during this period, and the 1998 and 2001 seasons were drier than the 1999 season (Dharmakeerthi et al., 2004). Because silking, an important corn development stage, is coincident with the latter part of July, any stresses occurring during this period should have the greatest consequences on corn yield. Therefore, the 1998 and 2001 seasons were categorized as drier seasons.


Figure 1
View larger version (81K):
[in this window]
[in a new window]
  		Fig. 1. Accumulation of rainfall and crop heat units during each month from May to October in the 1997, 1998, 1999, and 2001 growing seasons. The 13-yr average rainfall is from 1967 to 1980.

 
Seasonal Effect on Nitrogen Uptake
Nitrogen uptake approximated a sigmoid pattern: N uptake was very slow until the V6 stage and then increased at an increasing rate until the V12 stage and at a decreasing rate after silking (Fig. 2 ). Toward physiologic maturity (maximum kernel dry weight), N uptake virtually stopped. These observations are in agreement with the N uptake patterns described by Magdoff (1991).


Figure 2
View larger version (29K):
[in this window]
[in a new window]
  		Fig. 2. Variation in N uptake by corn at three contrasting landscape positions (averaged over management treatments in the 0N plots) during the 1997, 1998, 1999, and 2001 growing seasons. The vertical bars indicate the standard error.

 
Although the pattern of N uptake was consistent in all four seasons, the N uptake rates were different among the growing seasons (Fig. 2). For instance, N uptake rates between the second and third plant samplings (the period in which uptake was often greatest) were on average 0.17, 0.14, 0.18, and 0.13 kg N ha–1 CHU–1 (3.8, 3.2, 4.3, and 3.1 kg N ha–1 d–1) in the 0N treatments for 1997, 1998, 1999, and 2001, respectively. A decrease in the rate of N uptake before silking could be observed in several data sets, especially in the 1997 and 1999 seasons. Although Karlen et al. (1988) observed a short cessation of N uptake during the changeover from vegetative to reproductive growth, slowing of N uptake rates just before silking in the present study also coincided with the driest period of the season.

Spatio-Temporal Variability of Nitrogen Uptake
For simplicity and to reduce the sampling variability, the N uptake data were studied at three growth stages of corn: (i) around V6 stage (average of samplings 1 and 2, except in 1997 where sampling 1 data were not available), (ii) around silking (average of samplings 3 and 4), and (iii) around physiologic maturity (average of last two samplings). Sampling dates and the amount of CHU accumulated from planting up to each sampling date are given in Table 1.

Analysis of variance conducted on the N uptake data in the 1999 and 2001 seasons identified a significant management treatment effect, fertilizer N effect, and landscape position effect (Table 2) on N uptake at the three growth stages. Similar management treatment and landscape position effects were observed when the analyses were extended for all four seasons using only the 0N plots. Because the management by landscape position interaction effect was not significant most of the time, the effect of management treatment on N uptake seems to be similar across the entire landscape. The effect of fertilizer N addition on N uptake was also not influenced by the landscape position. The spatial variability and effects of management on N uptake were further studied in detail using multiple regression analysis. The coefficients of these effects in the regressions models (Table 3) should indicate the magnitude and the nature of these effects on N uptake at different growth stages.


View this table:
[in this window]
[in a new window]
  		Table 2. ANOVA of the N uptake data at three growth stages during the 1999 and 2001 growing seasons.

 

View this table:
[in this window]
[in a new window]
  		Table 3. Results of the stepwise regression analyses relating the effect of tillage, legume, fertilizer N (N), and organic carbon (g kg–1) (OC) to the cumulative N uptake (kg N ha–1) at different growth stages.

 
Spatial Variability
There was a significant landscape position effect on N uptake at all growth stages (Table 2). The lowest N uptake was observed at the backslope position, and the highest was observed at the toeslope or foot slope positions in all seasons (Fig. 2). These differences were attributed to differences in N availability and biomass production in these slope positions. Nitrogen availability was lowest at the backslope position and highest in the toeslope position (Dharmakeerthi et al., 2005). Biomass production was also lowest in the backslope position and highest in the footslope or toeslope positions (data not presented).

Using OC content as a surrogate for landscape position, the regression analyses (Table 3) showed that N uptake increased with OC content through the entire growing period. Generally, the plant N uptake increased with the OC content in a quadratic manner, with maximum uptake occurring at OC contents about 26 g kg–1. This quadratic relationship between N uptake and OC could be related to the variation in N availability with organic matter content because the rate of net N mineralization slows down with the increasing organic matter contents (Broadbent, 1984; Magdoff, 1991). We also have observed that the increase in plant available N with the increase in OC content decreased at high OC content in the landscape (Dharmakeerthi et al., 2005).

A sensitivity analysis using the regression equations given in Table 3 was performed with data from the Barley-NT (0N) plots to illustrate the change in plant N uptake as the OC content increased from 10 to 20 g kg–1 across the landscape (Table 4). The amount of N taken up by plants for this increase in the OC in the landscape varied from one season to the other and increased with growth stage. At crop maturity, the increase was lowest in 1998 (19.2 kg N ha–1), where the soil water content was most limiting. In the other 3 yr, the increase was significantly higher, ranging from 67.4 to 115.0 kg N ha–1.


View this table:
[in this window]
[in a new window]
  		Table 4. The change in N uptake at three growth stages of corn with a change in OC from 10 to 20 g kg–1 across the landscape in 0N plots in the Barley-NT treatment. Analysis was based on the regression equations given in the Table 3.

 
Effect of Tillage
A significant positive tillage effect on plant N uptake could often be observed during the early parts of the growing season, but the magnitude of this effect varied from one season to the other. This positive tillage effect diminished toward the end of the season (Table 3). The largest increase in plant N uptake due to tillage, 30.8 kg N ha–1 in the 0N plots, was observed in the 1999 season around silking. The amount of N taken up during the period between the V6 stage and silking was not significantly affected by the tillage, and therefore the influence of tillage on N uptake should have occurred by the V6 stage (Fig. 3 ). Galvez et al. (2001) observed that N uptake was significantly higher in moldboard plow plots than that in NT plots at the eight-leaf stage and at silking, but the differences were not significant at maturity. They speculated that the N availability in the moldboard plow plots could have been greater than that in the NT plots. However, no significant N fertilizer level by tillage interaction effect was observed in the 1999 or in the 2001 seasons during any growth stage (Table 3). Therefore, the increased N uptake in CT plots at early growth stages could not be related to N availability in this study. The DM production around V6 stage was 18 to 76% greater in the CT plots than that in the NT plots. The observed tillage effect on the N uptake may be attributed to the differences in crop growth rates because N uptake is governed by N availability and crop growth rate (Devienne-Barret et al., 2000).


Figure 3
View larger version (29K):
[in this window]
[in a new window]
  		Fig. 3. Variation in N uptake by corn in the three management treatments: (i) barley followed by corn under no-tillage (Barley-NT), (ii) barley followed by corn under a spring plowing and secondary tillage operation (Barley-CT), (iii) barley under-seeded with red clover that was plowed down in the following spring and plots planted to corn (Barley + RC-CT) (averaged over landscape positions in the 0N plots) during the 1997, 1998, 1999, and 2001 growing seasons. The vertical bars indicate the standard error.

 
Nitrogen uptake can be decreased due to tillage if weather conditions are not favorable for plant growth. In areas where the OC contents were low (<16.5 g kg–1), the N uptake at silking was significantly lower in CT plots than that in the NT plots in 1998 (Table 3). Plants in the 1998 season experienced below-average rainfall throughout the growing period (Fig. 1). Conventionally tilled soil is more vulnerable to drying than NT; therefore, N movement due to mass flow and diffusion within the soil could have been restricted in the CT plots, making N less available for plant uptake. We observed that the water-filled pore space in CT soils could be lower than that in NT soils (Dharmakeerthi et al., 2004). A marginally significant negative tillage effect (P < 0.1) on DM production was observed in low OC areas at silking in this year (data not presented). Mineralization of organic N may also have been reduced. Except for this incident and a positive tillage by OC interaction effect at the V6 stage in 1997, the effect of tillage on plant N uptake was generally the same across the range of OC contents in this study.

Effect of Legume
Incorporation of red clover residues had a positive influence on N uptake throughout the growing season (Fig. 3), but statistical significance was consistent among the four growing seasons only at plant maturity. At the end of the season, the increase in plant N uptake due to legume in 0N plots, averaged across all positions in the landscape, ranged from 27.5 kg N ha–1 in 1999 to 59.0 kg N ha–1 in 1998 (Table 3). The effect of legume was statistically significant three out of four seasons at the V6 stage and at silking. Dry matter values of red clover before incorporation, averaged across all positions in the landscape, were 0.96, 0.48, and 1.83 Mg ha–1 for the 1998, 1999, and 2001 seasons, respectively. Comparable red clover DM data for the 1997 season were not available. Lack of significant correlation between red clover DM and subsequent N uptake in different years likely means that variation in weather during the growing season had a bigger effect than variation in preceding red clover DM yields.

A consistent significant interaction effect between the soil OC content and legume was not observed. The effect of incorporation of red clover residues on plant N uptake varied with OC content only in three regression models: in one case the interaction effect was positive, whereas in the other two cases the interaction effects were negative (Table 3).

Effect of Fertilizer Nitrogen
Plant N uptake in the +N treatments was measured only in the 1999 and 2001 seasons. Plants in +N plots took up significantly more N than in the 0N counterpart at almost all growth stages (Table 3). The effect of fertilizer N was not significant at the V6 stage in 1999, whereas in 2001 the fertilizer N effect was reduced in the legume treatment at this stage. The amount of N taken up by plants as a result of fertilizer N addition varied between the two seasons. At silking and at maturity, the fertilizer N effect was larger in the 1999 season than in the 2001 season. These differences may be attributable, in part, to the better growing conditions in 1999 and the resulting higher crop growth rates in 1999 compared with those in the 2001 season. Slow movement of N within the soil under dry weather conditions in 2001 (Dharmakeerthi et al., 2004) could also be responsible for the differences.

The effect of fertilizer N addition became prominent during the rapid growth period before silking. These differences were carried forward until the end of the growing season (Fig. 4 ). Barley-NT and barley-CT management treatments under 0N exhibited smaller N uptake than the +N treatment through the period of active vegetative growth. However, the plants in the red clover-incorporated 0N plots took up amounts of N similar to those in the +N plots (Fig. 4 and Table 3); this confirms the findings of others (Stute and Posner, 1995; Tiffin and Hesterman, 1998; Vyn et al., 1999).


Figure 4
View larger version (47K):
[in this window]
[in a new window]
  		Fig. 4. Variation in N uptake by corn as influenced by the fertilizer N application (averaged over landscape positions) in the three management treatments: (i) barley followed by corn under no-tillage (Barley-NT), (ii) barley followed by corn under a spring plowing and secondary tillage operation (Barley-CT), (iii) barley under-seeded with red clover that was plowed down in the following spring and plots planted to corn (Barley + RC-CT) during the 1999 and 2001 growing seasons. The vertical bars indicate the standard error.

 
In the 1999 and 2001 seasons, the effect of fertilizer N did not vary with the spatial variation in OC content (the OC by N interaction effect was not significant). It is surprising that the fertilizer N effect did not decrease with the increase of OC content. Nitrogen availability increased as OC content increased in both seasons (Dharmakeerthi et al., 2005). In addition, Tiffin and Hesterman (1998) observed that corn following red clover continued to accumulate N in the aboveground biomass as the fertilizer N rate increased from 0 to 200 kg N ha–1.

Spatial Variation in Nitrogen Nutritional Status of Plants
The critical N dilution curve proposed by Plenet and Lemaire (1999) was used to quantify the nutritional status of plants and thereby determine areas where excess N uptake occurred in the landscape and how the incidence of excess uptake changed during the growing season. The relation between the DM accumulation and the N concentration in DM (Ni) relative to the critical dilution curve is shown in Fig. 5 . The Ni decreased as the plant accumulated more DM with time. Fig. 5 also indicates the relationship between Ni and the Nc for the same DM in the four seasons. The ratio between Ni and Nc for a given DM at a given time was calculated. The influence of tillage, legume incorporation, fertilizer N addition, and OC on Ni/Nc were determined using stepwise multiple regression analysis and the results for the 1999 and 2001 seasons given in Table 5.


Figure 5
View larger version (35K):
[in this window]
[in a new window]
  		Fig. 5. Relation between the aerial dry matter (DM) accumulation and N concentration in DM (Ni) in 0N and +N treatments in the 1997, 1998, 1999, and 2001 growing seasons with respect to the critical N concentration developed by Plenet and Lemaire (1999).

 

View this table:
[in this window]
[in a new window]
  		Table 5. Results of the stepwise regression analyses relating the effect of tillage, legume, fertilizer N (N), and organic carbon (g kg–1) (OC) to the Ni/Nc ratio at different sampling times in the 1999 and 2001 growing seasons.

 
Although the R2 values for the regression equations for Ni/Nc (Table 5) were generally larger than those for N uptake (Table 3), the effects of tillage, legume, and OC on Ni/Nc were similar to the effects of these variables on N uptake. A positive tillage effect was generally evident up to and including silking. A consistently positive legume effect in 0N plots could also be observed during the entire period. A significant OC effect on Ni/Nc was observed at all stages except at the V6 stage. At the silking and R3 stages, the relationship was quadratic, with the maximum Ni/Nc occurring at an OC content of about 26 g kg–1. However, unlike N uptake data, several important interaction effects became significant in this analysis. The effect of fertilizer N on the Ni/Nc ratio at the silking and R3 stages was less in legume-incorporated plots than that in nonlegume plots in both years and was less in high-OC–content areas than in low-OC–content areas in 1999 but not in the drier year (2001). This could be due, in part, to the differences in the amount of N released from soil organic matter due to mineralization under different weather conditions. Dharmakeerthi et al. (2005) observed that organic matter released more N in the wet season than in the dry season, especially by the end of the growing season.

The Ni/Nc ratio was greater than 1 at the V6 stage in all situations, indicating that the N availability was not limiting and that plants were experiencing luxury consumption (Fig. 6 ). The ratio decreased as the growing season progressed, but the rate of decrease was faster in areas where the OC content was low. Larger grain yield reduction results from water stress and nutrient deficiencies during 2 wk before and after silking, and the largest yield reduction results from stresses at silking (Ritchie et al., 1997). Plants grown with the legume treatment had Ni/Nc ≥ 1 at silking in both years across most of the range in OC contents. However, in the dryer season (2001), plants grown in areas with OC contents less than 15 g kg–1 and without red clover had a ratio less than 1 at this growth stage, even in the +N treatments. Retarded crop growth rate, poor mobility of N within the soil, or both could have occurred in these parts of the landscape.


Figure 6
View larger version (32K):
[in this window]
[in a new window]
  		Fig. 6. Effect of management treatments: (i) barley followed by corn under no-tillage (Barley-NT), (ii) barley followed by corn under a spring plowing and secondary tillage operation (Barley-CT), (iii) barley under-seeded with red clover that was plowed down in the following spring and plots planted to corn (Barley + RC-CT) on the ratio between Ni and Nc concentration in the aerial dry matter at three growth stages and its variation with organic C content. The solid horizontal line indicates the minimum N concentration in the aerial dry matter for the maximum growth (i.e., Nc). Other lines were generated using the regression models given in Table 5.

 
The ratio of Ni to Nc decreased with time at all OC contents (Fig. 6). If conditions through the entire growing period were as favorable as at the V6 stage, the ratio would have been expected to remain the same. However, the ratio decreased from silking to R3 stage even in plots with, theoretically, the greatest available N (e.g., legume incorporated plus 140 kg N ha–1 with OC content of 25 g kg–1). Under southern Ontario conditions, the surface soils (0–0.30 m) are normally driest during this period (Dharmakeerthi et al., 2004). Transpiration requirements of the plants are met primarily from water taken up from deeper in the profile under these situations. Nitrogen concentrations at greater depth are smaller than those of the surface soils. Therefore, N taken up by the plants during this period could be low. Plants that had excess N accumulated would be advantaged during this period because stored N could be remobilized to the sink (cob) without deleteriously affecting the N concentration in the source (leaves). We speculate that plants grown in areas with high organic matter (e.g., OC > 20 g kg–1) would be less susceptible to N stress at low water content in the surface soil than those in low organic matter content areas (e.g., OC < 15 g kg–1). However, further research is necessary to assess the role of water stress on Ni/Nc and the subsequent impact on grain yield and yield response to N.

Site-specific N management requires an indicator of the N status of the soil–plant system that can be used to identify parts of the field in which N supplementation would be effective and to estimate the additional N required. Although an indicator based on the N concentration of the plant tissue offers several desirable features (e.g., it integrates the amount of plant available soil N, weather and management and can be readily adapted to on-the-go measurement), the relations demonstrated in Fig. 6 suggest that measurements made early in the growing season exhibit spatial patterns in the adequacy and excess of N that bear little relation to measurements later in the season. The data substantiate the assertion by Schröder et al. (2000) in a review of the state of the art on N indicators that "there are good reasons to question whether a young crop properly predicts the status of that crop over the entire season."


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The dynamics of spatial and temporal variation in N uptake by corn was studied across a variable landscape under three crop management treatments over four growing seasons. The influences of management practices such as tillage, legume incorporation, and fertilizer N addition on the N uptake were similar across the entire landscape. Plants grown after red clover incorporation in the spring with 0N took up as much N as plants grown with the recommended fertilizer N, often across the range of OC content in the landscape. A significant relationship between OC content and N uptake was observed across the landscape, but the magnitude and the nature of the relationship varied with climate and the crop growth stage. The relationship between OC and N uptake was generally quadratic, especially at plant maturity. Maximum uptake occurred at an OC content of about 26 g kg–1.

The N nutritional status of the crop, as reflected in Ni/Nc, indicated that plants took up more N than required for maximum DM production, especially during the early growth stages. However, Ni/Nc declined as the growing season progressed across the range of OC content and under all crop management treatments. This suggests that the N supply to the root surface was restricted as the soil dried through late July and early August. The decrease in Ni/Nc was faster in areas with low OC content (e.g., OC < 15 g kg–1). Because of the diminishing N supply as the season progressed, plants that stored excess N could remobilize this excess N during the grain-filling period. Consequently, plants grown in areas with OC content > 20 g kg–1 with legumes or fertilizer N were able to maintain maximum or near-maximum growth even 3 wk after silking. These temporal and spatial changes in Ni/Nc suggest that indicators based on plant N measurements early in the season cannot be used in site-specific N management to accurately identify areas of adequate or excess N later in the growing season.

Although this study was based on a single landscape, there are two important implications for the spatial variability in N dynamics that warrant further study. First, the role of decreasing water content in the A horizon on Ni/Nc must be confirmed. The role of decreasing soil water content must be considered from the perspective of variation in water content across the landscape and decreasing water content through the growing season. Second, the potential impact on yield and yield response to fertilizer-N of decreases in Ni/Nc caused by drought needs to be assessed.


   ACKNOWLEDGMENTS
 
The Ontario Corn Producers' Association, Agriculture and Agri-Food Canada, the Natural Sciences and Engineering Research Council of Canada, and the Ontario Ministry of Agriculture and Food provided financial support for this research. The Canadian Commonwealth Scholarship and Fellowship Program provided financial support for R.S. Dharmakeerthi during his postgraduate studies leading to the Ph.D. degree at the University of Guelph. The assistance of J. Ferguson with fieldwork and R. Pararajasingham with laboratory work is also gratefully acknowledged.


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




This article has been cited by other articles:


Home page
 Agron. J.Home page
K.-I. Kim, D. E. Clay, C. G. Carlson, S. A. Clay, and T. Trooien
Do Synergistic Relationships between Nitrogen and Water Influence the Ability of Corn to Use Nitrogen Derived from Fertilizer and Soil?
Agron. J., May 7, 2008; 100(3): 551 - 556.
[Abstract] [Full Text] [PDF]

Home page
 Agron. J.Home page
M. Al-Kaisi and D. Kwaw-Mensah
Effect of Tillage and Nitrogen Rate on Corn Yield and Nitrogen and Phosphorus Uptake in a Corn-Soybean Rotation
Agron. J., October 15, 2007; 99(6): 1548 - 1558.
[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 (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Agricola
Right arrow Articles by Dharmakeerthi, R. S.
Right arrow Articles by Beauchamp, E. G.
Related Collections
Right arrow Nitrogen
Right arrow Plant Nutrition
Right arrow Tillage

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