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NITROGEN MANAGEMENT

Nitrogen Use Efficiency and Nitrogen Uptake of juncea Canola under Diverse Environments

^a Semiarid Prairie Agric. Res. Centre, Agric. and Agri-Food Canada, P.O. Box 1030, Airport Rd. East, Swift Current, SK S9H 3X2, Canada
^b Melfort Res. Farm, Agric. and Agri-Food Canada, Box 1240, Melfort, SK S0E 1A0, Canada
^c Scott Res. Farm, Agric. and Agri-Food Canada, Box 10, Scott, SK S0K 4A0, Canada
^d Saskatoon Res. Centre, Agric. and Agri-Food Canada, 107 Science Pl., Saskatoon, SK S7N 0X2, Canada
^e 142 Rogers Rd., Saskatoon, SK S7N 3T6, Canada

^* Corresponding author (gan{at}agr.gc.ca).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Nitrogen accounts for the largest energy input in oilseed production. Understanding N use characteristics of oilseed crops will help improve N use efficiency and minimize production costs. This study determined nitrogen use efficiency (NUE, defined as seed yield produced per unit of N supply), nitrogen fertilizer use efficiency (NFUE, defined as seed yield produced per unit of fertilizer N), and crop N uptake for oilseed crops under diverse environments. Five oilseed crops, namely Brassica juncea, B. napus, and B. rapa canolas, and B. juncea and Sinapis alba mustards, were grown at seven rates of N fertilizer (0, 25, 50, 100, 150, 200, and 250 kg N ha^–1), at 11 sites (year x location combinations) in Saskatchewan from 2003 to 2005. At sites with low soil N supply or low rainfall, alba mustard, juncea canola, and rapa canola had lower NUE and NFUE than juncea mustard and napus canola. At sites with high soil N supply or rainfall, napus canola had the greatest NUE and was the most sensitive to the gradient of productivity among the five oilseeds. All oilseed species responded to N fertilizer rates in a similar manner; both NUE and NFUE decreased as N fertilizer rate increased. The minimum NUE and NFUE were obtained with N fertilizer rate greater than 150 kg N ha^–1. At sites with low soil N supply or rainfall, alba mustard had the least NUE or NFUE response to increasing N fertilizer rates and napus canola the greatest. At sites with high soil N supply or rainfall, juncea mustard had the least NUE and NFUE response to increasing N fertilizer rates and rapa canola the greatest. On average, seed N uptake was greatest for juncea canola and juncea mustard and least for alba and rapa canola. The five oilseed species had similar response patterns of seed N uptake to N fertilizer rates, while the magnitude of response varied among species. Improving NUE in oilseed production systems requires optimizing rates of N fertilizer which vary depending on environmental conditions, and soil N supply and rainfall during the critical growth period of the oilseed crops play an important role in affecting NUE.

Abbreviations: NFUE, nitrogen fertilizer use efficiency • NUE, nitrogen use efficiency • NUTE, nitrogen utilization efficiency.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication July 3, 2007.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Canola quality Brassica juncea or juncea canola is a relatively new canola crop that is developed by crossing B. juncea oriental mustard with conventional B. napus canola (Woods et al., 1991). Oil characteristics of juncea canola are equivalent to that of the napus canola and the meal of juncea canola contains low glucosinolate, differing from oriental mustard (Burton et al., 2003). The benefits of growing juncea canola have been increasingly recognized in semiarid environments such as northern Montana, southwestern Saskatchewan, and southeastern Alberta of the northern Great Plains of North America. Main advantages of juncea canola over napus and rapa canola species include vigorous seedling growth, greater degree of tolerance to heat and drought stresses, and enhanced resistance to blackleg, a common disease caused by Leptosphaeria maculans (Desm.) Ces. Et de Not. (Woods et al., 1991; Burton et al., 2003). Under conditions with high temperature and water stresses, the juncea canola usually yields better than the napus and rapa canola species (Gan et al., 2004). Therefore, this nongenetically modified canola species may provide oilseed growers with options for diversifying their production systems (Potts et al., 2003).

Brassica oilseed crops respond to N fertilizer positively even when N fertilizer is applied at rates as high as 180 kg N ha^–1 (Brandt et al., 2002). Amounts of N fertilizer required for maximum yield of oilseed species vary, depending on environmental conditions (Gan et al., 2007). Under environments with low-yielding potential, the N fertilizer required for maximum seed yield was around 160 kg N ha^–1 for mustard species and 120 kg N ha^–1 for canola species. As moving to high-yielding environments from the low, the optimum N fertilizer requirement increased by 34 kg N ha^–1 for mustard species, but stayed the same for hybrid cultivars of napus canola. However, information on N responses of juncea canola to diverse environments is limited, and little is known about N use efficiency and N uptake of juncea canola under various growing conditions.

Nitrogen use efficiency may have different definitions (Sowers et al., 1994), but it is commonly defined as seed yield produced per unit of N supplied (Moll et al., 1982). The amount of N supply is quantified as the sum of the N from fertilizer applied (Nf) plus the N uptake of aboveground plant tissues (Nt) in plots with no N fertilizer applied (Limon-Ortega et al., 2000). With this definition, NUE can be used to assess crop management practices that are supposed to affect the amount of residual soil N, mineralized N, and the responses to applied fertilizer N. The NUE typically decreases with progressively greater rates of N fertilizer applied (Fageria and Baligar, 2005). At a level crop plants are approaching physiological inefficiencies of N use, N losses may occur with further increases of N supply (Limon-Ortega et al., 2000). The NUE may interact with crop species and growing conditions. For example, Hocking et al. (2002) demonstrated that extra kilograms of seed yield obtained from extra N application (relative to unfertilized checks) was greater for mustard than for canola at locations when low rates of N fertilizer were applied. However, this trend of differences between crop species was not shown at locations where high rates of N fertilizer were applied.

In grain cropping systems, N balance is the bookkeeping of N inputs relative to N losses (Racz et al., 1965). Nitrogen losses associated with N exports from harvested grains represent an important component of the N balance (Harbison et al., 1986), and the amount of N exported through grains may vary depending on crop species. For example, N lost through the harvest of napus canola was about 34 kg N ha^–1 (Hocking et al., 2002), whereas it was in the range of 40 (Hocking et al., 2002) to 48 kg N ha^–1 (Singh and Singh, 1984) for Indian mustard. Greater N removal by Indian mustard was attributable to greater N concentration in the mustard seed compared to napus canola. In some cases, the quantity of N lost through grain harvest exceeds the amount of N applied through fertilizer (Hocking et al., 2002), suggesting that N balance in oilseed production is influenced by residual soil N supply.

Nitrogen accounts for the largest energy use and input expenses in oilseed production systems (Zentner et al., 2002). The improvement of NUE is the key strategy for the development of sustainable agricultural systems that allow maximizing productivity with minimum energy inputs and N losses. An understanding of the N use characteristics of juncea canola will ensure that this new oilseed species is properly adapted to target production areas. We hypothesized that the N use characteristics of juncea canola would be similar to those of more popular napus and rapa canola as well as juncea mustard species. The objective of this study was to determine NUE and N uptake of juncea canola in comparison with napus and rapa canola species and alba and juncea mustard species under varying soil and climatic conditions with low-, average-, and high-yielding potentials.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Site Description and Experimental Design
Field experiments were conducted at four Saskatchewan locations: Melfort, Saskatoon, Scott, and Swift Current, from 2003 to 2005. Crops at Scott in 2005 were destroyed by hail during late flowering, thus no results from this location-year were obtained. The characteristics of the experimental locations, soil types, and field conditions are summarized in Table 1 . Detailed information on the experimental design and crop management practices used in this study has been reported previously (Gan et al., 2007). In brief, five oilseed species, S. alba yellow mustard (cv. AC Base); Brassica juncea canola (cv. Amulet); B. juncea condiment mustard (cv. Cutlass); B. rapa canola (cv. Hysyn 110); and B. napus hybrid canola (cv. InVigor 2663), were evaluated in a factorial combination with seven rates of N fertilizer (0, 25, 50, 100, 150, 200, and 250 kg N ha^–1) using a randomized complete block design with four replicates. Plot size (experimental unit) was between 4.8 and 12 m², varying among locations due to equipment. Plots were seeded between 30 April and 30 May varying from year to year, and seeding rates were adjusted for seed size and pre-seed germination of the species and cultivars to target a plant stand of 80 to 100 plants m^–2. The amounts of available nutrients in soil were determined (Hamm et al., 1970) before seeding at each location each year (Table 2 ).

Measurements and Calculations
Aboveground plant biomass was harvested from one 0.5 to 1.0 m² area per plot at maturity. Plants samples were oven dried at 50 to 70°C for 7 to 10 d and weighed to determine total aboveground dry weight. Entire plots were swathed or desiccated with a preharvest application of glyphosate at label recommendations before being harvested with a plot-scale combine. Seed moisture was near 11% at harvest. Infrared spectroscopy was used to measure total N concentration of seed samples and of straw plus chaff samples. To minimize analytical costs, the total N concentration of each treatment was measured on two sets of subsamples; the first set was assembled by bulking the seed samples from replicates 1 and 2, and the second from replicates 3 and 4.

Nitrogen use efficiency was calculated as seed yield (kg seed ha^–1) produced per unit of N supply (kg N ha^–1), that is, NUE = seed yield/(Nt + Nf), where Nt equals N derived from soil as determined by N uptake in seed + straw in control plots where zero-N fertilizer was applied, and Nf equals amount of N from fertilizer. The NUE was further partitioned into: (i) NFUE which was defined as seed yield produced per unit of fertilizer N [(seed yield with applied fertilizer N) – (seed yield in zero-N control)]/Nf, and (ii) nitrogen utilization efficiency (NUTE) which was defined as seed yield produced per unit of N derived from soil without fertilizer N (NUTE = seed yield/Nt). These different determinations of N use characteristics allow better understanding the responses of oilseed crops to various sources of N under diverse environmental conditions.

Statistical Analysis
Data were analyzed using the PROC MIXED procedure of SAS (Littel et al., 1996) with applied treatments (i.e., crop type, N rates) as fixed effects, and replicates and sites (the term site refers to location x year combinations and is used throughout the entire text of the paper) as random effects. With sites designated as random effects, inferences regarding the N management of different oilseed crops can be extended to the other similar areas of canola–mustard growing regions. Because of the complex nature of the interactions between crop, N fertilizer rate, and diverse environments encountered in this study, an extension of the statistical model was used to further explore important site x treatment interactions (Littel et al., 2002). The analysis included three covariables (i) overall mean response for seed yield, (ii) overall mean response for Nt, and (iii) the effect of rainfall during the June–July period. Rainfall during the June–July period was most critical for canola and mustard crops in the northern Great Plains (Angadi et al., 2000; Miller et al., 2001). The weather data were retrieved from Environment Canada weather stations near plot areas at each location. The three covariables were incorporated in the statistical model to determine the effects of crop type, N fertilizer rate, and crop type x N rate interactions. The most "informative" interaction was the one that was: (i) statistically significant, (ii) provided greatest reduction for variance estimate of the site x crop x N rate interaction, and (iii) resulted in an improved model fit (AICC: corrected Akaike's information model fit criterion) relative to standard analysis of variance. Means were estimated for the treatment effect associated with the informative interaction at low (mean seed yield at 1196 kg ha^–1), average (1700 kg ha^–1), and high (2263 kg ha^–1) levels of productivity. Treatment effects and variance estimates were declared significant at P < 0.05.

A grouping methodology, as described by Francis and Kannenberg (1978), was used to explore random variability or stability for the responses of a given treatment (i.e., crop type) or treatment combination (i.e., crop type x N rate combination) to diverse environments. Means and coefficients of variation (CV) for each crop x N rate combination were estimated across the various levels of treatment means. Each variable was plotted against its corresponding CVs, producing a biplot. The biplot, together with the scatter of data points, was used to identify four response categories: Group I: high means (i.e., NUE or NFUE) with low variability (optimal); Group II: high means with high variability; Group III: low means with high variability (poor); and Group IV: low means with low variability. These biplots gave an indication of the relative variability or stability of a crop type across various N rates and diverse environments.

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Straw and seed yields differed significantly among the five oilseed species and the magnitude of the difference was influenced by environmental conditions (Table 3 ). Also, there were significant linear and quadratic responses of yields to varying rates of N fertilizer for all oilseed species. Detailed results on the yield responses to N fertilizer rates and their variability across diverse environments have been discussed in a previous report (Gan et al., 2007). In the present paper, yield data were used only for determination of NUE and N uptake characteristics (Table 4 ) as well as variance estimate for treatment and random effects (Table 5 ).

Crop Species and Sites
Averaged across 11 sites, the napus canola and juncea mustard had the greatest NUE, NFUE, and NUTE, the alba mustard the lowest, and the juncea canola intermediate (Table 4). The difference among crop species in N use characteristics was closely linked to seed yield. At high-yielding sites, all oilseed species had greater NUE and NFUE compared to low-yielding sites. Among the three N use variables, NUTE > NFUE > NUE, and the ranking was similar for all crop species.

There were significant crop species x site interactions for NUE and NFUE (Table 3). The interaction was explored using covariance analysis where yield, Nt, and June–July rainfall were considered as covariables (Table 5). The analysis revealed that at sites with low- to average-yielding potentials, the alba mustard, juncea canola, and rapa canola had a similarly lower NUE than the juncea mustard and napus canola (Table 6 ). At sites with high-yielding potential, the napus canola had the NUE value that was 5.5 kg ha^–1 per kg N ha^–1 greater than the NUE obtained for the juncea mustard; both being greater than the three other oilseed species. The hybrid cultivar of the napus canola was the most sensitive to the gradient of productivity. The response of NFUE to the gradient of productivity followed a similar trend as the responses of NUE with a few exceptions (data not shown).

At sites with average (64 kg ha^–1) Nt:

At sites with high (180 kg ha^–1) Nt:

where y is NUE, and x is N fertilizer rate.

View larger version (14K): [in this window] [in a new window]   Fig. 1. Polynomial regression trend lines of N use efficiency (NUE = seed yield/(Nt + Nf); where Nt equals N derived from soil as determined by N uptake in seed + straw in zero-N control, and Nf equals amount of N from applied fertilizer) on N fertilizer rates at environmental sites with (A) low, average, and high levels of Nt and (B) low, average, and high June–July rainfall. The unit of yield is in kg seed ha–1 and the unit of N supply is in kg N ha–1 (this is also applicable to Fig. 2, 3, and 4). The data were averages of five oilseed crops across 11 sites (location x year combinations) in Saskatchewan, Canada, 2003 to 2005.

View larger version (29K): [in this window] [in a new window]   Fig. 2. Polynomial regression trend lines for N use efficiency (NUE = seed yield/(Nt + Nf); where Nt equals N derived from soil as determined by N uptake in seed + straw in zero-N control, and Nf equals amount of N from applied fertilizer) on N fertilizer rates at environmental sites with (Left) low, average, and high levels of Nt and (Right) low, average, and high June–July rainfall, for the five oilseed crops across 11 sites (location x year combinations) in Saskatchewan, Canada, 2003 to 2005.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Polynomial regression trend lines for the seed N uptake variable at sites with low-, average-, and high-levels of Nt (graphs on the left) and of June–July rainfall (graphs on the right), for five oilseed crops across 11 sites (location x year combinations) in Saskatchewan, Canada, 2003 to 2005.

View larger version (39K): [in this window] [in a new window]   Fig. 4. Biplot (mean vs. CV) for crop species x N fertilizer rate combination treatments for data collected across 11 sites (location x year combinations) in Saskatchewan, Canada, 2003 to 2005. The letter on the label of the data point indicates crop cultivars (AC = AC-Base alba mustard, Am = Amulet juncea canola, C = Cultlass juncea mustard, H = Hysyn 110 rapa canola, and I = InVigor 2633 napus canola) and the following number indicates the N fertilizer rate. For example, the data point ‘H50’ means the cv. Hysyn 110 at 50 kg N ha–1. A number closely clustered or clustered near the origin were not labeled.

Similarly, the effect of N fertilizer rates on NUE was interacted with the levels of June–July rainfall (Fig. 1B). The regression responses were described as follows:At low (100 mm) rainfall sites:

At moderate (178 mm) rainfall sites:

At high (250 mm) rainfall sites:

where y is NUE, and x is N fertilizer rate. The five oilseed species exhibited a similar NUE response pattern to N fertilizer rates, while the magnitude of the responses differed among crop species and were interacted with sites (Fig. 2 ). At sites with low to average soil N supply or rainfall, the alba mustard had the least NUE response to the changes of N fertilizer rates and the napus canola the greatest. At sites with high soil N supply or rainfall, the juncea mustard had the least response of NUE to the changes of N fertilizer rates and the rapa canola the greatest.

Overall, response patterns of NFUE to the changes of N fertilizer rates and their interactions with environmental conditions were similar to the NUE response patterns described above (data not presented). Only difference was that NUE decreased all the way as the N fertilizer rates increased to 250 kg N ha^–1, whereas the NFUE decreased as fertilizer N rates increased to near 130 kg ha^–1, and thereafter, the NFUE response was leveled off or became slightly more positive. For NUTE, the variance estimate for interaction between crop and N rate was statistically not important and the proportion of variance explained by this interaction relative to the overall variation among sites was relatively low (Table 3).

Nitrogen Uptake
Correlation analysis showed that total N uptake was highly related to seed and straw yields and total N concentrations (Table 7 ). Also, the N uptake in seed was closely associated with yields and seed N concentrations. However, seed N uptake was not directly associated with straw N concentration. The magnitude of the associations was interacted with both crop species and N fertilizer rates.

Similarly, the rainfall by N fertilizer rate interaction for seed N uptake was reflected by the declining linear and quadratic coefficients in size when the rainfall increased from low to high (Fig. 3). All crop species showed similar aforementioned trends at sites with low (100 mm) to average (178 mm) rainfall. However, at sites where there was high rainfall (250 mm), the rapa and napus canola had the lowest seed N uptake responses to increased N fertilizer rates and the juncea mustard the highest. For the juncea canola and the alba mustard, the response of seed N uptake to the increased N fertilizer increased until N rates approaching 150 kg ha^–1 and then declined.

Variability
The variability of the N use characteristics was assessed using biplots where the mean values of a variable (i.e., NUE) were plotted against their corresponding CVs with each data point being the crop type by N rate combination (Fig. 4 ). For example, the data point "jm50" means the juncea mustard at 50 kg N ha^–1. For each variable, the biplot categorized variability responses into four groups (i.e., Group I, high means with low variability; Group II, high means with high variability; Group III, low means with high variability; and Group IV, low means with low variability). The biplots showed clear trends for the NUE, NUTE, straw N uptake, and total N uptake variables. Increasing NUE mean values increased the variability of NUE, with highest NUE values being coupled with highest variability. The variability of NUE was affected mostly by N fertilizer rates and was less affected by crop species. The highest NUE variability was obtained with those treatments receiving none to low rates of N fertilizer. The pattern of NUTE in variability was similar to the pattern of NUE. Conversely, the biplots showed that increasing mean values in straw N and total N uptake decreased their variability. Crops having the highest N uptake had the lowest variability. However, both NFUE and seed N uptake did not show any trend in variability; all data points scattered all over the four variability groups.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Only one cultivar was selectively used for each of the five oilseed species in the present study; this was mainly due to consideration of the size of the experiment. Also, previous studies have shown that differences between cultivars within a canola or mustard species are always smaller than differences between oilseed species (Angadi et al., 2000; Gan et al., 2004). For example, regardless of choice of cultivars, alba mustard always matures about 1 to 2 wk earlier than juncea mustard under the same growing conditions in the northern Great Plains. Therefore, in this paper, we tend to discuss differences among the five oilseed species rather than specific cultivars. Additionally, fertilizer application was implemented using conventional tillage at Melfort and no-till systems at all other locations. However, preliminary analysis showed no significant interactions of tillage and treatment effects (data not shown), therefore, the difference in tillage was ignored in all statistical analyses.

The juncea canola is developed to have the drought tolerance trait of oriental mustard, yet has seed quality attributes of the napus and rapa canola (Burton et al., 2003), thus allowing producers in the northern Great Plains more options to produce oilseed with a marketing choice of species. Previous studies have shown that N fertilizer accounts for the most costly inputs in oilseed production, and the reduction of energy input costs is the key in development of sustainable agriculture (Zentner et al., 2002). Considering the relative greater N demands of oilseed crops relative to crops like spring wheat, it is particularly important to understand differences in N use characteristics and corresponding N fertilizer management for different oilseed species.

Nitrogen Use Efficiency
The results of this study showed that there was a general trend of decreasing NUE with increasing N fertilizer rate for all the five oilseed species studied. This is in agreement with previous findings by others (Hocking et al., 2002; Fageria and Baligar, 2005). Our study also showed that the magnitude of decrease in NUE with increasing rates of N fertilizer was interactively affected by soil N supply aside from fertilization. At any given N fertilizer rate <100 kg N ha^–1, the NUE was high as soil N supply was low. The preceding effect likely reflects better match of soil N supply with physiological N demand of the oilseed plants. It is also possible that soil N losses via denitrification or alike are minimum when fertilizer N is low and that mineral N pool is smaller (Nyborg et al., 1999).

The magnitude of decrease in NUE with increasing rates of N fertilizer was interactively affected by rainfall during the months of June–July when canola and mustard are in their vigorous vegetative growth and flowering period. At any given N fertilizer rate <100 kg N ha^–1, the NUE was greater at sites where June–July rainfall was lower compared to sites with more rainfall. The preceding trends were consistent among the five oilseed crops, indicating that N use attributes of oilseed crops are largely influenced by environmental conditions rather than by genotypic or phenotypic differences. June–July rainfall was considered as an environmental covariable in this study, because the variance of analysis revealed that this covariable explained a substantially large portion of the site by N fertilizer rate interaction that reflected on seed yield and NUE. Previous studies have shown that water supply during the period of vigorous vegetative growth and flowering is critical for oilseed production (Morrison and Stewart, 2002; Gan et al., 2004).

There was a general trend of decreasing NFUE with increasing N fertilizer rate. The NFUE improved more when levels of soil N supply were <100 kg N ha^–1. The hybrid cultivar of the napus canola and the juncea mustard had the greatest NFUE responses to N fertilizer rate and were the most sensitive to soil N availability. The genetic make up of these two species may allow more elastic responses to environmental and N fertility conditions than the three other oilseed species. Coincidently, the napus hybrid canola and juncea mustard were the highest yield producers, leading us to speculate that there might be some associations between mechanisms controlling yield and NFUE.

Nitrogen Uptake
Seed N uptake increased with increasing N fertilizer rate in canola and mustard species in most cases; this is in agreement with previous reports (Malhi and Gill, 2004). Our study also showed that the magnitude of changes in seed N uptake among oilseed species varied with soil N supply and rainfall during the June–July period. The maximum N uptake in seed was lowest (60 kg N ha^–1) and its response to applied N fertilizer rate was greatest when soils contained available N <20 kg N ha^–1. This was probably due to less contribution of N from soil toward N uptake in seed, resulting in greatest response of N uptake to applied N fertilizer at low N soils. On the other hand, in soils with available N of 180 kg N ha^–1, the maximum seed N uptake averaged about 120 kg N ha^–1. The response of seed N uptake to N fertilizer rates was low, although it varied among oilseed species. This suggests that in soils with high N supply, there will be less contribution of N fertilizer toward N uptake in seed because of greater availability of N from soil to the crop, thus reducing N uptake response to applied N fertilizer.

Under high soil N conditions, the juncea mustard had the greatest N uptake in seed and also the greatest response to increasing N fertilizer rates. This indicates that juncea mustard is the most efficient oilseed species in making best use of both soil and fertilizer N. For alba mustard, rapa canola, and napus canola, the lower N uptake in seed and relatively flat response patterns to N fertilizer rate when soil N supply was high, suggesting that these oilseed species are poor users of N both from soil and fertilizer. For juncea canola, seed N uptake and response pattern indicated that this oilseed species is intermediate in using soil and fertilizer N between the juncea mustard and the other oilseed species. The order of performance of oilseed species in taking up N and their response to fertilizer N can be generated as juncea mustard > alba mustard > napus canola > juncea canola > rapa canola under average soil N conditions, and while napus canola = alba mustard = juncea mustard > juncea canola = rapa canola under low soil N conditions. These results indicated that rapa canola was the least efficient user of soil and fertilizer N regardless of soil N supply conditions.

Crop N uptake response to increasing rates of N fertilizer decreased and varied among oilseed species when rainfall during June to July increased from low to high. With a few exceptions, all oilseed species showed similar N uptake response trends at sites with low (100 mm) to average (178 mm) rainfall. Under high rainfall (250 mm) conditions, the rapa and napus canola had the lowest responses to increasing N fertilizer rates, and the juncea mustard the highest. These results indicate that rapa canola is the poorest user of soil and fertilizer N regardless of soil moisture conditions and juncea mustard is the best or near best user of N from soil and fertilizer under high soil moisture conditions.

A reasonable N balance associated with seed N export during harvest is an important attribute of a sustainable cropping system. A system with an inordinate amount of N exported may create a significant "N leak", which has both long-term economic consequences (Nyborg et al., 1999). Our results showed that the two juncea species exported up to 28 kg N ha^–1 more than conventional canola and mustard species when soil N availability was high. Seed N uptake was highly correlated with seed yield, indicating that the practical implications of resolving N losses associated with oilseed crop harvest will not be easy, especially when seed yield is economic cornerstone of canola and juncea production. Genetic enhancement and best crop management to minimize N concentration in oilseed seed may decrease N losses via seed export with minimum yield penalty, particularly for the juncea canola and juncea mustard. However, this may decrease protein concentration in oilseed meal which is often used for animal feed.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Maximum NUE and NFUE were achieved at N fertilizer rates <100 kg N ha^–1, less than the rates necessary to maximize seed yield which required N fertilizer rates near 130 kg N ha^–1. High-yielding hybrid cultivars of napus canola and the juncea mustard had greater NFUE and N uptake at lower N fertilizer rates (<50 kg N ha^–1) than other oilseed crops, especially under more productive environments. These two oilseed species may be better scavengers of soil and fertilizer N. It was noteworthy that juncea canola required more N to achieve optimum seed yield and exported more N via seed harvest especially when soil N availability was high. Therefore, an optimal N fertilizer rate for NUE and seed yield may be difficult to reconcile for juncea mustard and juncea canola species. It may be wise to determine optimal N fertilizer rates for the preceding oilseed crops based largely on economics rather than NUE and N uptake. Additionally, the refinement of N fertilizer management (e.g, split and timing of N applications) may be necessary for juncea canola to achieve maximum seed yield while minimizing N losses via seed harvest.

	ACKNOWLEDGMENTS

 
The authors acknowledge the funding from the Saskatchewan Canola Development Commission, Agricultural Development Fund, and Agriculture and Agri-Food Canada Matching Investment Initiative. We also acknowledge the expert technical assistance of Cal McDonald, Greg Ford, Larry Spensor, Darwin Leach, Don Rode, Cliff Powlowski, and Donna Chen.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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