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Regional Evaluation of Critical Nitrogen Concentrations in Winter Wheat Production of the North China Plain

^a Dep. of Plant Nutrition, College of Resources and Environ. Sci., China Agricultural Univ., Beijing 100094, China
^b School of Veterinary Medicine, Univ. of Pennsylvania, Kennett Square, PA 19348
^c College of Resources and Environ. Sci., Qingdao Agricultural Univ., Qingdao 266109, China
^d College of Resources and Environ. Sci., Henan Agricultural Univ., Zhengzhou 450000, China
^e Inst. of Soil Sci. and Fertilizer, Shanxi Academy of Agric. Sci., Taiyuan 030031, China
^f College of Resources and Environ. Sci., Shandong Agricultural Univ., Taian 271018, China
^g Institute of Soil Sci. and Fertilizer, Henan Academy of Agricultural Sci., Zhengzhou, 450000, China

^* Corresponding author (chenxp{at}cau.edu.cn).

	ABSTRACT

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

 
Investigating critical nitrogen concentration (CNC) in grain and straw provides insights into N nutrition, and can serve as a guide to improved agricultural practice. This regional study evaluated the relationship between N fertilization rate and grain yield, N concentration, potential N loss, and determined critical grain and straw nitrogen concentrations (CGNC and CSNC) for winter wheat (Triticum aestivum L.) production in China. At the economically optimum nitrogen rate (EONR), grain N concentration was similar to the maximum value calculated using a linear plus plateau model, while straw N concentration was significantly less than the relevant maximum value. Soil nitrate N content after harvest and apparent N loss for maximum straw N concentration increased by 19 and 9 kg N ha^–1 compared to values at the EONR. Based on nine field experiments, CGNC and CSNC corresponding to optimal N rate were established to be 21.9 g kg^–1 (20.8–23.0 g kg^–1) and 6.8 g kg^–1 (6.5–7.1 g kg^–1), respectively. An evaluation of CGNC and CSNC across 111 on-farm sites indicated that while many sites had grain and straw N concentrations falling within the CGNC and CSNC, a substantial portion of the sites had grain and straw N concentrations falling outside of the CGNC and CSNC or falling within the critical ranges when N supply was deficient (0 N control) or excess (at farmer's N practice). This region-wide study provided evidence for the usefulness of CSNC, and particularly CGNC, as indicators of N deficiencies in wheat production; however, neither indicator provided information about excess N fertilization.

Abbreviations: CGNC, critical grain N concentration • CNC, critical N concentration • CSNC, critical straw N concentration • EONR, economically optimal N rate • NCP, North China Plain

Received for publication March 31, 2008.

	INTRODUCTION

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

 
THE INTENSIFICATION of agricultural production has been accompanied by decreased N use efficiency and increased environmental pollution in many regions (Matson et al., 1997; Shanahan et al., 2008). In China, recovery N efficiency in cereal grain production decreased from 30 to 35% in the 1980s (Zhu, 1998) to <20% today (Cui et al., 2008; Wang, 2007), the latter is substantially less than the world average of 33% (Raun and Johnson, 1999). Increased N fertilizer application and decreased N recovery are associated with a widespread problem of groundwater contamination by nitrate. Nitrogen emission to the atmosphere also increased, which in turn results in substantial N deposition. In the Beijing region, bulk deposition from rain ranged from 26.6 to 38.5 kg N ha^–1 yr^–1 and averaged 30.6 kg N ha^–1 yr^–1 during 1998 to 2004 (Liu et al., 2006).

Sustainable agricultural development must address relevant environmental challenges while endeavoring to meet growing food demand with high crop yield. Toward this, agronomically sound and environmentally acceptable N management practices are essential. The European Union recommends N balances as a useful tool for on-farm N management. However, it was suggested that the N balance approach should be complemented by test-based indicators to obtain insight into N dynamics (Öborn et al., 2003; Oenema et al., 2003). Developing N status indicators for particular production systems has received much attention, for example, the chlorophyll meter (Singh et al., 2002) or remote-sensing (Hong et al., 2006) technologies for determining the need of N sidedress for cereal crops. These two methods may be effective in preventing N deficiency, but they are of limited use in detecting luxury N uptake (Dwyer et al., 1995). Soil inorganic N (NH₄⁺-N plus NO₃–N) or nitrate N test has proven useful for assessing N status during the wheat growing season (Magdoff et al., 1990). However, a shortcoming of this approach is that considerable effort is required in sampling and sample processing, which for practical purposes prohibits its routine use as a regional monitoring tool.

Previous research has indicated a close relationship between N or protein concentration in the grain and soil N supply, which provided the basis for the idea of using grain N concentration as an indicator of N status (Pierre et al., 1977; Egelkraut et al., 2004). The term CNC was introduced, defined as the minimal N concentration in grain at maximum grain yield. However, studies have shown CNC to be highly variable (Glenn et al., 1985; Selles and Zentner, 2001), depending on the region (environment), cereal species, even genotypes within species (Fowler et al., 1990). Herrmann and Taube (2005) made a case for studying CNC at the geographical scale to improve its relevance as a predictive tool.

The North China Plain (NCP), located in northeastern China, is one of the most important areas for cereal crop production in the country. The region has a dominate winter wheat (October–June) and summer maize (June–September) rotation system, accounting for 61 and 33% of the nation's annual production of wheat and maize (Zea mays L.). During the last two decades, applying large amount of N fertilizer (typically >350 kg N ha^–1) to winter wheat has become a common practice. Such a magnitude more than doubles the amount of N required for maximum grain yield in most cases (Cui, 2005). Excess N application has been shown to result in nitrate accumulation in the soil profile and increased N loss to ground and surface waters (Ju et al., 2006). To minimize N loss while sustaining crop yield, a diagnostic tool is needed for the assessment of N status of winter wheat that can guide farmers in adjusting their N fertilization practices.

Typically, crop N uptake responding to N fertilizers can be described by a curve characterized by a minimum percentage, a ‘poverty adjustment’, and luxury consumption zone (Macy, 1936; Fowler, 2003). However, studies on luxury N consumption and potential environmental consequences have not been well described, particularly under on-farm conditions. The objectives of this study were to: (i) establish relationships between N application rate and winter wheat grain yield, plant N status, and potential N loss through intensive fertilizer N rate studies at nine experimental sites; (ii) evaluate critical grain and straw N concentrations at harvest through extensive field trials conducted at 111 sites across three provinces.

	MATERIALS AND METHODS

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The climate in the NCP is warm-temperate subhumid continental monsoon with cold winters and hot summers. The annual cumulative mean temperature for days with mean temperatures above 10°C is 4000 to 5000°C, and the annual frost-free period is 175 to 220 d. Annual precipitation is 500 to 700 mm, with about 70% of the rainfall occurring during the summer maize growing season (Zhang and You, 1996). The amount and distribution pattern of rainfall vary widely from year to year, affected by the continental monsoon climate. Depending on the weather, winter wheat typically receives three irrigations—before winter, at shooting stage during which stem elongation occurs, and in flowering stage (Li et al., 2005; Sun et al., 2006). Fertilization for winter wheat usually includes two applications, one at planting and the other around shooting stage (Cui, 2005; Cui et al., 2008).

The Intensive Experiments
Nine field experiments were conducted on-farm (i.e., in farmers' fields) during 2003–2005, in Huimin County (HM), Shandong Province. Soil texture and other selected soil test parameters (nitrate N in 0–90 cm soil layer; OM, total N, Olsen-P and NH₄OAc-K in 0–30 cm soil layer) were shown in Table 1 .

Optimal N rate for each experimental site was determined according to an in-season N management strategy developed by Chen et al. (2006), Zhao et al. (2006), and Cui et al. (2008). Following this strategy, N target values are established for two stages during winter wheat growth and necessary adjustments made according to soil nitrate N test results. For the stage of planting to shooting, N target value is 85 kg N ha^–1 which is adjusted by subtracting the amount of nitrate N in 0 to 30 cm soil; for the stage of shooting to harvest, the N target value is 170 kg N ha^–1 which is also adjusted by subtracting the amount of nitrate N in 0 to 90 cm soil profile.

A randomized complete block design was employed with four replications; each plot measured 40 m². All plots received 90 kg P₂O₅ ha^–1 as triple superphosphate [Ca(H₂PO₄) ₂·H₂O] and 60 kg K₂O ha^–1 as potassium chloride (K₂SO₄) before planting. Urea [CO(NH₂)₂] application was made before winter wheat planting and again at shooting stage according to the N treatments.

The Extensive Experiments
A total of 121 sites were selected in seven counties of three provinces (Table 2 ), all within the NCP region. Each site was located within a farmer's field and was managed by the farmer with winter wheat-summer maize rotation system. The experiments were conducted during 2003–2005. Soil nitrate N in 0 to 90 cm soil layer and selected chemical properties in 0 to 30 cm soil layer were given in Table 2.

Plot size was 40 m² for 32 sites with three or four replications. The mean values per N treatment were used for data analysis in this study. For the remaining 89 sites, plot size was 120 m² with no replication. Based on soil P and K test results, all plots received appropriate amounts of triple superphosphate (from 0–150 kg P₂O₅ ha^–1) and potassium chloride (from 0–120 kg K₂O ha^–1) before planting. Urea application was made before winter wheat planting and again at shooting stage according to the N treatments. Except for fertilizer application and harvest, the plots were managed in the same manner as the rest of the field by the individual farmer using standard field equipment. Different varieties of winter wheat were used among the experimental sites, as selection of variety was made by the respective farmers. Nevertheless, all varieties used had high yield potential (>7 t ha^–1).

Sampling and Laboratory Procedures
Soil samples from all experimental sites were obtained three times: before planting at the beginning of October, near shooting before N fertilization in early April, and at harvest in the middle of June. Each sampling included five cores per plot taken to a depth of 90 cm at 30-cm increment. Composite samples before planting were extracted with 1:10 ratio of soil: 0.01 mol L^–1 CaCl₂ and analyzed for NH₄⁺-N and NO₃^–-N using Continuous Flow Analysis (TRAACS 2000). Soil water content was measured by oven drying at 105°C with 24 h. Subsamples from the 0 to 30 cm soil layer collected before planting were air-dried, sieved, and used to measure organic matter (Walkley, 1947), total N (Bremner, 1996), Olsen-P (0.5 M NaHCO₃; Olsen et al., 1954) and NH₄OAc-K(1 M ammonium acetate at pH 7; van Reeuwijk, 1992).

Soil inorganic N was also measured for every plot at harvest using Continuous Flow Analysis (TRAACS 2000), and used to estimate the N balance. For soils obtained before planting and near shooting, subsamples were extracted with 1:1 ratio of soil to distilled water, and soil nitrate N concentrations were determined using nitrate N-test strips and a reflectometer. The results were used to calculate optimal N rate by difference between N target value and the measured soil nitrate N content. The analytical procedure for these samples can be easily performed in the field. A good correlation between these two methods of soil nitrate N testing using Continuous Flow Analysis and nitrate N-test strips was reported for the NCP region in China (Cui et al., 2005) and in Germany (Schmidhalter, 2005). Soil inorganic N and nitrate N (unit, kg N ha^–1) was calculated by average bulk density in the NCP of 1.33, 1.41, and 1.43 g cm^–3 for 0 to 30 cm, 30 to 60 cm, and 60 to 90 cm soil layers, respectively.

At maturity, wheat plants in an area of 3 by 1 m² in the middle of each plot were harvested manually. Dry weights of stems and grain were determined after separation and being oven-dried at 60°C at forced draft oven. Subsamples passing through a 1-mm screen in a sample mill were mineralized using H₂SO₄–H₂O₂ and N was measured using the standard Kjeldahl method (Horowitz, 1970).

Data Analysis
Wheat grain yield, grain N and straw N concentration response curves to N rate at each of the nine sites were generated using the NLIN procedure in SAS (SAS Institute, 1998). Three response models were evaluated in this study: quadratic, quadratic with plateau, and linear with plateau (Cerrato and Blackmer, 1990). The linear with plateau model produced the best fit, and thus the results were reported here. The EONR was calculated using 0.375 $ kg^–1 for wheat grain and 0.5 $ kg^–1 for N fertilizer (Chen et al., 2004). Grain and straw N concentrations at the EONR were calculated using the fitted response functions.

For each experimental site, relative grain yield was calculated as:

[1]

where GY_treatment stands for wheat grain yield in each N treatment, GY_max for the maximum grain yield at that site.

Apparent N loss was estimated following the procedure used by Zhao et al. (2006):

[2]

where initial soil N_inorganic stands for the amount of inorganic N in 0 to 90 cm soil layer before planting, residual soil N_inorganic for the amount of inorganic N in 0 to 90 cm soil layer at harvest, N_fer for fertilizer N rate, and N_organic for apparent N mineralization during the wheat growing season. The amount of N_organic was estimated by the sum of residual soil inorganic N and crop N uptake subtracted by initial soil inorganic N for the 0 N control. The same procedure has been used by Cabrera and Kissel (1988) and Olfs et al. (2005).

The relationship between N rate and soil residual nitrate N content at harvest or apparent N loss was simulated by linear and exponential models. In most cases, the linear model fit the data better for the relationship between N rate and soil residual nitrate N content, while the exponential model was superior in describing the relationship between N rate and apparent N loss.

For all experimental sites, we used experimental sites as replicates, and data were analyzed following analysis of variance using One-way ANOVA in SAS (SAS Institute, 1998), and means of N treatment were compared based on least significant difference (LSD) at the 0.05 level of probability.

	RESULTS AND DISCUSSION

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Relationships between Nitrogen Rate and Grain Yield, Grain Nitrogen and Straw Nitrogen Concentrations, and Potential Nitrogen Loss
Results from the intensive experiments with six N rates and nine sites indicated that grain yield and grain N and straw N concentrations as a function of N rate fit linear with plateau models across all experimental sites (P < 0.01). Calculated EONR averaged 132 kg N ha^–1 with a range from 92 to 189 kg N ha^–1. The minimum N rate corresponding to maximum grain N concentration ranged from 101 to 164 kg N ha^–1 with a mean of 137 kg N ha^–1, which is similar to EONR. For maximum straw N concentrations, however, the corresponding N rate ranged from 99 to 184 kg N ha^–1 with a mean of 177 kg N ha^–1, which is substantially greater than the EONR. For individual sites, the EONR were similar to the minimum N rates for maximum grain N concentration but substantially lower than that for maximum straw N concentrations (Table 3 ). This indicates that when luxury N consumption occurred, the excess N did not lead to greater grain yield or greater grain N concentration but accumulated in the straw.

Pooling data from all nine sites together, the response in wheat grain yield, grain N and straw N concentrations to fertilizer N rate also fit to a linear-plateau model (P < 0.01, Fig. 1 ). Minimum N rate needed to achieve maximum grain yield, maximum grain N or maximum straw N concentration as calculated from the models was 132, 141, and 173 kg N ha^–1, respectively. Based on these models, calculated maximum grain N and maximum straw N concentrations were 22.0 and 7.4 g kg^–1, respectively, and grain N and straw N concentrations at the minimum N rate for maximum grain yield were 22.0 and 6.9 g kg^–1, respectively. Based on the relationship between N rate and soil residual nitrate N content or apparent N loss, calculated residual soil nitrate N was 102, 106, and 121 kg N ha^–1 and apparent N loss was 24, 26, and 33 kg N ha^–1 for the minimum N rate for maximum grain yield, maximum grain N or straw N concentration, respectively. These results with all experimental sites pooled together were similar to the data obtained for individual sites (Tables 3 and 4).

View larger version (24K): [in this window] [in a new window]   Fig. 1. (a) Relative grain yield, (b) grain N concentration, (c) straw N concentration, (d) residual soil nitrate N at harvest, and (e) apparent N loss as functions of fertilizer N rate. Data were pooled from nine field experiments (n = 54).

Critical Grain and Straw Nitrogen Concentrations
Grain and straw N concentrations at the EONR were assessed and designated as CGNC and CSNC for the purpose of improved N management. The average CGNC was 21.9 g kg^–1 ( ± 5% range = 20.8–23.0 g kg^–1) and CSNC was 6.8 g kg^–1 (6.5–7.1 g kg^–1) from the nine experiments.

The utility of the CGNC and CSNC was evaluated using data from the extensive experiments at 121 field sites. Of those sites, 111 had optimal N rate less than the farmer's N practice rates; for unknown reasons 10 sites were in the opposite (i.e., optimal N rate greater than the farmer's N rate). Data from the 10 sites were excluded from the analysis.

Across the 111 experiments at the optimal N rate, grain N concentrations averaged 21.5 g kg^–1 and straw N concentrations averaged 6.5 g kg^–1 (Table 5 ), which were comparable to the CGNC (21.9 g kg^–1) and CSNC (6.8 g kg^–1) established through the intensive experiments discussed earlier. Considered individually, however, 55% of the 111 sites had grain N concentrations falling within the CGNC ± 5% range (20.8–23.0 g kg^–1) while 36% of the sites had grain N concentrations less than and 9% of the sites had grain N concentrations greater than the CGNC range at the optimal N rates. For straw N concentration, the optimal N treatment had 48% of the sites falling within the CSNC range (6.5–7.1 g kg^–1) while 37 and 15% of the sites were less or greater than the CSNC range. In the meanwhile, at the rate of farmer's N practice, only 23% of the sites had grain N concentration exceeding the CGNC range whereas 25% of the sites had grain N concentration below the CGNC range; the respective proportion for straw N concentration was 50% exceeding and 23% below the CSNC range. Where no N was applied (0 N control), 32% of the sites had grain N concentrations either falling within or exceeding the CGNC range (Table 6 ); straw N concentrations fell within or exceeded the CSNC range at 14% of the sites (Table 7 ).

View this table: [in this window] [in a new window]   Table 5. Number of field experimental sites, fertilizer N rate, wheat grain yield, grain and straw N concentrations in 111 field experimental sites.

We note that grain and straw N concentrations varied considerably across the experimental region, presumably due to environmental factors such as temperature or water status, soil characteristics and yield potential (Fowler et al., 1990). Although it is difficult to pinpoint the specific reasons contributing to the spatial variation in grain and straw N concentrations, we did observe high grain yield associated with moderate grain and straw N concentrations in the present study. Under the optimal N treatment, TA County had the highest mean grain yield (8.8 t ha^–1) and the lowest grain N (20.4 g kg^–1) and straw N concentrations (6.0 g kg^–1) (Table 5). Apparently, high grain yield had a dilution effect on the N concentrations, which was consistent with earlier observations (Triboi and Triboi-Blondel, 2002; Fowler, 2003). Oury et al. (2003) reported that grain N concentration decreased by approximately 10 mg g^–1 for an increase of 1 t grain yield.

Overall we expected grain yield and grain N concentration to follow one of four patterns: low yield-low N concentration (LL), low yield-high N concentration (LH), high yield-low N concentration (HL) and high yield-high N concentration (HH). The LL pattern typically results from severe N deficit for crop growth, HH may reflect luxury N consumption due to excess N supply. The LH pattern generally occurs when N uptake is plentiful but crop carbon assimilation is suppressed–particularly in the later part of the growing season-due to growth-limiting factors such as drought, high temperature, or edaphic characteristics (Lawlor, 2002; Triboi and Triboi-Blondel, 2002). When grain yield increases at a faster rate than the N supply can sustain, the HL pattern may occur (Triboi and Triboi-Blondel, 2002; Oury et al., 2003; Fowler, 2003). The HL and LH patterns are apparent in the present study for TA counties. Such patterns have been reported previously in wheat, for example, high yield potential in the northern and western regions of Europe (>8 Mg ha^–1) with low protein concentration (10–12%), and lower yield potential ( <5.0 Mg ha^–1) with higher protein concentration (15%) in southern and eastern regions (Triboi et al., 2006).

Taking into consideration all of the observations presented here, grain N and straw N concentrations appear to be a useful indicator of N deficiency status in winter wheat, however, these parameters were not generally indicative of luxury N consumption and N fertilizer over supply.

	CONCLUSIONS

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

 
In this study, grain N concentration at the EONR was similar to the calculated maximum value based on linear plus plateau model, while straw N concentration at EONR was significantly less than the maximum value. At the fertilizer N rate that produced maximum straw N, residual nitrate N content and apparent N losses were increased by 19 and 9 kg N ha^–1, respectively, compared to the values at the EONR. This shows that grain yield and environmental impacts can be optimized at the same time, N loss potential increased with maximum straw N concentration. The two scales used in this study, intensive research trial sites and extensive studies conducted on-farm across three provinces, illustrated that large variation in grain and straw N concentrations is present, and this poses significant challenges to development of an indicator of N status to guide agricultural practice.

	ACKNOWLEDGMENTS

 
We thank the National Basic Research Program of China (973 Program: 2009CB118606), "863" National Plan (2006AA10A303).

	NOTES

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