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Published online 5 June 2006
Published in Agron J 98:938-945 (2006)
DOI: 10.2134/agronj2005.0157
© 2006 American Society of Agronomy
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Production Papers

Fertilization and Nitrogen Balance in a Wheat–Maize Rotation System in North China

Rong-Fang Zhaoa, Xin-Ping Chena,*, Fu-Suo Zhanga, Hailin Zhangb, Jackie Schroderb and Volker Römheldc

a Dep. of Plant Nutrition, China Agricultural Univ., Beijing, 100094, P. R. China
b Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078, USA
c Institute of Plant Nutrition, Univ. of Hohenheim, 70593 Stuttgart, Germany

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

Received for publication May 23, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Over N fertilization is a common problem for the winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) rotation system in the North China Plain. A field experiment which included control (no N), conventional N (Con. N) fertilization, and optimized N (Nmin) fertilization treatments, was conducted from 1999 to 2003 near Beijing, China. Soil nitrate (NO3) dynamics were measured and N balance was calculated for the period of the eight successive cropping seasons. Soil NO3–N in the 0- to 90-cm profile for the Con. N treatment ranged from 157 to 700 kg ha–1 during the eight successive cropping seasons, much greater than those in the no N and optimized N treatments. Large amounts of soil NO3–N were detected in the 90- to 200-cm layer under the conventional N fertilization treatment, especially in the summer maize season. For the Nmin treatment, the total amount of N applied was 511 kg N ha–1 in the eight successive crops as compared with 2400 kg N ha–1 of the Con. N treatment. Grain yields were not different between the fertilized treatments except for maize in 2003. Soil NO3–N in the root zone under conditions of optimized N fertilization was maintained at a relatively low level as compared with the Con. N treatment, therefore dramatically decreasing NO3–N movement to deeper soil profile. This study indicates that soil NO3 movement out of the effective crop root zone is an important pathway of N losses in this winter wheat–summer maize rotation system in the North China Plain and the optimized N fertilization by an improved Nmin method shows high potential of reducing N-leaching losses.

Abbreviations: ANOVA, analysis of variance • NUE, nitrogen-use efficiency • PASWC, plant-available soil water content • TDR, time domain reflectometry


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
WINTER WHEAT–SUMMER MAIZE ROTATION is the most important agricultural production system in the North China Plain. This system consists of growing two crops, winter wheat and summer maize in 1 yr. Excessive mineral N fertilization is considered as a common problem in this system. For instance, an investigation conducted in Shunyi and Tongzhou Counties (Beijing) showed that the average amount of N fertilizer application was 309 kg N ha–1 yr–1 for winter wheat (n = 150) and 256 kg N ha–1 yr–1 for summer maize (n = 100) (Zhao, 1997). Gao et al. (1999) reported application rates of 587 and 652 kg N ha–1 yr–1 in two high-yielding counties, Wenxian, Henan Province and Huantai, Shandong Province, respectively; and 514 kg N ha–1 yr–1 in a middle-yielding county, Kaifeng, Henan Province. Those rates are much higher than those recommended in many developed countries (Zhao, 1997; Gao et al., 1999).

Excessive N fertilization may result in low NUE and potentially exerts more pressure on the environment. Nitrogen-use efficiency maybe defined as follows: NUE = (N uptake of N fertilization treatment – N uptake of no N treatment)/N fertilizer rate). A 15N experiment showed the recovery of fertilizer N by crops where conventional fertilization rates (300 kg N ha–1 for each crop) were used was approximately 25%, and that 30 to 50% of the applied N was lost in the winter wheat–summer maize rotation system on the North China Plain (Pan, 2001). Large amounts of fertilizer N loss to the environment could cause a serious environmental problem such as groundwater NO3 contamination (Zhang et al., 1996; Chen et al., 2000). In spite of this concern, N leaching has always been ignored as a main N loss pathway in the North China Plain because of low precipitation and high transpiration. Conversely, ammonia (NH3) volatilization has been considered the major pathway of N losses in this system due to high soil pH and the form of N fertilizer commonly applied (urea and NH4HCO3). Recently, investigations have shown good relationships exist between high NO3 concentration in the groundwater and high N fertilization in the North China Plain (Chen et al., 2000; Gao et al., 1999). There are numerous reports of severe NO3 accumulation from other parts of the world when fertilizer N was applied at higher quantity than crop requirements (Roth and Fox, 1990; Karlen et al., 1998; Elmi et al., 2000; Andraski et al., 2000). In the North China Plain, the rates of N fertilizer applied to winter wheat and summer maize were much higher than the crop N demand (Gao et al., 1999), therefore large amounts of NO3 accumulated in soil profiles (Ju et al., 2004; Liu et al., 2003). Hence, the importance of N leaching in the fate of N fertilizer should be reconsidered to better protect water quality.

Researchers have shown that the N rate for winter wheat can be reduced to <180 kg N ha–1 without any risk of yield decrease when yield response curve method or soil NO3 testing was used (Chen et al., 2004; Jia et al., 2001). Recently, an improved Nmin method which considers the synchronization of crop N demand and soil N supply, was developed and demonstrated great potential for improved NUE and minimized loss (Chen et al., 2001, 2005). However, the N loss of the optimized N fertilization technique needs to be compared with N loss due to the conventional N management practice. The objectives of this research were: (i) to investigate whether soil NO3 movement out of the effective root zone is a main pathway of N losses in the winter wheat–summer maize rotation system over a 4-yr study period, and (ii) to evaluate the potential of reducing N losses by the optimized N fertilization using an improved Nmin method in the winter wheat–summer maize rotation system in the North China Plain.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
A field experiment was conducted from September 1999 to September 2003 in Dongbeiwang Town, Beijing (40.0° N, 116.2° E) on a calcareous alluvial soil (calcareous cambisol, FAO Classification) (FAO, 2002), typical for the North China Plain. Soil texture was loamy, with a bulk density of 1.33 g cm–3 at 0- to 30-cm layer and 1.45 g cm–3 at both 30 to 60 cm and 60- to 90-cm layers. Other soil test parameters for the 0- to 30-cm sample were: pH (1:2.5, soil/water) 8.0, total N 1.17 g kg–1 (Bremner, 1996), Olsen P (the molybdate–ascorbic acid method, Fixen and Grove, 1990) 34.6 mg kg–1, exchangeable K (the ammonium acetate Baker method, Haby et al., 1990), 145 mg kg–1 and organic matter 21.4 g kg–1 (Yeomans and Bremner, 1988).

A winter wheat–summer maize rotation system was used for the study, which involved planting and harvesting of two crops (wheat and maize) per year from 1999 to 2003. The eight successive crops were grown on the same plots over the 4-yr period. Winter wheat (var. Jingdong 8) was planted in mid-October with a row spacing of 15 cm and a seeding rate of 300 kg ha–1 and harvested at mid-June the following year. After winter wheat harvest, summer maize (var. Jingkeng 114 in 2000–2002, and Nonghua 103 in 2003) was planted with a row spacing of 70 cm and a seeding rate of 37.5 kg ha–1and harvested in early October each year. The experimental design was a randomized complete block with four replications and the plot size was 15 by 20 m.

Three N treatments, no N as a control, conventional N fertilization and optimized N fertilization, were used in the experiment. The conventional N fertilization treatment represents a typical farmer's practice in most high-yielding areas of the North China Plain. The conventional N fertilization treatment received 150 kg N ha–1 (NH4HCO3) at planting (incorporation after broadcasting) and another 150 kg N ha–1 (urea) as top-dress at shooting stage (broadcast followed by irrigation) for winter wheat; The conventional N fertilization treatment for summer maize received 100 kg N ha–1 (NH4HCO3) as first top-dressing fertilizer at three-leaf stage (banded in the 3–5 cm deep furrow with subsequent cover with soil in 2000 or broadcast followed by irrigation in 2001–2003) and another 200 kg N ha–1 (urea) as second top-dressing at 10-extended leaf stage (broadcast before rain in 2000 and broadcast followed by irrigation in 2001–2003).

The rate and time of N fertilization in the optimized N fertilization treatment was based on an improved Nmin method, which considered the synchronization of crop N demand and soil N supply (Chen et al., 2001, 2005). The plant N demand at three different growth periods included both shoot and root N demand as shown in Fig. 1 . In the first year (1999/2000), for winter wheat, the Nmin target value from regreening to shooting was 120 kg N ha–1 and from shooting to harvest was 125 kg N ha–1. In the years of 2000/2001, 2001/2002, and 2002/2003, the Nmin target value from regreening to shooting was adjusted to 90 kg N ha–1 and the Nmin target value from shooting to harvest was adjusted to 110 kg ha–1. The target values for summer maize and all years were 30 kg N ha–1 from sowing to three leaf stage, 90 kg N ha–1 from three leaf stage to 10 leaf stage and 110 kg N ha–1 from 10 leaf stage to harvest. Nmin target values were dependent on target yield (6 t ha–1 for each crop in this study) (Chen et al., 2001, 2005). The soil mineral N (Nmin = NH4–N + NO3–N) in the effective root depth changed with roots development. Soil samples (five cores from each plot) were taken from 0 to 30 cm for the first growth period, 0 to 60 cm for the second growth period, and 0 to 90 cm for the third growth period at the beginning of the periods and were measured for Nmin. The rate of N applied for each growth period was equal to the differences between the target N demand and the measured soil Nmin. After each crop harvest, soil samples in 0 to 90 cm were taken and analyzed for soil Nmin. Five 90-cm deep soil cores were taken from each plot, separated into 0- to 30-cm, 30- to 60-cm, and 60- to 90-cm depths. For evaluation of a possible NO3 movement to deeper soil layer, 90- to 200-cm soil samples were taken at the same time and separated into 90- to 120-cm, 120- to 150-cm, and 150- to 200-cm depths. All soil samples were sieved to pass a 2.0-mm screen, mixed, and extracted with 0.01 M CaCl2 solution. Extracted samples were analyzed for ammonium (NH4)–N and NO3–N using Continuous Flow Analysis (TRAACS 2000 system, Bran and Luebbe, Norderstedt, Germany). Soil water content was also determined at the same time of mineral N extraction. Table 1 shows the amount and time of applied N fertilizer in the optimized N fertilization treatment.


Figure 1
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  		Fig. 1. Method of optimizing N fertilization for winter wheat (left) and summer maize (right) is based on the plant N demands and soil N supply at three growth stages. Soil Nmin was tested in the effective root depth at different growth periods. Nmin target value included N demand by wheat/maize shoot and N demand by root. In the first year (1999–2000), for winter wheat, Nmin target value from regreening to shooting was 120 kg N ha–1, and from shooting to harvest was 125 kg N ha–1. In the years of 2000–2001, 2001–2002, and 2002–2003, the Nmin target value from regreening to shooting was adjusted to 90 kg N ha–1 and the Nmin target value from shooting to harvest was adjusted to 110 kg ha–1.

 

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  		Table 1. The amount and time of N fertilization (kg N ha–1) for the optimized N fertilization treatment in 4 experimental yr.

 
Sprinkler irrigation was used to keep plant-available soil water content (PASWC) between 45 and 80%. Soil water content was measured by TDR (time domain reflectometry, type E. S. I./MP-97, Mack, 2005) every 4 d. A total of 319, 310, 249, and 172 mm water was applied in seven, six, four, and four irrigations, respectively for winter wheat from 1999 to 2003. For summer maize, no irrigation was applied in the Year 2000 but a total of 40, 24, and 65 mm was sprinkled in split applications after fertilization in the Year 2001, 2002, and 2003, respectively. The natural rainfall amounts during the 4-yr study period were lower than the historic average of 629 mm yr–1 (1955–1996, data from the Meteorological Observatory of Haidian District, Beijing City). Rainfall totaled 416 mm from October 1999 to September 2000, 336 mm from October 2000 to September 2001, 475 mm from October 2001 to September 2002 and 436 mm from October 2002 to September 2003 in Dongbeiwang, Beijing (Table 2).


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  		Table 2. The total amount of irrigation and rainfall in whole growth period of winter wheat and summer maize (mm).

 
Phosphorus and K fertilizers were applied only once per rotation (before winter wheat planting). All plots received 78.5 kg P ha–1 as triple superphosphate and 75 kg K ha–1 as KCl before winter wheat sowing in the rotation of 1999/2000, but reduced to 55 kg P ha–1 and no K afterward because soil test indicated adequate available P and K in the soils.

Aboveground biomass was measured manually on all plots. For winter wheat final harvest, three separate areas (each 3 m2) in the middle of each winter wheat plot were harvested to determine fresh grain yield. Grain and straw samples were for oven-dried at 60°C till weight constancy for the determination of dry matter. For summer maize final harvest, 39.2 m2 (seven rows, 8-m length) in the middle of each summer maize plots with N fertilizer application and 16.8 m2 (three rows, 8-m length) in the middle of each summer maize plots without N fertilizer application were harvested to determine fresh cob and stover yield. Six subsamples were randomly selected from the harvested summer maize plants and the stover and cob were separated and weighed. Whole cob and stover fraction samples were oven-dried at 60°C till weight constancy for the determination of dry matter weight. Subsamples of grain and stover were analyzed for N content by the Kjedahl method (Bremner, 1996). The wheat straw and maize stover were removed from the plots at each harvest and straw and stover were included in the calculation of N uptake. The differences among the treatments were determined using analysis of variance (ANOVA). Statistical analyses were performed using DUNCAN procedures of the SAS software package (SAS Institute, 1996).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Crop Yields
There was no significant difference of crop yields between the optimized and conventional N fertilization treatments in all successive wheat and maize crops except for the 2003 maize season (Fig. 2 ). A higher-yielding maize variety (Nonghua 103) was used in 2003 instead of the preceding variety (Jingkeng 114) but without changing the target N supply value, hence the grain yield in the optimized N treatment was lower than that in the conventional treatment. The grain yield in the control treatment (no N) was significantly less than those in the optimized and conventional N fertilization treatments for all years (Fig. 2).


Figure 2
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  		Fig. 2. Effect of N fertilization treatments (No N: no N fertilization; Con. N: conventional N fertilization; and Opt. N: optimized N fertilization) on grain yields of winter wheat and summer maize in the 4 experimental yr. Different letters indicate significant differences among treatments at P = 0.05.

 
Dynamics of Nitrate-Nitrogen and Ammonium-Nitrogen in 0- to 90-cm Soils
During all eight successive crop seasons, soil NO3–N in 0- to 90-cm profile in the conventional N fertilization treatment ranged from 157 to 700 kg ha–1, much greater than those in the no N and optimized N fertilization treatments (Fig. 3 ). However, soil NO3–N in 0- to 90-cm profile of the optimized N fertilization treatment (24–153 kg ha–1) was about the same as that of the no N treatment (13–144 kg ha–1) (Fig. 3).


Figure 3
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  		Fig. 3. Nitrogen fertilization strategies and concentrations of nitrate (NO3)–N in soil depths of 0 to 30, 30 to 60, 60 to 90, and 0 to 90 cm during the 4 experimental yr. No N, Con. N, and Opt. N represent no N fertilization, conventional N fertilization and optimized N fertilization treatment, respectively.

 
For the conventional N treatment, significant NO3–N accumulation was found in 30- to 60-cm soil layer but not in 60- to 90-cm layer in the first crop season (winter wheat 1999/2000). In the second crop season (summer maize 2000), soil NO3–N in 30- to 60-cm soil layer significantly decreased, but significantly increased in 60- to 90-cm soil layer. Similarly, significant NO3–N accumulation was also found in both 30- to 60-cm and 60- to 90-cm soil layers in the fifth crop growth season (winter wheat 2001/2002), and then dramatically decreased in the sixth crop growth season (summer maize 2002) (Fig. 3).

Soil NH4–N in 0- to 90-cm profile in the conventional N fertilization treatments ranged from 3 to 36 kg ha–1 and did not change with time during the successive crop seasons. It was the same as that of the no N treatment (2–34 kg ha–1) and the optimized N treatments (3–32 kg ha–1).

Accumulation of Nitrate-Nitrogen in 0- to 200-cm Soils
No obvious accumulation of NO3–N was found in the 90- to 200-cm soil layer for the conventional N fertilization treatment right after the wheat harvest of the first growing season (winter wheat 1999/2000) (Fig. 4 ). But after second crop (summer maize 2000) harvest, high accumulation of soil NO3–N was observed in the 90- to 200-cm soil layer for the conventional N fertilization treatment. Soil NO3–N accumulation in the 150- to 200-cm soil layer was found in the third crop (winter wheat 2000/2001) season. High accumulation of soil NO3–N occurred in 90- to 200-cm soil layer in the fourth, sixth, and eight crop seasons (summer maize 2001, 2002, and 2003). The increased amounts of soil NO3–N in the 90- to 200-cm layer (in the four summer maize crop seasons were 209, 60, 256, and 47 kg ha–1, respectively) equal to 90- to 200-cm soil NO3–N after harvest minus 90- to 200-cm NO3–N before sowing in each summer maize crop season (Fig. 4). In contrast, the amount of precipitation in these four summer maize growth seasons were 339, 214, 342, and 235 mm, respectively (Table 2). The amount of precipitation seems to be related to soil NO3 movement in the soil profile.


Figure 4
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  		Fig. 4. Distribution of nitrate (NO3)–N in 0- to 200-cm soil profile before winter wheat planting in 1999/2000 and after winter wheat and summer maize harvest from 1999 to 2003. No N, Con. N and Opt. N represent no N fertilization, conventional N fertilization, and optimized N fertilization treatment, respectively.

 
No significant differences in NO3–N accumulation in 90- to 200-cm soil layer were found in all eight successive crop seasons between no N treatment and the optimized N fertilization treatment (Fig. 4).

Nitrogen Balance
Soil N supplying capacity significantly decreased over time in this experiment for both the continuous no N fertilizer application and the optimized N fertilization treatment (Table 3). The N uptake by crops in the no N treatment was 187, 149, 122, and 97 kg ha–1, for each of the four experimental years (each year had two crops). The total N uptake by eight crops in the no N treatments (555 kg N ha–1), was contributed by 47 kg N ha–1 soil Nmin, and other unknown sources (508 kg N ha–1), probably from soil N mineralization and N deposition.


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  		Table 3. The calculated N balances for three N treatments in successive eight crop seasons (kg N ha–1). No N, Con. N, and Opt. N represent no N fertilization, conventional N fertilization, and optimized N fertilization treatments, respectively.

 
For the conventional N fertilization treatment, the total amount of N fertilizer application during the successive eight crops was 2400 kg N ha–1, while the total N uptake was only 1145 kg N ha–1, therefore the NUE was only 24.5%. Considering the contribution of soil N mineralization and N deposition (508 kg N ha–1), there was a total N surplus of 1763 kg N ha–1, while 234 kg N ha–1 contributed to the increasing of 0- to 90-cm soil Nmin, other 1529 kg N ha–1 could be lost through the soil-crop system. The calculated apparent N losses were 201, 559, 487, and 282 kg ha–1, for each of the four experimental years (Table 3).

For the optimized N fertilization treatment, the total amount of N fertilizer applied in the successive eight crops was only 511 kg N ha–1, while the total N uptake was 1041 kg N ha–1, therefore the NUE was 95.1% (Table 3). This high rate of NUE is probably due to the utilization of elevated levels of residual N in the soil. Considering the contribution of soil N mineralization and N deposition (508 kg N ha–1) and soil Nmin (original soil Nmin minus final soil Nmin = 69 kg N ha–1), the total N losses was only 47 kg N ha–1, which could be separated as –10, 95, 0, and –38 into the each experimental year (Table 3).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Farmers' conventional N fertilization practice usually causes high N losses because of excessive N input. For example, Karlen et al. (1998) found that about 50% of the fertilizer N applied at conventional rates was not accounted for by crop removal. The calculated N balance in this experiment indicated that for the conventional treatment a high N fertilization (2400 kg N ha–1) caused high N surplus (1763 kg N ha–1) and high N losses (1529 kg N ha–1) in successive eight crop seasons.

It is accepted by many that N leaching is not an important pathway of N losses in the winter wheat–summer maize rotation system in the North China Plain because of low precipitation and high transpiration (Zhu and Wen, 1992). However, there is increasing evidence showing NO3 leaching occurring in low-rainfall regions under certain circumstances (Strong, 1995). Such leaching occurs episodically during extraordinarily wet periods (Campbell et al., 1984). The continuous 4-yr observation in this experiment showed that elevated concentrations of soil NO3–N were found in the 90- to 200-cm layer for the conventional N fertilization treatment, especially after the summer maize season. The increased levels of soil NO3–N in 90- to 200-cm layer during summer maize season in the four experimental years were 209, 60, 256, and 47 kg N ha–1, respectively. Researchers have shown that a very small portion of the total N taken up by maize occurs at soil depths >90 cm (Huang et al., 1996; Zhang, 2004). The total accumulation of 572 kg N ha–1 accounts for 37% of the overall N losses. Therefore, soil NO3–N movement to deeper soil layers (out of the crop rooting depth) should be considered an important pathway of N losses in this winter wheat–summer maize rotation system.

The movement of NO3 out of the root zone depends on the soil hydraulic properties, the amount of irrigation and/or precipitation, the amount of N applied, the form of N in the fertilizer and the time of application (Cameira et al., 2003). In our study, excessive N fertilization caused extremely large amounts of NO3 to accumulate in soil profiles after harvest. There are numerous reports on severe NO3 accumulation when fertilizer N was applied at higher quantity than crop requirements (Roth and Fox, 1990; Karlen et al., 1998; Elmi et al., 2000; Andraski et al., 2000). In the North China Plain, the rates of N fertilizer applied to winter wheat and summer maize were much higher than the crop N demand (Gao et al., 1999), therefore large amounts of NO3 accumulate in soil profiles of farmers' fields as well as in experimental fields (Ju et al., 2004; Liu et al., 2003). This not only wastes valuable resources but also poses more threat to groundwater quality in the region.

Nitrate movement depends much on water movement below the rooting depth of the crops. Higher water supply than the crop requirement (by rain and/or irrigation) is a very simple indicator for leaching potential. Commonly, irrigated land has a high potential for NO3 leaching, especially in combination with high N fertilizer application rates (Diez et al., 2000). In our experiment, the amount of soil NO3–N accumulation in 90- to 200-cm layer during winter wheat season did not increase significantly in four experimental years because the amount and time of irrigation were controlled well by soil water content testing. However, heavy rainfall events always happened in summer in the North China Plain. For instance, in these four experimental years, the precipitation in summer maize season was 339, 214, 342, and 235 mm, respectively, while the total amount for each experimental year were 416, 336, 475, and 436 mm, respectively. This explains the accumulation of soil NO3–N in 90- to 200-cm layer during the summer maize season. Hence, assessing NO3 movement considering only the winter wheat season is unjustified. This continuous field experiment also showed a good relationship between precipitation and increased soil NO3–N accumulation in 90- to 200-cm layer during summer maize season. The precipitations were 339 and 342 mm while the increased amounts of soil NO3–N accumulation in 90- to 200-cm layer were 209 and 257 kg N ha–1 following the summer maize seasons of 2000 and 2002. On the other hand, the precipitations were only 214 and 235 mm while the increased amounts of soil NO3–N accumulation in 90- to 200-cm layer were 60 and 47 kg N ha–1 for the other two summer seasons. Thus, in our study, excessive N fertilization combined with elevated precipitation during the summer maize season resulted in increased leaching of N.

Reduction of the applied N fertilizer rate to an optimized rate can reduce soil NO3 leaching (Power et al., 2000; Sogbedji et al., 2000; Diez et al., 2000; Guillard et al., 1999). Optimized N fertilization usually involves applying N fertilizer in split applications to match crops needs (Ottman and Pope, 2000). Nitrogen-use efficiency has been shown to be the greatest with split N application (Alcoz et al., 1993; Ayoub et al., 1995), and such split applications will presumably lower the potential for NO3 leaching. In this study, an improved Nmin method, which considered the synchronization of crop N demand and soil N supply was used for the optimized N fertilization treatment. The results of our study indicate that soil NO3–N in the root zone under conditions of optimized N fertilization was maintained at a relatively low level as compared with the conventional N fertilization treatment. However, the optimized N fertilization treatment did not reduce yield with the exception of summer maize yield in 2003 when a higher-yielding maize variety was used without changing the target N supply value. The optimized N fertilization treatment dramatically decreased soil NO3–N movement to deeper soil layers.

This continuous 4-yr experiment showed that the soil-available N supply from the mineralization of organic-bound N decreased with time, and the rate of N fertilizer in the optimized N fertilization treatments increased gradually. Therefore, the reduced amount of N fertilizer at the beginning of the experiment can not be maintained for a long time without impacting yields. The question is how much the potential ability of reducing N fertilizer in the optimized N fertilization system is in a long period. According to Engels and Kuhlmann (1993), in the optimum N treatment, the N mineralization and immobilization will finally reach a balance point. It means N supply will be equal to the N removal by crops if the N losses by fertilizer application can be controlled well and N deposition can be ignored. In the case of the middle-yield area (approximately 6 t ha–1 target yield for winter wheat as well as for summer maize) in the North China Plain, rough calculations show that the optimized N rate should be about 180 kg ha–1 for each crop of winter wheat–summer maize rotation system under the condition of no straw return and no organic fertilizer application. Comparing this strategy with farmers' current practices shows that approximately 40% of N fertilizer can be saved over the first few years.


   ACKNOWLEDGMENTS
 
This study was supported financially by NSFC, P. R. China No. 30390084, NSFC in Beijing City, P. R. China No. 601001, and BMBF Germany No. 0339712A. We would like to thank Dr. Liu X., Dr. Ju X., Dr. Jia L., Dr. Böning-Zilkens, Mr. Liao X., Mr. Wu J., and Mr. Yang J. to assist in part of experimental work and thank Dr. Claupein and Dr. Schulz for their scientific contribution.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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