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Published in Agron J 100:517-525 (2008)
DOI: 10.2134/agronj2007.0194
© 2008 American Society of Agronomy
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FERTILIZER MANAGEMENT

On-Farm Evaluation of the Improved Soil Nmin–based Nitrogen Management for Summer Maize in North China Plain

Zhenling Cuia, Xinping Chena, Yuxin Miaoa, Fusuo Zhanga,*, Qinping Suna, Jackie Schroderb, Hailin Zhangb, Junliang Lic, Liwei Shic, Jiufei Xuc, Youliang Yed, Chunsheng Liue, Zhiping Yangf, Qiang Zhangf, Shaomin Huangg and Dejun Baog

a Dep. of Plant Nutrition, College of Resource and Environ. Sci., China Agricultural Univ., Beijing 100094, China
b Dep. of Plant and Soil Sci., Oklahoma State Univ., Stillwater, OK 74708
c Dep. of Agron., Qingdao Agric. Univ., Qingdao 266023, China
d College of Resource and Environ. Sci., Henan Agric. Univ., Zhengzhou 450000, China
e College of Resource and Environ. Sci., Shandong Agricultural Univ., Taian 271018, China
f Inst. of Soil Sci. and Fertilizer, Shanxi Acad. of Agric. Sci.. Taiyuan, 030031, China
g Inst. of Soil Sci. and Fertilizer, Henan Academy of Agric. Sci., Zhengzhou, 450000, China

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


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The improved soil Nmin–based N management is a promising approach to precision N management, which determines the optimum side-dress N rates based on N target values and measured soil nitrate N content in the root soil layer at different growth stages. A total of 148 on-farm N-response experiments, in seven key summer maize (Zea mays L.) production regions of North China Plain (NCP) from 2003 to 2005, were conducted to evaluate the Nmin–based N management compared to traditional farmer's N practices. The recommended N rates based on the improved soil Nmin method were not significantly different ( ≤31 kg N ha–1) from those determined by yield response curves (n = 13). The average N rate determined with the soil Nmin method (157 kg N ha–1) was significantly lower than farmer's practice (263 kg N ha–1), while maize grain yield was 0.4 Mg ha–1 higher than farmer's N practice (8.5 Mg ha–1) across all sites (n = 148). As a result, the improved soil Nmin–based N management significantly increased net economic gains by $202 ha–1, reduced residual nitrate N content and N losses by 44 kg N ha–1 and 65 kg N ha–1, respectively, and improved recovery N efficiency, agronomic N efficiency and N partial factor productivity by 16%, 6 kg kg–1 and 36 kg kg–1, respectively, compared with farmer's N practice. We conclude that the improved soil Nmin–based N management can be applied for summer maize production in NCP for improved N use efficiency and reduced environmental contamination.

Abbreviations: AEN, agronomic nitrogen efficiency • EONR, economically optimum nitrogen rates • NCP, North China Plain • PFPN, nitrogen partial factor productivity • REN, recovery nitrogen 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 June 4, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SUMMER MAIZE is one of the staple grain crops in the NCP, and its planting area and total grain production accounted for 32 and 31% of the China crop in 2002, respectively. As an "insurance," an excessive amount of N fertilizer has usually been applied for summer maize production in this region in last two decades (Gao et al., 1999; Zhao, 1997). A recent investigation showed an average application of 249 kg N ha–1 as mineral fertilizer (56–600 kg N ha–1) to summer maize (n = 370) in Shandong Province, exceeding crop requirements for maximum grain yield (Cui, 2005). Excessive N application has been shown to lead to the high accumulation of soil nitrate N and groundwater pollution. Ju et al. (2006) observed that the residual soil nitrate N after maize harvest was 275 kg N ha–1 in the top 90 cm soil profile and 213 kg N ha–1 at 90 to 180 cm soil depth in this region. An investigation of groundwater nitrate N content in Shouguang County of NCP showed 29% of 80 samples exceeded 45 mg NO3 L–1 in 1998, while the percentage reached 49% in 2001 (Chen et al., 2000; Zhu, 2002).

Due to the huge food demand, increasing crop yield by fertilization and reducing negative environmental impacts from fertilization is urgent for sustaining agricultural development in China, especially in the NCP. Therefore, the objective of N fertilization management in this region is to achieve high yield and minimize N losses. The key to achieving such goals is to develop a feasible N recommendation method that can harmonize N requirement and environmental protection in the high yielding crop rotation system for this region.

Since the 1980s, N recommendation for summer maize production in China has relied extensively on a yield-based strategy that was developed to represent regional average values (Li, 1983; Sun and Liu, 1989). Implicit to yield-based N recommendations is the presumption that crop yield response to N fertilizer was similar both among and within fields. However, the great variability of crop N responses to applied N fertilizer was reported both among and within fields, which has been attributed to differences in soil N supply and losses (Scharf and Alley, 1994), crop demand (Fiez et al., 1995) and N use efficiency (Meisinger, 1984). Therefore, approaches considering yield-based systems were originally intended as a first approximation in making generalized fertilizer N recommendation for long-term periods on a regional scale, but have been applied indiscriminately to fertilize individual fields in a particular growing season.

Soil Nmin (NO3–N + NH4–N) testing to determine N fertilizer recommendation has been a high priority for soil scientists in past decades (Wehrmann and Scharpf, 1979; Soper and Huang, 1962). In the original soil Nmin method, a so-called "N target value" was defined from which soil Nmin found in 0 to 90 cm was subtracted to calculate the necessary N fertilizer rate (Wehrmann et al., 1988). This "N target value" was less than total N uptake of the crop because of potential net N mineralization during the crop growth season. However, the variability of net N mineralization among fields had limited this method for site (or field)-specific N recommendation. In addition, it was impossible to estimate N immobilization and losses after one soil test before planting. To better estimate soil net N mineralization rate and match crop needs, several versions of presidedress soil nitrate nitrogen test (PSNT) (Magdoff et al., 1984; Fox et al., 1989; Magdoff et al., 1990) or modifications such as the late-spring nitrate N test (Blackmer et al., 1989) have been developed. However, most research only studied the critical indicator value above which response to sidedress N was not likely, only a few scientists had made quantitative recommendations for sidedress N application (Schröeder et al., 2000).

In NCP, an improved Nmin method considering N target value in different crop growth periods and measured soil nitrate N values at different soil depths was developed for winter wheat–summer maize rotation system (Chen et al., 2006). In this method, crop growth was divided into several periods, and the recommended sidedress N rate was calculated by deducting the measured soil nitrate N content from N target value in each period. The results from eight successive crop seasons in winter wheat-summer maize rotation system showed that 79% of N fertilizer could be saved using this method without crop yield losses, compared with farmer's N practice (Zhao et al., 2006). Two important questions that have not been answered are (i) Is the N recommendation rate based on the improved soil Nmin method economically optimum (EONR)? (ii) Can these results be extended to the whole NCP? So far, no on-farm experiments have been conducted to thoroughly evaluate this N management in farmers' fields at different regions of NCP, which is necessary before wide application of this approach.

Our objectives of this study were to (i) compare the recommended N rates based on the improved soil Nmin method with the EONR determined with crop response curves, and (ii) evaluate agronomic performances and potential environmental impacts of the improved soil Nmin–based N management against farmer's N practices in different regions of NCP.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
An improved soil Nmin method was developed and used for the optimum N treatment in each experiment. In this treatment, the growth of summer maize was divided into three periods: (i) from planting to three leaf stage; (ii) 3 to 10 leaf stage; and (iii) 10 leaf stage to harvest. The optimum N rate in each maize growth period was determined by deducting measured soil nitrate N content in root layers (0–30, 0–60, and 0–90 cm for the three periods, respectively) from N target value, which was the sum of N uptake by shoot and root estimated by target yield and N content based on reference. A more detailed description about N target value can be found elsewhere (Chen et al., 2006; Zhao et al., 2006). In this study, N target values in different maize growth periods at different regions are given in Table 1 .


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  		Table 1. Nitrogen target value in different maize growth periods at different locations.

 
To determine whether the N rate recommend by this improved soil Nmin method was EONR, 12 field experiments with six N rates (sites 1–12) and one field experiment with five N rates (site 13) were conducted from 2003 to 2005, in Huimin (HM) County, Shandong Province. Soil Nmin in the top 90 cm of soil profile, OM, total N, Olsen-P, and NH4OAc-K in the each experiment before planting are shown in Table 2 .


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  		Table 2. Soil Nmin (NO3–N + NH4–N) in the top 90-cm soil profile and selected chemical properties in the top 30-cm layer at the 13 experimental sites in Huimin County, Shandong province.

 
At sites 2, 3, 4, 6, 8, 10, and 12, the six N treatments were control (N0), optimum N (based on the improved soil Nmin test), farmer's N practice, 40, 70, and 130% of optimum N. In sites 1, 5, 7, 9, and 11, the six N treatments included a control (N0), optimum N, farmer's N practice, 30 and 60 kg N ha–1 more and 30 kg N ha–1 less than optimum N rate. Site 13 had five N treatments: control (N0), optimum N, farmer's N practice, 50 and 150% of optimum N.

The below-optimum N treatments at all locations, including 40, 70, and 50% of optimum N and 30 kg N ha–1 less than optimum N rate, were designed to evaluate the potential of further reducing N fertilizer input. The above-optimum N treatments, including 130 and 150% of optimum N rate and 30 and 60 kg N ha–1 more than optimum N rate, were designed to determine the potential of further increasing grain yield. An average N rate of 244 kg N ha–1 (100 and 144 kg N ha–1 was applied in the 3 and 10 leaf stages, respectively) represented a typical farmer's practice in this region. Nitrogen application rates for each N treatment at the 3 and 10 leaf stages for the 13 experiments are presented in Table 3 .


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  		Table 3. Nitrogen application rates (kg N ha–1) for different N treatments for the 13 experimental sites.

 
To evaluate agronomic performances and potential environmental impacts of the improved soil Nmin–based N management, on-farm experiments at 148 sites in seven key summer maize regions of NCP were conducted from 2003 to 2005. All experimental sites practiced winter wheat–summer maize rotation system, including Huimin (HM), Yanzhou (YZ), and Taian (TA) County in Shandong Province, Yongji (YJ) and Hongtong (HT) County regions in Shanxi Province, and Suiping (SP) and Xinxing (XX) County regions in Henan Province. The number of experimental sites, soil Nmin in the top 90 cm of soil profile, OM, total N, Olsen-P, and NH4OAc-K in each region are shown in Table 4 . Each experiment had at least three N treatments: control (N0), optimum N and farmer's N practice. Nitrogen fertilization with farmer's practice was decided by each individual producer.


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  		Table 4. The number of experimental sites, soil Nmin in the top 90 cm of soil profile, organic matter (OM), Total N, Olsen-P, and NH4OAc–K in the top 30-cm layer in different experimental locations.

 
In all field experiments, N fertilizer as urea was applied for maize at the 3 and 10 leaf stage, and no N fertilizer was applied before planting at any experimental site because of high soil nitrate N content in the top 30 cm of soil profile. The plot size was from 40 m2 (5 by 8 m) to 120 m2 (15 by 8 m). All plots received 0 to 150 kg P2O5 ha–1 as triple superphosphate and 0 to 120 kg K2O ha–1 as potassium chloride at the three leaf stage based on soil P and K test results. No organic manure was applied at any experimental site. Except for fertilizer application and grain yield harvest, each field was managed using the individual producer's current management practices. Summer maize was immediately planted without tillage after winter wheat harvest in the middle of June and harvested at the end of September.

At least five soil samples in every plot were taken to a depth of 90 cm at 30-cm increment before planting, N application, and after harvest at each site. Soil samples, before planting and after harvest, were extracted with 0.01 mol L–1 CaCl2, and analyzed for NH4–N and NO3–N using Continuous Flow Analysis (TRAACS 2000, Bran and Luebbe, Norderstedt, Germany) in the laboratory. Soil water content was measured by oven drying at 105°C. Soil samples before N application were extracted with 1:1 ratio of soil to distilled water. Nitrate-N-test strips and reflectometer were used to analyze soil nitrate N content and calculate the optimum N rate in the field. Soil water content was measured by alcohol burning method (Cui et al., 2005). A good correlation between these two methods of soil nitrate N testing had been reported in NCP (Cui et al., 2005) and Germany (Schmidhalter, 2005). Soil samples at 0 to 30 soil layer collected before planting was air-dried and sieved through a 0.2 mm mesh to remove undecomposed plant materials. The sieved samples were used to measure OM, total N, Olsen-P, and NH4OAc-K.

Aboveground biomass was measured by hand in all plots. Eight m2 (two rows, about 8-m length) in the middle of each plot was harvested to determine fresh cob and stover yield. Six subsamples were randomly selected from the harvested summer maize plants and separated into stover and cob, which were oven-dried at 60°C for the determination of dry matter weight. Subsamples of grain and stover were analyzed for N content using the Kjedahl method.

Maize yield response curves to N rate at each of the 13 sites with five or six N treatments 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. In most cases, linear with plateau model fitted the data best, and was chosen for all the sites (Cerrato and Blackmer, 1990). The EONR was calculated using $0.375 and $0.5 for corn grain and N prices, respectively. Corn yield at EONR was calculated using the fitted response functions.

Apparent N mineralization rate during maize growth season (Norganic) was calculated as the difference between N output (plant N uptake plus residual soil Nmin in 0 to 90 cm soil layers) and the initial soil Nmin in 0 to 90 cm soil layers in control (No N) plots (Cabrera and Kissel, 1988; Olfs et al., 2005).

Formula 1[1]
Apparent N loss during maize growing season (Nloss) was calculated as the difference between N input (N fertilizer application rate plus initial soil Nmin plus apparent N mineralization) and N output (plant N uptake plus residual soil Nmin) in plots with N application (Zhao et al., 2006).

Formula 2[2]
where Soil Nmin (start) and (end) are the mineral N content within the top 90 cm of soil profile before planting and after harvest, respectively; Crop N uptake represents N accumulation in aboveground biomass; Nfer is N from the applied fertilizer.

Recovery nitrogen efficiency (REN), agronomic nitrogen efficiency (AEN), and nitrogen partial factor productivity (PFPN) were calculated using Equations [3] to [5]. The REN is the efficiency of N recovery from applied N fertilizer, and AEN is the yield increase per unit of N applied. The PFPN is the most important index for farmers because it integrates the use efficiency of both indigenous and applied N resources.

Formula 3[3]

Formula 4[4]

Formula 5[5]
where UN and U0 are crop N uptake in applied N plots and N0 plots, YN and Y0 are grain yield in N application plots and N0 plots, and N is N from the applied fertilizer.

Data were analyzed following analysis of variance by SPSS 13.0 (SPSS Inc., Chicago, IL), and means of N treatments were compared based on least significant difference (LSD) at 0.05 level of probability.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Comparison of Optimum Nitrogen Rates based on the Improved Soil Nmin Test and Yield Response Curve Method
Maize yield response to N rate was similar among the 13 experimental sites with five or six N treatments in HM County, Shandong Province (Fig. 1 ). The EONRN based on grain yield response curve and the corresponding total recommended optimum N rates based on the improved soil Nmin method were given in Table 5 . The optimum N rates determined with these two methods were not significantly different (P < 0.05) across the 13 sites (Table 5).


Figure 1
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  		Fig. 1. Grain yield as a function of increasing N rate for all sites: (a) Site 1 to 5 in 2003, (b) Site 6 to 11 in 2004, and (c) site 12 to 13 in 2005.

 

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  		Table 5. Comparison of N rate and maize grain yield between the improved Nmin method and yield response curve method on 13 sites in Huimin County, Shandong Province.

 
Across all 13 sites, grain yield in the optimum N treatment ranged from 6.5 to 11.1 Mg ha–1 with a mean of 8.8 Mg ha–1. Optimum N grain yields were not significantly different from the grain yield with EONR (average: 8.6 Mg ha–1, from 6.5–10.8 Mg ha–1) (P < 0.05). Overall, the optimum N treatment increased maize yield at eight sites of the 13 sites (62%), produced similar yield at four sites (31%), and reduced yield at one site (7%), when compared with grain yield with EONR based on crop response curve (Table 5).

Compared with the optimum N rate based-on the improved soil Nmin method (average: 144 kg N ha–1), the below-optimum N treatment (average: 96 kg N ha–1) significantly decreased maize grain yield from 8.7 to 7.9 Mg ha–1 (P < 0.05), and similar N losses were observed between these two N treatments. The above-optimum N treatment (average: 202 kg N ha–1) did not significantly increase maize grain yield, but significantly increased N losses during maize growing season from 52 to 83 kg N ha–1 (P < 0.05) (Table 6 ).


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  		Table 6. Comparison of N rate, grain yield and nitrogen losses for below-optimum N (BON), optimum nitrogen (Opt. N) and above-optimum nitrogen (AON) treatments at 13 sites in Huimin County, Shandong Province.

 
Agronomic Performances and Potential Environmental Impacts of the Improved Soil Nmin–Based Nitrogen Management
Data from 148 experimental sites located in seven regions of NCP collected from 2003 to 2005 were used to evaluate the agronomic performances and potential environmental impacts of the improved soil Nmin–based N management strategy. In these regions, farmers generally apply N fertilizers from 96 to 482 kg N ha–1(with a mean of 263 kg N ha–1), and summer maize yield ranges from 3.2 to 11.7 Mg ha–1 (with a mean of 8.5 Mg ha–1), which are common in NCP (Table 7 ).


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  		Table 7. Nitrogen application rates, grain yields, crop N uptake, and economic gains for control (N0), optimum N, and farmer's N practice on 148 sites in the seven key summer maize domains of the North China Plain.

 
Across all 148 sites, N fertilizer application significantly increased maize grain yield, with the optimum N treatment and farmer's N practice increasing 1.5 Mg ha–1 (20%) and 1.1 Mg ha–1 (15%), respectively, as compared with control (N0) (P < 0.05) (Table 7). Compared with farmer's N practice, the optimum N treatment significantly increased maize yield (0.4 Mg ha–1) (P < 0.05), although an average of 107 less N ha–1 was used. The optimum N treatment increased maize yield over farmer's N practice in 105 sites (71%), with 61 sites (41%) and 44 sites (29%) having yield increase of more than 0.5 Mg ha–1and 1.0 Mg ha–1, respectively.

Compared with the farmer's N practice treatment, the optimum N treatment applied less N fertilizer in 122 (82%) and the remaining 26 (18%) sites received more N fertilizer. On average, the optimum N treatment used 157 kg N ha–1 (ranging from 37–280 kg N ha–1), which was 107 kg N ha–1 (41%) less than farmer's N practice (263 kg N ha–1, ranging from 96 to 482 kg N ha–1) (Table 7). The difference between optimum N management and farmer's N practice was not only reflected in the total amount of N application, but also in the proportions of N at different crop growth stages. At 73 of the 148 sites (49%), N fertilizer was applied once in the five or six leaf stage by farmer, with a mean N application rate of 220 kg N ha–1 (96–483 kg N ha–1). At the other 75 sites (51%), farmers applied an average of 126 and 175 kg N ha–1 at three to four leaf and 10 to 11 leaf stages, respectively. With the optimum N treatment, an average of 50 and 106 kg N ha–1 was applied at 3 and 10 leaf stages, respectively. The ratio of basal: sidedressing N rate was 1:2.1 and 1:1.4 for optimum N treatment and farmer's N practice, respectively (Fig. 2 ).


Figure 2
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  		Fig. 2. The N application rate for base-dressing (ONBD) and top-dressing (ONTD) in the optimum N treatment, and base-dressing (CNBD), top-dressing (CNTD), and once fertilization (CNOT) in farmer's N practice. The outliers were cases with values between 1.5 and 3 box lengths from the upper or lower edge of the box. The extremes were cases with values more than three box lengths from the upper or lower edge of the box. The box length is the interquartile range.

 
As a consequence of increased grain yield and reduced N rates, the optimum N treatment significantly increased net economic gains by $202 ha–1 and $478 ha–1 as compared with farmer's N practice and control, respectively (P < 0.05) (Table 7). Farmer's N practice resulted in higher soil nitrate N accumulation, compared with optimum N treatment, and obvious N losses occurred (Table 8 , Fig. 3 ). Soil nitrate N accumulation after harvest ranged from 40 to 418 kg N ha–1 with an mean of 147 kg N ha–1 and from 16 to 276 kg N ha–1 with an average of 103 kg N ha–1 for farmer's N practice and optimum N treatments, respectively. Mean soil nitrate N content in the 60 to 90 cm cm soil layer was 27, 31, and 52 kg N ha–1 for control (No N), optimum N and farmer's N practice, respectively. Compared with the optimum N treatment, apparent N losses in farmer's N practice significantly increased by 65 kg N ha–1 from 86 to 151 kg N ha–1 (P < 0.05).


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  		Table 8. The calculated N balance (kg N ha–1) for three N treatments in all 148 sites. No N, optimum N, and N represent no N fertilization, Optimum N treatment, and conventional N treatments, respectively.

 

Figure 3
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  		Fig. 3. Nitrate–N distribution in the top 90 cm soil profile after maize harvest with N0, optimum N, and farmer's N practice across all sites. Mean with the different letter are significantly different with different N treatment (P < 0.05)

 
The optimum N treatment significantly increased N use efficiency indices (Fig. 4 ). The average REN, AEN, and PFPN for the optimum N treatment were 31%, 11 kg kg–1, and 72 kg kg–1, respectively, which were 102, 122, and 99% higher than those of farmer's N practice, respectively (P < 0.05). Sites with high ( >50%) and low ( <10%) REN in the optimum N treatment were 13 and 6% of all sites, respectively, as compared with 1 and 34% in farmer's N practice, respectively. The REN exceeded 60% in 8% of sites with optimum N treatment, while no sites had REN more than 60% with farmer's N practice. The proportion of sites with high AEN ( >20 kg kg–1) in optimum N treatment increased to 11% as compared with 2% in farmer's N practice, while the proportion of sites with low AEN (<5 kg kg–1) in optimum N treatment decreased to 23% as compared with 61% in farmer's N practice. In comparison with farmer's N practice, 126 sites (85% of all the sites) had higher PFPN in the optimum treatments, with 44 sites >36 kg kg–1 and 36 sites >50 kg kg–1.


Figure 4
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  		Fig. 4. Nitrogen use efficiency indices including (A) recovery N efficiency, (B) agronomy N efficiency, and (C) N partial factor productivity with optimum N and farmer's N practice across all sites. The symbols are the same as those in Fig. 2. Mean with the different letter are significantly different with different N treatment (P < 0.05).

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Implicit to yield-based N recommendations is the presumption that mineralization is a negligible source of N, which implies that yield in the absence of applied N will be substantially lower than that with N fertilizer application (Mulvaney et al., 2006). In this study, maize N uptake in control ranged from 79 to 218 kg N ha–1 with a mean of 147 kg N ha–1, which was 77% of crop N uptake for the optimum N treatment (Table 8). These results provide good reason to question the validity of yield-based N recommendations, and may partly explain why there is no significant correlation between optimum N and crop N uptake (Vanotti and Bundy, 1994; Kachanoski et al., 1996; Lory and Scharf, 2003). Therefore, N recommendations based on yield response data may provide an erroneous N recommendation because of the strong influences of site-specific soil-crop-climate conditions on soil N supply.

Earlier work (Andraski and Bundy, 2002; Scharf and Alley, 1994; Delin et al., 2005; Cui et al., 2006) showed that yield response to applied N was strongly affected by soil N supply. Bundy and Andraski (2004) also reported that the strongest relationship among diagnostic tests and optimum N rate was preplant soil nitrate N at the 90-cm depth. During summer maize growing season in the NCP, an average of 127 kg N ha–1 from organic-N mineralization and the environment (e.g., dry and wet N deposition, N fixation) were used by summer maize. Therefore, an average of 107 kg N ha–1 was saved using the improved soil Nmin–based N management without causing grain yield loss, compared with farmer's N practice.

Recent literature on improving N management in crop production systems has emphasized the need for greater synchronization between crop N demand and N supply from all sources through the growing season (Fox et al., 1989; Ayoub et al., 1995; Campbell et al., 1995; Cassman et al., 2002). Flowers et al. (2004) indicated that in-season optimization of N rate was more important than site-specific management for grain yield based on the result from soft red winter wheat (Triticum aestivum L.) in the North Carolina coastal plain and piedmont.

In NCP, maize grows in summer, and high temperature and humidity have encouraged high net N mineralization (an average of 127 kg N ha–1 across all sites). During the first several weeks after emergence, summer maize N demand can be met by soil N supply from net N mineralization and initial soil Nmin in root layer. Therefore, no N fertilizer before planting was applied in the soil Nmin–based N management. The ratio of 1:2.1 N fertilizer rates between 3 and 10 leaf stages was similar to that of crop N uptake at 10 leaf stage (about 60 kg N ha–1) and 10 leaf stage to harvest (about 130 kg N ha–1). More N was applied near the time of peak crop N demand, which may increase grain yield, N use efficiency and decrease the opportunity for soil N losses due to leaching or denitrification (Welch et al., 1971; Russelle et al., 1981; Fox et al., 1986). With farmer's N practice, 49% farmers applied all the N fertilizer at five or six leaf stage. This may result in insufficient N supply before five leaf stage or after 10 leaf stage. The other 51% of the farmers applied excessive N at 126 kg N ha–1 and 175 kg N ha–1 during the 3 and 10 leaf stage, respectively. As a consequence, N losses increased to 151 kg N ha–1 as compared with 86 kg N ha–1 using the improved soil Nmin–based N management. Randall and Mulla (2001) reported that annual losses of nitrate N in tile drainage were 36% higher in fall application than in spring application of N for corn production in Minnesota. Sainz Rozas et al. (2004) also found that high N use efficiency could be obtained when N was applied nearest to the time of the crop's N uptake peak because of low gaseous N losses and nitrate N leaching. In conclusion, synchronization of soil N supply (Nmin in the root layer), fertilizer N application and subsequent crop N demand was another reason for the improved soil Nmin–based N management to use lower N rate, increase grain yield and N use efficiency.

It is difficult to measure N cycle processes, including mineralization, immobilization and N losses (e.g., NH3 volatilization, nitrate N leaching, denitrification) under field conditions. As a result of N cycle between soil and plant, the amount of soil Nmin in the crop root layer can be measured by soil testing (Stanford, 1982). In the improved soil Nmin–based N management, the optimum N rate in different crop growth periods was calculated by N target value minus measured nitrate N value in root layer. The result of N cycle after soil testing was considered in the next N recommendation. When N balance was negative, the residual soil nitrate N content would be decreased, and N recommendation rate would be increased, and vice versa. In this study, the improved Nmin method considered the soil nitrate N levels at three different growth stages and provided different N recommendations accordingly. Therefore, synchronization of soil N supply (nitrate N in the root layer), fertilizer N application with subsequent crop demand on N is improved in this improved soil Nmin–based N management strategy.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In conclusion, the great spatial and temporal variation in optimum N rates among fields is common in summer maize production in the NCP. This variation can be explained by initial soil Nmin or soil N supply, but not crop N uptake. Therefore, yield-based N recommendations based on crop response to N rates may not be an appropriate method for N management in summer maize production in this region.

As an "insurance," a high rate of N fertilizer (96–482 kg N ha–1) has usually been applied by farmers for summer maize production in NCP. However, higher N fertilizer input did not increase maize grain yield but reduced significantly farmer's net economic gain and increased N losses to the environment, compared with optimum N management using the improved soil Nmin method. The improved soil Nmin–based N management significantly increased maize yield (0.4 Mg ha–1), although an average of 107 less N ha–1 was used in this region, compared with farmer's N practice, due to greater synchronization between crop N demand and N supply from all sources in the crop growing season. The average REN, AEN, PFPN in the optimum N treatment were 31%, 11 and 72 kg kg–1, respectively, which were all significantly higher than farmer's N practice (REN, 15%; AEN, 5 kg kg–1; PFPN, 36 kg kg–1). As a result, the improved soil Nmin–based N management significantly increased economic gains by $202 ha–1, reduced residual nitrate N content in the top 90 cm soil layer and N losses by 44 kg N ha–1 and 65 kg N ha–1, respectively, compared with farmer's N practice. Therefore, there would be great benefits in terms of farmers' economic returns and potentially negative environmental impacts if the Nmin–based N management can be widely adopted for summer maize production in NCP


   ACKNOWLEDGMENTS
 
We thank the National Natural Science Foundation of China (30390084) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT0511) for their financial support.

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 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
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 Proc. Natl. Acad. Sci. USAHome page
X.-T. Ju, G.-X. Xing, X.-P. Chen, S.-L. Zhang, L.-J. Zhang, X.-J. Liu, Z.-L. Cui, B. Yin, P. Christie, Z.-L. Zhu, et al.
From the Cover: Reducing environmental risk by improving N management in intensive Chinese agricultural systems
PNAS, March 3, 2009; 106(9): 3041 - 3046.
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