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NITROGEN MANAGEMENT

The Ecologically Optimum Application of Nitrogen in Wheat Season of Rice–Wheat Cropping System

^a Dep. of Environmental Engineering, College of Natural Resources and Environmental Science, Zhejiang Univ., Hangzhou, 310029, PR China
^b Dep. of Resource and Environment, Huangshan Univ., Huangshan, 245000, PR China

^* Corresponding author (liang410{at}zju.edu.cn).

	ABSTRACT

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

 
Because excessive application of N fertilizer for crop production leads to environmental pollution and low N utility efficiency, a better understanding of the effects of N application rates on crop yields and NO₃–N leaching is required for developing optimum ecological N management that reduces NO₃–N leaching while keeping crop yield. Field experiments at two sites in the Taihu region of China were conducted to study the ecologically optimum application of N in wheat (Triticum aestivum L.) season of rice (Oryza sativa L.)–wheat cropping system. The experiment at either site had five N rates on wheat (0–360 kg N ha^–1 in 90-kg increments) and NO₃–N in leachate were collected by wedge-shaped fiberglass wick lysimeters. At either site, the N-wheat yield quadratic response curve was fitted quite well and a significantly linear relationship between N rates and seasonal NO₃–N masses in leachate was also found. The calculated economically optimum N rate for wheat was more site related than depending on changing growing conditions from year to year, while the ecologically optimum N rate was significantly different both at sites and growing conditions (P = 0.01). The results suggest that applying the ecologically optimum N rates of 120–180 kg N ha^–1 to wheat is beneficial for maximally reducing NO₃–N leaching loss but minimally decreasing yield.

Abbreviations: CVR, cost:value ratio • R-W, rice and wheat

The Ecologically Optimum Application of Nitrogen in Wheat Season of Rice–Wheat Cropping System

^a Dep. of Environmental Engineering, College of Natural Resources and Environmental Science, Zhejiang Univ., Hangzhou, 310029, PR China
^b Dep. of Resource and Environment, Huangshan Univ., Huangshan, 245000, PR China

^* Corresponding author (liang410{at}zju.edu.cn).

Received for publication June 29, 2006.
Because excessive application of N fertilizer for crop production leads to environmental pollution and low N utility efficiency, a better understanding of the effects of N application rates on crop yields and NO₃–N leaching is required for developing optimum ecological N management that reduces NO₃–N leaching while keeping crop yield. Field experiments at two sites in the Taihu region of China were conducted to study the ecologically optimum application of N in wheat (Triticum aestivum L.) season of rice (Oryza sativa L.)–wheat cropping system. The experiment at either site had five N rates on wheat (0–360 kg N ha^–1 in 90-kg increments) and NO₃–N in leachate were collected by wedge-shaped fiberglass wick lysimeters. At either site, the N-wheat yield quadratic response curve was fitted quite well and a significantly linear relationship between N rates and seasonal NO₃–N masses in leachate was also found. The calculated economically optimum N rate for wheat was more site related than depending on changing growing conditions from year to year, while the ecologically optimum N rate was significantly different both at sites and growing conditions (P = 0.01). The results suggest that applying the ecologically optimum N rates of 120–180 kg N ha^–1 to wheat is beneficial for maximally reducing NO₃–N leaching loss but minimally decreasing yield.

Abbreviations: CVR, cost:value ratio • R-W, rice and wheat

	INTRODUCTION

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

 
RICE AND WHEAT (R-W) are two of the most important cereal crops across the world, contributing 45% of the digestible energy and 30% of total protein in the human diet, as well as a substantial contribution to feeding livestock (Evans, 1993). Most R-W systems are located in South and East Asia within subtropical to warm-temperate climates. The major producers are India (10 million ha) and China (13 million ha). Other contries include Packistan (2.2 million ha), Bangladesh (0.5 million ha), and Nepal (0.6 million ha) (Timsina and Connor, 2001). They extend across the Indo-Gangetic Plains into the Himalayan foothills, spanning a vast area from Pakistan's Swat Valley in the north to India's Maharashtra State in the south, and from the mountainous Hindu Kush of Afghanistan in the west, to the Brahmaputra flood plains of Bangladesh in the east. In China, the systems are practiced widely in the provinces of Jiangsu, Zhejiang, Hubei, Guizhou, Yunnan, Sichuan, and Anhui, along the Yangtse River Basin. Though most R-W areas in China are concentrated below 35°N latitude in the plains and below 28°N in the highlands, they are also found as far as 40°N across the Huihe and Yellow Rivers (Lianzheng and Yixian, 1994; Jiaguo, 2000).

Since the 1960s, there has been substantial increase in the area cultivated to R-W rotation in South and East Asia by the development of two crops cultivated within a short duration. The driving force for expansion, however, remains the increasing demand for food that must be met by more intensive production systems because limited land is available for expansion of agriculture. In many places the pressure to intensify agriculture is exacerbated, as agricultural land is lost due to degradation and urbanization. Farmers have started to adopt greater than recommended rates of chemical fertilizers, particularly N fertilizers, to maintain the yield levels previously attained with relatively less fertilizer. Recent diagnostic surveys on nutrient management practices prevailing in R-W dominated areas of western Uttar Pradesh revealed that nearly one-third of R-W growing farmers apply as much as 180 kg N ha^–1 to each rice and wheat crop as against the local recommendation of 120 kg N ha^–1 (Dwivedi and Upadhyay, 2001).

Such emerging trends of indiscriminate use of fertilizer N have to be curbed because of two major concerns. First, fertilizer use in some areas has already reached very high levels, and further increase is unlikely to give economic returns (Yadav, 1998). Second, as the efficiency of fertilizer N in paddy soils is quite low due to excessive N losses, increase in fertilizer N application rates may also enhance the intensity of nitrate-leaching and sequently lead to pollution of groundwater (Singh, 1995). Hence, the efficiency of applied fertilizers rather than rate of fertilization needs to be increased to sustain the productivity of R-W system, as well as to minimize the environmental hazards.

The Taihu region is located in the center of the Yangtze River Delta at an altitude of 3 to 4 m above sea level. This region is one of the most densely populated and intensive crop growing areas in China, and about 75% of arable lands in this region are used for rice growing in summer and wheat (or oil rape [Brassica napus L.]) growing in winter. The current deteriorated water quality is commonly due to nonpoint-source pollution form agricultural system, after identification and reduction of water pollution from industrial and commercial activities since the 1980s. The N application rates in the region presently are 550 to 650 kg N ha^–1 per year to two crops (Zhu et al., 2003), much higher than the Chinese national average of about 300 kg N ha^–1, implying that nonpoint-source pollution from agricultural ecosystems plays an important role in water quality degradation. Previous researchers observed that N leaching occurred mainly in the wheat season of R-W cropping system in the region and NO₃–N was the main form of N leached. With leaching-pond and plot experiments, Huang et al. (2005) found NO₃–N load to water bodies from wheat fields were as high as 18.4 kg N ha^–1, and 14.0 kg N ha^–1 via leaching to shallow groundwater. Using lysimeter experiment, Lian and Wang (2004) observed that 36.2 to 99.1% of total leached N from wheat fields, with average of 82.9%, was in the form of NO₃–N. Previous researchers also highlighted that NO₃–N concentrations in leachate and its associated loss masses in wheat seasons were apparently affected by N application rates. Wang et al. (2004) observed that NO₃–N concentrations in leachate rose to 5.4 to 21.3 mg N L^–1 during wheat seasons when the urea N application rate reached 225 kg N ha^–1, with about 60% of the leachate samples determined contained NO₃–N beyond the directive for drinking water (NO₃–N 10 mg N L^–1) by the World Health Organization (WHO). At 150 kg N ha^–1 urea N rate, Lian and Wang found the leached NO₃–N mass in wheat season was only 3.2 kg N ha^–1, but reached 14.3 kg N ha^–1 as receiving the rate of 300 kg N ha^–1. Therefore, NO₃–N leaching posed a great threat to the local groundwater quality in the Taihu region, but until now few efforts had been taken toward ecologically optimum N management. Wang et al. (2004) reported that the economically optimum N rates for wheat in the region were in the range of 180 to 225 kg N ha^–1, however, they failed to get an ecologically optimum N rate for lacking field data on NO₃–N masses in leachate under different N rates. At the ecologically optimum rate, crop yields should be kept as much as possible but N losses should be reduced to a safe level for environmental quality, wildlife welfare, and human health.

Therefore, the objectives of this study are to quantitatively assess the NO₃–N leaching losses during wheat seasons from typical paddy soils of the Taihu region under different urea N application rates and analyze the effects of economically and ecologically optimum N fertilization on wheat yields and NO₃–N masses in leachate.

	MATERIALS AND METHODS

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

 
Two typical sites located at Jiaxing (JX) Agricultural Research Station (30° 50' N, 120°40' E) and Yuhang (YH) Agricultural Research Station (30°30' N, 120°18' E) are selected from the rice production areas in Taihu region of China. This region prevails R-W rotation and possesses the characteristics of typical climate and terrace of the coastal plain area. Table 1 shows the physic-chemical characteristics of soils at the JX and YH sites. The JX soil is a gleyed paddy soil (clay loam, mixed, mesic Mollic Endoaquepts) and the YH soil is a hydragic paddy soil (silt loam, mixed, mesic Mollic Endoaquepts). The JX soil has the lower pH (6.78) and sand content (12.1%) but has the higher total N (2.65 g kg^–1), total P (1.53 g kg^–1), organic matter (35.0 g kg^–1), bulk density (1.33 g cm^–3), and clay content (51.7%) than YH. The soil pH, total N, total P, organic matter, bulk density, sand content, and clay content at the YH are 7.17, 2.34 g kg^–1, 0.93 g kg^–1, 19.7 g kg^–1, 1.04 g cm^–3, 15.2%, and 37.0%, respectively. Notably, the soil infiltration rate at the YH (0.786 cm d^–1) is about 1.5-folds higher than JX (0.424 cm d^–1).

Six months before the start of the experiment at the SQ in 2004, wedge-shaped fiberglass wick lysimeters were installed in all plots where winter wheat received 0, 90, 180, 270, or 360 kg N ha^–1 to monitor NO₃^– leaching as a function of N fertilizer rate. The wick lysimeters were installed 0.6 m below the soil surface of plots. In 2005, the lysimeters were also installed at the YH site with the same method as the SQ.

The wick lysimeter was specially designed in our experiments (Fig. 1 ). It has a 40 by 40 cm top surface fixed with an inversely bounded frame. Cleaned wick fiber cloth is evenly spread on the top surface. In the lysimeter, the upper part includes three 2 cm layers, a quartz sand layer, a gravel layer, and a pebble layer. The left bottom part is a buffer space for leachate discharged by a tube. The other tube is laterally inserted into the pebble layer for connecting air outside. Tunnels for lysimeter installation were excavated 100 cm into soil at 0.6 m below the soil surface to accommodate the lysimeters. The vertical distance from the top surface to the bottom of the lysimeter was 10 cm, which created up to 10 cm of water tension on the soil. Because the lysimeters were installed in plots with 0, 90, 180, 270, and 360 kg ha^–1 N rates with three replications, a total of 15 plots were installed with lysimeters at both field experimental sites.

View larger version (28K): [in this window] [in a new window]   Fig. 1. Configuration of the wedge-shaped fiberglass wick lysimeter.

At the end of the growing seasons, the crop yields were determined on the inner 20 m² (4 by 5 m) of each plot and the grains were dried in sunny days and weighted. For each growing season, we fitted quadratic curves to express the crop grain yield responses to the applied urea N rates, and fitted general lines to express the NO₃–N leached mass responses to the N rates, separately.

The marginal grain yield response to applied urea N was determined by calculating the first derivative of the grain yield response curves (Bullock and Bullock, 1994). The economically optimum N fertilization was then determined as the N rate at which the yield response dropped to the cost/value ratio (CVR), defined as the ratio of the cost of 1 kg of urea N to the purchase price of 1 kg grain of winter wheat (Neeteson and Wadman, 1987; Schlegel et al., 1996). According to Chinese local data, we applied a cost price of 4.0 Yuan per kg of urea N and a purchase price of 1.6 Yuan for 1 kg of winter wheat grain. This resulted in a CVR = 2.5.

People in the Taihu region are used to using shallow groundwater as drinking water by digging wells. The ecologically optimum urea N fertilization was determined as the N rate at which the flow-weighted NO₃–N concentration dropped to the WHO's nitrate N directive for drinking water (<10 mg N L^–1).

Three double ring infiltrometers equipped with a marriotte buret were installed at field sites to determine soil infiltrate rate (Chen and Liu, 2002). Plow-layer (0–20 cm) soil samples were taken at both sites before the start of experiments. The soil samples were air-dried and ground to a 2-mm sieve. Soil total N was determined by the Kjeldahl digestion-distillation method. Soil total P was determined by pretreatment of H₂SO₄–HClO₄ digestion, then analyzed by the molybdenum blue color method. Soil organic matter in the soil was measured by oxidation with potassium dichromate.

The differences in flow-weighted NO₃–N concentrations and NO₃–N masses in leachate between sites and seasons were analyzed using the paired t test in SPSS software. The differences in flow-weighted NO₃–N concentrations in leachate between N rates were analyzed with the General Linear Model in ANOVA in the SPSS software, and the resulting error mean squares were used to calculate the LSD values for mean separation.

	RESULTS AND DISCUSSION

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

 
Grain Yield Responses to Nitrogen Rates
There is a clear N contribution effect on winter grain yields: when N supply is supra-optimal, no advantage of urea was observed. For each season at SQ and YH sites, the crop grain yield significantly increased with the increase of N rates from 0 to 270 kg N ha^–1 at P = 0.01 level (Table 2 ). However, at 360 kg N ha^–1, the urea had a negative effect on grain yields: minus 5% in SQ-2004, minus 9% in SQ-2005, and minus 7% in YH-2005 as compared with 270 kg N ha^–1. Additionally, there was no significant difference in grain yields between the 180 kg N ha^–1 and 360 kg N ha^–1. Table 2 showed the N-wheat yield quadratic response curves were fitted quite well for each season at the SQ and YH sites.

Flow-weighted Nitrate–Nitrogen Concentration in Leachate
At the same site (SQ), seasonal flow-weighted NO₃–N concentrations in leachate in 2005 were significantly higher than in 2004 at P = 0.01 level for all N rates, mainly due to different precipitation during either season (Table 3 ). Meanwhile, at the same season (2005), the seasonal flow-weighted NO₃–N concentrations in leachate for YH were significantly higher than SQ at P = 0.01 level for all N rates, resulting from the different soil texture with different soil infiltrate rate at two sites (Table 1).

All the leachate flow-weighted NO₃–N concentrations in SQ-2004, SQ-2005, and YH-2005 significantly increased with the increase of N rates (Table 2). At or above 180 kg N ha^–1, flow-weighted NO₃–N concentrations in leachate identically exceeded 10 mg N L^–1 of NO₃–N directive for drinking water. At 360 kg N ha^–1 in the SQ-2005 and YH-2005, the flow-weighted NO₃–N concentrations in leachate were even as high as 23.2 and 25.8 mg N L^–1 exceeding NO₃–N directive by 132.4 and 158.0%, which created significant nitrate contamination to the local groundwater.

Nitrate–Nitrogen Mass in Leachate
Nitrogen fertilizer management may have a significant impact on NO₃–N leaching through the soil profile, which considered as the most controllable factor that had the greatest impact on NO₃–N losses (Rao, 1996; Zhu et al., 2000; Zhu and Fox, 2003; Delgado and Bausch, 2005; Delgado et al., 2005). The seasonal NO₃–N masses in leachate in SQ-2004 varied from 3.6 to 31.7 kg N ha^–1 over the N rates from 0 to 360 kg N ha^–1, which were significantly lower than those in SQ-2005 at P = 0.01 level. It indicates that the seasonal NO₃–N mass in leachate is dependent on the growing condition, and mostly on the rainfall amount in this study (Table 4 ). Meanwhile, the seasonal NO₃–N masses in leachate in YH-2005 varied from 9.9 to 60.9 kg N ha^–1 over the N rates from 0 to 360 kg N ha^–1, which were significantly higher than those in the SQ-2005 at P = 0.01 level. This mostly attributes to relative high soil infiltration rate at the YH (Table 1). These results suggest that NO₃–N leaching potentials are affected by both the spatial and temporal variability and agree with other researchers (Chen et al., 2006; Dobermann et al., 2003; Wang et al., 2001). So, site-specific nutrient management has been proposed widely for reducing NO₃–N leaching in recent years. Delgado et al. (2005b) mapped site-specific management zones based on soil color, topography, and the producer's past management experiences that affect NO₃–N leaching, and they found that NO₃–N leaching losses was cut by 25% during the first year after a site-specific nutrient management plan.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Relationship between N application rates and NO3–N masses in leachate during winter wheat seasons at experimental sites of SQ and YH in 2004 and 2005.

View larger version (27K): [in this window] [in a new window]   Fig. 3. Decreases of N rates, NO3–N leached, and yields from economically optimal fertilization to ecologically optimal fertilization. Percentage of decrease of N rate is deferred as: (the economically optimal N rate minus the ecologically optimal N rate)/the economically optimal N rate x 100%. The calculation of percentages of decreases of NO3–N leached and yields are similar as the N rate.

By comparison of economically and ecologically optimum N fertilization, it could be found that the economically N_opt and NO₃–N mass in leachate every season at SQ or YH were significantly higher than the ecologically N_opt and the corresponding NO₃–N mass in leachate at P = 0.01 level, but the corresponding yields were only lower at P = 0.05 level for all seasons. Fig. 3 showed the percentages of decreases of N rates, NO₃–N masses in leachate, and yields from economically to ecologically optimum N fertilization. In the SQ-2004, the yield under ecologically optimum fertilization was only cut by 3.6%, while the N rate and NO₃–N mass in leachate were cut by 10.3 and 15.0%, respectively. In the SQ-2005, the yield decreased by 10.3% but the N rate and NO₃–N mass in leachate decreased as high as 37.7 and 29.8%, respectively. In the YH-2005, the decreases of N rate and NO₃–N mass in leachate reached 40.1 and 31.6% following a small decrease of yield by 12.9%. Wang et al.(2004) observed that the economically N_opt for wheat in the Taihu region ranged from 180 to 225 kg N ha^–1, whereas about 60% of the leachate samples at those N rates contained NO₃–N beyond 10 mg N L^–1. Results of this and previous study indicate that wheat yield maximization but NO₃–N leaching mass minimization can be realized when wheat field received the ecologically N_opt (about 120 to 180 kg N ha^–1).

	CONCLUSIONS

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

 
There was a clear N contribution effect on winter grain yields: when N supply was supra-optimal, no advantage of urea was observed. The N-wheat yield quadratic response curve was fitted quite well at either SQ or YH site. An economically optimum yield was available by applying more urea N fertilizer, but it would take a greater NO₃–N mass in leachate. Leachate flow-weighted NO₃–N concentrations in wheat seasons were significantly affected by increasing N rates, and there was a significantly linear relationship between N rates and seasonal NO₃–N masses in leachate at either SQ or YH site. The average ratios of N leached to total N applied were as high as 0.178, 0.129, and 0.074 in YH-2005, SQ-2005, and SQ-2004, respectively. The ecologically optimum N rate and NO₃–N mass in leachate every season at SQ or YH were significantly lower than the economically optimum N rate and the corresponding NO₃–N mass in leachate at P = 0.01 level, but the corresponding yields were only lower at P = 0.05 level for all the seasons. Besides, it was found that the economically optimum N rate was more site related than depending on changing growing conditions from season to season, while the flow-weighted NO₃–N concentrations, and seasonal NO₃–N masses in leachate, and its associated ecologically optimum N rates for wheat are significantly different at P = 0.01 level both at sites and growing conditions. The ecologically optimum N rates of 120–180 kg N ha^–1 were suggested to apply to wheat season of a R-W rotation, because the wheat grain yields at those rates only reduced about 4 to 13% but the NO₃–N masses in leachate were cut by about 15 to 30% as compared with economically optimum N fertilization.

	ACKNOWLEDGMENTS

 
The authors wish to thank the National Key Basic Research Project of China (2002CB410807) for funding this study.

	NOTES

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

 
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