Leaching losses of nitrate nitrogen and dissolved

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Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under mo.... Article in Nutrient Cycling in Agroecosystems · September 2011 DOI: 10.1007/s10705-011-9447-z

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Nutr Cycl Agroecosyst (2011) 91:77–89 DOI 10.1007/s10705-011-9447-z

ORIGINAL ARTICLE

Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations Manxiang Huang • Tao Liang • Zhu Ou-Yang Lingqing Wang • Chaosheng Zhang • Chenghu Zhou

•

Received: 29 March 2011 / Accepted: 20 July 2011 / Published online: 2 August 2011 Ó Springer Science+Business Media B.V. 2011

Abstract A large amount of nitrogen (N) fertilizers applied to the winter wheat–summer maize double cropping systems in the North China Plain (NCP) contributes largely to N leaching to the groundwater. A series of field experiments were carried out during October 2004 and September 2007 in a lysimeter field to reveal the temporal changes of N leaching losses below 2-m depth from this land system as well as the effects of N fertilizer application rates on N leaching. Four N rates (0, 180, 260, and 360 kg N ha-1 as urea) were applied in the study area. Seasonal leachate volumes were 87 and 72 mm in the first and second maize season, respectively, and 13 and 4 mm during the winter wheat and maize season in the third rotational year, respectively. The average seasonal flow-weighted NO3-N concentrations in leachate for the four N fertilizer application rates ranged from 8.1 to 103.7 mg N l-1, and seasonal flow-weighted dissolved organic nitrogen (DON) concentrations in leachate varied from 0.8 to 6.0 mg N l-1. Total amounts of NO3-N leaching lost

M. Huang T. Liang (&) Z. Ou-Yang L. Wang C. Zhou Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China e-mail: [email protected] C. Zhang Department of Geography and Environmental Change Institute, National University of Ireland, Galway, Ireland

throughout the 3 years were in the range of 14.6 to 177.8 kg ha-1 for the four N application rates, corresponding to N leaching losses in the range of 4.0–7.6% of the fertilizers applied. DON losses throughout the 3 years were 1.4, 2.1, 3.6, and 6.3 kg N ha-1 for the four corresponding fertilization rates. The application rate of 180 kg N ha-1 was recommended based on the balance between reducing N leaching and maintaining crop yields. The results indicated that there is a potential risk of N leaching during the winter wheat season, and over-fertilization of chemical N can result in substantial N leaching losses by high-intensity rainfalls in summer. Keywords Dissolved organic nitrogen Nitrate nitrogen Leaching losses

Introduction In recent years, shallow ground water quality in the North China Plain (NCP) has been impaired by the presence of nitrogen (N) compounds and pesticides with concentrations exceeding the drinking–water standards. Studies across the Beijing–Tianjin-Tangshan region found elevated NO3-N levels in the groundwater adjacent to crop fields and a positive correlation with fertilizer N application rates (Zhang et al. 1996; Li et al. 2001a). The NO3-N concentrations of up to 300 mg l-1 have been detected in subsurface water drained from agricultural lands (Zhang et al. 1995).

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Elevated NO3-N concentrations in groundwater associated with excessive N fertilization have caused increased concerns in the NCP (Xing and Zhu 2000; Ju et al. 2004, 2006 Hu et al. 2006; Fang et al. 2006). The growth habit and management techniques commonly used in the NCP may have contributed to increased potential for NO3-N leaching from agricultural lands. There are large acreages of crops with high fertilizer nitrogen inputs and intensive food production without intercultivation periods in this region. For example, winter wheat-summer maize double cropping is a common farming practice in an area of [14 million ha in this region and produces about 45% of the total cereal production in China (China Agricultural Yearbook 2001). In the NCP, an average application rate of 150–180 kg N ha-1 was regarded as the economically optimum nitrogen fertilizer application rate (EONR) for a single crop (Zhu 1998). The conventional N application rates are 256–280 kg N ha-1 per crop (Zhao et al. 1997; He et al. 2009). However, farmers tend to apply high rates of fertilizer N to their fields because of national production subsidies and readily accessible supply of urea. Over-fertilization of 309–450 kg N ha-1 per crop during the last two decades has prevailed in this region (Ma 1999; He et al. 2009). Clearly, such high N application rates greatly exceed the N requirements of crops and will inevitably cause reduced N use efficiency and increased risk of NO3-N leaching to groundwater. Several recent studies have focused on effects of N application rates on soil nitrate accumulation and dynamics of NO3-N in soil solution (Liu et al. 2003; Ju et al. 2004; Fang et al. 2006). There is still little understanding of the overall patterns of NO3-N leaching from winter wheat (Triticum aesstivum L.) -summer maize (Zea mays L.) rotation system in the NCP. Another factor affecting the NO3- downward movement in the wheat-maize rotation system is the N fertilization practices and water inputs including precipitation and irrigation. The climate in the NCP belongs to the Asian monsoon category, and heavy rainfall events often occur in summer (Liu and Wei 1989). Nitrogen management for the non-tillage maize cropping usually involves high fertilizer N rates applied at the V3 and V10 vegetative growth periods before heavy rainfall. Poorly timed N applications during the rainy growth season can lead to lower N recovery and substantial N loss from the soil

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Nutr Cycl Agroecosyst (2011) 91:77–89

system (Baker and Timmons, 1994; Blackmer and Sanchez 1988; Timmons and Cruse 1990). Little information is available concerning the timing of NO3-N leaching losses during the maize cropping season in the NCP. Water supply from soil water storages is essential to the growth of winter wheat and high cereal production in the NCP (Li et al. 2001b; Zhang et al. 2004). The scarcity of precipitation in the winter season necessitates irrigation to meet water demand of wheat (Li et al. 1999; Fan et al. 2001). Traditionally, farmers increase N content and soil moisture in deeper soils to induce deep-root development of wheat by incorporating basal fertilizer (N, P, K) with previous maize stubble left in the soil before sowing of winter wheat. Full irrigation is provided to more than 100% of soil water-holding capacity throughout the 2-m soil profile after topdressing of fertilizer N in winter wheat season. These practices obviously contribute to the high moisture and soil N accumulation in the deep soil layers. Although it has been recommended that an average of 75 mm of water is needed for a supplement irrigation without drainage from a 1-m soil profile by scientists and advisory services since 1994 (Lu et al. 1994; Lan and Zhou 1995), unforeseen precipitation soon after irrigation may maximize water and nitrogen deep seepage. Therefore, there are some uncertainties with the risk of N and water leaching out of the 2-m soil profile during winter wheat seasons. The existence of dissolved organic nitrogen (DON) in soil solution and its leaching losses from agricultural fields have been recognized for more than 125 year (Kessel et al. 2009). Leaching of DON into water bodies can lead to eutrophication and acidification, and pose a potential risk to human health (Jordan et al. 1997). DON significantly contributes to N leaching losses from forest ecosystems (Campbell et al. 2000; Qualls et al. 2000; Perakis and Hedin 2002), and may be an important form of N losses from agricultural systems (Murphy et al. 2000; McDowell 2003; Siemens and kaupenjohann 2002; Saarijarvi et al. 2007). Compared to NO3-N, there are only a limited number of studies on DON leaching losses from agricultural soils (Ghani et al. 2007; Kessel et al. 2009). On average, the amount of DON leaching losses from agricultural systems was estimated to be approximately one-third of that of NO3-N (Kessel et al. 2009). Oelmann et al. (2007) found that the species had little effect on DON losses but the presence of legumes led to


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an increase in DON losses. It was also found that there were more DON losses in a pasture with multiple grass species than that with a single grass species under application of inorganic fertilizers. The Rothamsted experiment indicated that fertilization increased loss of DON (Murphy et al. 2000). The effects of cropping rotations on DON concentration and its losses have been investigated by few studies (Chantigny 2003). There is no report on DON as a potential form of N losses from the wheat-maize double cropping system in the NCP. Since the increasing area used for double cropping in the NCP may cause an increase of N leaching, baseline leaching data are needed to make sure that the land use systems are both economically and environmentally sustainable. The objectives of this study were: (1) to identify periods that are particularly prone to leaching of nitrogenous compounds (NO3-N and DON) and to quantify the leaching losses of urea-N applied to winter wheat-summer maize rotation systems in the NCP; (2) to determine whether DON losses are significant and to quantify the contribution of DON to N leaching; (3) to investigate the yield responses to different N fertilizer rates and to determine the optimal fertilization rate that minimizes N leaching losses while maintaining crop yields under the conventional irrigation and management systems in the NCP.

Materials and methods A 3-year study was carried out at a site of 667 m2 in Shunyi County (30 km northeast of Beijing City, China). The average annual rainfall for the area is 640 mm. Average annual temperature is 11°C, with the highest and lowest monthly mean temperatures being 26°C(July) and -12°C (January), respectively. The average annual evapotranspitration is about 1,500 mm, with the highest seasonal mean evapotranspitration in summer and lowest in winter. Generally, evapotranspitration demand exceeds precipitation except in summer (Beijing Agricultural Department 1984). The soil is a typical cinnamon soil derived from the loess parent material. The soil profile of the experimental area was divided into four layers: a loam topsoil (0–0.65 m), a clay loam upper subsoil (0.65–1.0 m), a fine silt sandy clay median subsoil (1.0–1.27 m) and a clay loam lower subsoil (1.27–2.0 m) (Anonymous 1961). The average available soil water-holding capacity (0.03–1.5 MPa) was measured to be 0.153 m3m-3 for a 2-m soil profile at this site. Irrigation water requirement needed for a supplement to available soil water (0.15–0.1 M Pa) was estimated at 75 mm for a 1-m soil profile (Lu et al. 1994). The water table was below a depth of 14 m. The main soil physico-chemical properties are shown in Table 1. Prior to the experiment, a winter

Table 1 Selected properties of the soil used in lysimeter experiment Soil depth (cm) 0–27

27–65

65–100

100–127

127–160

160–200

Bulk density, Mg m-3

1.32

1.46

1.50

1.46

1.46

1.48

Textural classa(Sa/Si/Cl), %

36.1/44.7/19.2

33.5/44.7/21.8

21.0/46.2/32.8

7.0/47.5/45.5

32.0/40.0/28.0

31.0/39.4/29.6

pH

8.3

8.2

8.1

8.2

8.0

8.0

Organic C, %

2.67

1.58

0.96

0.77

0.60

0.44

Total N, g kg-1

1.43

0.79

0.39

0.42

0.34

0.28

Olsen-P, mg kg-1

37.8

24.1

9.8

NH4OAC-K, mg kg-1

117.8

88.7

79.2

Available water holding capacity, m3 m-3 between 0.03 and 15 M Pa

0.197

0.159

0.140

0.119

0.152

0.152

Saturated hydraulic conductivity, mm h-1b

12.2

10.6

8.1

7.6

5.6

3.9

a

According to the USDA soil classification system (Sa sand, 0.05–2 mm, Si silt, 0.002–0.05 mm, Cl clay, \0.002 mm)

b

Measured on laboratory

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wheat –summer maize double cropping rotation had been applied for many years, with N fertilizer application rate of 75 kg N ha-1 per crop from 1991 to 1997, and 120 kg N ha-1 per crop from 1998 to 2002 with chicken manure 2,000 kg ha-1 as basal application for winter wheat. The lysimeter field consisted of 12 in situ repacked concrete–lined and gravity-drained lysimeters installed during 2002–2003. The repacked lysimeters provide an alternative means to quantify total water flow and N movement through soil (Baker and Timmons 1994; Martin et al. 1994; Kalmoach 1995; Rasse et al. 1999; Huang et al. 1999; Derby et al. 2002; Nyamangara et al. 2003). Each lysimeter was 2 m long, 2 m wide, and 2 m deep. Winter wheat roots extend up to 2.0 m in the winter wheat- summer maize rotation in the NCP (Zhou et al. 2008). Therefore, the 2-m deep lysimeter is required to accommodate this deep-rooted crop (Evett et al. 2009). The details of lysimeter installation about excavation and replacement of soil horizons, type and installation of the lysimeter liner, and installation of drainage tubes were the same as those in Baker and Timmons (1994); Kalmoach (1995); Rasse et al. (1999). The soil layers were repacked to their original density following the soil profile identified during site characterization. Because the soil was severely disturbed during installation of the lysimeters, summer maize was planted without fertilization in 2004 to promote reestablishment of the undisturbed soil profile properties. A typical winter wheat–summer maize double cropping system was adopted in the experiment. Four N application rates of 0, 180, 260, and 360 kg N ha-1 per crop (noted as N0, N180, N260, and N360) were chosen to be representative of the unfertilized control, the EONR fertilizer rate, the conventional N rate, and current prevalent N rate (based on the farmers’ practice) in the NCP. The N fertilizer treatments were arranged randomly among the lysimeters with three replications. The four N fertilizer treatments were combined with the conventional cultivation practices in the area. In early October each year, the amount of 35% of total urea-N applied was used as basal fertilizer to the lysimeters. Basal fertilizers of 71 kg P2O5 ha-1 as triple superphosphate (TSP 0-46-0) and 60 kg K2O ha-1 as KCl (0-0-60) were also surface applied to the all lysimeters (Cao et al. 1997; Liu et al. 2003). Then the lysimeters were rototilled by repeatedly

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pulling a small rototiller across the width of each lysimeter. The ‘‘Jimei 22’’ winter wheat cultivar sown at a rate of 180 kg seeds ha-1 with 0.20 m row spacing. The amount of 15% of total urea-N applied was top dressed in early December with irrigation of 75 mm. The remainder fertilizer N (50% of total N applied) was top dressed before irrigation of 75 mm in early April. An isobutyl ester formulation of 2,4-D (2,4-dichlorophenoxyacetic acid) of 0.6 kg a.i ha-1 was applied in-mid April for weed control. An additional irrigation of 75 mm was applied in mid–May. Irrigation water was applied using the surface flood method with plastic pipes of 50 mm in diameter, where a water meter was equipped to measure the irrigation amounts. The winter wheat plants of each plot were harvested manually and the aboveground straw except stubbles were removed from the plots. Subsamples of grains and straw were collected and adjusted for moisture prior to measurement of N contents. The grain and straw samples were digested in H2SO4-H2O2 solution and total N contents were determined using the microKjeldahl method (Bremner and Mulvaney 1982). The aboveground biomass was obtained as the sum of grain and straw biomass, and the aboveground N accumulation was calculated based on the sum of grain and straw N accumulation. The grain/straw N accumulation ratios were determined as the product of grain/ straw biomasses times their N contents. Apparent N recovery fraction (NF) is defined as the total N uptake from a given fertilizer N rate treatment minus the total N uptake from the control treatment, divided by the amount of the added fertilizer N. All the wheat residues were manually removed after the harvest of winter wheat, a hybrid maize cultivar ‘‘Denghai 661’’ was seeded at 5 cm depth by hand with no tillage in rows of 0.6-m spacing with 0.25-m distance between every pair of adjacent plants (24 plants per lysimeter) between 16 and 26 June in 2005, 2006, and 2007, respectively. In early July each year, when the maize had reached stage V3-4, N fertilizer was top-dressed before rainfall at a rate of half of total N applied. The remaining N fertilizer was applied in V10 stage (early August). Small weeds were removed by hand from each lysimeter whenever it appears. Within 2 weeks after planting, 71 kg P2O5 ha-1 as triple superphosphate (0-46-0) and 60 kg K2O ha-1 as KCl (0-0-60) were surface applied to each lysimeter. Maize was irrigated only at the 3rd-leaf stage (40 mm) and the 10th-leaf stage (60 mm) in the 2007.


The maize plants were hand harvested and six samples were collected from each plot. The samples were separated into stovers (including cobs) and grains. The maize samples were oven dried at 65°C, weighed for grain and stover dry matter yields, and finely ground. Analyses of total N content, aboveground biomass and N accumulation, and NF were determined using the same methods as those for wheat samples. Each lysimeter was equipped with an access chamber of 130 cm long, 130 cm wide and 180 cm deep. The access chamber was installed 50 cm deeper than the bottom of the lysimeter to allow a collection container to be placed under the drainage pipe. Sampling was conducted manually once every 2 week except during dry periods when no leaching occurred. The amount of leachate was recorded and subsamples were acidified (pH B 2) and stored frozen until analysis. To monitor the DON concentrations at different soil depths, suction cups made from borosilicate glass (Wessel-Bothe et al. 2000) were buried at the first replication lysimeters in late October 2005. Seven suction cups were placed at 0.2, 0.4, 0.6, 0.8, 1.0, 1.4, and 1.8-m soil depths in each replication lysimeter. Samples of the soil solution were taken at an interval of about 2 weeks from April 2006 to September 2007. Subsamples of leachate were analyzed for inorganic and organic forms of nitrogen. Total inorganic nitrogen (TIN) concentrations were the sum of NH4N, NO2-N, and NO3-N concentrations. Inorganic nitrogen concentrations were determined with the phenate and cadmium reduction-diazotization method on a flow-injection analyzer. The concentration of NH4-N and NO2-N in sample was negligible. Dissolved organic nitrogen (DON) in leachate or soil solution was calculated as the difference between the TIN concentration and the NO3-N concentration determined after alkaline persulfate digestion of leachate samples (Cabrera and Beare 1993; Shuster et al. 2003). A flow–weighted average concentration value was the weighted average based on total N transport and total leachate volume for that period. Weather data was recorded at a meteorological station located 500 m away from the lysimeter site. The crop-percolate year was defined as October through September of the following year. Data were analyzed and presented based on treatment seasons. Seasonal and annual nitrogen transport amounts were

81

the sum of all of the transport for the corresponding time periods. The loss percentage of N fertilizer by leaching is defined as the total N leaching loss from a given N fertilizer rate treatment minus the total N leaching loss from the control treatment, divided by the applied N rate. The data obtained included precipitation, irrigation, leachate volumes from the lysimeters, NO3-N concentrations, DON concentrations, NO3-N leaching losses and DON leaching losses in the lysimeter outflow, crop yields and N removal by crop harvest. The data were analyzed as a function of N treatment by crop season. Statistical analyses of the data were accomplished using standard analysis of variance (ANOVA) and mean values were compared using least significant difference method at the 5% significance level using the SAS software package (SAS Institute 2002).

Results and discussion Weather and percolation Results of precipitation and percolation (averaged over all lysimeters) are shown in Fig. 1 for the 3 year of field experiments. Total water inputs were 369.8 and 335.5 mm in the 2004–2005 and 2005–2006 winter wheat seasons, respectively. There was no drainage during the 2004–2005 and 2005–2006 winter wheat growing seasons. Rainfall amount was 487.4 mm in the 2005 summer maize season, 73% of which occurred during late-June through July, bringing the total seasonal leachate volume to 87 mm. Total rainfall in the 2006 maize season was 544.4 mm, 90.6% of which occurred during July to August, resulting in a leachate volume of 72 mm. In contrast to the two former rotational years, total water input was 471 mm during 2006–2007 winter wheat season, with a leachate volume of 12.8 mm. Total water input in 2007 summer maize season was of 312.7 mm (This was supplemented with 100 mm of irrigation). The average amount of soil water draining to the lysimeter bottoms during the same period was only 3.6 mm. No measurable percolation occurred from October to the March of the subsequent year due to dry and freezing weather conditions during the experiment period. Main drainage episodes occurred as a

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leachate (mm)

percolation (mm)

82 120 80 40 0 60 2004-2005

2005-2006

2006-2007

40 20 0

O N D J FM AM J J A S O N D J FM AM J J A S O N D J FM AM J J A S

month Fig. 1 Leaching losses of dissolved organic nitrogen and nitrate nitrogen from a wheat-maize rotation system. Distribution of rainfall (including irrigation) and leachate volumes from the 2-m lysimeters (vertical arrows denote irrigation)

consequence of successive episodes of heavy rainfalls in summer. Because the amount of precipitation varied greatly between the summer maize seasons, a strong cause and effect relationship was more easily observed between precipitation and percolation at the bottoms of the lysimeters. It illustrates the strong influence of summer weather conditions on the potential percolation of water in the NCP. There were only small differences in the volumes of leachate between the different lysimeters and the differences were insignificant (P [ 0.05). Similar results have been reported elsewhere (Baker and Timmons 1994; Kamukondiwa and Bergstrom 1994; Bergstrom and Kirchmann 1999; Nyamangara et al. 2003). Therefore, the average leachate volume of all lysimeters was used for estimation of N leaching losses. Nitrate nitrogen in leachate As expected, seasonal leachate flow-weighted NO3-N concentrations increased with increasing N application rates over all years of the study (Table 2). The 3-year average seasonal flow-weighted NO3-N concentrations were computed to be 8.1, 26.5, 53.1, and 103.7 mg l-1 for the N0, N180, N260, and N360 treatments, respectively. Leachate NO3-N concentrations were consistently low (\10 mg l-1) from the control lysimeters, ranging from 5.8 to 9.9 mg l-1. The NO3-N concentrations in leachate from all lysimeters with fertilizer N

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Table 2 Summary of seasonal flow-weighed N concentration and N leaching losses from lysimeters during 3 years N0

N180

N260

N360

NO3-N concentration, mg l-1 99.5a

2005 maize

8.8d

37.4c

61.9b

2006 maize

7.7d

22.3c

50.0b

2006/2007 wheat

7.9d

23.0c

48.3b

103.5a

2007 maize

7.83d

23.2c

52.1b

108.9a

102.9a

DON concentration, mg N l-1 2005 maize

0.70c

0.92c

1.65b

2.76a

2006 maize

0.87c

1.47cb

2.28b

3.25a

2006/2007 wheat

0.80c

1.46cb

2.91b

6.77a

2007 maize

0.80c

1.72c

5.27b

11.94a

NO3-N leaching loss, kg ha-1 2005 maize

7.7d

32.5c

53.8b

86.4a

2006 maize

5.5d

16.1c

36.0b

74.1a

2006/2007 wheat

1.0d

3.0c

6.2b

13.3a

2007 maize 0.3d 0.9c DON leaching loss, kg N ha-1

1.9b

4.0a

2005 maize

0.61c

0.80c

1.43b

2.40a

2006 maize

0.62d

1.03c

1.64b

2.75a

2006/2007 wheat

0.10d

0.19c

0.35b

0.72a

2007 maize

0.03c

0.06c

0.19b

0.44a

Values within the same row by crop growth season followed by the different letter are significantly different (P \ 0.05) using the least significant difference test

applications exceeded the maximum contaminant level (MCL) of 11.3 mg NO3-N l-1 for drinking water quality in the European Union (EU) over the


83

-1

leachate DON (mg N L )

-1

leachate NO3-N (mgL )

three rotation years and displayed strong variation, fluctuating between 11.8 and 49.9 mg l-1 for the N180 treatment, 24.6–75.5 mg l-1 for the N260 treatment, and 84.9–129.1 mg l-1 for the N360 treatment over the three years. The flow-weighted NO3-N concentrations from lysimeters were also significantly different between treatments within sampling dates (Fig. 2). The dotted lines are drawn Only for convenience to read and do not have any physical significance. The NO3-N concentrations in leachate ranged from 18.1 to 52.5 and 91.1–140 mg l-1 for the N180 and N360 lysimeters, respectively, during the 2007 winter wheat season. These were comparable with the NO3N concentrations in the soil solution at 2-m depth under lower soil moisture conditions at 200 and 300 kg N ha-1 application rates for winter wheat, respectively (Fang et al. 2006). No NO3-N loss occurred during the first two winter wheat seasons of the study in 2005 and 2006. This implied that rainfall and irrigation episodes during the first two winter wheat seasons had little effect on water seepage at the 2.0-m soil layer. Field observation and modeling in the NCP also suggested that no leakage occurred even under the recommended amount of irrigation (B75 mm) during the winter wheat season assuming 1-m as the rooting zone (Fan et al. 2001; Gong and Li 1995; Yuan et al. 1995). Nitrate N leaching losses were observed during the 2007 winter wheat season (Fig. 2; Table 2). The

monthly precipitation amounts in Oct of 2006, Jan, Feb, Apr, and May of 2007 were more than the 30-year averages for the same periods, respectively. The total amount of water inputs from October 2006 to March 2007 was 120.2 mm. The supplementary irrigation before soil freezing in early Dec-2006 and snowfalls in Jan, Feb-2007 retained in the soil profile caused a higher soil moisture by soil thawing in lateMarch 2007. Meanwhile, irrigation and unforeseen rainfalls in April and May triggered deep water percolation and NO3-N leaching losses. The leaching loss during the period from April to June 2007 probably represented a worst-case scenario. The results suggest that a potential risk for NO3-N leaching losses does exist during the winter wheat growth season. The amounts of nitrate N leaching losses were significantly different between treatments for each crop season during the 3-year study period, with high NO3-N leaching losses in lysimeters with high N fertilizer application rates. Though leaching losses were much lower during the 2007 winter wheat and maize seasons than those during the other maize seasons, the N fertilizer effect was still significant (P \ 0.05) (Table 2). However, the annual variations of NO3-N leaching losses within treatments were even greater than the variations between treatments (Table 2). The annual changes of NO3-N losses during the maize seasons showed the same trend as precipitation. The losses across all lysimeters with N applications were the

150 120 90 60 30 0 25 20 15 10

NO N180 N260 N360

5 0 2005/3/30

2005/9/30

2006/3/30

2006/9/30

2007/3/30

2007/9/30

date

Fig. 2 Dissolved organic nitrogen and nitrate nitrogen leaching loss from a wheat-maize rotation. Flow-weighted DON and NO3-N concentrations in leachate from lysimeters fertilized at 0, 180, 260, and 360 kg N ha-1 rates as urea

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84

lowest in 2007. Meanwhile, considerably less leachate was collected in the 2007 maize season than the other maize seasons as discussed earlier. Similarly, the NO3-N leaching losses for all lysimeters with N applications during the winter wheat season were higher than those during the maize season in the 2006–2007 rotational year. This indicated that the seasonal NO3-N leaching loss depended more on drainage but less on variation of nitrate N concentrations. In 2005 maize season, the loss percentages of N fertilizer leached were 13.8, 17.7 and 21.9% for the N180, N260, and N360 treatment, respectively, showing that nitrate N leaching loss in a wet year was an important mechanism for N loss of higher fertilizer N applied to crops in the NCP. The loss percentages of N fertilizer leached were 0.1, 0.6 and 1% for the N180, N260, and N360 treatment in the 2007 maize season, respectively. The percentages of seasonal NO3-N leaching losses significantly decreased as seasonal drainage decreased in the NCP (Yuan et al. 1995). The total amount of NO3-N leaching loss over 3 years varied greatly from 14.6 to 177.8 kg ha-1 among the four N application rates. The annual average percentage of N fertilizer leaching loss accounted for 4.0, 5.3, and 7.6% for the N180, N260, and N360 treatment, respectively. Xing and Zhu (2000) estimated that the N loss through leaching from uplands in the NCP accounted for 0.5–4.2% of the fertilizer N applied. Residual soil nitrate N data from the study by Liu et al. (2003) indicated that a total of 11.4–16.6% of fertilizer N applied was leached annually below 2 m under the single-year winter wheat—summer maize rotation on Meadow Aqualf receiving 120–360 kg N ha-1 per crop. The higher loss in Meadow Aqualf was partly due to the higher water conductivity than that of Cinnamon soil in the NCP (Beijing Agricultural Department 1984). Nitrogen leaching losses for summer maize mainly occurring before the fully closed canopy phase is probably a distinctive feature of double cropping system in the NCP which can be related to a number of reasons. Firstly, during the no-crop period (between wheat harvest and seedling emergence for maize), uptake of N and soil water drops to near zero while residual NO3-N following harvest of winter wheat may build up in the 2-m soil profile. Secondly, uptake of N and soil water is minimal during the canopy expansion phase of the maize with the first

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application of half of fertilizer N at V3 stage (early July). Thirdly, the second fertilization before rainstorm at V10 (early August) still provides an opportunity for N leaching before complete canopy closure. In the NCP, rainfall events generally occur from late-June through August. While delays between wheat harvest and the sowing of maize were minimized, little can be done to shorten the time required for maize seedling emergence and the development of a closed canopy and extensive root system. It was reported that about 85% of the total N in corn plants (aboveground parts) accumulated between the V10 and R6 stages (Ritchie and Hanway 1982) and only about 6% of the total plant N was taken up by the maize during the expansion phase (Bennett et al. 1989). Therefore, guidelines for the rate and timing of fertilizer N application in the NCP must consider maize N requirements while ensuring the protection of the groundwater environment. Nitrate nitrogen leaching losses were observed in several discrete, narrow time intervals from April to September, mainly during the summer maize season. This is similar to the pattern of NO3-N leaching losses during the maize season (November–April) in the tropical region of Africa (Nyamangara et al. 2003) and the temperate continental climatic region of Iowa (Baker and Timmons 1994). However, the pattern under Asia monsoon climate in the NCP is quite different from other places. Studies on corn in the Scandinavian climatic regions of southern Sweden, the humid-temperate climatic regions of Pennsylvania State, and the continental climatic area of Ohio showed that most of percolation and NO3-N transport occurs during the noncropping season (Toth and Fox 1998; Bergstrom and Kirchmann 1999; Owens et al. 2000). Dissolved organic nitrogen concentrations and fluxes Vertical distributions of soil solution DON concentrations are shown by the median values (Fig. 3). The median DON concentrations were between 1.0 and 2.0 mg N l-1 for the N0 treatment. Except at the depth of 1.8 m, DON concentrations for the control treatment did not show a strong variation along the soil profile. Application of urea N led to increases in soil solution DON concentrations within 2.0 m of the soil profiles. The median DON concentrations were between 1.2 and


85

25

25

20

20

15

15

10

10

5

5

0

0

25

25

N260 20

20

15

15

10

10

5

5

0

N180

N360

0 0.2

0.4

0.6

0.8

1.0

1.4

1.8

0.2

0.4

0.6

0.8

1.0

1.4

1.8

Fig. 3 Dissolved organic nitrogen and nitrate nitrogen leaching loss from a wheat-maize rotation. Vertical distributions of DON concentrations. Whiskers show 5 and 95% percentiles.

Boxes denote 25 and 75% percentiles. The medians are shown as lines splitting boxes in two parts

2.2 mg N l-1 for the N180 treatment, with the highest value at 0.4 m depth. The median DON concentrations varied between 1.8 and 5.1 mg N l-1 for the N260 treatment, with the highest median value of above 5.0 mg N l-1 at 0.8 m depth. The median DON concentrations were found to be between 1.6 and 5.3 mg N l-1 for the N360 treatment. Clearly, high rates of fertilizer N applied significantly affected soil solution DON concentration within the 0–2.0 m soil profile. Leachate DON concentrations from the lysimeters increased with the increase of N fertilization rates (Fig. 2; Table 2). Leachate DON concentrations for the N260 and N360 treatments were significantly higher than those of the control and N180 treatments. However, the DON concentration for the N180 treament was not significantly different from that of the control treatment. Depending on the applied N rate and season, seasonal flow-weighted DON concentrations ranged from 0.7 to 11.9 mg N l-1. With exception of the N180 treatment in the 2005 maize season, seasonal flowweighted DON under fertilized lysimeters surpassed the allowable Kjeldahl N concentration of 1.0 mg in drinking water in the EU (European Community 1980).

The 3-year annual averages for the N0, N180, N260, and N360 treatments were 0.7, 1.4, 3.0, and 6.0 mg N l-1, respectively. It is also necessary to note that the 3-year flow-weighted DON concentrations ranged from 0.8 to 3.6 mg N l-1 for all N rates. Kessel et al. (2009) found that 90% of DON concentrations in leachates from the cropping systems ranged between 0.2 and 3.5 mg N l-1 at the 0.9-m soil depth. Strong temporal variations of leachate DON concentrations was also observed. Except the N0 treatment (P = 0.742), seasonal flow-weighted DON concentrations in leachate for the three fertilization treatments were also significantly different among seasons (Table 2; P \ 0.01). The dynamic of DON concentrations may reflect to some extent the carbon dynamics and chemistry in the soil (Burton and beauchamp 1985; Brye et al. 2001). The relatively high DON concentrations. in the 2007 cropping seasons were probably due to less leachate volumes, which may have increased variability. However, the temporal variation of the DON concentrations in leachate was weaker than that of NO3-N during the period of study (Fig. 2). A

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similar observation was also reported by Siemens et al. (2002). As was found with the amount of NO3-N leaching loss, fluxes of DON significantly increased with the increase of fertilizer application rates (Table 2). On the other hand, seasonal DON leaching losses ranged from 0.03 to 2.75 kg N ha-1. In the 2007 maize season DON leaching losses across all N rates were the lowest. In the 2006 maize season DON leaching losses were significantly higher across all N rates than those found in the 2005 and 2007 maize seasons and in the 2007 winter wheat season. The DON proportions of TN (NO3-N ? DON) flux (%) were 8.6, 3.5, 3.6, and 3.4% for the N0, N180, N260, and N360 treatment, respectively. There were no differences among the three fertilization lysimeters. Gundersen et al. (1998); Solinger et al. (2001) found that the contribution of DON to N leaching from forest ecosystems was commonly reduced to \10% if N input was more than 10 kg N ha-1 year-1 and total N leaching was [19 kg ha-1 year-1.

Nutr Cycl Agroecosyst (2011) 91:77–89 Table 3 Grain yield (GY), plant N in above-ground drymatter, apparent recovery fraction of applied fertilizer N (NF) in the wheat-maize rotation system from 2004 to 2007 Treatments

Plant N (kg ha-1)

NF (%)

Winter wheat 2004–2005 N0

3.74(0.192)b

73.7(12.3)c

N180

6.90(0.0.07)a

173.7(13.5)b

55.6(18.1)a

N260

7.09(0.191)a

185.2(14.3)a

42.9(14.0)b

N360

7.06(0.140)a

190.3(11.1)a

32.4(5.6)c

Summer maize 2005 N0 4.52(0.338)b

73.2(15.7)c

N180

7.91(0.043)a

152.1(12.8)b

44.0(10.8)a

N260

8.08(0.220)a

173.6(17.8)a

38.6(12.3)a

N360

8.16(0.676)a

179.2(11.3)a

29.5(4.07)b

Winter wheat 2005–2006 N0

3.40(0.215)b

64.7(13.5)c

N180

7.32(0.119)a

179.0(13.8)b

63.5(11.8)a

N260

7.43(0.040)a

197.7(14.6)a

51.2(12.3)b

N360

7.37(0.111)a

201.4(13.3)a

38.0(15.0)c

Summer maize 2006

Grain yield and nitrogen uptake

N0

In all the three cycles of the rotation, both grain yields and N uptakes of winter wheat and summer maize significantly responded to applied N rate (P \ 0.05) (Table 3). However, yields were not significantly different among the three N application rates (N180, N260, and N360) during the 3 years. Fertilizer treatments significantly increased the crop N uptakes. The crop N uptakes with the N260 and N360 treatments were significantly higher than the N180 treatment but the difference between the N260 treatment and the N360 treatment was not significant with an exception of the 2007 maize season. A severe drought during the 2007 maize season led to a significant decline in maize yields and a reduced N response compared with those of the 2005 and 2006 maize seasons under normal rainfall conditions. These suggested that N application exceeding 180 kg N ha-1 did not further increase crop yields but induced excess of N accumulation in crops. This result is consistent with other researches. Studies on optimum N application rates for winter wheat and summer maize in the NCP showed that the yield do not decreased with increasing N fertilizer rate when the fertilizer rate exceeded 180 kg N ha-1 but keep as a plateau (Chen et al. 2000). The analysis of the relationship between plant N and rates of N fertilization also

N260 N360

N180

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GY (Mg ha-1)

4.58(0.254)b

63.0(15.7)c

7.89(0.243)a

167.8(12.7)b

8.19(0.139)a

187.7(13.1)a

48.0(10.6)b

8.25(0.295)a

194.6(14.4)a

36.68(12.2)c

58.2(14.6)a

Winter wheat 2006–2007 N0 N180

2.83(0.106)b 7.66(0.158)a

54.8(13.60)c 183.8(10.2)b

71.7(6.3)a

N260

7.73(0.252)a

203.6(15.50)a

57.2(13.5)b

N360

7.55(0.068)a

206.0(17.50)a

42.0(12.3)c

Summer maize 2007 N0

3.82(0.143)b

57.8(13.36)d

N180

5.94(0.238)a

151.9(10.3)c

52.2(5.6)a

N260

5.87(0.188)a

161.0(18.0)b

39.7(9.1)b

N360

5.97(0.128)a

171.7(15.0)a

31.6(14.4)c

Values within a column by crop growth season followed by the same letters are not significantly different based on the least significant difference test

indicates there is significant correlation (R = 0.8778– 0.9406) for winter wheat in the different growing stages but only from full canopy cover stage through the mature stage for maize (R = 0.8402) (Zeng 1999). The differences of NF between treatments were significant (P \ 0.05) (Table 3). There was a significant decrease in NF for both of winter wheat and maize crops with increasing N fertilizer inputs. This is in line with the results from another study in the


NCP (Fang et al. 2006).Several published studies in the NCP indicate that the high N application rates used in the area can augment substantially more N loss to the environment via gaseous N or accumulation in soil profile besides greater N leaching losses and result in a decreasing NF by crop (Xing and Zhu 2000; Zhu and Chen 2002; Wang et al. 2002; Liu et al. 2003; Wu et al. 2009). The higher NF values of winter wheat than those of maize indicated that winter wheat had higher fertilizer N use efficiency than maize. Chen et al. (2004) reported a similar trend. Our experiment explained in part the reason. The NO3-N leaching losses may have contributed to the lower NF values of maize, especially in the case of higher N fertilization rates. Therefore, it was concluded that the recommended N fertilization rate of 180 kg N ha-1 is sufficient for an optimal yield and limiting NO3-N leaching in this agroecosystem with our climate conditions.

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management scheme for maize usually coinciding with peak rainfall resulted in the majority of leaching losses of NO3-N before the complete canopy closure of maize. While DON concentration in deep drainage under N fertilization was [1 mg l-1, contribution of DON to N lost was small. Our results enhance that an application rate of 180 kg N ha-1 would make a lot of sense (agronomically sound and environmentally acceptable). Future research will well focus on moisture dynamics through in situ moisture measurements in the lysimeters during the year and modeling crop uptake of N as well as coupled moisture and mineral N transport through the soil. Acknowledgments This study was sponsored by the National Natural Scientific Foundation of China (No. 4047110 and 40871225) and the National Basic Research Program of China (973 Program) (2005CB121100). We also thank two anonymous referees for their valuable comments on the manuscript.

References Conclusion Crop yields significantly responded to N fertilizer application but were not significantly different among the three N application rates in the 3 years of the study. NF for both of winter wheat and maize reduced as N fertilizer applied rates increased from 180 kg N ha-1. Although the N fertilization rate of 180 kg N ha-1 seemed to be high for both of the average NO3-N and DON concentration to meet the allowable concentration for the drinking water quality, it was relatively effective in maintaining the approximately maximum grain yields in a winter wheat-summer maize rotation while reducing N leaching losses, compared to the conventional N rate and prevalent N rate in the study area. The climatic conditions together with the timing of the N fertilization determined the relative importance of N leaching losses in this area. Seasonal nitrogen leaching losses occurred from April to September. A potential risk for NO3-N leaching losses during the winter wheat season exists when large volumes of unforeseen precipitation plus irrigation water are provided. Nitrogen leaching losses for summer maize varied with amount and distribution of precipitation under the Asia monsoon climatic conditions. The nitrogen fertilization of the traditional nitrogen

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