Science of the Total Environment 610–611 (2018) 1020–1028

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Nitrate accumulation and leaching potential reduced by coupled water and nitrogen management in the Huang-Huai-Hai Plain Ping Huang a,b,c, Jiabao Zhang a,⁎, Anning Zhu a, Xiaopeng Li a, Donghao Ma a, Xiuli Xin a, Congzhi Zhang a, Shengjun Wu b, Gina Garland c, Engil Isadora Pujol Pereira c a b c

Fengqiu Agro-ecological Experimental Station, State Key Laboratory of Soil Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, PR China Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China Department of Environmental Systems Science, Swiss Federal Institute of Technology, ETH-Zurich, Zurich 8092, Switzerland

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Increase in nitrogen application rates promoted nitrate accumulation and leaching. • Increase in irrigation amounts reduced nitrate accumulation. • Continuous torrential rainfall was the main cause for nitrate leaching. • Groundwater nitrate concentration was minimally affected by treatments. • Reasonable water and nitrogen managements were attainable and recommended.

a r t i c l e

i n f o

Article history: Received 27 April 2017 Received in revised form 28 July 2017 Accepted 13 August 2017 Available online xxxx Keywords: Nitrate leaching Nitrate storage Coupled water and nitrogen management Groundwater Soil profile

⁎ Corresponding author. E-mail address: [email protected] (J. Zhang).

http://dx.doi.org/10.1016/j.scitotenv.2017.08.127 0048-9697/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t Irrigation and nitrogen (N) fertilization in excess of crop requirements are responsible for substantial nitrate accumulation in the soil profile and contamination of groundwater by nitrate leaching during intensive agricultural production. In this on-farm field trial, we compared 16 different water and N treatments on nitrate accumulation and its distribution in the soil profile (0–180 cm), nitrate leaching potential, and groundwater nitrate concentration within a summermaize (Zea mays L.) and winter-wheat (Triticum aestivum L.) rotation system in the Huang-Huai-Hai Plain over five cropping cycles (2006–2010). The results indicated that nitrate remaining in the soil profile after crop harvest and nitrate concentration of soil solutions at two depths (80 cm and 180 cm) declined with increasing irrigation amounts and increased greatly with increasing N application rates, especially for seasonal N application rates higher than 190 kg N ha−1. During the experimental period, continuous torrential rainfall was the main cause for nitrate leaching beyond the root zone (180 cm), which could pose potential risks for contamination of groundwater. Nitrate concentration of groundwater varied from 0.2 to 2.9 mg L−1, which was lower than the limit of 10 mg L−1 as the maximum safe level for drinking water. In view of the balance between grain production and environmental consequences, seasonal N application rates of 190 kg N ha−1 and 150 kg N ha−1 were recommended for winter wheat and summer maize, respectively. Irrigation to the field capacity of 0–40 cm and 0–60 cm soil depth could be appropriate for maize and wheat, respectively. Therefore, taking grain yields, mineral N accumulation in the soil profile, nitrate leaching potential, and groundwater quality into account, coupled water and N management could provide an opportunity to promote grain production while reducing negative environmental impacts in this region. © 2017 Elsevier B.V. All rights reserved.

P. Huang et al. / Science of the Total Environment 610–611 (2018) 1020–1028

1. Introduction Nitrogen (N) is of significant importance for crop production in the agricultural sector (Galloway et al., 2004). Nitrate, a form of N that is available for plant uptake, is highly soluble and easily lost through leaching as water moves below the root zone of the soil profile (Nielsen and Jensen, 1990; Zhang et al., 1996; Li et al., 2009). As a result, the leached nitrate may enter water-saturated zones and lead to groundwater contamination. Nitrate contamination of surface and groundwater resulting from intensive agriculture, high population densities and high atmospheric N deposition have been well reported in many regions of the world (Oenema et al., 1998; Hoffmann et al., 2000; Xing and Zhu, 2000; Jenkinson, 2001; Jussy et al., 2004). Besides eutrophication, large amounts of nitrate can lead to an accelerated loss of biological diversity (Baron et al., 2000), and there are also serious concerns that elevated nitrate concentrations in drinking water pose a health risk to humans, especially to infants and pregnant women, by causing illnesses such as infantile methaemoglobinaemia or cancer of the digestive tract (Lord et al., 2002). In dry lands, nitrate is the major N form existing in soil and also the major form taken up by plants due to the high soil pH buffering capacity (Li et al., 2009; Huang et al., 2015a). In arid and semi-arid regions, agriculture is heavily dependent on groundwater resources for irrigation and therefore its quality is of great importance (Grogan et al., 2015). Nitrate leaching rarely happens in rain-fed agricultural areas with arid or semi-arid conditions due to higher rates of evaporation compared to precipitation, but the introduction of irrigation makes the situation different, especially for improper irrigation methods, such as over-flooding (Zhang et al., 1996; Spalding et al., 2001). Irrigation and N fertilizer application in excess of crop requirements tend to increase the potential risk of nitrate contamination to groundwater, especially for welldrained soils and intensive production of shallow-rooted crops under irrigated or torrential rainfall conditions, which can lead to considerable losses of nitrate through leaching (Zhao et al., 2007; Li et al., 2009; Mueller et al., 2012). The Huang-Huai-Hai Plain, of strategic importance for food security in China, accounts for over 20% of the total national grain production (Zhu et al., 2005; Huang et al., 2015a). Within this region, winter wheat (Triticum aestivum L.) followed by summer maize (Zea mays L.) is the prevailing crop rotation system. A sandy loam soil with high hydraulic conductivity is the dominant soil type (Huang et al., 2015a), and its permeable properties make the region susceptible to groundwater contamination by nitrate leaching (Zhao et al., 2007). Under conventional cultivation practices, increasing nitrate concentration in groundwater was monitored with an average annual N application rate of 500 kg N ha−1, while over-flooding irrigation, heavy rainfall during maize seasons, and N application in excess of crop demand were found to be responsible for the groundwater contamination by high nitrate leaching (Zhu et al., 2005; Zhao et al., 2007). Nitrate leaching represented a substantial loss of N fertilization and lowered the N use efficiency as a result, and nitrate concentration in groundwater has increased steadily during recent decades in some sites of the region (Zhang et al., 1996; Zhang et al., 2005). Therefore, alternative practices which are able to synchronize fertilizer application with plant growth, such as decreasing N application rates, splitting N fertilizer incorporations, and optimizing irrigation methods, were proposed as practices which could substantially reduce nitrate leaching (Zhang et al., 2005; Zhao et al., 2007; Ju et al., 2009; Chen et al., 2014; Zhou and Butterbach-Bahl, 2014). Combined management of fertilization and irrigation were found to be effective in promoting grain yields and waterand nutrient-use efficiencies (Fang et al., 2006; Ju et al., 2009; Mueller et al., 2012). However, the recommendation of proper N application rates and irrigation amounts has not been made with balanced considerations of grain production and nitrate leaching. A long-term field trial on coupled water and N management was established in the hinterland of Huang-Huai-Hai Plain in 2005, with

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the purpose of evaluating the economic and environmental consequences of different combinations of irrigation and N application in order to find the proper combination(s) suitable for agricultural production in this region. Based on our preliminary findings, coupled water and N management had considerable potential in enhancing grain yields and mitigating gaseous N losses (Huang et al., 2015b). However, the dynamics of nitrate accumulation and its distribution throughout the soil profile as well as groundwater contamination potential are yet unknown under coupled water and N management. In this study, we assessed nitrate accumulation and its distribution in the soil profile after crop harvest, nitrate leaching potential, and groundwater nitrate concentrations under 16 different water and N treatments. The main objectives of this project were to evaluate potential risks of groundwater contamination by nitrate leaching under different irrigation and fertilization management practices and to optimize appropriate water and N management with balanced considerations between grain production and environmental costs. 2. Methods and materials 2.1. Site description This work was conducted in the Fengqiu Agro-ecological Experimental Station (114°24′ E, 35°00′ N), Chinese Academy of Sciences, and further details and a map of the site can be found in (Huang et al., 2016). The Station, located in the hinterland of the Huang-Huai-Hai Plain, has a typical monsoon climate with an annual mean temperature of 13.9 °C and rainfall of approximately 615 mm, two thirds of which take place between June and October. This region has a flat terrain with an average altitude of 67.5 m above sea level. Sandy loam soil, mainly composed of vermiculite and montmorillonite and derived from alluvial sediments of the Yellow river, accounts for N98% of the total arable land in this region (Zhu et al., 2005). The depth of alluvial sediments ranged from 40,000 to 50,000 cm. Clay contents in the soil depths of 0–30 cm, 30–80 cm, and 80–180 cm were 137, 359, and 103 g kg−1, while 10.2, 7.1, and 3.2 g kg−1 for soil organic matter contents, and 7.88, 14.35, and 5.27 cmol·kg−1 for cation exchange capacity, respectively. Daily precipitation and average air temperature during the experimental period and more basic soil physico-chemical properties were described by Huang et al. (2015b). 2.2. Plot design and field management A long-term field experiment (Fig. 1) was established in late 2005, in which all plots were subjected to a winter wheat growing season with identical management in order to minimize baseline heterogeneities among field plots. The application of different water and N treatments commenced in the maize season of 2006, following a typical rotation of winter wheat and summer maize. For the experiment, 48 rectangular plots (length 800 cm; width 600 cm) were evenly arranged in four parallel rows (each row with 12 plots) perpendicular to the prevailing wind direction (south-north), and adjacent plots were separated by concrete walls (depth 100 cm; width 10 cm) with 20 cm above the ground. To minimize edge effects, the experimental field was surrounded with protective strips as buffer zones over 100 cm wide (Fig. 1). The experiment consisted of 16 treatments, including five application rates of N fertilizer, 0 (F0), 150 (F150), 190 (F190), 230 (F230), and 270 kg N ha−1 (F270) for each crop season, and three irrigation rates, i.e., so as to reach the field capacities of 0–20 cm (W20), 0–40 cm (W40), and 0– 60 cm (W60) soil depth in the main irrigation events during crop growth. In addition, there was a rain-fed treatment for F190. Each treatment was replicated three times. Triple superphosphate (P2O5, 46.1%, 80 kg ha−1) and potassium sulfate (K2O, 49.6%, 200 kg ha−1) were applied once in each crop season before plowing (0–20 cm), and urea (N, 46.3%) was split into two applications. In wheat seasons, basal N fertilization accounted for 60% of the total application rate, while it was 40%

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analysis within one week. During each sampling of soil solutions, the water storage in the soil profile (0–170 cm) was measured by a neutron probe (503B, CNC, China) at intervals of 10 cm, and the groundwater level was recorded (Fig. 2). The aforementioned measurements were conducted according to the protocols described by Zhu et al. (2005). The sampling and measurements were conducted continuously from the commencement of treatments in 2006 until the end of maize growing season in 2010. Prior to the analysis of soil solutions, samples were thawed at room + temperature. Nitrate (NO− 3 -N) and ammonium (NH4 -N) concentrations of soil solutions were measured by the dual-wavelength (UV1800, Shimadzu, Japan) method (Norman et al., 1985) and the indophenol blue colorimetric method (Ti et al., 2012), respectively.

Aug-10

Jun-10

Apr-10

Dec-09

Sep-09

Jul-09

May-09

Mar-09

Dec-08

Sep-08

Jul-08

May-08

Mar-08

Dec-07

Sep-07

Jul-07

Jun-07

Apr-07

0

Jan-07

Analysis of variance (one-way ANOVA) and least significant difference (LSD) calculations at P = 0.05 were performed to identify statistically significant differences of nitrate remaining in the soil profile after crop harvests, grain yields, and aboveground N uptake by crops among different water-N treatments using the SPSS 15.0 software package for Windows. All figures were processed by Origin 8.0.

Oct-06

In the center of each plot, a neutron probe pipe (5 cm i.d.) was inserted 175 cm underground and four sets of ceramic suction cups (2 cm i.d.) with depths of 20, 40, 80, and 180 cm were installed before the initiation of the experiment. The ceramic suction cups were installed with a manual auger of the same diameter, with the head placed 5 cm below the targeted depth. They were disposed along a line perpendicular to the tillage direction, with a minimum distance of 20 cm between two adjacent cups. For each treatment, an observation well (5 cm i.d.) was installed at a depth of 1,200 cm and 100 cm away from the neutron probe pipe in order to monitor the water table and nitrate concentration. Two irrigation wells (60 cm i.d.), 10,000 cm away from the field, were simultaneously monitored to validate the groundwater table in order to avoid possible capillary effects resulting from the relatively small field observation pipes (5 cm i.d.). Soil solution sampling was generally performed every five days, and the frequency was reasonably increased after irrigation, fertilization, and rainfall (N 20 mm). In winter, the sampling time intervals were changed to 7–10 days, and the sampling of soil solutions at depths of 20 cm and 40 cm was suspended when the soil was frozen. Soil solutions of different depths were labeled and stored at 4 °C for further

2.5. Data analysis

Aug-06

2.3. Experimental facilities and monitoring

Background soil samples were taken before the initiation of treatments in 2006. Samples were collected at a 20 cm depth resolution from the top 180 cm of the profile using a standard hand auger (2.5 cm i.d.). Each plot had two soil cores taken diagonally, and samples of the same depth were mixed and homogenized for each plot. Soil samples were stored at 4 °C for further analysis. Right before analysis, soil samples were thawed at room temperature and then analyzed for + NO− 3 -N, NH4 -N contents, and gravimetric moisture within two weeks of storage. Ten grams of soil samples were extracted by 50 mL KCl solution (2 mol L−1, one-hour shaking), and the extracts were used to deter+ mine NO− 3 -N and NH4 -N contents by the dual-wavelength (UV1800, Shimadzu, Japan) method (Norman et al., 1985) and the indophenol blue colorimetric method (Ti et al., 2012), respectively. Gravimetric soil moisture was measured by the oven-drying (BAO-250A, STIK, U.S.A.) method (105 °C, 12 h) and used to correct the results of NO− 3 N, NH+ 4 -N contents. Such sampling and measuring activities were performed after wheat harvest in 2008 and 2010, and maize harvest in 2010.

Jul-06

for the maize seasons; the remaining amount was broadcast at the elongation stages of each crop during their respective growing seasons. Fertilization and harvest dates are provided by Huang et al. (2015b). During crop growth, irrigation was conducted when the water deficit (the difference between water storage and field capacity) in the soil profile (0–170 cm) exceeded 100 mm. Irrigation amounts for each treatment were calculated by the difference between the field capacity of targeted soil depth and the actual soil moisture. Water from the neighboring well was pumped to the field and irrigated evenly by a hose (8 cm, inner diameter, i.d). Flow meters (FR80, KEWILL, Germany) were calibrated and used to record irrigation amounts. If the soil was too dry to sow, all plots were irrigated identically (20.8 mm) before plowing. After supplemental fertilization, irrigation was performed if no heavy rain occurred (b20 mm). Huang et al. (2015b) presented the details of each irrigation event during the experimental period. Prior to crop sowing, the topsoil (0–20 cm) of plots was manually ploughed immediately after basal fertilization. Maize was manually sown with a row spacing of 60 cm and an interplant spacing of 20 cm. After 3 weeks, some seedlings were removed so as to have 305 plants per plot. Wheat was equally sown at a row distance of 20 cm, amounting to 0.77 kg seeds per plot.

2.4. Mineral N remaining in soil profile after harvest

Torrential rainfall

-200

Depth of water table (cm)

Fig. 1. Aerial (A) and horizontal (B) views of the field plots with different irrigation and fertilization treatments in the crop growing seasons (photo by A. Zhu and X. Li). Great variations of wheat growth status were presented among different treatments, and green-yellow plots indicate N-deficiency. Near the experimental field in the east is the meteorological observation enclosure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

-400

-600

Maize

Wheat Maize

Wheat Maize

Wheat Maize Wheat

-800

Maize

Fig. 2. Average depth of the water table for all the treatments during the experimental period. A continuous torrential rainfall (139 mm) occurred in July 2010. Bars in the graph indicate standard deviation of the mean (n = 16).

P. Huang et al. / Science of the Total Environment 610–611 (2018) 1020–1028

3. Results 3.1. Nitrate storage and distribution in the soil profile After two years of continuous cultivation, great variations in nitrate distribution along the soil profile were demonstrated between the different fertilization and irrigation treatments (Fig. 3A). However, nitrate accumulation was not observed after the wheat harvest in 2008, except in the plots receiving N application rates higher than 190 kg N ha− 1 (Fig. 3A). Nitrate storage in the soil profile enhanced with increasing N application rates, and a downward movement of nitrate was observed in the 0–80 cm depth especially for the high irrigation treatments (W60). On the other hand, nitrate content in the topsoil (0–20 cm) was similar for all treatments (~10 kg N ha−1) after wheat harvest in 2008 (Fig. 3A). After four years of crop rotation cycles (Fig. 3B), the nitrate distribution in the soil profile was similar to the two-year scenario, albeit more conspicuous, especially for the treatments with N application rates over 190 kg N ha−1. However, nitrate storage in the soil profile decreased under the seasonal N application rates b190 kg N ha−1 (Table 1). Nitrate mainly accumulated between the soil depth of 40–120 cm, with a peak at ~80 cm. Irrigation appreciably reshaped the nitrate distribution in the soil profile for the plots receiving N fertilization, and high irrigation (W60) smoothed the peak and moved the nitrate downward in contrast to the rain-fed (W0) and low irrigation (W20) treatments (Fig. 3B), probably due to the promotion of root growth and nitrate uptake in

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deeper soil layers with favorable soil water regimes after high irrigation. Plots receiving a N application rate of 190 kg N ha−1 showed no obvious nitrate accumulation, except for the rain-fed (F190W0) treatment (Fig. 3B). The plots experienced a torrential rainfall (139 mm) during the maturing stage of maize growth in 2010 (Huang et al., 2015b) and after the maize harvest of that year, the peak of nitrate distribution in the soil profile moved downwards from ~80 cm to 120–140 cm (Fig. 3C). The nitrate storage and distribution was minimally impacted for plots receiving N application rates b 190 kg N ha−1, however, the continuous torrential rain completely modified the picture of vertical nitrate distribution in the soil profile for plots receiving N application rates over 190 kg N ha−1 and resulted in high nitrate loss via water infiltration (Fig. 3C, Table 1). 3.2. Nitrate concentration of soil solutions at different depths The nitrate concentration of the soil solutions at 80 cm depth was appreciably impacted by N application and irrigation during the monitoring period (Fig. 4A). Nitrate concentration of the soil solution was generally increased with increasing N application rates and decreasing irrigation amounts. N application rates lower than 190 kg N ha−1 hardly increased nitrate concentration at a depth of 80 cm, while a steady increase in nitrate concentration was observed for treatments receiving N application rates higher than 190 kg N ha−1 before the continuous torrential rainfall in 2010 (Fig. 4A). After the continuous rainfall, nitrate

Fig. 3. Nitrate distribution in the soil profile after the wheat harvest in 2008 (A), the wheat harvest in 2010 (B), and the maize harvest in 2010 (C). F0W20, no N application and irrigated to the field capacity of 0–20 cm depth during each irrigation treatment; F150W20, a seasonal N application rate of 150 kg N ha−1 and irrigated to the field capacity of 0–20 cm depth during each irrigation treatment; F190W0, a seasonal N application rate of 190 kg N ha−1 and rain-fed; the rest of the treatments can be deduced by analogy. Bars in the graphs represent standard deviation of the mean (n = 3).

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Table 1 Cumulative nitrate remaining (kg N ha−1) in soil profile (0–180 cm) after harvests of different cropping seasons compared to the background before the commencement of the field trial in 2006. F0W20, no N application and irrigated to the field capacity of 0–20 cm depth during each irrigation treatment; F150W20, a seasonal N application rate of 150 kg N ha−1 and irrigated to the field capacity of 0–20 cm depth during each irrigation treatment; F190W0, a seasonal N application rate of 190 kg N ha−1 and rain-fed; the rest of the treatments can be deduced by analogy. Treatment

Background

2008 wheat

2010 wheat

2010 maize

F0W20 F0W40 F0W60 F150W20 F150W40 F150W60 F190W0 F190W20 F190W40 F190W60 F230W20 F230W40 F230W60 F270W20 F270W40 F270W60

382 (127) a 306 (106) a 168 (72) a 284 (14) a 243 (85) a 241 (17) a 314 (79) a 345 (73) a 348 (110) a 262 (68) a 271 (83) a 316 (86) a 320 (82) a 324 (92) a 267 (58) a 215 (65) a

−289 (121) a −206 (96) a −103 (71) a −105 (34) a −70 (37) a −82 (53) a −30 (80) b −32 (56) b −27 (81) b 15 (47) bc 47 (66) c 15 (86) bc 29 (94) c 227 (129) d 178 (60) d 245 (47) d

−309 (113) a −253 (110) a −127 (67) a −150 (42) a −95 (34) a −162 (26) a 142 (55) bc 37 (99) b −26 (56) b −49 (45) ab 166 (74) c 193 (71) c 186 (55) c 444 (147) d 375 (84) d 385 (92) d

−343 (104) a −271 (72) a −136 (71) a −153 (61) a −122 (53) a −158 (15) a 235 (73) d −9 (33) b −30 (65) b −78 (96) ab 110 (75) cd 93 (52) c 23 (51) c 767 (146) d 299 (112) d 392 (80) d

Numbers in the parentheses represent standard deviation of the mean (n = 3). Different letters following values in the same column denote significant differences between different treatments (P b 0.05).

concentrations decreased rapidly for treatments receiving N application rates b 190 kg N ha−1 and were only slightly influenced for plots with the N application rates N190 kg N ha−1, which mirrored the vertical distribution of residual nitrate in the soil profile after crop harvest (Fig. 3). Compared with the nitrate concentration of the soil solution at 80 cm depth, the impacts of N application and irrigation on nitrate concentration at 180 cm depth (Fig. 4B) were smaller during the monitoring period because of relatively low root distribution and activities as

well as the lagging effect of irrigation and fertilization on nitrate content in the deeper soil layers. Nitrate concentration of the soil solution generally increased with increasing N application rates, but the effect of irrigation on nitrate concentration was low, perhaps due to the influences of background nitrate residue and the limited effects of irrigation treatments on deeper soil layers (N 80 cm) in the present study. The nitrate concentration of the soil solution tended to decrease over time in plots receiving N application rates b 190 kg N ha−1, while the reverse trend was demonstrated in N 190 kg N ha−1 plots. Nitrate concentration of the soil solution for plots receiving a N application rate of 190 kg N ha−1 maintained relatively stable during the five-year continuous experiment (Fig. 4B). Nitrate concentration of the soil solution at 180 cm depth decreased with increasing irrigation, but such effect may be diluted or even covered by the influence of unequal background nitrate contents for different plots (Table 1), and the background residual effect could be reduced by increasing cultivation years.

3.3. Dynamics of water deficits in the soil profile and nitrate leaching potential A water deficit was generally maintained within 100 mm during crop growth, except for the rain-fed treatment (Fig. 5). This deficit increased during winter wheat seasons due to low precipitation during these periods, and intensive rainfall in summer maize greatly supplied the water storage in the soil profile and also raised the water table (Fig. 2). Dynamics of the water regime in the soil profile was similar for each irrigation treatment, so only the treatment with a N application rate of 190 kg N ha−1 was shown (Fig. 5). If it is assumed that deep drainage only occurred when water storage in soil profile surpassed field capacity, then irrigation did not induce water percolation beyond the root zone in the present study, and instead the torrential rainfall between June and August was the dominant driving factor for nitrate leaching (Figs. 4 and 5). In the 2007 maize season, drainage induced by water surplus obviously increased the nitrate

Fig. 4. Nitrate concentrations of the soil solution at different depths of 80 cm (A) and 180 cm (B) during the experimental period.

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Fig. 5. Water deficits (differences between water storage and the field capacity) in the soil profile (0–170 cm) during crop growth. Field capacity of the soil profile (0–170 cm) equaled 679 mm.

concentration of the soil solution at the depth of 80 cm (Figs. 4A and 5), but no conspicuous changes were observed for 180 cm (Fig. 4B), probably because the nitrate transport did not reach the depth of 180 cm. A small surplus of water above field capacity occurred in the 2008 wheat season, mostly due to an abrupt rainfall two days after irrigation. 4. Discussion 4.1. Ammonium and nitrate accumulation and its leaching potential Ammonium storage in the soil profile (0–180 cm) was relatively low and stable after each harvest (data not shown). Furthermore, no significant differences in the ammonium content were found among the different soil layers and treatments, with a value of 8–12 kg N ha−1 for each layer (20 cm intervals). Ammonium concentration of the soil solution varied greatly in the topsoil, but the concentration was kept extremely low (0.01–0.03 mg L−1) in the deeper soil layer (180 cm) during the experimental period. Ammonium could be retained by the exchange sites of minerals such as vermiculite and montmorillonite and the functional groups on the soil organic matter. For these reasons we concluded that changes in ammonium storage in the soil profile were not the main factor driving the differences of mineral N pool after each crop harvest and ammonium leaching was negligible in this study. Nitrate was the major mineral N form existing in the tested soil. Nitrate content remained stable in the surface soil of plots receiving no N fertilizer after each harvest (Fig. 3), due to the balance between N removal by crop harvest and N supplement from mineralization and atmospheric deposition (Huang et al., 2016). For the fertilized plots, mineral N in the topsoil was more susceptible to root uptake, ammonia volatilization, denitrification and downward movement than the deeper soils (Huang et al., 2015b), which led to low nitrate concentration of topsoil after crop harvest. Before the continuous torrential rainfall in 2010, nitrate accumulated greatly around the soil depth of 80 cm (Fig. 3), mainly because of the horizontal distribution of a clayey layer at the depth of 30–80 cm (Huang et al., 2015b), which could have retarded water drainage and nitrate leaching. However, nitrate deeper than 80 cm could be more susceptible to leaching, because of the permeable property of loamy silt in the soil profile below 80 cm, which

would pose great risks of nitrate contamination for groundwater if torrential rain occurs or irrigation substantially exceeds field capacity. Nitrate concentration along the soil profile was high and although it varied spatially before the initiation of the field trial, there were no significant differences in residual nitrate concentrations among different treatments (Table 1). This was mainly due to conventional field management prior to the trial and natural soil variability (Finke, 1993), even though a winter-wheat season with identical management for all plots was performed to make the field homogenized before irrigation and fertilization treatments. Nitrate accumulation in the soil profile largely exceeds the limit set by European countries (90– 100 kg N ha−1 in the top 0–100 cm) (Zhao et al., 2007), especially the plots receiving seasonal N application rates higher than 190 kg N ha−1 (Table 1). Therefore, a seasonal N application rate of 190 kg N ha−1 could be a threshold for balanced mineral N in the soil profile under current cultivation practices. 4.2. Water drainage and irrigation management The measurement of water percolation beyond the rooting zone is essential for estimating nitrate leaching (Addiscott, 1996; Aschonitis et al., 2012). In the current investigation, the lack of long-term actual evapo-transpiration (ET) data with corresponding irrigation treatments challenged the calculation of drainage for each plot. However, nitrate accumulation and concentration of the soil solution in deeper soil layers (180 cm) remained stable for the treatments receiving N fertilization before the torrential rainfall in 2010 (Figs. 3, 4), indicating that nitrate leaching was very small before, and only happened after continuous torrential rain for the investigated practices (Fig. 5). Therefore, nitrate leaching beyond the root zone would be a cumulative process driven by different extents of drainage. The uneven distribution of seasonal rainfall and inter-annual variation of precipitation were responsible for the differences in drainage during the different cropping seasons, which was comparable to the results reported by Zhang et al. (2005) and Zhu et al. (2005). Simulation modeling could quantify those drainages and is considered as a powerful tool to assist in the future investigations on nitrate leaching (Addiscott, 1996; Aschonitis et al., 2012). Reducing the nitrate storage in soil profile or minimizing water drainage beyond root zones would be promising solutions to address

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the problem of nitrate leaching (Table 1). These results show that it is possible to optimize the N application rate to enhance N use efficiency and to reduce N losses and nitrate remaining in soil profile (Li et al., 2007; Ju et al., 2009; Yang et al., 2015). However, considerable nitrate accumulation in the soil profile, high gaseous N losses (Huang et al., 2015b), and relatively low grain yield for the rain-fed plots (F190W0, Fig. 3B, C) implied that merely reducing the N application rate could not achieve the goal of high yields and low environmental risks associated with nitrate leaching in this region. As water use efficiency needs to be improved in this region (Zhang et al., 2017), improving irrigation methods and clearing drainage systems could be effective ways to avoid deep water drainage and to retain nitrate in the root zone during the crop growing seasons, as well as to reduce the flush of water through the soil profile. Therefore, coupled water and N management could be an appropriate strategy for mitigating nitrate accumulation and leaching in the soil profile and enhancing the water use efficiency in this region. Due to the clayey layer at the soil depth of 30–80 cm (clay content, 359 g kg−1), nitrate could be reserved in this layer with the strong reduction of water percolation. There was no obvious nitrate leaching before the continuous torrential rain in 2010 (Figs. 3, 4). Irrigation to the field capacity of 0–40 cm could be conducive to root spreading in the top 40 cm soil layer, effectively reducing gaseous N losses (Huang et al., 2015b) and nitrate accumulation in soil profile (Fig. 3), while still maintaining grain yield without significant difference between W40 and W60 (Table 2). However, we recommend irrigation to the field capacity of 0–60 cm for wheat due to lower precipitation and a deeper water table during wheat seasons than maize (Fig. 2), and higher potentials to increase wheat yields by increasing irrigation rates (Table 2), as well as deeper root distribution of wheat in the soil profile than that of maize (Zhao et al., 2007; Zhou et al., 2008). Inter-annual variations of precipitation would make irrigation situations complicated (Mueller et al., 2012), especially for the increasing extreme weather events (e.g., drought and torrential rainfall) reported in this region in recent decades (Howden et al., 2007; Huang et al., 2014), further emphasizing the need for irrigation to be performed based on crop growth, water regime in the soil profile, and weather forecast. Advisory agencies could play an important role in providing helpful recommendations and instructions during the process of agricultural production.

Table 2 Average grain yields and aboveground N uptake for wheat (four seasons) and maize (five seasons) under different water and N treatments during the experimental period (2006– 2010). Treatment

F0W20 F0W40 F0W60 F150W20 F150W40 F150W60 F190W0 F190W20 F190W40 F190W60 F230W20 F230W40 F230W60 F270W20 F270W40 F270W60

Grain yield (Mg ha−1)a

Crop N uptake (kg N ha−1)

Wheat

Maize

Wheat

Maize

2.60 (0.45) a 2.09 (0.78) a 2.06 (0.39) a 5.60 (0.37) c 5.63 (0.07) c 5.60 (0.56) c 4.76 (0.17) b 5.43 (0.18) c 5.38 (0.09) c 5.99 (0.31) c 5.68 (0.13) c 5.49 (0.44) c 5.85 (0.07) c 5.76 (0.33) c 5.95 (0.44) c 5.89 (0.27) c

4.62 (0.41) a 4.35 (0.34) a 4.23 (0.32) a 9.05 (0.09) cd 9.27 (0.17) cde 9.14 (0.13) cde 8.51 (0.14) b 8.85 (0.38) bc 9.35 (0.12) de 9.40 (0.26) de 8.85 (0.08) bc 9.20 (0.19) cde 9.44 (0.13) de 9.13 (0.11) cde 9.10 (0.19) cde 9.54 (0.23) e

30 (5) a 25 (2) a 25 (3) a 128 (5) cde 124 (6) bcd 119 (9) bc 112 (8) b 128 (5) cde 125 (2) bcde 141 (9) ef 134 (4) cdef 131 (16) cde 136 (4) def 139 (12) def 149 (16) f 148 (9) f

44 (8) a 42 (5) a 42 (3) a 141 (1) c 140 (0) c 139 (4) c 129 (1) b 140 (2) c 144 (4) cd 150 (4) de 138 (4) c 147 (10) cd 155 (7) e 152 (2) de 151 (2) de 154 (6) e

Numbers in the parentheses represent standard deviation of the mean (n = 15 and 12 for maize and wheat, respectively). Different letters following values in the same column denote significant differences between different treatments (P b 0.05). a Grain yield data were normalized to a water content of 14% from constant dry weight.

4.3. Groundwater and nitrate concentration Nitrate concentration in the groundwater varied from 0.2 to 2.9 mg L−1 for all the treatments from the beginning of maize season in 2006 to the end of maize season in 2010, which was lower than the limit of 10 mg L− 1 as the maximum safe level for drinking water (Zhao et al., 2007). Because of the limited depth (100 cm) of the concrete walls separating plots and the free groundwater flow and exchange, nitrate concentration of the groundwater samples demonstrated negligible differences between different treatments. Meanwhile, nitrate in excess of crop needs basically accumulated in the soil profile instead of leaching into the groundwater in this investigation (Fig. 3). However, a large nitrate accumulation in the soil profile resulting from high N applications posed potential risks to groundwater contamination. Therefore, informed decision-making on groundwater resource management requires substantial data tracking of the fate of nitrate transport across the root zone to the groundwater, in addition to monitoring the nitrate concentration of groundwater. The water table would impact the water budget of the agroecosystem in the arid and semiarid areas, including irrigation, drainage, and evapo-transpiration, and relatively more irrigation water percolating into groundwater at the agricultural site with a shallow water table (Guo et al., 2006). In this study, disturbance of the water table rise above root zone (180 cm) made the situation complicated in the maize seasons. The shallowest water table was 50 cm (Fig. 2), but such a situation lasted only several days and the concrete walls could block horizontal water flow among different plots. More attention needs to be focused on the rise of the water table, as this may lead to secondary salinization due to high salt contents of groundwater in this region (Hu et al., 2005). Due to the rising of the groundwater level to the root zone, nitrate transport can be facilitated, making uncertainties for estimating nitrate leaching, and thus deserves further investigation. 4.4. Mass N balance and its implications Heavy N fertilization (600–800 kg N ha−1 yr−1) for intensive agriculture has been performed for several decades in the Huang-HuaiHai Plain, and conventional over-flooding irrigation methods induced nitrate leaching or accumulating in the root zone (Fang et al., 2006). Remaining mineral N in the soil profile made it possible to reduce N application rate due to its availability to the following crops (Ju et al., 2007; Zhou et al., 2008), so fertilizer application rates should take the residual N in soil profile into consideration. Although mineral N storage in the soil profile could be utilized by crops, high contents of nitrate remaining in the soil profile could lead to great leaching potential induced by torrential rain or the rise of the water table above the root zone (Figs. 2, 3C) as well as subsoil denitrification (Jahangir et al., 2012), which would lower N use efficiency and result in greenhouse gas emissions and soil acidification (Bateman and Baggs, 2005; Huang et al., 2015a). With a seasonal N application rate of 150 kg N ha− 1 for summer maize and winter wheat rotation in this area, a high yield could be attained in the first 5 years of the trial because of high contents of residual background N (Tables 1, 2). However, the situation is not stable and sustainable due to the depleting mineral N storage and deceasing nitrate concentration of the soil solution in the root zone (Figs. 3, 4). For the applications of 190 kg N ha−1, nitrate accumulated slightly in the soil profile with reasonable irrigation (Figs. 3 and 4, Table 1). With the targeted total grain yield of 15 Mg ha− 1 including both wheat and maize (Table 2), the aboveground N uptake (grain and stalk) of normal growing crops (except for the plots receiving zero N application and the rain-fed plots) ranged from 258 kg N ha−1 (F150W60) to 302 kg N ha−1 (F270W60), and the annual N application rate could be recommended as 300–380 kg N ha− 1, taking the annual root N assimilation (15– 19 kg N ha−1) and the exogenous load from atmospheric N deposition (around 30 kg N ha−1 yr−1) into account (Huang et al., 2016). The contribution of N fertilization to wheat yield was higher than to maize, and

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Liu et al. (2010) suggested that wheat could be given priority over maize when recommending N application rates mainly due to its longer growing period, as well as lower precipitation and temperature during wheat growth. Relatively high N contents of the soil may compensate for the disadvantages, such as low temperature and soil moisture, for wheat growth in contrast with maize. The nitrate remaining in the soil profile after wheat harvest could be utilized by maize (Ju et al., 2007), and N losses in wheat seasons were much lower than those in maize seasons (Cai et al., 2002; Huang et al., 2015b). Therefore, the N application rate could be recommended as 150 kg N ha− 1 for maize and 190 kg N ha−1 for wheat, which would reduce conventional N fertilization by over 30% in this region, which was comparable to the results from the Integrated Soil-Crop System management (Chen et al., 2014) and the long-term fertilization trial (Liu et al., 2010). Maintaining the delicate balance between productive agriculture and a healthy environment requires a broad perspective over different time and dimensional scales (Hu et al., 2005). While agricultural productivity is measured on a timescale of seasons, usually several months in this region, its final impact on environment is a long-term cumulative process with a timescale of years to decades. Therefore, long-term observations on grain production, environmental impacts, and soil fertility under coupled water and N management were necessary in the future studies. Different practices from smallholders of farmlands challenge the regional integrated implementation of optimal nutrient and water management (Chen et al., 2014), so the government could contribute more to the coordination of agricultural production and environmental protection. For instance, the status of nitrate storage and distribution in the soil profile is still unknown in this region, which hinders the reasonable recommendation of N application rates. Fertilization practice has been improved gradually with the recommendation of scientists and local governments as well as the efforts made by fertilizer manufacturers in recent years (Chen et al., 2014; Zhang et al., 2017). The increasing economic cost for fertilizers also makes the recommended N application rate applicable. However, water management is the most challenging work during grain production in this region due to the farmers' willful irrigation and lack of guidance or directions provided by the administrative departments or nongovernmental organizations. Therefore, irrigation quota for each field deployed by the local advisory body could be a promising solution to improving water use efficiency and reducing nitrate leaching. As torrential rainfall was the main cause for nitrate leaching, a regional information and management system, including soil properties, crop growth status, real-time monitoring network for soil moisture and groundwater, weather forecast etc., is urgently needed for the guidance of agricultural production. In recent years, the increased incorporation of crop straw, mechanization and reduced tillage made the conventional cultivation changed (Huang et al., 2015a; Zhang et al., 2017), and the recommendation of proper water and N management considering these comprehensive practices and issues requires further investigations. 5. Conclusion High fertilization rates generally resulted in great nitrate loadings in the soil profile and posed potential risks to groundwater contamination by nitrate leaching. Even with the recommended fertilization rate, the rain-fed plots (F190W0) witnessed high nitrate accumulation in the soil profile, thus highlighting the fact that water management is also important for crop production and environmental protection. Therefore, a proper N application rate combined with reasonable irrigation rates should be implemented in order to reduce residual nitrate build-up in the soil profile while maintaining high grain yields. This five-year field experiment revealed that coupled water and N management could enhance crop yields, improve the recovery of N fertilizers, minimize environmental hazards, and save water resources and production costs in the Huang-Huai-Hai Plain. Considering agricultural production and environmental impacts, seasonal N application rates of 190 kg N ha−1 and

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150 kg N ha−1 were recommended for winter wheat and summer maize, respectively. Irrigation to the field capacity of 0–40 cm and 0– 60 cm soil depth could be appropriate for crop growth and reduction of N losses for maize and wheat production, respectively, yet specific irrigation rates should be conducted according to crop requirement, water deficit, and precipitation. Therefore, the challenging balance between grain production and environmental protection could be attainable through coupled water and N management in this region. Although there is great potential to reduce the traditional N application rates based on the current results, many smallholder farmers regard such high N input as a requirement for high yield. Therefore, additional actions beyond field experiments must be taken to persuade policymakers and farmers to cultivate in a more eco-friendly manner.

Acknowledgments This study was financially supported by the National Key Research and Development Program of China (No. 2016YFD0300802 and 2016YFD0200107), National Natural Science Foundation of China (41401243), the Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2017391), the China Agriculture Research System (No. CARS-02A), and Research Fund of State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences (No. Y412201401). Special thanks to Mr. Qi'ao Jiang, Prof. Lingyun Zhou, Prof. Weixin Ding, Prof. Shengwu Qin, Mr. Jian Liu, and Prof. Jinfang Wang for their helpful advice and valuable assistance. We also extend our gratitude to Mrs. Xianrui Chang, Mr. Tian'en Liu, Mrs. Fengmei Sun, and Mrs. Chunli Sun for their excellent technical assistance. We are grateful to the editors and anonymous reviewers for their valuable comments, which greatly improved the quality of the manuscript. References Addiscott, T.M., 1996. Measuring and modelling nitrogen leaching: parallel problems. Plant Soil 181, 1–6. Aschonitis, V.G., Mastrocicco, M., Colombani, N., Salemi, E., Kazakis, N., Voudouris, K., Castaldelli, G., 2012. Assessment of the intrinsic vulnerability of agricultural land to water and nitrogen losses via deterministic approach and regression analysis. Water Air Soil Pollut. 223, 1605–1614. Baron, J.S., Rueth, H.M., Wolfe, A.M., Nydick, K.R., Allstott, E.J., Minear, J.T., Moraska, B., 2000. Ecosystem responses to nitrogen deposition in the Colorado Front Range. Ecosystems 3, 352–368. Bateman, E.J., Baggs, E.M., 2005. Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space. Biol. Fertil. Soils 41, 379–388. Cai, G.X., Chen, D.L., Ding, H., Pacholski, A., Fan, X.H., Zhu, Z.L., 2002. Nitrogen losses from fertilizers applied to maize, wheat and rice in the North China Plain. Nutr. Cycl. Agroecosyst. 63, 187–195. Chen, X., Cui, Z., Fan, M., Vitousek, P., Zhao, M., Ma, W., Wang, Z., Zhang, W., Yan, X., Yang, J., Deng, X., Gao, Q., Zhang, Q., Guo, S., Ren, J., Li, S., Ye, Y., Wang, Z., Huang, J., Tang, Q., Sun, Y., Peng, X., Zhang, J., He, M., Zhu, Y., Xue, J., Wang, G., Wu, L., An, N., Wu, L., Ma, L., Zhang, W., Zhang, F., 2014. Producing more grain with lower environmental costs. Nature 514, 486–489. Fang, Q.X., Yu, Q., Wang, E.L., Chen, Y.H., Zhang, G.L., Wang, J., Li, L.H., 2006. Soil nitrate accumulation, leaching and crop nitrogen use as influenced by fertilization and irrigation in an intensive wheat-maize double cropping system in the North China Plain. Plant Soil 284, 335–350. Finke, P.A., 1993. Field-scale variability of soil-structure and its impact on crop growth and nitrate leaching in the analysis of fertilizing scenarios. Geoderma 60, 89–107. Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, A.R., Vorosmarty, C.J., 2004. Nitrogen cycles: past, present, and future. Biogeochemistry 70, 153–226. Grogan, D.S., Zhang, F., Prusevich, A., Lammers, R.B., Wisser, D., Glidden, S., Li, C., Frolking, S., 2015. Quantifying the link between crop production and mined groundwater irrigation in China. Sci. Total Environ. 511, 161–175. Guo, H.M., Li, G.H., Zhang, D.Y., Zhang, X., Lu, C.A., 2006. Effects of water table and fertilization management on nitrogen loading to groundwater. Agric. Water Manag. 82, 86–98. Hoffmann, M., Johnsson, H., Gustafson, A., Grimvall, A., 2000. Leaching of nitrogen in Swedish agriculture - a historical perspective. Agric. Ecosyst. Environ. 80, 277–290. Howden, S.M., Soussana, J.F., Tubiello, F.N., Chhetri, N., Dunlop, M., Meinke, H., 2007. Adapting agriculture to climate change. Proc. Natl. Acad. Sci. U. S. A. 104, 19691–19696.

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