Optimizing the synthetic nitrogen rate to balance residual nitrate and crop yield in a leguminous green-manured wheat cropping system

Science of the Total Environment 631–632 (2018) 1234–1242

Contents lists available at ScienceDirect

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

Optimizing the synthetic nitrogen rate to balance residual nitrate and crop yield in a leguminous green-manured wheat cropping system Zhiyuan Yao a, Dabin Zhang a, Pengwei Yao a, Na Zhao b, Yangyang Li c, Suiqi Zhang c, Bingnian Zhai a, Donglin Huang a, Aisheng Ma a, Yajie Zuo a, Weidong Cao d, Yajun Gao a,e,⁎ a

College of Natural Resources and Environment, Northwest A&F University, 712100 Yangling, Shaanxi, China Bayannaoer Academy of Agricultural and Animal Sciences, 015000 Bayannaoer, Inner Mongolia, China Institute of Soil and Water Conservation, CAS & MWR, 712100 Yangling, Shaanxi, China d Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, 100081 Beijing, China e Key Laboratory of Plant Nutrition and Agro-environment in Northwest China, Ministry of Agriculture, 712100 Yangling, Shaanxi, China b c

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

• Total N threshold for highest wheat yield was 141 kg N ha−1. • N from LGM was less prone for leaching than synthetic N. • Root-zone soil nitrate-holding capacity for LGM treatments were 104–117 kg N ha−1. • Corresponding synthetic N limits for LGM treatments were 97–130 kg N ha−1. • Optimal synthetic N rates were 50–68% lower than the traditional practice.

a r t i c l e

i n f o

Article history: Received 14 November 2017 Received in revised form 8 March 2018 Accepted 10 March 2018 Available online xxxx Editor: Charlotte Poschenrieder Keywords: Cover crop Catch crop Legume Nitrate accumulation Winter wheat Root zone

a b s t r a c t Nitrate that originates from agriculture is linked to a series of deleterious environmental consequences that are closely related to human health. Therefore, it is vital to design cropping systems that can produce acceptable crop yields while minimizing the impact of surplus soil nitrate. To develop quantitative estimations, data from 2008 to 2016 were evaluated using multiple regression models. A split-plot field experiment was conducted, with the main treatments of growing Huai bean, soybean and mung bean in summer as leguminous green manure (LGM) while fallow as the control. Four synthetic N rates (0, 108, 135 and 162 kg ha−1) were applied as sub-treatments at wheat seeding. The N accumulation for LGMs ranged from 61 to 90 kg ha−1, and that of Huai bean was 46% higher than the average value of soybean and mung bean (P b 0.05). The threshold of total N for wheat to produce the highest yields was 141 kg ha−1. For the LGM treatments, residual nitrate accumulated below the root-zone soil was not significantly increased even when their total N inputs were higher than that of fallow with 162 kg ha−1 of synthetic N. The estimated nitrate-holding capacity of the root-zone soil for the LGM treatments ranged from 104 to 117 kg ha−1, and the corresponding synthetic N limits were 97–130 kg ha−1. Considering the target of producing high wheat yields while keeping the residual nitrate in the root-zone soil, the optimal synthetic N rates for LGM treatments were 52–80 kg ha−1. In conclusion, growing LGMs can maintain high

⁎ Corresponding author at: College of Natural Resources and Environment, Northwest A&F University, 712100 Yangling, Shaanxi, China. E-mail address: [email protected] (Y. Gao).

https://doi.org/10.1016/j.scitotenv.2018.03.115 0048-9697/© 2018 Elsevier B.V. All rights reserved.

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

1235

crop yield and mitigate the environmental impact of residual nitrate by substantially replacing the synthetic N to avoid nitrate leaching to deeper soils. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Agriculture faces a great challenge to ensure global food security by propelling crop yields while mitigating environmental costs (Foley et al., 2011). To feed its ever-growing population, China consumed N30% of the annual global synthetic fertilizer for an increased crop yield (Zhang et al., 2012a; IFA, 2016; Jiao et al., 2016). However, the great consumption of synthetic fertilizer has caused a cascade of environmental issues, such as increased greenhouse gas emissions (Yao et al., 2017; Yue et al., 2017), air/water pollution (Galloway et al., 2008; Ju et al., 2009; Liu et al., 2013; Gu et al., 2013) and soil acidification (Guo et al., 2010). To mitigate these impacts, in 2015, the central government launched a policy plan called “Zero Growth in Synthetic Fertilizers after 2020” (Strokal et al., 2017). China's population is predicted to peak at 1.47 billion by the mid-2030s; thus, the corresponding total grain production must increase by at least 50% for China to maintain food self-sufficiency (Cui et al., 2014). Considering environmental protection and food security, it is imperative to optimize fertilization strategies. Nitrogen (N) is a major plant nutrient that is essential to and affects plant growth (Wienhold et al., 1995). Owing to the limited supply of plant-available N by soil, synthetic N has been widely applied to enhance crop production. However, the residual nitrate in soil profile due to the application of synthetic N is susceptible to lose (Cheng et al., 2017). An assessment of global N flows in cropland shows that 16% (ca. 24 Tg) of the N was lost through leaching (Liu et al., 2010). This loss can lead to eutrophication in surface waters and increased nitrate concentrations in groundwater (Perego et al., 2012; Ma et al., 2013). Nitrate leaching raises great concerns because its environmental impacts are often closely related to human welfare. Therefore, for environmentally friendly crop production, soil nitrate accumulation should be managed at an acceptable level, especially at the end of the growing season, to minimize the risk of leaching loss (Dinnes et al., 2002). However, the lack of knowledge among Chinese farmers, the ineffective services for soil testing, the ignorance of harsh environmental consequences and the affordability of synthetic N resulted in the severe overuse of synthetic N (Miao et al., 2010; Lu et al., 2015; Yang et al., 2017). The average amount of synthetic N applied by farmers in the North China Plain for a maize-wheat rotation ranged from 550 to 600 kg ha−1 annually, which is much higher than the amount of N removed from the field through the harvest (Vitousek et al., 2009; Zhang et al., 2016a). The excessive nutrients that are uncoupled both spatially and temporally from plant demands can exacerbate potential leakage out of the root zone (Drinkwater and Snapp, 2007; Shen et al., 2012). Similarly, the overuse of synthetic N can strongly enhance nitrate leaching beyond the root zone and become a potential pollutant (Perego et al., 2012; Dai et al., 2015; Zhou et al., 2016). Growing green manure crops (also known as cover/catch crops) is a feasible way to control nitrate leaching. These crops are planted between periods when cash crops are produced to increase the biomass inputs to the soil, decrease erosion, disrupt pest cycles, and retain mineral N (Thorup-Kristensen et al., 2003; McDaniel et al., 2014). Studies suggest that both legume and non-legume crops can reduce nitrate leaching significantly, although high N demand non-legume crops such as oil radish, ryegrass and barley have been shown to be more efficient (Möller and Reents, 2009; Askegaard et al., 2011; Tosti et al., 2014; Couëdel et al., 2018). Nitrate leaching is positively correlated with surplus N in the soil (Chen et al., 2014; Zhou et al., 2016); therefore, another widely recognized way to avoid nitrate leaching is to consciously reduce the rates at which synthetic N is applied to crops.

Studies in different locations of China indicate that synthetic N can be reduced by 30–60% and still maintain or even increase the yield of the main crops (Yao et al., 2017; Zhang et al., 2016c; Cui et al., 2014). For China, growing green manure to recycle the residual nitrate after harvesting the main crops and fully exploiting the synthetic N reduction potential will become an integral part of a sustainable portfolio to control the accumulation of residual nitrate and lower the risk of nitrate leaching. The precipitation can have a strong effect on nitrate leaching by influencing the cycle of N among the crops and agroecosystem. The annual precipitation on the Loess Plateau of China fluctuates widely and its effects on wheat yield, residual nitrate accumulation and the leaching loss can be very different among the years (Dai et al., 2015; He et al., 2016; Yao et al., 2018). Therefore, since the annual precipitation and its variation are still unpredictable, estimating the general patterns of residual nitrate accumulation in the soil profile will not only benefit researchers with theoretical values, but will also provide farmers a practicable guideline to optimize their cropping systems for sustainable crop production. Elucidating the effects of different combinations of leguminous green manure (LGM) species and synthetic N rates on residual nitrate storage and the potential for synthetic N reduction will be helpful to deepen the understanding of their environmental impacts and to further explore sustainable fertilization strategies on the Loess Plateau of China. In this study, we investigated 1) the impact of different combinations of LGMs and synthetic N rates on residual nitrate storage in the soil profile after wheat harvest; 2) whether the synthetic N rate can be further reduced for cropping systems with LGMs to lower the risk of nitrate leaching beyond the root-zone soil while maintaining the high yield of winter wheat; and 3) the optimal suite of LGMs and synthetic N rates that is practicable for farmers and policymakers to achieve high crop yields with minimal environmental impacts. 2. Materials and methods 2.1. Site description The experiment was conducted at the Agricultural Technology Demonstration Centre of Changwu County, Shaanxi Province (35°12′ N, 107°44′ E, altitude of 1220 m a.s.l), on the southern Loess Plateau of China. The experimental site is located in a semiarid, continental monsoon climate and receives 2230 h of average annual sunlight each year. The average annual temperature is 9.1 °C, with an average 171 frost-free days each year. The annual precipitation of the study area is 570–590 mm with 50–60% of which occurs from June to September. Agricultural production in this region is completely dependent on the natural precipitation. The average monthly precipitation of the study (2008–2016) and the long-term records can be found in Supplementary Fig. 1. The soil at the experimental site is classified as a Cumuli-Ustic Isohumosol according to the Chinese Soil Taxonomy (Gong et al., 2007) (Cumulic Haplustolls according to the USDA Soil Taxonomy). The initial soil properties of the plough layer (0–20 cm) prior to the experiment were as follows: pH 8.11, organic C 8.32 g kg−1, total N 0.95 g kg−1, total P 0.66 g kg−1, field capacity 22.4% and clay content 24%. 2.2. Experimental design The long-term experiment was initiated in June 2008 and was arranged in a split-plot design with 3 replications. There were four main

1236

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

treatments: (i) summer fallow–winter wheat (Triticum aestivum L.) (control), (ii) Huai bean (Glycine soja Sieb. et Zucc, also called Glycine ussuriensis Regel et Maack)–winter wheat, (iii) soybean (G. max (L.) Merr.)–winter wheat and (iv) mung bean (Vigna radiata (Linn.) Wilczek, also called Phaseolus radiatus L.)–winter wheat. The sub-treatments included four rates of synthetic N, 0, 108, 135 and 162 kg ha−1, applied at wheat seeding. Among them, 162 kg ha−1 is the typical synthetic N rate of local farmers, so summer fallow–winter wheat with 162 kg ha−1 of synthetic N is the traditional practice. Each sub-plot was 6 × 5 m2.

every year to determine the residual nitrate storage. The filtrate of each sample was extracted from 5.0 g of fresh soil with 50 ml of 1 M potassium chloride (KCl) (Henriksen and Selmer-Olsen, 1970) and then measured using a high-resolution digital colorimeter Auto Analyzer 3 (AA3, Bran + Luebbe, Germany) to determine the nitrate concentration. 2.5. Data analysis NLGM (kg ha−1) represents the N accumulation of LGM that is available to be incorporated into the soil, and was calculated as follows:

2.3. Field management The wheat was harvested in late June, after which the wheat straw was removed from the plots. Each plot was ploughed (0–20 cm) with a rotavator to prepare the seedbed for LGMs. The Huai bean, soybean and mung bean seeding rates were 165, 150, and 135 kg ha−1, respectively. They were seeded via a planter in late June to early July after wheat harvest, depending on the weather conditions. Seven to eight weeks after LGM seeding (late August to early September), fresh samples were manually collected at full-bloom stage; the rest of the plot was then manually mowed, and the LGM was spread evenly across the corresponding plot. The plant residues were then chopped into small pieces with farm machinery and immediately incorporated into the soil to a depth of 20 cm using a rotavator. After the residues had decomposed for 2 to 3 weeks, the four synthetic N sub-treatments were applied as urea (46% N) along with 120 kg ha−1 of P2O5 (triple superphosphate, 46% P2O5) as basal fertilizer to each treatment in late September or early October (Fig. 1), then the winter wheat was sown at 180 kg ha−1 with a row space of 20 cm. 2.4. Sampling and laboratory measurement The fresh aboveground biomass of winter wheat was determined by manually weighing samples from 10 rows of each plot (for an area of 10 m2) during harvest in late June. The wheat samples for laboratory analysis were collected from four 1-m-long randomized rows in each plot. They were dried at 95 °C for 0.5 h and at 65 °C for 24 h to a constant weight to determine the water content and calculate the different parts of the aboveground dry biomass, including straw and grain yield; the sub-samples were ground for further analysis. The fresh aboveground yield of the LGMs was determined manually in early September from the whole plot because of their relatively low biomass compared with wheat; the samples were collected and processed following the procedure for wheat samples. The ground sub-samples of LGMs were used to determine the N concentration following the Kjeldahl method (Bremner, 1996). The soil was sampled every 20 cm through a 0–200 cm soil profile using a soil auger (inner diameter 4.0 cm) after the wheat harvest

NLGM ¼

  33:8%  DMLGM−a  nLGM−a DMLGM−a  nLGM−a þ 1−33:8%  10−3

where DMLGM-a (kg ha−1) is the aboveground dry matter (DM) of the LGM and nLGM-a is the measured N content (g kg−1) of the aboveground parts of the LGM (Supplementary Table 1), while 33.8% is the proportion of N in the belowground parts (recoverable root + rhizodeposits) of the LGM relative to total plant N according to Arcand et al. (2014). RN represents the residual nitrate storage (kg ha−1) in the soil layer after wheat harvest. It was calculated based on the equation below: RN ¼ CN  d  ρ  10−1 where CN is the concentration of soil nitrate (mg kg−1), d is the thickness of each soil layer (in this study, d = 20 cm), ρ is the bulk density (g cm−3) of the corresponding soil layer and 10-1 is a conversion coefficient. 2.6. Statistical analysis All data were compiled and analysed with Microsoft Excel 2016, and the statistical analyses were conducted using the SAS software package, version 9.1 (SAS Institute, Cary, USA). Analysis of variance (ANOVA) was conducted to determine whether significant differences existed in the biomass and N accumulation of the 3 LGMs, the soil nitrate storage in each layer from 0 to 200 cm, and the sum of the soil nitrate storage in the 0–100 cm and 100–200 cm soil profile between the LGM and fallow treatments. When the ANOVA results were significant, the LSD test was used to determine the significant differences between the means at a significance level of P b 0.05. A regression analysis was performed to determine the best-fit model between 1) the total N inputs and wheat yield; 2) the synthetic N rate and residual nitrate accumulated in the 0–100 cm soil profile of the main treatments; and 3) the synthetic N rate and the difference in residual nitrate accumulated in the 100–200 cm soil profile between the LGM and fallow treatments. 3. Results 3.1. Dry matter and N incorporated into the field via LGM

Fig. 1. The time sequence of field management.

The dry matter and N incorporated into the field via the 3 LGMs were significantly different. Specifically, the dry matter of Huai bean (2616 kg ha−1) was 41% and 16% higher than that measured for the soybean and mung bean treatments (P b 0.05), respectively, with soybean dry matter accounting for 1853 kg ha−1, the lowest of the 3 LGMs (Table 1). The N incorporated into the soil was also the highest for Huai bean (90 kg ha−1), which was, on average, 46% higher than that of soybean (62 kg ha−1) and mung bean (61 kg ha−1) (P b 0.05). However, the difference in N incorporation between soybean and mung bean was minimal and insignificant. Furthermore, different synthetic N rates applied at wheat seeding exerted no effect on the dry matter or the N returned to the soil for the 3 LGMs (Table 1).

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242 Table 1 The 8-year average dry matter (DMLGM) and N (NLGM) accumulation for the Huai bean, soybean and mung bean at different synthetic N rates applied at wheat seeding. Different lowercase letters within a column indicate significant differences for dry matter or N accumulation of an LGM at various synthetic N rates, and different uppercase letters within a row indicate significant differences of average dry matter or N accumulation over the 4 synthetic N rates for the 3 LGMs. Treatment Synthetic N

0 108 135 162 Average

Leguminous green manure Huai bean

Soybean

Mung bean

DMLGM

NLGM

DMLGM

NLGM

DMLGM

NLGM

2636 a 2577 a 2616 a 2634 a 2616 A

90 a 88 a 90 a 90 a 90 A

1912 a 1847 a 1854 a 1797 a 1853 C

64 a 62 a 62 a 60 a 62 B

2401 a 2200 a 2217 a 2209 a 2257 B

65 a 60 a 60 a 59 a 61 B

3.2. Total N inputs and their effect on wheat yield. The N within LGMs can easily decompose and become available for wheat due to the low C:N of legume crops; therefore, LGMs are considered to have a comparable effect on crop yield to that of synthetic N. The total N inputs from both synthetic and LGM N for the 4 main treatments ranged from 0 to 252 kg ha−1, and the corresponding wheat yield was within the range of 2709–4244 kg ha−1. The regression analysis suggests that a linear + plateau model can properly delineate the quantitative correlation between total N inputs and wheat yield (Fig. 2). The model shows that the total N threshold to achieve the greatest wheat yield was 141 kg ha−1. When the total N supply was lower than this value, the yield increased linearly, and each kg of N improved the wheat yield by approximately 9 kg. However, when the total N inputs exceeded 141 kg ha−1, the yield remained constant at 3924 kg ha−1 and did not respond to the increased N supply (Fig. 2).

1237

the difference of residual nitrate storage between LGM and fallow treatments in the 0–20 and 20–40 cm soil layers increased from 7.46 and 6.02 kg ha−1 to 12.00 and 17.27 kg ha−1, respectively. In addition, residual nitrate storage in the 0–100 cm soil profile of LGM treatments was also significantly higher than that of fallow when the synthetic N rate increased from 0 to 108 kg ha−1 (Fig. 3b). Further increasing the synthetic N rate from 108 to 135 and 162 kg ha−1, the residual nitrate for LGM treatments gradually moved to deeper soil layers (80–100 cm) (Fig. 3c) and finally accumulated beyond the root-zone soil (0–100 cm) compared with fallow treatment with the corresponding synthetic N rate (Fig. 3d). As a result, growing LGMs significantly increased the residual nitrate accumulated within the non-root-zone soil profile (100–200 cm) (by 64.43 kg ha −1) only when the synthetic N rate was 162 kg ha−1 (Fig. 3d). The average total N inputs for LGM treatments with 108 kg ha−1 of synthetic N were close to that of the fallow treatment with 162 kg ha−1 of synthetic N, and comparing them can indicate whether the LGMs induced extra nitrate accumulation. The results indicate that when synthetic N was applied at 108 kg ha−1 to the LGM treatments, no significant difference in residual nitrate was observed in any of the soil layers or in the residual nitrate stored in the 0–100 or 100–200 cm soil profiles compared to fallow soil with 162 kg ha−1 of synthetic N (Fig. 4a). Furthermore, when the synthetic N at 135 kg ha−1 was used for LGM treatments (for which the total N ranged from 195 to 225 kg ha−1), although the residual nitrate that accumulated in the 20 and 80 cm soil layers was significantly increased compared to the fallow treatment with 162 kg ha−1 of synthetic N, no significant difference was observed in the residual nitrate that accumulated in the 0–100 or 100–200 cm soil profiles (Fig. 4b). These results indicate that the N from LGMs did not induce extra residual nitrate accumulated within the non-root-zone soil profile relative to the synthetic N applications. 3.4. Root zone soil nitrate-holding capacity & optimal synthetic N rates for LGM treatments

3.3. Soil residual nitrate storage for different combinations of LGMs and synthetic N rates LGM incorporation and increasing the synthetic N rate led to higher residual nitrate storage in the soil profile. When the synthetic N was 0 kg ha−1, LGMs only significantly increased residual nitrate storage of the 0–20 and 20–40 cm soil layers (Fig. 3a); the trend was the same when the synthetic N rate was raised to 108 kg ha−1 (Fig. 3b). However,

Fig. 2. The 8-year average total N inputs (from synthetic N + LGM N) and wheat yield; the bars associated with the dots represent the standard error (n = 3). The solid line is the regression curve (linear + plateau model) of total N inputs and wheat yield, and the vertical and horizontal dashed lines indicate the total N threshold to produce the highest wheat yield and the corresponding wheat yield, respectively.

Residual nitrate storage in the 0–100 cm soil profile increased exponentially for the 4 main treatments when synthetic N increased from 0 to 162 kg ha−1. The residual nitrate storage for the Huai bean treatment ranged from 61 to 202 kg ha−1 and was always significantly higher than that in the fallow treatment (39–124 kg ha−1) at the corresponding synthetic N rate (Fig. 5a). For the fallow treatment, the residual nitrate within the 100–200 cm soil profile did not vary significantly with the different synthetic N rates; thus, fertilization did not appear to have induced additional residual nitrate accumulation beyond the root-zone soil in this treatment. Therefore, by comparing whether the residual nitrate difference between the 3 LGM and fallow treatments was higher than 0, it could be determined if the LGM treatments caused extra residual nitrate accumulation in the non-root-zone soil at a specific synthetic N rate. The data show that for the Huai bean, soybean and mung bean treatments, when the synthetic N rates were b98, 119 and 130 kg ha−1, respectively, extra residual nitrate accumulation within the non-root-zone soil could be avoided (Fig. 5b). The corresponding residual nitrate storage (117, 104 and 112 kg ha−1) within the 0–100 cm soil profile were then considered the root-zone soil nitrate-holding capacity for Huai bean, soybean and mung bean treatments, respectively (Fig. 5a). The N from Huai bean, soybean and mung bean were 90, 62 and 61 kg ha−1, respectively (Table 1), and they were considered equally efficient as synthetic N on wheat yield. Therefore, based on the estimated total N threshold (141 kg ha−1) for winter wheat in the study area to reach the maximum yield (Fig. 2), the synthetic N requirements for Huai bean, soybean and mung bean treatments were 52, 79 and 80 kg ha−1, respectively, as obtained by subtracting the corresponding LGM N supply from the total N threshold. When also considering the root-zone soil nitrate-holding capacity, the acceptable ranges of synthetic N application rate were 52–98, 79–119 and 80–130 kg ha−1,

1238

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

Fig. 3. Residual nitrate storage distribution in the soil profile for different combinations of LGMs and synthetic N rates after wheat harvest; Fig. 3-a, b, c and d represent synthetic N rate at 0, 108, 135 and 162 kg ha−1, respectively. LGM represents the average residual nitrate storage of the 3 LGMs; bars show standard error (n = 3) and asterisks indicate significant differences between the LGM and fallow treatments at each soil layer (P b 0.05). The inserts show nitrate storage in the 0–100 and 100–200 cm soil profiles. Different lowercase letters with the same colour indicate significant differences in soil nitrate stored in the 0–100 and 100–200 cm soil profiles.

respectively, for Huai bean, soybean and mung bean treatments (Fig. 5). Since the lower limits of the acceptable ranges of synthetic N rate achieved both the highest wheat yield and the lowest residual nitrate accumulation below the root-zone soil, 52, 79 and 80 kg ha−1 were considered the optimal synthetic N rates for Huai bean, soybean and mung

bean treatments, respectively. These values were 52%, 27% and 26% lower than the lowest designed synthetic N rate (108 kg ha−1, which was 33% lower than traditionally applied). The Huai bean treatment was the most effective LGM at reducing the amount of synthetic N needed.

Fig. 4. Residual nitrate storage distribution in the soil profile for the fallow treatment with 162 kg ha−1 and the average of 3 LGMs with 108 kg ha−1 (a) and 135 kg ha−1 (b) of synthetic N after wheat harvest; bars show standard error (n = 3) and asterisks indicate significant differences between the LGM and fallow treatments at each soil layer (P b 0.05). The inserts show nitrate storage in the 0–100 and 100–200 cm soil profiles. Different lowercase letters with the same colour indicate significant differences in soil nitrate stored in the 0–100 and 100–200 cm soil profiles.

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

1239

faba beans can be as high as 5.07% but that in pigeon peas reach only 2.35% (Unkovich et al., 2010). The N from the LGMs is also very important to the yield of winter wheat, because after a LGM is incorporated into the field, it releases considerable N in time for next season's crop due to the low C:N and lignin content (Cicek et al., 2015; Masunga et al., 2016). Moreover, the non-N benefits of introducing legumes into a cropping system, such as the mobilization of phosphorus, improvements in soil aggregate structure and greater microbial activity and diversity in soil, also contribute to improved crop growth (Peoples et al., 2009). Consequently, although LGMs cannot fully decompose to release all of their N during the growth stage of the following crop, studies show that using LGM as a primary N source can achieve competitive crop yields with respect to conventional practices where the same amount of synthetic N was applied (Tonitto et al., 2006; Forte et al., 2017). These studies support the idea that LGM-based N is equally effective at improving crop yield as synthetic N. The total N threshold at which the best wheat yields were achieved in this study was 141 kg ha−1 according to the linear + plateau model (Fig. 2). A study also suggests that the same model properly depicted the responses of wheat yield to total N input for 10 sites within a county and found that the total N threshold to produce the highest wheat yield (5.5–7.1 Mg ha−1) ranged from 71 to 189 kg ha−1 (Cui et al., 2010b). By comparison, the result of our study (141 kg ha−1) was within that range, but the wheat yield was far lower. One of the most important factors to explain the different wheat yield was that the growth of winter wheat in our study depended solely on the limited natural rainfall, whereas the study of Cui et al. (2010b) had a significantly higher amount of water through irrigation. 4.2. Synthetic N rates and LGM on the distribution of residual nitrate storage after wheat harvest Fig. 5. The correlation between the synthetic N rate and the 8-year average residual nitrate storage within the 0–100 cm soil profile of the four main treatments (a) and the correlation between the synthetic N rate and the difference of the 8-year average residual nitrate storage between the 3 LGM and fallow treatments within the 100–200 cm soil profile (b). The area with the same colour as the legend indicates the acceptable range of synthetic N rate for the LGM treatment that can achieve the highest wheat yield while avoid extra residual nitrate accumulation within the 100–200 cm soil profile relative to the fallow treatment. The corresponding equations are y = 36.678e0.0064x (R2 = 0.89*), y = 59.4e0.0069x (R2 = 0.97*), y = 51.437e0.0059x (R2 = 0.99*) and y = 45.41e0.007x (R2 = 0.90*) for fallow, Huai bean, soybean and mung bean treatments, respectively (a); y = 0.0118x2 − 1.2793x + 12.306 (R2 = 0.99*), y = 0.007x2 − 0.9132x + 9.5996 (R2 = 0.98*) and y = 0.0065x2 − 0.8347x − 0.9111 (R2 = 0.89*) for Huai bean, soybean and mung bean treatments, respectively (b). Bars represent standard error (n = 3) and asterisks indicate significant differences between the main treatments at a synthetic N rate (P b 0.05).

4. Discussion 4.1. Dry matter, N accumulation of LGM and winter wheat yield For legume species, where they grow can significantly affect parameters such as biomass, N2 fixation rate and total N accumulation (Peoples et al., 2009). Therefore, the species' adaptability to the local environment may explain the different biomass and N accumulation of the 3 LGMs of this study. Huai bean is a local legume, and mung bean is also widely planted in Northern Shaanxi, besides their adaptation to the abiotic conditions, the amounts of N derived from the atmosphere (Ndfa) via symbiotic N2 fixation for Huai bean and mung bean were on average 75% and 86%, which were significantly higher than that of soybean (48%) (Supplementary Fig. 2). Benefited from their adapatabilty and higher N2 fixation rate, Huai bean and mung bean produced an increased amount of dry matter compared with soybean (Table 1). However, the N accumulation of soybean was the same as mung bean because of the former's higher N concentration (Supplementary Table 1). This variation in legume N concentrations between distinct species is common: one study revealed that the N concentrations in

As the amount of synthetic N increased, residual nitrate storage in both the fallow and LGM treatments increased (Fig. 3), which reflects the strong impact of synthetic N quantity. In addition, the N uptake by crops is also a determinant of the residual nitrate accumulation and the related leaching. For places where crop growth is restricted by environmental factors such as precipitation, only a small amount of N can be assimilated efficiently (e.g., this study). As a result, synthetic N application rate should consider the local environmental conditions to avoid nitrate leaching into deep soil layers. Our results support previous studies on the Loess Plateau and in other countries, which showed that excessive synthetic N leads to increased nitrate accumulation and/or leaching (Dai et al., 2015; Dai et al., 2016; Lenka et al., 2013; Delin and Stenberg, 2014). Similarly, in the semi-humid croplands of China, the nitrate accumulation in the 0–4 m soil profile for wheat and maize fields were found to be 453 and 749 kg ha−1, respectively, and reached 2155 kg ha−1 for orchard soils as the long-term synthetic N application rates increased (Zhou et al., 2016). One important cause of this fact is that current synthetic N application rates has already gone past a point of diminishing returns (Cui et al., 2016). Therefore, avoiding overuse of synthetic N has been identified as a valid strategy to control surplus nitrate accumulation in the soil profile. To achieve this, the farmers in China should be taught about the tactics for synthetic N reduction based on scientific research because the knowledge gap among farmers is now identified as one of the main causes of over fertilization (Cui et al., 2018; Miao et al., 2010). When the synthetic N rate for LGM treatments was higher than 108 kg ha−1, the residual nitrate accumulated in deeper soil layers (Fig. 3c, d) because under the local climate, the surplus N was not efficiently assimilated by winter wheat. An adjacent field experiment also suggests that when the synthetic N was applied at 138 or 150 kg ha− 1 , the incorporation of Huai bean as an LGM increased residual nitrate accumulation below the root-zone soil (N100 cm) (Li et al., 2015). However, when comparing the residual nitrate storage in the soil profile between the fallow treatment with 162 kg ha−1 and LGM with 108 or

1240

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

135 kg ha−1 synthetic N (total N inputs for the treatments were 162 vs 178 or 206 kg ha−1, respectively), no significant differences were found in the root- and non-root-zone soil profiles (Fig. 4). These results suggest that LGM N is less susceptible to induce superfluous residual nitrate accumulation or leaching, and the reasons can be attributed to several causes. Growing LGM after a cash crop is an effective way to recycle residual nitrate, which reduces its accumulation in the soil (Gabriel et al., 2012; Zhang et al., 2016b). LGM may also limit residual nitrate accumulation by improving crop N uptake because studies suggest that the addition of C from different organic sources can reduce nitrate leaching by propelling a better match of N supply to crop N demand (Liang et al., 2013; Xia et al., 2017; Cheng et al., 2017). In addition, LGM can increase soil N retention without a significant increase in nitrate leaching by enhancing organically bound soil N (Quemada et al., 2013) (Supplementary Fig. 3). 4.3. Nitrate-holding capacity of the root-zone soil and optimal synthetic N rates for LGM treatments Quantifying the residual nitrate-holding capacity of the root-zone soil provides a standard by which cropping systems with low residual nitrate accumulation or leaching can be designed. Previous studies focused on obtaining the ‘appropriate’ residual nitrate levels within the root-zone soil through a semi-quantitative approach and were primarily based on the goal of achieving relatively low residual nitrate accumulation levels and acceptable crop yields. An early study suggests that on the North China Plain, residual nitrate in the 0–90 cm soil profile after maize or wheat harvest should not exceed 150 kg ha−1 when taking environmental endurance to N and relatively high crop yields into account (Zhong et al., 2006). Based on this standard, a study on the Loess Plateau shows that the corresponding synthetic N rate was 226 kg ha−1, and the grain yield was high (6493 kg ha−1); however, a synthetic N rate N 160 kg ha−1 induced substantial residual nitrate accumulation in soil layers deeper than 100 cm (Dai et al., 2015). In contrast, a study that covered several experiments on the Loess Plateau of China suggests that residual nitrate storage within the top 100 cm soil profile after wheat harvest should not exceed 55 kg ha−1 to avoid significant leaching during the summer fallow period (Zhang et al., 2012b). A review based on experiences in both China and Europe suggests that the residual nitrate in the top 90 cm soil profile should be approximately 90 kg ha−1 to avoid high nitrate leaching and ensure proper crop yields of the next season (Cui et al., 2010a). These varied results indicate that acceptable residual nitrate levels within the root-zone soil profile should be estimated quantitatively and locally specific. Our results show that the residual nitrate-holding capacity of the root-zone soil profile after the winter wheat harvest for the Huai bean, soybean and mung bean treatments were 117, 104 and 112 kg ha−1, respectively (Fig. 5). These values are higher than the previously estimate of 55 kg ha−1 for the Loess Plateau but lower than that of 150 kg ha−1 on the North China Plain. When the corresponding synthetic N was lower than 98, 119 and 130 ha−1, nitrate accumulation in deeper soil layers could be avoided (Fig. 5). The quantitatively appraised rootzone soil nitrate-holding capacity and the corresponding upper limit of synthetic N inputs for the LGM treatments will be useful for establishing LGM-based cropping systems to avoid nitrate leaching below the root-zone soil profile, thus reducing the potential environmental threats from residual nitrate accumulation. Matching N supply to crop N demand is the key to reducing nitrate leaching and maintaining crop yields (Fang et al., 2006). It is common for famers in China to apply synthetic N in a single basal fertilizer application at wheat seeding to reduce labour and field management costs. However, the N demand for winter wheat is low from seeding to the elongation stage, which accounts for 36% of total crop N uptake (Cui et al., 2010b). The asynchrony between N supply and crop N demand is an important pathway for nitrate leaching. Since LGM N is less susceptible to leaching and can be considered to be as efficient as synthetic N in

increasing crop yields as clarified previously, using it to partly replace the total N threshold (141 kg ha−1) at wheat seeding may abate the possible nitrate leaching while ensuring high wheat yields. Accordingly, the synthetic N rates required for Huai bean, soybean and mung bean treatments to reach the highest yield were 52, 79 and 80 kg ha−1. The reactive N from agriculture has caused severe damage to human health and ecosystem services (Bodirsky et al., 2014), and the adverse impacts of nitrate leaching are profound (Xia et al., 2016). Therefore, the ideal synthetic N rates for LGM treatments should also be able to minimize the nitrate accumulation in the non-root-zone soil profile. By comparing the acceptable ranges of synthetic N application rates (Fig. 5), the calculated values also had the lowest residual nitrate accumulation within the non-root-zone soil and can thus be considered the optimal synthetic N rates for the 3 investigated LGMs. Compared with the local typical synthetic N rate, the optimal rates for the 3 LGM treatments were 50–68% lower. Although studies in South China and India also suggest that the incorporation of LGM can reduce the synthetic N by up to 50% of the local practice without decreasing crop yields, the residual nitrate in the soil profile was not explicitly assessed (Xie et al., 2016; Singh et al., 2017). In contrast, the optimal synthetic N rates for the LGM treatments of this study are more reliable because the estimation was based on both the crop yield and the residual nitrate distribution in the soil profile. Synthetic N topdressing is useful for decreasing its application rate owing to the increased crop yield and NUE (Chen et al., 2015; Chen et al., 2017). Therefore, considering the central government of China is pooling the farmland resources via the farmland transfer policy to facilitate the modernization of large-scale crop production, our future study should also quantify the synthetic N reduction potential when dressing fertilization is applied to adapt to the new trend. 5. Conclusions LGMs can return an appreciable amount of N to the soil and the total N threshold for wheat to produce the highest yield was 141 kg ha−1 when both synthetic and LGM N were considered. LGM treatments with 108 or 135 kg ha−1 of synthetic N did not induce residual nitrate accumulated in deeper soils even though their total N inputs were higher than that of the fallow treatment with 162 kg ha−1 of synthetic N, which suggests that the N from LGMs was less susceptible to leaching than the synthetic N was. The nitrate-holding capacity of the root-zone soil and the corresponding synthetic N rates for LGM treatments were useful guidelines to avoid nitrate leaching. Considering the total N threshold to reach the maximum wheat yield, the N from LGM and the synthetic N limitation to avoid residual nitrate accumulation being greater than the root-zone soil holding capacity, the synthetic N for the LGM treatments can be further lowered by 26–52% compared with the designed lowest synthetic N rate (108 kg ha−1) and Huai bean is the LGM with the highest synthetic N reduction potential. The quantified optimal synthetic N rates of the present research for cropping systems with LGM filled the gap of our previous studies, and the results have paved the way for farmers to design cropping systems that are beneficial for long-term sustainable production via efficiently controlling nitrate leaching and sustaining high crop yields. Acknowledgements This study was supported by the Key Project of the National Science & Technology Support Plan (2015BAD22B01, 2015BAD23B04), the Special Fund for Agro-scientific Research in the Public Interest (201503124, 201103005), the China Agricultural Research System (CARS-03-1-31), the Chinese National Natural Science Foundation (41401330) and the Fundamental Research Funds for the Central Universities (2452017042). The author also want to thank Mrs Yao Bai, Prof. Rong Yan, Dr. Qiusheng Yao and Master student Qian Xu for their kind help duing the revision of the manuscript.

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.03.115.

References Arcand, M.M., Lemke, R., Farrell, R.E., Knight, J.D., 2014. Nitrogen supply from belowground residues of lentil and wheat to a subsequent wheat crop. Biol. Fertil. Soils 50, 507–515. Askegaard, M., Olesen, J.E., Rasmussen, I.A., Kristensen, K., 2011. Nitrate leaching from organic arable crop rotations is mostly determined by autumn field management. Agric. Ecosyst. Environ. 142 (3), 149–160. Bodirsky, B.L., Popp, A., Lotze-Campen, H., Dietrich, J.P., Rolinski, S., Weindl, I., Schmitz, C., Müller, C., Bonsch, M., Humpenöder, F., Biewald, A., 2014. Reactive nitrogen requirements to feed the world in 2050 and potential to mitigate nitrogen pollution. Nat. Commun. 5, 3858. Bremner, J.M., 1996. Nitrogen-total. In: Sparks, A.L., Page, A.L., Helmke, P.A., Loeppert, R.H., Soltanpour, P.N., Tabatabai, M.A., Johnston, C.T., Sumner, M.E. (Eds.), Methods of Soil Analysis. Part 3. Chemical Methods. SSSA and ASA, Madison, WI, pp. 1085–1121. Chen, X., Cui, Z., Fan, M., Vitousek, P., Zhao, M., Ma, W., Wang, Z., Zhang, W., Yan, X., Yang, J., Deng, X., 2014. Producing more grain with lower environmental costs. Nature 514, 486–489. Chen, Y., Peng, J., Wang, J., Fu, P., Hou, Y., Zhang, C., Fahad, S., Peng, S., Cui, K., Nie, L., Huang, J., 2015. Crop management based on multi-split topdressing enhances grain yield and nitrogen use efficiency in irrigated rice in China. Field Crop Res. 184, 50–57. Chen, F., Ameen, A., Tang, C.C., Du, F., Yang, X.L., Xie, G., 2017. Effects of nitrogen fertilization on soil nitrogen for energy sorghum on marginal land in China. Agron. J. 109, 1–10. Cheng, Y., Wang, J., Wang, J., Chang, S.X., Wang, S., 2017. The quality and quantity of exogenous organic carbon input control microbial NO− 3 immobilization: a metaanalysis. Soil Biol. Biochem. 115, 357–363. Cicek, H., Martens, J., Bamford, K., Entz, M., 2015. Late-season catch crops reduce nitrate leaching risk after grazed green manures but release N slower than wheat demand. Agric. Ecosyst. Environ. 202, 31–41. Couëdel, A., Alletto, L., Tribouillois, H., Justes, É., 2018. Cover crop crucifer-legume mixtures provide effective nitrate catch crop and nitrogen green manure ecosystem services. Agric. Ecosyst. Environ. 254, 50–59. Cui, Z., Chen, X., Zhang, F., 2010a. Current nitrogen management status and measures to improve the intensive wheat–maize system in China. Ambio 39 (5–6), 376–384. Cui, Z., Zhang, F., Chen, X., Dou, Z., Li, J., 2010b. In-season nitrogen management strategy for winter wheat: maximizing yields, minimizing environmental impact in an overfertilization context. Field Crop Res. 116, 140–146. Cui, Z., Dou, Z., Chen, X., Ju, X., Zhang, F., 2014. Managing agricultural nutrients for food security in China: past, present, and future. Agron. J. 106, 191–198. Cui, Z., Vitousek, P.M., Zhang, F., Chen, X., 2016. Strengthening agronomy research for food security and environmental quality. Environ. Sci. Technol. 50 (4), 1639–1641. Cui, Z., Zhang, H., Chen, X., Zhang, C., Ma, W., Huang, C., Zhang, W., Mi, G., Miao, Y., Li, X., Gao, Q., Yang, J., Wang, Z., Ye, Y., Guo, S., Lu, J., Huang, J., Lv, S., Sun, Y., Liu, Y., Peng, X., Ren, J., Li, S., Deng, X., Shi, X., Zhang, Q., Yang, Z., Tang, L., Wei, C., Jia, L., Zhang, J., He, M., Tong, Y., Tang, Q., Zhong, X., Liu, Z., Cao, N., Kou, C., Ying, H., Yin, Y., Jiao, X., Zhang, Q., Fan, M., Jiang, R., Zhang, F., Dou, Z., 2018. Pursuing sustainable productivity with millions of smallholder farmers. Nature https://doi.org/10.1038/nature25785. Dai, J., Wang, Z., Li, F., He, G., Wang, S., Li, Q., Cao, H., Luo, L., Zan, Y., Meng, X., Zhang, W., 2015. Optimizing nitrogen input by balancing winter wheat yield and residual nitrate-N in soil in a long-term dryland field experiment in the Loess Plateau of China. Field Crop Res. 181, 32–41. Dai, J., Wang, Z., Li, M., He, G., Li, Q., Cao, H., Wang, S., Gao, Y., Hui, X., 2016. Winter wheat grain yield and summer nitrate leaching: long-term effects of nitrogen and phosphorus rates on the Loess Plateau of China. Field Crop Res. 196, 180–190. Delin, S., Stenberg, M., 2014. Effect of nitrogen fertilization on nitrate leaching in relation to grain yield response on loamy sand in Sweden. Eur. J. Agron. 52, 291–296. Dinnes, D.L., Karlen, D.L., Jaynes, D.B., Kaspar, T.C., Hatfield, J.L., Colvin, T.S., Cambardella, C.A., 2002. Nitrogen management strategies to reduce nitrate leaching in tiledrained Midwestern soils. Agron. J. 94 (1), 153–171. Drinkwater, L.E., Snapp, S.S., 2007. Nutrients in agroecosystems: rethinking the management paradigm. Adv. Agron. 92, 163–186. Fang, Q., Yu, Q., Wang, E., Chen, Y., Zhang, G., Wang, J., Li, L., 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 (1), 335–350. Foley, J.A., Ramankutty, N., Brauman, K.A., Cassidy, E.S., Gerber, J.S., Johnston, M., Mueller, N.D., O'Connell, C., Ray, D.K., West, P.C., Balzer, C., 2011. Solutions for a cultivated planet. Nature 478 (7369), 337–342. Forte, A., Fagnano, M., Fierro, A., 2017. Potential role of compost and green manure amendment to mitigate soil GHGs emissions in Mediterranean drip irrigated maize production systems. J. Environ. Manag. 192, 68–78. Gabriel, J.L., Muñoz-Carpena, R., Quemada, M., 2012. The role of cover crops in irrigated systems: water balance, nitrate leaching and soil mineral nitrogen accumulation. Agric. Ecosyst. Environ. 155, 50–61. Galloway, J.N., Townsend, A.R., Erisman, J.W., Bekunda, M., Cai, Z., Freney, J.R., Martinelli, L.A., Seitzinger, S.P., Sutton, M.A., 2008. Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320 (5878), 889–892.

1241

Gong, Z.T., Zhang, G.L., Chen, Z.C. (Eds.), 2007. Pedogenesis and Soil Taxonomy. Science Press Publishing, Beijing (in Chinese). Gu, B., Ge, Y., Chang, S.X., Luo, W., Chang, J., 2013. Nitrate in groundwater of China: sources and driving forces. Glob. Environ. Chang. 23 (5), 1112–1121. Guo, J.H., Liu, X.J., Zhang, Y., Shen, J.L., Han, W.X., Zhang, W.F., Christie, P., Goulding, K.W.T., Vitousek, P.M., Zhang, F.S., 2010. Significant acidification in major Chinese croplands. Science 327 (5968), 1008–1010. He, G., Wang, Z., Li, F., Dai, J., Ma, X., Li, Q., Xue, C., Cao, H., Wang, S., Liu, H., Luo, L., Huang, M., Malhi, S., 2016. Soil nitrate–n residue, loss and accumulation affected by soil surface management and precipitation in a winter wheat-summer fallow system on dryland. Nutr. Cycl. Agroecosyst. 106 (1), 1–16. Henriksen, A., Selmer-Olsen, A.R., 1970. Automatic methods for determining nitrate and nitrite in water and soil extracts. Analyst 95 (1130), 514–518. IFA, 2016. IFADATA–International Fertilizer Industry Association. http://ifadata.fertilizer. org/ucSearch.aspx, Accessed date: March 2018. Jiao, X., Lyu, Y., Wu, X., Li, H., Cheng, L., Zhang, C., Yuan, L., Jiang, R., Jiang, B., Rengel, Z., Zhang, F., 2016. Grain production versus resource and environmental costs: towards increasing sustainability of nutrient use in China. J. Exp. Bot. 67 (17), 4935–4949. Ju, X.T., Xing, G.X., Chen, X.P., Zhang, S.L., Zhang, L.J., Liu, X.J., Cui, Z.L., Yin, B., Christie, P., Zhu, Z.L., Zhang, F.S., 2009. Reducing environmental risk by improving N management in intensive Chinese agricultural systems. Proc. Natl. Acad. Sci. U. S. A. 106, 3041–3046. Lenka, S., Singh, A.K., Lenka, N.K., 2013. Soil water and nitrogen interaction effect on residual soil nitrate and crop nitrogen recovery under maize–wheat cropping system in the semi-arid region of northern India. Agric. Ecosyst. Environ. 179, 108–115. Li, F., Wang, Z., Dai, J., Li, Q., Wang, X., Xue, C., Liu, H., He, G., 2015. Fate of nitrogen from green manure, straw, and fertilizer applied to wheat under different summer fallow management strategies in dryland. Biol. Fertil. Soils 51 (7), 769–780. Liang, B., Zhao, W., Yang, X., Zhou, J., 2013. Fate of nitrogen-15 as influenced by soil and nutrient management history in a 19-year wheat–maize experiment. Field Crop Res. 144, 126–134. Liu, J., You, L., Amini, M., Obersteiner, M., Herrero, M., Zehnder, A.J., Yang, H., 2010. A highresolution assessment on global nitrogen flows in cropland. Proc. Natl. Acad. Sci. U. S. A. 107 (17), 8035–8040. Liu, X., Zhang, Y., Han, W., Tang, A., Shen, J., Cui, Z., Vitousek, P., Erisman, J.W., Goulding, K., Christie, P., Fangmeier, A., 2013. Enhanced nitrogen deposition over China. Nature 494, 459–462. Lu, Y., Chadwick, D., Norse, D., Powlson, D., Shi, W., 2015. Sustainable intensification of China's agriculture: the key role of nutrient management and climate change mitigation and adaptation. Agric. Ecosyst. Environ. 209, 1–4. Ma, L., Wang, F., Zhang, W., Ma, W., Velthof, G., Qin, W., Oenema, O., Zhang, F., 2013. Environmental assessment of management options for nutrient flows in the food chain in China. Environ. Sci. Technol. 47 (13), 7260–7268. Masunga, R.H., Uzokwe, V.N., Mlay, P.D., Odeh, I., Singh, A., Buchan, D., De Neve, S., 2016. Nitrogen mineralization dynamics of different valuable organic amendments commonly used in agriculture. Appl. Soil Ecol. 101, 185–193. McDaniel, M.D., Tiemann, L.K., Grandy, A.S., 2014. Does agricultural crop diversity enhance soil microbial biomass and organic matter dynamics? A meta-analysis. Ecol. Appl. 24 (3), 560–570. Miao, Y., Stewart, B.A., Zhang, F., 2010. Long-term experiments for sustainable nutrient management in China. A review. Agron. Sustain. Dev. 31, 397–414. Möller, K., Reents, H.J., 2009. Effects of various cover crops after peas on nitrate leaching and nitrogen supply to succeeding winter wheat or potato crops. J. Plant Nutr. Soil Sci. 172 (2), 277–287. Peoples, M.B., Brockwell, J., Herridge, D.F., Rochester, I.J., Alves, B.J.R., Urquiaga, S., Boddey, R.M., Dakora, F.D., Bhattarai, S., Maskey, S.L., Sampet, C., 2009. The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems. Symbiosis 48 (1–3), 1–17. Perego, A., Basile, A., Bonfante, A., De Mascellis, R., Terribile, F., Brenna, S., Acutis, M., 2012. Nitrate leaching under maize cropping systems in Po Valley (Italy). Agric. Ecosyst. Environ. 147, 57–65. Quemada, M., Baranski, M., Nobel-de Lange, M.N.J., Vallejo, A., Cooper, J.M., 2013. Metaanalysis of strategies to control nitrate leaching in irrigated agricultural systems and their effects on crop yield. Agric. Ecosyst. Environ. 174, 1–10. Shen, J., Li, C., Mi, G., Li, L., Yuan, L., Jiang, R., Zhang, F., 2012. Maximizing root/rhizosphere efficiency to improve crop productivity and nutrient use efficiency in intensive agriculture of China. J. Exp. Bot. 64 (5), 1181–1192. Singh, R.J., Ghosh, B.N., Sharma, N.K., Patra, S., Dadhwal, K.S., Meena, V.S., Deshwal, J.S., Mishra, P.K., 2017. Effect of seven years of nutrient supplementation through organic and inorganic sources on productivity, soil and water conservation, and soil fertility changes of maize-wheat rotation in north-western Indian Himalayas. Agric. Ecosyst. Environ. 249, 177–186. Strokal, M., Kroeze, C., Wang, M., Ma, L., 2017. Reducing future river export of nutrients to coastal waters of China in optimistic scenarios. Sci. Total Environ. 579, 517–528. Thorup-Kristensen, K., Magid, J., Jensen, L.S., 2003. Catch crops and green manures as biological tools in nitrogen management in temperate zones. Adv. Agron. 79, 227–302. Tonitto, C., David, M.B., Drinkwater, L.E., 2006. Replacing bare fallows with cover crops in fertilizer-intensive cropping systems: a meta-analysis of crop yield and N dynamics. Agric. Ecosyst. Environ. 112 (1), 58–72. Tosti, G., Benincasa, P., Farneselli, M., Tei, F., Guiducci, M., 2014. Barley–hairy vetch mixture as cover crop for green manuring and the mitigation of N leaching risk. Eur. J. Agron. 54, 34–39. Unkovich, M.J., Baldock, J., Peoples, M.B., 2010. Prospects and problems of simple linear models for estimating symbiotic N2 fixation by crop and pasture legumes. Plant Soil 329 (1–2), 75–89.

1242

Z. Yao et al. / Science of the Total Environment 631–632 (2018) 1234–1242

Vitousek, P.M., Naylor, R., Crews, T., David, M.B., Drinkwater, L.E., Holland, E., Johnes, P.J., Katzenberger, J., Martinelli, L.A., Matson, P.A., Nziguheba, G., 2009. Nutrient imbalances in agricultural development. Science 324 (5934), 1519–1520. Wienhold, B.J., Trooien, T.P., Reichman, G.A., 1995. Yield and nitrogen use efficiency of irrigated corn in the northern Great Plains. Agron. J. 87 (5), 842–846. Xia, L., Ti, C., Li, B., Xia, Y., Yan, X., 2016. Greenhouse gas emissions and reactive nitrogen releases during the life-cycles of staple food production in China and their mitigation potential. Sci. Total Environ. 556, 116–125. Xia, L., Lam, S.K., Yan, X., Chen, D., 2017. How does recycling of livestock manure in agroecosystems affect crop productivity, reactive nitrogen losses and soil carbon balance? Environ. Sci. Technol. 51 (13), 7450–7457. Xie, Z., Tu, S., Shah, F., Xu, C., Chen, J., Han, D., Liu, G., Li, H., Muhammad, I., Cao, W., 2016. Substitution of fertilizer-N by green manure improves the sustainability of yield in double-rice cropping system in south China. Field Crop Res. 188, 142–149. Yang, X., Lu, Y., Ding, Y., Yin, X., Raza, S., Tong, Y.A., 2017. Optimising nitrogen fertilisation: a key to improving nitrogen-use efficiency and minimising nitrate leaching losses in an intensive wheat/maize rotation (2008–2014). Field Crop Res. 206, 1–10. Yao, Z.Y., Zhang, D.B., Yao, P.W., Zhao, N., Liu, N., Zhai, B.N., Zhang, S.Q., Li, Y.Y., Huang, D.L., Cao, W.D., Gao, Y.J., 2017. Coupling life-cycle assessment and the RothC model to estimate the carbon footprint of green manure-based wheat production in China. Sci. Total Environ. 607–608, 433–442. Yao, Z.Y., Wang, Z., Li, J., Bedoussac, L., Zhang, S.Q., Li, Y.Y., Cao, W.D., Zhai, B.N., Wang, Z.H., Gao, Y.J., 2018. Screen for sustainable cropping systems in the rain-fed area on the Loess Plateau of China. Soil Tillage Res. 176, 26–35.

Yue, Q., Xu, X., Hillier, J., Cheng, K., Pan, G., 2017. Mitigating greenhouse gas emissions in agriculture: from farm production to food consumption. J. Clean. Prod. 149, 1011–1019. Zhang, F., Cui, Z., Chen, X., Ju, X., Shen, J., Chen, Q., Liu, X., Zhang, W., Mi, G., Fan, M., Jiang, R., 2012a. Chapter one–integrated nutrient management for food security and environmental quality in China. Adv. Agron. 116, 1–40. Zhang, Z.L., Liu, J.S., Wang, Z.H., Zhao, H.B., Yang, N., Yang, R., Cao, H.B., 2012b. Nitrogen recommendation for dryland winter wheat by monitoring nitrate in 1m soil and based on nitrogen balance. Plant Nutr. Fertil. Sci. 18, 1387–1396 (in Chinese). Zhang, Y., Li, C., Wang, Y., Hu, Y., Christie, P., Zhang, J., Li, X., 2016a. Maize yield and soil fertility with combined use of compost and inorganic fertilizers on a calcareous soil on the North China Plain. Soil Tillage Res. 155, 85–94. Zhang, D.B., Yao, P.W., Zhao, N., Yu, C.W., Cao, W.D., Gao, Y.J., 2016b. Contribution of green manure legumes to nitrogen dynamics in traditional winter wheat cropping system in the Loess Plateau of China. Eur. J. Agron. 72, 47–55. Zhang, W., Cao, G., Li, X., Zhang, H., Wang, C., Liu, Q., Chen, X., Cui, Z., Shen, J., Jiang, R., Mi, G., 2016c. Closing yield gaps in China by empowering smallholder farmers. Nature 537, 671–674. Zhong, Q., Ju, X.T., Zhang, F.S., 2006. Analysis of environmental endurance of winter wheat/summer maize rotation system to nitrogen in North China Plain. Plant Nutr. Fertil. Sci. 12, 285–293 (in Chinese). Zhou, J., Gu, B., Schlesinger, W.H., Ju, X., 2016. Significant accumulation of nitrate in Chinese semi-humid croplands. Sci. Rep. 6, 25088.