Nitrous oxide emissions from maize-wheat field during four successive years in the North China Plain

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Introduction
Emissions of nitrous oxide (N 2 O) to the atmosphere have attracted much attention because of its significance for greenhouse effect and depletion of stratospheric ozone (Crutzen, 1970;Bolle et al., 1986).Agricultural soil has been recognized as a main source of anthropogenic N 2 O emissions to the atmosphere (Khalil et al., 2006) and contributes about 65 % of total anthropogenic N 2 O emission (Smith, 1997).It is well known that N 2 O is a by-product in microbial nitrification and an intermediate in denitrification process (Firestone and Davidson, 1989).carbon availability (Smith et al., 2003;Ruser et al., 2006), crop type and residue management (Raich and Tufekcioglu, 2000;Huang et al., 2004;Chen et al., 2008), and the management of N fertilizer (Hao et al., 2001;Bouwman et al., 2002).Among the various influence factors, fertilization, soil temperature and moisture play important roles on N 2 O emission.Fertilization directly provides substrate for soil nitrifying and denitrifying microbes, and soil temperature and moisture have major impacts on soil microorganisms (Smith et al., 2003).The microbial process generally increases exponentially with soil temperature when other factors are not limiting (Meixner and Yang, 2006).Soil water content plays important roles not only on the substrate supply for the microorganisms (Meixner and Yang, 2006) but also on gas diffusivity (Smith et al., 2003).Increasing soil moisture is conducive to produce anaerobic condition and thus promotes N 2 O formation via denitrification (Dobbie and Smith, 2001).Large temporalspatial variation of N 2 O emission from agricultural fields could be expected due to the changes of the various influence factors, e.g.there are great uncertainties in N 2 O emission from agricultural fields with the reported emission factors (EFs) of 0-7 % for mineral soils (Bouwman, 1996).Therefore, it is necessary to conduct long-term N 2 O flux measurements from different agricultural field to reduce the uncertainty of N 2 O estimation (Barton et al., 2008;Scheer et al., 2008).North China Plain (NCP) is one of the greatest grain production areas in China.Maize and wheat, the main grain crops in this region, provide 39 % and 48 % of the total maize and wheat yields in China, respectively (Liu and Mu, 1993).The NCP has a cultivated land area of 17.95 million ha, which accounts for 18.6 % of the total agricultural area in China (Liu et al., 2001), and consumes about 30 % of the total national N-fertilizer (Zhang et al., 2004).As N-fertilizer is the necessary substrate for soil nitrification and denitrification, the huge amounts of N-fertilizer applications in this region can greatly stimulate N 2 O emission.Therefore, N 2 O emissions from the agricultural fields in the NCP have been investigated intensively (Zeng et al., 1995;Dong et al., 2000;Meng et al., 2005;Ding et al., 2007;Sun et al., 2008;Wang et al., 2008;Wang et al., 2009;Li et al., 2010;Cui et al., 2012;Cai et al., 2013).However, among the ten re-Introduction

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Full ports on N 2 O emissions from the fields, nine studies conducted one year, and only the study of Cai et al. (2013) implemented the N 2 O measurement for three years (2004)(2005)(2006)(2007).According to these treatment-site-year data, large differences of N 2 O emissions (ranging from 0.77 to 6.0 kg N ha −1 yr −1 ) and EFs (in the range of 0.10-1.4%) from the agricultural fields in the NCP were obtained.
In this study, the N 2 O flux from maize-wheat rotation system in the NCP was investigated from 2008 to 2012.The objectives of this study were: (1) to understand the interannual variation characters of N 2 O emission, (2) to determine the key influence factors on N 2 O emission and (3) to assess the total N 2 O emission from the maizewheat field in the NCP.

Field experiment
This study was conducted in a summer maize (Zea mays L.) and winter wheat (Triticum aestivum L.) rotation system in Wangdu County (38 • 71 N, 115 • 15 E), Baoding City, Hebei Province, China.The detail information about the experiment field had been mentioned in our previous papers (Zhang et al., 2011(Zhang et al., , 2012)).
The field experiment was conducted with two different treatments: control plot (CK, without fertilization) and chemical N fertilizer plot (NP).Only with the exception of fertilization, the two plots were identically managed.Each plot (6.5 × 3.5 m 2 ) was separated by a 1.2 m broad zone to prevent nutrient transfer between treatments.Maize and wheat were planted in June and October each year, respectively, and the field was tilled before wheat sowing.Field managements including fertilization, irrigation, herbicide and pesticide applications strictly followed the cultivating manner of local farmers.
The detail information about fertilizer management is listed in  (Zhang et al., 2011(Zhang et al., , 2012)).The N 2 O flux (F , ng N m −2 s −1 ) was calculated by the following equation: where H is the chamber headspace height (m), ∆C/∆t is the slope (ppbv s −1 ) of the linear regression of N 2 O concentration in the chamber with time (R 2 > 0.85), P is the atmospheric pressure (atm) measured in the field, R is the gas constant (0.082 atm L K −1 mol −1 ), T is the ambient air temperature (K) and M N is the molecular weight of N 2 O-N (28 g mol −1 ).

Measurement of soil characteristics
Four soil samples in each plot were collected from 0-10 cm soil layer using a stainless steel soil sampler and were mixed carefully for the analysis of soil mineral N (NH + 4 -N and NO − 3 -N concentrations).Soil water-filled pore space (WFPS) was determined at 5 cm depth by a ring sampler (100 cm 3 ) (Zhang et al., 2011(Zhang et al., , 2012)).Soil temper-Introduction

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Full soil temperatures in CK plot were record in 2008.The data of precipitation were from http://www.wunderground.com.

Date calculation and statistical analysis
The statistical analysis was conducted by Origin 8.0 (Origin Lab Corporation, USA) and SPSS 13.0 software (SPSS Inc., Chicago, USA).Prior to analysis, normal distributions of N 2 O fluxes and driving factors were tested using the Shapiro-Wilk test and data were log-transformed as needed to normalize the distributions.Paired-samples T test was adopted to analyze the difference between CK and NP treatments during the no fertilization periods.Stepwise linear regression analysis was performed to examine the relationships between N 2 O fluxes and important driving factors, and only the regression equations that have statistical significance are listed in this study.Significance of all tests was accepted at P < 0.05.
The N 2 O fluxes presented in the figures are the arithmetic means of the replications in each treatment.The cumulative N 2 O emission from each treatment was estimated by linear interpolations between the sampling days.The EFs during the investigation periods were calculated as the difference between the cumulative N 2 O-N emission in the fertilized plot and control plot divided by the amount of N fertilizer applied.

Environmental variables
The variations of soil moisture were mainly regulated by precipitation and irrigation.

N 2 O fluxes and key influence factors
The  -2009, 2009-2010, 2010-2011 and 2011-2012 wheat seasons, respectively.The N 2 O emission peaks after basal or supplemental fertilizer application were usually higher during the maize seasons than during the wheat seasons, which might be due to the relatively low soil temperature in the wheat seasons (Fig. 1e-h).However, the maximal peak of N 2 O emission (624 ng N m −2 s −1 ) from the NP plot among the four investigated years appeared in the 2012 wheat season after the supplemental fertilization, which was 4-8 times higher than those in other wheat seasons.During the period of the N 2 O peak emission from the NP plot in the 2012 wheat season, the soil WFPS (78 %) was evidently higher than those in 2009 (66 %) and 2011 (60 %) wheat seasons (Fig. 1a, c and d).Although higher WFPS (82 %) was observed after the supplemental fertilizer application in the wheat season of 2010, the obvious low soil temperature (10 • C) compared with that (15.5 • C) in 2012 greatly restricted the activities of soil microorganisms (Meixner and Yang, 2006).Therefore, the higher N 2 O emission in the wheat season of 2012 was due to the synergistic effect of appropriate soil temperature (15.5 • C) and WFPS (78 %), which could build the soil micro environment in favor of denitrification, and thus promote the N 2 O emission (Dobbie and Smith, 2001).
To elucidate the influence of various influencing factors on N 2 O emission, the regression analysis between N 2 O fluxes and important driving factors was conducted as shown in Table 2. Evidently, soil mineral N, temperature and WFPS were positively correlated with N 2 O emission.Soil temperature and WFPS could explain 27-52 % and 18-31 % of the total N 2 O emission, respectively.However, not all influence factors displayed the significant relationships with the N 2 O emission in each year and treatment, Introduction

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Full and the similar conclusion has been drawn by other studies (Wang et al., 2005;Rowlings et al., 2012).

Cumulative N 2 O emissions and emission factors
The cumulative N 2 O emissions and EFs are listed in Table 3.The lowest emissions always occurred in the CK plot, with the 4 yr mean fluxes of 0.6 kg N ha −1 in the maize season, 1.3 kg N ha −1 in the wheat season and 1.9 kg N ha The annual EFs were 2.4 %, 0.6 %, 1.1 % and 2.9 % in 2008-2012 maize-wheat seasons, respectively (Table 3).The mean 4 yr EF in the maize season (2.2 %) was 1.8 times higher than that in the wheat season.

Interannual variation of N 2 O emission
The above results well revealed evident interannual variation of N 2 O emissions from the agricultural field during the four successive years.Considering the nearly identical Nfertilization rates and similar irrigation operations in each year, the interannual variation of N 2 O emissions was mainly ascribed to the changes of meteorological condition that affected the soil temperature and moisture.The annual cumulative N 2 O emissions (F 1 , kg N ha −1 ) from the CK plot significantly correlated with the annual total rainfall (X 1 , mm), and the relationship fitted the following equation: It indicated that rainfall was a dominant factor for controlling N 2 O emission from the agricultural field without fertilization.As for the NP plot, there was no evident correlation between the annual cumulative N 2 O emissions and total rainfall.However, there was a significant linear correlation (N = 4, R 2 = 0.99241, P = 0.00253) between the annual fertilizer-induced N 2 O emissions (F 2 , kg N ha −1 ) during the periods of 10 days after fertilization and the amounts of rainfall (X 2 , mm) 4 days before-and 10 days after each fertilization in each year.The equation could be expressed as: Because the cumulative N 2 O emission from the NP plot during the periods of 10 days after fertilization accounted for ∼ 50 % of the total cumulative N 2 O emissions, the rainfall events far from fertilization events only made modest contribution to the total cumulative N 2 O emissions.The rainfall events just before and after fertilization might favor fostering the community of microorganism and promoting N 2 O formation.Therefore, it is easy to understand why the strong correlation only limited between the yearly fertilizer-induced N 2 O emissions during the periods of 10 days after fertilizations and the amounts of rainfall around fertilization events in each year.There were no significant differences between the CK and NP treatments during the no fertilization periods in the four years (T test, P = 0.056-0.177)and the yearly management (fertilization rate and irrigation) of the agricultural field was almost the same in recent years.Only based on the rainfall, the correlations between the N 2 O emissions and the rainfall could be useful to estimate the cumulative N 2 O emissions (F , kg N ha −1 ) from the agricultural field: It should be mentioned that the above estimation could be only applied to limit area where the rainfall is nearly identical and fertilization rate is similar with this study.Because the cumulative N 2 O emissions usually linearly correlate with fertilization rates Introduction

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Full  (Henault et al., 1998;De Klein et al., 2006;Halvorson et al., 2008), the above experiential algorithm for estimating the annual cumulative N 2 O emissions (Y , kg N ha −1 ) from agricultural fields could be expressed in more general form: where n, A, B and C are constants which can be derived from the correlation of field measurements, and F is the application rate of N-fertilizer.To verify the applicability of the above algorithm, more field studies in various agricultural fields are needed.If the applicability of the above algorithm was verified, the global annual cumulative N 2 O emissions from agricultural fields could be easily estimated just based on fertilization rates and rainfall in different regions.

Comparison with previous studies and assessing the total N 2 O emission in the NCP
The results of studies from maize-wheat fields in the NCP are shown in Table 4. irrigation at the agricultural field of this study (data not shown), and no remarkable difference of N 2 O emissions from the four agricultural soils was found in comparison with the uncertainty of the triplet treatments for the agricultural soil investigated in this study.Therefore, the very large temporal-spatial variations of the cumulative N 2 O emissions and EFs from the agricultural fields in the NCP might also be partially ascribed to the different weather conditions (especially rainfall as mentioned above) in different areas and years during the investigations.To some extent, the field simulation experiment confirmed that the results investigated at any agricultural fields in the NCP could be applied for estimating the annual cumulative N 2 O emission and the fertilizer-induced N 2 O emission from the agricultural field in the NCP.The estimation would be more representative based on the average value of many years' investigations, because the multi-years rainfall in one small region might partially reflect the uneven distribution of rainfall in different areas of the NCP.
The NCP has a cultivation area of 17.95 million ha (Liu et al., 2001) and the average fertilization rate is about 350 kg N ha −1 yr −1 (Table 4).Based on the four years' average    Full N 2 O emissions from soils are strongly affected by many factors, e.g.soil temperature and moisture, soil aeration status and Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | maize-wheat seasons, respectively, and the precipitation in the maize season accounted for 75 %, 62 %, 64 % and 79 % of the Introduction Discussion Paper | Discussion Paper | Discussion Paper | temporal variations of N 2 O fluxes from the CK and NP plots during the four years are illustrated in Fig. 2. N 2 O emissions from the CK plot were in the range of −37-70 ng N m −2 s −1 , and obvious emission pulses occasionally occur after irrigation and rainfall events.As for the NP plot, the relatively high N 2 O emissions (75-624 ng N m −2 s −1 ) usually occurred after fertilization, and the N 2 O emission was from −19 to 33 ng N m −2 s −1 during the periods of pre-and post-fertilizer application.Negative N 2 O fluxes (uptake, i.e. fluxes from the atmosphere to the soil) were occasionally observed in the CK and NP plots in this study, which accounted for 4-10 % of total investigation data in each maize-wheat season.The lowest detectable flux of the GC-ECD is 0.57 ng N m −2 s −1 in this study, and thus the extremely low N 2 O uptakes are probably caused by the fluctuations of instrument.Nevertheless, the larger N 2 O uptakes (−7 to −37 ng N m −2 s −1 in the CK plot; −7 to −19 ng N m −2 s −1 in the NP plot) can be ascribed to denitrification and nitrifier denitrification by reduction of N 2 O to N 2 (Chapuis-lardy et al., 2007).Yamulki et al. (1995) and Mahmood et al. (1998) also reported evident negative N 2 O fluxes from agricultural fields.As shown in Fig. 2, N 2 O emission peaks induced by fertilization usually occurred at the 1st-5th day after fertilization following irrigation in each growing season, while they delayed 1-2 days when rainfall events occurred just after fertilizations, e.g. on 21 August 2008, 5 July and 5 August 2009 (Fig. 2a and b).Generally, the N 2 O peaks only lasted for one day and then decreased quickly, while the high N 2 O emissions Discussion Paper | Discussion Paper | Discussion Paper | (about 550 ng N m −2 s −1 ) sustained 3 days after basal fertilization following showers (from 1 July to 2 July) in 2011 maize season (Fig. 2d).Therefore, precipitation coincided with the fertilization would probably promote N 2 O emission because of the substrate supplement and development of anaerobic soil condition.The N 2 O emission peaks from the NP plot were 294, 142, 503 and 558 ng N m −2 s −1 in 2008, 2009, 2010 and 2011 maize seasons, respectively, and were 75, 100, 147 and 624 ng N m −2 s −1 in 2008 Discussion Paper | Discussion Paper | Discussion Paper |

F
1 = 5.6 × 10 −10 X 3.7 1 , N = 4, R 2 = 0.754, P = 0.013.(Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | It is evident that there are very large temporal-spatial variations of the cumulative N 2 O emissions and EFs reported in the NCP.With only the exception of the data in 2009-2011 maize-wheat seasons, the cumulative N 2 O emissions from the NP plot in this study were 33-108 % greater than the upper limit value reported in the literatures.The EF value of 0.60 % in 2009-2010 was in good agreement with the values reported by Dong et al. (2000) and Ding et al. (2007), and of 1.1 % in 2010-2011 was in line with the values reported by Li et al. (2010), Sun et al. (2008) and Cai et al. (2013).The EFs from the NP plot in 2008-2009 and 2011-2012 were two times greater than the upper limit value reported in the NCP, but were still within the uncertainty range recommended by the IPCC (0.3-3 %, De Klein et al., 2006).To check the possible influence of the soils from different areas in the NCP on N 2 O emission, soil samples were collected from four sampling sites (Fengqiu, Luancheng, Yucheng and Beijing) where N 2 O emissions have been investigated.N 2 O emissions from the four fields were simultaneously measured under the same fertilization and Discussion Paper | Discussion Paper | Discussion Paper | cumulative N 2 O emission from the CK plot and EF obtained by this study, the annual total N 2 O emission and fertilizer-induced N 2 O emission from the agricultural fields in the NCP were estimated to be 144 Gg N and 110 Gg N, respectively.5 Conclusions Large interannual variations of N 2 O emissions were observed from the maize-wheat field in the NCP during the four successive years.Precipitation was primarily responsible for the temporal-spatial variation of N 2 O emission.The significant correlation between cumulative N 2 O emission and precipitation obtained in this study may be used to estimate N 2 O emission from the area where the rainfall and fertilization rate are similar with this study.Discussion Paper | Discussion Paper | Discussion Paper | of Sciences (No. XDB05010100), the Chinese National Natural Science Foundation (No. 41075094 and 21177140), and the National Basic Research and the Development Program 973 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Fig. 1.Precipitation, soil WFPS (a, b, c, d) and soil temperatures (e, f, g, h) in the CK and NP plots during 2008-2012 maize-wheat seasons.Dash arrows show irrigation events.

Table 1 .
The amount, date and type of nitrogen fertilizer application in the experiment field.

Table 4 .
Summary of N 2 O emissions from maize-wheat soils in the NCP.