Nitrous oxide emissions from irrigated and fertilized spring maize in semi-arid northern China

Agriculture, Ecosystems and Environment 141 (2011) 287–295

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Nitrous oxide emissions from irrigated and fertilized spring maize in semi-arid northern China Y.T. Liu a,b , Y.E. Li a,b,∗ , Y.F. Wan a,b , D.L. Chen c , Q.Z. Gao a,b , Y. Li d , X.B. Qin a,b a

Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China The Key Laboratory for Agro-Environment and Climate Change, Ministry of Agriculture, Beijing 100081, China Department of Resource Management and Geography, Melbourne School of Land and Environment, The University of Melbourne, Victoria 3010, Australia d Key Laboratory of Agro-ecological Processes in Subtropical Region, Institute of Subtropical Agriculture, The Chinese Academy of Sciences, Hunan 410125, China b c

a r t i c l e

i n f o

Article history: Received 15 December 2010 Received in revised form 1 March 2011 Accepted 1 March 2011 Available online 20 April 2011 Keywords: Automated measurement Nitrous oxide emissions Nitrous oxide uptake Emission factor Fertilizer rate

a b s t r a c t As maize requires a high input of fertilizer nitrogen, it is likely to be an important source of nitrous oxide (N2 O). Detailed information on N2 O emissions over long time periods, and management practices that aim to reduce N2 O emissions from spring maize fields in China is lacking. Consequently we measured the emissions of N2 O from a spring maize field continuously from 2007 to 2009 at Yuci, Shanxi Province, China using newly developed automated chambers and explored strategies to reduce N2 O emissions. The results showed that the Optimal fertilizer treatment (120 kg N ha−1 y−1 ) produced the same yield of grain as the Traditional fertilizer treatment (330 kg N ha−1 y−1 ), and significantly reduced N2 O emissions by 48%. Topdressing with urea was the main source of N2 O, which on average accounted for 58% of the total N2 O emissions each year. Uptake of N2 O occurred during the late stage of maize growth when soil mineral N content was less than 46.4 mg N kg−1 soil. The N2 O emission factors were lower than the IPCC default value. Nitrous oxide emissions could also be reduced if farmers did not apply fertilizer N during periods of heavy rainfall and did not irrigate immediately after fertilization. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Nitrous oxide (N2 O) contributes to rising atmospheric temperature with a global warming potential 296 times greater than CO2 for a 100-year time horizon and participates in the destruction of the stratospheric ozone layer (IPCC, 2000, 2006). The atmospheric N2 O concentration has increased from about 273 ppb in 1750 to 319 ppb in 2005 (IPCC, 2007). Approximately 57% of total global annual N2 O emissions emanate from soils, and 35% of these come from agricultural production (FAO/IFA, 2001). While N2 O is mainly produced in soil by the two microbially mediated processes, nitrification and denitrification (Granli and Bøckman, 1994) it may also be produced by other organisms. For example nitrate-respiring bacteria were found to produce N2 O by consuming NO3 − through a process that is apparently not restricted by soil NH4 + content or oxygen level (Bleakley and Tiedje, 1982). The activity of these microbial processes is strongly affected by environmental conditions such as soil temperature and moisture (Smith et al., 2003; Adviento-Borbe

∗ Corresponding author at: Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, China. Tel.: +86 10 82105615; fax: +86 10 82105615. E-mail address: [email protected] (Y.E. Li). 0167-8809/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2011.03.002

et al., 2007), but fertilizer management can also have an effect (Wagner-Riddle et al., 2007; Song and Zhang, 2009; Ma et al., 2010) as application of fertilizer nitrogen usually results in enhanced emission of nitrous oxide (Skiba et al., 1994). In China, the area sown to maize has increased from 19.6 million hectares in 1978 to 29.8 million hectares in 2008 (19.1% of the total cropped area; China Statistical Yearbook, 2009) and maize is the second most important cereal crop in China (FAO, 2010). Spring maize is mainly grown in Liaoning, Jilin, Heilongjiang, Inner Mongolia, Ningxia, Gansu, Shanxi, and Shaanxi provinces (Fig. 1). The sowing area of spring maize is almost 36% of the total maize area, and the yield accounts for 40% of the total maize yield (Xiao et al., 2010). In addition China’s fertilizer nitrogen (N) use has increased rapidly from 544 Gg N in 1961 to 32.4 Tg N in 2007. Since 1979 China has been the world’s biggest N fertilizer consumer, and since 2002 it has accounted for about 30% of the world’s N fertilizer use (FAO, 2010; Fig. 2). Maize requires more fertilizer N than other crops (IFA, 2009), but the N use efficiency for maize world wide is only ∼30% because much of the applied N is lost to the environment (Balasubramanian et al., 2004; Peoples et al., 2004). As large fertilizer N applications to croplands can result in very high N2 O emissions (Wagner-Riddle et al., 2007) maize may be an important source of atmospheric nitrous oxide. The use of a static chamber (Hutchinson and Mosier, 1981), and the traditional manual measurement frequency of one flux deter-

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Fig. 1. Area where spring maize is grown in China and location of the experiment at Yuci.

2. Materials and methods 2.1. Study site

Fig. 2. Nitrogen fertilizer used in China during the period 1961–2007.

mination of a single gas per 3–10 days may miss the peak emissions because fertilizer-induced N2 O emissions are often short-lived (Zheng et al., 2000). Low frequency measurements have produced annual estimates which differed widely from those based on continuous measurements (Liu et al., 2010). In order to save manpower and improve estimates of the contribution of terrestrial ecosystems to the global N2 O budget, automated chamber sampling systems have been developed (Conrad et al., 1983; Loftfield et al., 1997; Zheng et al., 1998; Breuer et al., 2000), modified (Zheng et al., 1998) and applied to rice (Bronson et al., 1997; Zheng et al., 1998; Seiichi et al., 2005), forests (Butterbach-Bahl et al., 1997, 1998; Breuer et al., 2000), potato (Flessa et al., 2002), wheat (Seiichi et al., 2005; Barton et al., 2008), cotton (Liu et al., 2010), summer maize (Wan et al., 2005) and canola (Barton et al., 2010). Although numerous studies have investigated N2 O emissions from agricultural soils cropped to maize, very few have used the automated chamber system to measure N2 O emissions from irrigated and N fertilized spring maize crops. The objectives of this study were: (i) to present a detailed description of a system for continuous automated monitoring of N2 O emissions, (ii) to compare the effects of fertilizer treatments on N2 O emissions, (iii) to identify the main environmental drivers of N2 O emissions from a spring maize field in northern China, and (iv) to devise feasible strategies to reduce N2 O emissions.

The study site was at Yuci in north China (37◦ 38 N, 112◦ 51 E, elevation 789 m) (Fig. 1). Field experiments were conducted during the maize growing periods from May to September from 2005 to 2009, and the measurement of N2 O fluxes from the two treatments commenced from 2007. Maize is the main crop in this area. Yuci is classified as a semi-arid climatic region with an average annual rainfall of 430 mm, which mainly falls during the summer months (June, July and August), and with average annual air temperature of 9.3 ◦ C (China Meteorological Data Sharing Service System, 2010). The soil at the site is classified as a meadow saline soil (National Soil Survey Office, 1998). The 0–20 cm topsoil has a bulk density of 1.33 g cm−3 , a pH of 8.4 (1 soil:2.5 water), and contains 19.9% clay, 16.9 g organic C kg−1 , 1.79 g N kg−1 , 1.58 g P2 O5 kg−1 , and 90.6 mg available K kg−1 . 2.2. Treatments The experiment was established to evaluate fertilizer-efficient management practices for achieving high maize yield and reduced N2 O emissions. A completely randomized design with three replicates was set up. Each plot had an area of 10 m × 6 m, separated by buffer zones 1 m wide, which were also planted with maize. There were two fertilizer treatments: (1) Traditional, with a basal dressing of nitrophosphate fertilizer (supplying 60 kg P2 O5 ha−1 and 145 kg N ha−1 , of which 75% is NH4 + -N), and a topdressing of urea (185 kg N ha−1 , 0.05 m deep in the row) at approximately 60 days after seeding, and (2) Optimal, recommended by the Soil and Fertilizer Research Institute of Shanxi Academy of Agricultural Sciences, with a basal dressing of superphosphate (100 kg P2 O5 ha−1 ) and potassium sulfate (100 kg K2 O ha−1 ) and a topdressing of urea (120 kg N ha−1 0.05 m deep in the row) on the same day as the Traditional treatment. The same land management practices were applied to both treatments (Table 1). Two flood irrigations were applied each year, one before tillage and one after top dressing, at the rate of 70 mm water for each event. Each year the basal fertiliz-

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Table 1 The agriculture practices and yields of the experimental treatments from 2007 to 2009. Year

Treatment

Basal fertilizer (kg ha−1 )

Top dressing (kg ha−1 )

Tillage date

Harvest date

Yield (t ha−1 )

1st

2nd

2007

T O

N185 Urea N120 Urea

25 Apr.

2008

T O

N185 Urea N120 Urea

2009

T O

N145 P60 NP P100 SSP K100 K2 SO4 N145 P60 NP P100 SSP K100 K2 SO4 N145 P60 NP P100 SSP K100 K2 SO4

13 Jul

2 May

17 Sep

11.9 ± 0.5 12.0 ± 0.4

23 Apr.

15 Jul

30 Apr.

19 Sep

11.8 ± 0.8 11.8 ± 0.6

N185 Urea N120 Urea

15 May

16 Jul

24 May

21 Sep

12.2 ± 0.5 12.3 ± 0.8

Irrigation date

T: Traditional treatment, O: Optimal treatment, NP: nitrophosphate, SSP: superphosphate; K2 SO4 : potassium sulfate; Nx Py Kz : N, P2 O5 , K2 O kg ha−1 ; Irrigation: 70 mm ha−1 every time; Tillage: 20 cm deep. Yield: the values shown are arithmetic means with standard errors.

ers for the two treatments were applied and the soil was tilled to a depth of 20 cm. About 2 h after tillage maize (Zea mays L., cv. Xianyu 335) was planted at the rate of 60,000 seeds ha−1 . The details of the treatments are summarized in Table 1. An area (0.5 m2 ) within the middle of each plot which included the plants, surrounding area and the fertilization line, 10 cm from the crop row, was designated for measuring N2 O emissions, while the remainder of the plot was used for soil and plant sampling.

2.3. Measurement of N2 O emissions Nitrous oxide emissions from the two treatments were measured from sowing to harvest in 2007–2009, using a six-chamber automated measuring system. This system (Fig. 3) was developed by the Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agriculture Science, and has two parts: Part A consists mainly of six translucent chambers, the covers of which could be opened and closed automatically by pneumatic devices and an air compressor installed in the field, and Part B includes the computer, gas chromatograph (GC5890, Agilent Technologies Inc., USA), sampling pump (DOA-P104-BN, GAST, USA), desiccant, flow rate indicator and four high-pressure cylinders. Fig. 4 shows a diagram of the chamber, which was 70 cm long × 70 cm wide × 140 cm high, and consisted an angular steel frame to which transparent polymethyl methacrylate plates were fixed. Rubber seals on the lid ensured gas-tightness when the chambers were closed. Every chamber was equipped with two temperature sensors (one to measure the air temperature inside the chamber, another to measure the soil temperature at 5 cm depth), one sensor to measure the soil volumetric water content at 10 cm depth and two fans to rapidly circulate the air inside the chamber when it was closed. Each chamber was also fitted with two pneumatic cylinders to automatically close and open the lid of the chamber, and a control box which included a five-way solenoid valve to control the opening and closing of the chamber, a three way valve to control the sampling of air from the chamber and a circuit board to control the action of the chamber and transfer the sensor signals to the computer. All six chambers were installed in a maize row and sealed by inserting their bases 10 cm into the soil. The air sampling procedure is described below. Each chamber was closed for 30 min, and the air inside was circulated by two 24 V DC fans, which stopped when the air sampling procedure was completed. The chambers remained open for 90 min between each measurement cycle to re-establish ambient conditions.The six chambers opened and closed every 2 h (12 times per day). In each air sampling procedure the chamber air sampling took place twice, once when the chambers had just closed, and again just before the chambers were opened. Before sampling the air, the three-

way valves on all six chambers were closed, and the Teflon tubes (about 30-m long, 2.17 mm of inner diameter), which connected the six three-way valves with the GC, were evacuated at a pressure of 150 Pa, then one of the six three-way valves was opened and air was pumped from one chamber through a desiccant trap and a flow meter to the GC at a flow rate of 1.5 L min−1 for 1 min to flush out the Teflon tubes to prevent contamination (Flessa et al., 2002). The pumping time is then extended 4 s to inject the air samples into the GC and a 3 ml gas sample was analyzed. Eventually, the pump stopped and the three-way valve was closed. The 3.2 L air samples taken during the two sampling processes in each measurement cycle accounted for only 0.5% of the chamber volume (637 L), and thus any effect of the flushing process on the pressure inside the chamber would have been very small. In each measurement cycle, the N2 O concentration of the air was automatically calibrated using a reference gas with a concentration of 0.32 ppmv N2 O in N2 (variance ±2%; Air Products and Chemicals, Inc., Beijing, China) as done by Barton et al. (2008). At the end of each day, the gas chromatograph was calibrated manually using three gases with concentrations of 0, 0.32 and 2.12 ppmv N2 O in N2 . The gas chromatograph was equipped with an electron capture detector (ECD), the columns were packed with Porapak Q, and the carrier gas was high-purity nitrogen (99.99%). The oven was operated at 55 ◦ C, and the ECD was operated at 330 ◦ C. The N2 O flux calculations were based on two N2 O concentration measurements and two inside air temperatures when the chamber closed and opened. The full details of the calculation are described as follows: f =

M × h × (C1 × 273 × P2 /(P0 × T2 ) − C1 × 273 × P1 /(P0 × T1 )) (t2 − t1 ) × 22.4 (1)

where f is the N2 O flux (mg m−2 h−1 ), h is the height of the chamber (m); M is the molecular weight of N2 O, t1 and t2 are the times at the beginning and end of sampling (h); C1 and C2 are the concentrations of N2 O (cm3 m−3 ), T1 and T2 are the absolute temperatures (K) and P1 and P2 are the actual atmospheric pressure inside the chamber (Pa) when the chamber is closing and opening; and P0 is the standard atmospheric pressure at 273 K (Pa). As only 0.5% of the chamber volume was sampled, we assumed that P2 = P1 which would equal P0 . We multiplied the arithmetic mean of the 36 measured hourly N2 O fluxes (12 measurements with three replicates each day) by 24 h to calculate the daily flux. In this automated system, the detection limit for N2 O was 4.5 × 10−9 kg N m−2 h−1 , and the precision was ±5.5 ppb for N2 O. When the daily measurement cycles were less than 8 due to power failure or other abnormal conditions of the system, and the daily fluxes less than the system detection limit (1.08 × 10−7 kg N m−2 d−1 ) during the monitoring period, a miss-

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Fig. 3. Diagram of the automated measurement system for N2 O emission.

ing value for such a day was assigned. There were 32 days with missing values, which were excluded from the calculation for the annual N2 O emission.

2.4. Auxiliary measurements The soil (at 5 cm depth) and air temperatures inside the chambers were measured by 18B20 temperature sensors, and the soil water at 10 cm depth was measured by an EC-5 sensor. Water-filled pore space (WFPS) was calculated from the volumetric water content, the determined bulk density of 1.33 g cm−3 and a theoretical particle density of 2.65 g cm−3 . All measurement data were saved to the computer when the chambers closed and opened automatically. At harvest, maize cobs were collected by hand from three random 4 m2 quadrants in each treatment. The grain was stripped manually from the cob, oven dried at 60 ◦ C for 1 week and weighed to determine the maize yield in kg ha−1 . Five samples of the topsoil (0–20 cm) from the two treatments were collected randomly every 10 days, before sowing and after harvest, and immediately after fertilization, irrigation, and signifi-

cant precipitation (>5 mm d−1 ). The sampling location was marked to avoid resampling at the same location. The soil (10.0 g) was extracted immediately after sampling with 100 mL of the 2 M KCl solution, filtered through a Whatman No 42 filter paper and the filtrates were frozen at −20 ◦ C, and 10 days later analyzed for ammonium (NH4 + ) and nitrate (NO3 − ) by a continuous-flow autoanalyzer (SKALAR, San Plus System, the Netherlands). The daily weather data were obtained from the China Meteorological Data Sharing Service System (http://cdc.cma.gov. cn/index.jsp).

2.5. Data analysis The Pearson correlation analysis was carried out to investigate the associations of daily N2 O fluxes with WFPS, soil temperature and mineral N content. The significance of difference between treatments was tested using Duncan’s Multiple Range Test. The Software R program (http://www.R-project.org) was used for the statistical analyses.

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291

Fig. 4. Chamber used for measuring nitrous oxide emission from soil.

3. Results 3.1. Climate and soil nitrogen The rainfall for 2007, 2008 and 2009 during the study period was 400, 222, and 479 mm, respectively (Fig. 5). In 2008 the rainfall was less than the long term average (355 mm). The WFPS (at 10 cm) varied from 15.9% (13–15 June 2008) to 90.6% (13 July 2007, 15 July 2008 and 17 July 2009), and the lowest soil water contents occurred in the year with the lowest precipitation in 2008 and the highest occurred every year after the second irrigation. During the maize growth period May to September in 2007–2009, the average air temperature was 21.9 ◦ C, which is about 1.3 ◦ C higher than the average for this period during 1951–2006. The soil temperature (5 cm) during the study ranged from14.0 to 28.5 ◦ C (Fig. 5). The NH4 + content of the topsoil (0–20 cm) from the Traditional and Optimal treatments ranged from 0.4 to 62 and 0.5–33 mg N kg−1 soil, respectively. Nitrate varied from 0.4 to 54 and 2.5–27 mg N kg−1 soil, respectively, for the two treatments (Fig. 5 and Table 2). The NH4 + and NO3 − contents in the Traditional treatment were significantly higher (p < 0.01) than those in

the Optimal treatment in 2008 and 2009. In 2009, the highest value for NH4 + , which occurred just after topdressing, was significantly greater than that in 2008 (p < 0.05; Table 2). This may have been caused by the different interval between topdressing and the second irrigation. The average value for NO3 − in 2008 was higher than that in 2009 (Fig. 5 and Table 2), and this may have been due to the low rainfall during the maize growth period in 2008 (Fig. 5), which restricted leaching and denitrification. 3.2. Nitrous oxide emission Emission of N2 O in the 3 years ranged from −5.9 to 221.6 g N ha−1 d−1 for the Traditional treatment and from −16.1 to 117 g N ha−1 d−1 for the Optimal treatment (Table 2). The cumulative emissions of N2 O from the Traditional treatment were calculated to be 1.46, 1.40 and 1.60 kg N ha−1 for 2007, 2008 and 2009, respectively, while those from the Optimal treatment were 0.78, 0.69 and 0.86 kg N ha−1 , respectively. During the 3 years N2 O emission from the Traditional treatment was significantly greater than that from the Optimal treatment (p < 0.01, Table 2). Most of the loss occurred after fertilization and irrigation, or after significant

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40 30 20

100 75

45

50

30

25

15

0 60 20

0.08

40

0.18

N2O Flux

0 60

N2O Flux 40

0.28

NH+4 − N NO−3 − N

0

20

0.08

0.18

Optimal treatment

−0.02

N2O flux (kg N ha−1 d−1)

Mineral N (mg N kg−1)

NH+4 − N NO−3 − N

Mineral N (mg N kg−1)

0.28

Traditional treatment

−0.01

N2O flux (kg N ha−1 d−1)

0

Precipitation (mm)

WFPS Precipitation

Water−filled pore space, WFPS

10

Temperature (oC)

Soil temperature(5cm) Air temperature

1−5−07

7−6−07

23−8−07

13−6−08

28−8−08

14−7−09

21−9−09

Date (Day−Month−Year) Fig. 5. Dynamics of N2 O fluxes under Traditional and Optimal treatments, and climate and environmental parameters. Solid and dashed arrows indicate the dates of base fertilizer and topdressing respectively.

rainfall (Fig. 5). The high emission period after topdressing and irrigation (15 days after the second irrigation each year) contributed, on average, 58% of the total N2 O emission each year. Negative values for N2 O emission were observed each year during the late stages of maize growth when the 0–20 cm soil mineral N content was less than 464 mg kg−1 soil. Negative fluxes of N2 O were apparent in both treatments (Fig. 5); in the Traditional treatment negative fluxes occurred for 48 days and ranged from

−5.9 to −0.03 g N ha−1 d−1 , whereas in the Optimal treatment they occurred on 70 days, and ranged from −16.1 to −0.15 g N ha−1 d−1 . When all data for the two treatments over 3 years were taken into account, the N2 O emission was weakly correlated to soil temperature (r = 0.11, p < 0.05, n = 736), WFPS (r = 0.34, p < 0.001, n = 736), soil NH4 + (r = 0.47, p < 0.01, n = 74) and precipitation plus irrigation (r = 0.40, p < 0.001, n = 255). Nitrous oxide emission was not correlated to soil NO3 − in this study (Table 3).

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Table 2 Soil mineral N content and N2 O flux for the two treatments (T and O) during 2007–2009. Year

Variable NO3 − (mg N kg−1 )

NH4 + (mg N kg−1 )

2007

2008

2009

Max Min Average Max Min Average Max Min Average

N2 O (g N ha−1 d−1 )

T

O

T

O

T

O

ND ND ND 54 ± 9.7 0.4 ± 0.1 15.2 ± 2.3 62 ± 10.5 0.5 ± 0.1 15.1 ± 2.5

ND ND ND 30 ± 6.9 0.45 ± 0.1 6.5 ± 1.1 33 ± 7.6 0.5 ± 0.1 6.2 ± 1.4

ND ND ND 54 ± 9.7 20 ± 5.1 39.7 ± 6.2 50 ± 5.8 3 ± 0.8 29.0 ± 6.5

ND ND ND 27 ± 5.7 5 ± 1.2 16.2 ± 3.9 25 ± 5.8 2.5 ± 0.5 15.5 ± 3.0

191.3 ± 10.0 −4.3 ± 0.5 11.1 ± 5.9A 178.4 ± 27.6 −5.9 ± 0.3 11.0 ± 5.9A 221.6 ± 63.5 −5.8 ± 0.6 14.5 ± 8.8A

106.3 ± 9.0 −5.7 ± 0.1 5.9 ± 1.2B 98.7 ± 17.6 −9.6 ± 0.2 5.6 ± 1.9B 117.0 ± 21.9 −16.1 ± 0.3 7.7 ± 2.5B

T: Traditional; O: Optimal; Values in a row followed by a different letter differ significantly at 0.01 level (Duncan’s Multiple Range Test); ND: No data; Values are means ± standard error. Table 3 Correlation coefficients (r) for the relationships between N2 O emission and environmental and soil variables during 2007–2009. Year

2007 2008 2009 All

T

NO3 −

NH4 +

WFPS

n

r

n

r

n

r

n

r

260 252 224 736

0.056 0.066 0.126 0.11*

260 252 224 736

0.249** 0.568** 0.283** 0.336**

40 34 74

0.576** 0.389* 0.470**

40 34 74

0.102 0.313 0.036

T: Soil temperature at −5 cm, WFPS in 0–10 cm soil layer (%); NH4 + : 0–20 cm (mg N kg−1 soil); NO3 − : 0–20 cm (mg N kg−1 soil); n: number of observations. * Significant at p < 0.05. ** Significant at p < 0.01.

4. Discussion 4.1. Nitrous oxide emission Measurements of N2 O emission from summer maize in northern China were carried out only by either the closed static chamber method (Huang et al., 1998; Ye et al., 2005; Wang et al., 2008; Sun et al., 2008; Li et al., 2010) or the automated method (Wan et al., 2005; Gao et al., 2005; Ju et al., 2011). No other studies use the automated method to continuously measure N2 O emissions from the spring maize. Thus, our study is the first attempt to continuously measure N2 O emissions from irrigated and fertilized spring maize field in the semi-arid northern China using automated chambers. The emission factor derived from the present study would be very helpful to quantify the contribution of spring maize to the national inventory of N2 O emission from agricultural soils in China. Nitrous oxide emission was significantly smaller by 48% when 120 kg N ha−1 was applied in the Optimal treatment (on average 0.78 kg N2 O-N ha−1 y−1 ) than when 330 kg N ha−1 was applied in the Traditional treatment (on average 1.49 kg N2 O-N ha−1 y−1 ). It was expected that the main driver for differences in emission between treatments would be the availability of NH4 + and NO3 − in the soil as substrates for nitrification and denitrification, but this was not reflected in the correlation analysis. The Pearson correlation analysis showed that the relationship between N2 O emissions and soil NH4 + content was weak. There was no correlation between N2 O emissions and soil NO3 − content (Table 3). The reduction in N2 O emission in the Optimal treatment was achieved without sacrificing spring maize yield. The results showed that the Optimal fertilization treatment, which involved the use of P and K as base fertilizers and the application of N according to the needs of the crop at the time of maximum uptake, produced the same yield of grain as the Traditional fertilization treatment over 3 years in the study (Table 1). This decrease in fertilizer-induced N2 O emission is in agreement with the results of previous studies (McSwiney and Robertson, 2005; Halvorson et al., 2008; Ma

et al., 2010). The spring maize mainly distributes in the northern China (Fig. 1), with similar semi-arid climatic conditions and management practices in Shanxi, Inner Mongolia, Ningxia, Gansu, and Shaanxi provinces (Xiao et al., 2010), thus the Optimal method could be applied to the most spring maize growing regions in the northern China. The IPCC (2006) advocates the use of an emission factor (1% of applied N) to estimate the amount of N2 O directly emitted from fertilizer applications for inventory purposes. The current study in a spring maize crop in a semi-arid area indicated that fertilizerinduced emission in 2009 accounted for only 0.42% and 0.72%, respectively, of the fertilizer N applied in the Traditional and Optimal treatments. This was considerably less than the IPCC (2006) recommended factor. The low emission factors found in this study may be due to the aridity of the region. Environment Canada (2007) used an aridity index for their calculation of Canada’s greenhouse gas inventory because of the finding that 0.16 and 0.8% of the fertilizer N was emitted as N2 O from two arid regions compared with 1.18% from wetter regions. Barton et al. (2008) also found a very low emission factor (0.1% of applied N) for a semi-arid soil in Western Australia. Reducing the amount of fertilizer N applied in the Optimal treatment reduced the total amount of N2 O emitted, but the proportion of fertilizer N lost as N2 O was greater than that in the Traditional treatment. As the yield of grain did not increase when more than 120 kg N ha−1 was applied, it is assumed that crop demands had been met at that level of application and that the extra N left in the soil of the Traditional treatment (Table 2) was available for metabolism by nitrifying and denitrifying organisms and that more N2 O would be emitted. The smaller proportion of fertilizer N emitted as N2 O in the Traditional treatment suggests that other organisms, such as immobilizers, were competing with the nitrifying and denitrifying organisms for the excess N. This suggestion is supported by the finding that, apart from the period immediately after fertilizer application, little NH4 + (the preferred substrate for immobilizing organisms) remained in the soil (Fig. 5). These results

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support the suggestion of McSwiney and Robertson (2005) that a recommended emission factor may only be appropriate when N is applied at a rate which is less than that required for maximum yield. The fertilizer-induced N2 O emissions were short lived, and the highest emissions occurred during the 15-day period just after topdressing with urea and the second irrigation, when soil NH4 + content was high (Fig. 5). This result is in line with other studies (e.g. Conrad et al., 1983; Parkin and Kaspar, 2006; Wagner-Riddle et al., 2007) which showed dramatic increases in N2 O evolution rates (up to 240 g N ha−1 d−1 ) after application of fertilizer N. We also found that the time interval between topdressing and irrigation or between fertilization and rainfall induced different patterns of N2 O emission, indicating the fertilizer should not be applied during periods of heavy rainfall and that irrigation should not follow immediately after fertilization (see also Smith et al., 1998; Barton et al., 2008). These results suggest that management options to reduce N2 O emissions from spring maize in northern China should focus on optimizing the timing, amount and method of N fertilization. 4.2. N2 O emission and environmental variables It is well known that emission of N2 O is controlled by soil temperature, available organic carbon and soil water content in addition to NH4 + and NO3 − content (e.g. Conrad et al., 1983; Davidson and Swank, 1986; Wagner-Riddle et al., 2007; Liu et al., 2010). Soil moisture is one of the key environmental factors which drives N2 O emission due to its effect on the production and transport of substrate and N2 O (Zheng et al., 2004; McSwiney and Robertson, 2005; Song and Zhang, 2009). In our study N2 O emission was only weakly correlated with WFPS (Table 3). We found that rainfall, even <10 mm, enhanced N2 O emission especially just after topdressing with urea. However, heavy rainfall did not always induce high N2 O emission, especially in the last month of each observation period when soil mineral N content was low, which is in agreement with the results of Wagner-Riddle et al. (2007), Barton et al. (2008) and Ma et al. (2010). Overall, the simple linear regression between N2 O emission and WFPS explained only 11% of the variance in the Traditional and Optimal treatments and there was no relationship between N2 O emission and soil temperature (Table 3). This is not unexpected as N2 O production is carried out by microorganisms, therefore the production of N2 O is governed by factors which control the growth and metabolism of microorganisms, viz. temperature, availability of water and N substrate (Breuer et al., 2000; Seiichi et al., 2005; Barton et al., 2008; Ma et al., 2010). 4.3. Nitrous oxide uptake On a number of occasions during the 3-year study emissions were negative which suggested that N2 O was taken up by the soil. Negative fluxes have been reported by others including Ryden (1981), Henault et al. (1998), Verchot et al. (1999), Glatzel and Stahr (2001), and Barton et al. (2008), and while we can’t unequivocally rule out measurement errors we concluded that these negative values were real. It is generally assumed that ‘traditional’ denitrification is responsible for N2 O uptake (Bremner, 1997), but other N2 O reducing processes such as nitrifier denitrification and aerobic denitrification may be involved (Poth, 1986; Schmidt et al., 2004; Chapuis-Lardy et al., 2007). The rate of N2 O uptake by soil seems to be controlled by a wide range of soil properties including mineral N, water content, temperature, oxygen, pH, redox potential, and the availability of labile organic C (Chapuis-Lardy et al., 2007). Our observations showed that the negative fluxes mainly occurred late in the measurement period of each year (Fig. 5), when the

0–20 cm top soil mineral N content was lower than 46.4 mg N kg−1 soil, which was higher than the level of 16–20 mg N kg−1 observed by Mahmood et al. (1998) and Ma et al. (2010), but confirms the conclusion of Chapuis-Lardy et al. (2007) that N2 O uptake may occur on occasions when the soil mineral N content was not low. 5. Conclusions The automated system of measuring N2 O flux from the spring maize field developed in this study not only saved manpower, but also captured key events such as high emissions after fertilization, irrigation and rainfall and the uptake of N2 O by the soil. The Optimal fertilizer treatment produced the same yield of grain as the Traditional fertilizer treatment, and but significantly reduced N2 O emissions by 48% on average. The WFPS and soil NH4 + content were identified as the major environmental factors controlling N2 O emissions from the soil every year. No application of N fertilizers during periods of heavy rainfalls and no irrigation immediately after fertilization were devised as the feasible strategies to reduce N2 O emissions from spring maize in north China. The N2 O emission factor for the spring maize cropping derived from the present study was lower than that recommended by IPCC (2006). N2 O uptake occurred mainly during the late stage of spring maize growth when the soil mineral N content was less than 46.4 mg kg−1 soil, and its undergoing mechanism needs further investigations. Our collective results indicated that reducing N2 O emissions from agricultural lands, at least the spring maize field, is technically achievable, but the implementation of such technologies is regarded very hard in some places, such as in China, where the food security is the first priority of the governmental administration. Acknowledgements We gratefully acknowledge financial support from the National Basic Research Program of China (2010CB951302); the National Key Project of Scientific and Technical Supporting Program of China (2008BAD95B13); and the Australian Centre for International Agricultural Research (LWR/2003/039). References Adviento-Borbe, M.A.A., Haddix, M.L., Binder, D.L., Walters, D.T., Dobermann, A., 2007. Soil greenhouse gas flues and global warming potential in four highyielding maize systems. Global Change Biol. 13, 1972–1988. Balasubramanian, V., Alves, B., Aulakh, M., Bekunda, M., Cai, Z.C., Drinkwater L., Mugendi, D., van Kessel, C., Oenema, O., 2004. Crop, environmental and management factors affecting nitrogen use efficiency. In: Mosier, A.R., Syers, J.K., Freney, J.R., Agriculture and the Nitrogen Cycle: Accessing the Impacts of Fertilizer Use on Food Production and the Environment. SCOPE (Series 65), Washington, DC, USA, pp. 19–33. Barton, L., Kiese, R., Gatter, D., Butterbach-Bahl, K., Buck, R., Hinz, C., Murphy, D.V., 2008. Nitrous oxide emissions from a cropped soil in a semi-arid climate. Global Change Biol. 14, 177–192. Barton, L., Murphy, D.V., Kiese, R., Butterbach-Bahl, K., 2010. Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate. Global Change Biol. Bioenergy 2, 1–15. Bleakley, B.H., Tiedje, J.M., 1982. Nitrous oxide production by organisms other than nitrifiers or denitrifiers. Appl. Environ. Microbiol. 44, 1342–1348. Bremner, J.M., 1997. Sources of nitrous oxide in soils. Nutr. Cycl. Agroecosys. 49, 7–16. Breuer, L., Papen, H., Butterbach-Bahl, K., 2000. N2 O emission from tropical forest soils of Australia. J. Geophys. Res. 105, 26353–26367. Bronson, K.F., Singh, U., Neue, H.U., Abao, E.B., 1997. Automated chamber measurements of methane and nitrous oxide flux in a flooded rice soil. II. Fallow period emissions. Soil Sci. Soc. Am. J. 61, 988–993. Butterbach-Bahl, K., Gasche, R., Breuer, L., Papen, H., 1997. Fluxes of NO and N2 O from temperate forest soils: impact of forest type. N deposition and of liming on the NO and N2 O emission. Nutr. Cycl. Agroecosys. 48, 79–90. Butterbach-Bahl, K., Gasche, R., Huber, C., Kreutzer, K., Papen, H., 1998. Impact of Ninput by wet deposition on N-trace gas fluxes and CH4 -oxidation in spruce forest ecosystems of the temperate zone in Europe. Atmos. Environ. 32, 559–564. Chapuis-Lardy, L., Wrage, N., Metay, A., Chotte, J.-L., Bernoux, M., 2007. Soils, a sink for N2 O? A review. Global Change Biol. 13, 1–17.

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