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Atmospheric Environment 72 (2013) 70e76

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Atmospheric Environment

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Measurements of N2O emissions from different vegetable fields on theNorth China Plain

Tiantian Diao a, Liyong Xie b,1, Liping Guo a,*, Hongliang Yan a, Miao Lin a,b, He Zhang a,b,Jia Lin c, Erda Lin a,2

aKey Lab for Agro-Environment, Ministry of Agriculture, Institute of Environment and Sustainable Development in Agriculture,Chinese Academy of Agricultural Sciences, Beijing 100081, ChinabCollege of agronomy, Shenyang Agricultural University, Shenyang, Liaoning 110866, ChinacCollege of Agronomy, Yangtze University, Hubei 434025, China

h i g h l i g h t s

< N2O emitted from vegetable fields and the typical cereal fields are compared.< Characteristics of N2O emissions from vegetable soils are studied.< Manure nitrogen shows lower emission factor than chemical nitrogen.< Importance of N2O from vegetable fields to the national N2O inventories is discussed.

a r t i c l e i n f o

Article history:Received 24 August 2012Received in revised form17 February 2013Accepted 22 February 2013

Keywords:Greenhouse gasN2OVegetable soilEmission factorGreenhouse gas inventory

* Corresponding author.E-mail addresses: Guolp@ami.ac.cn, lpg_mail@yah

1 The co-first author.2 The co-corresponding author.

1352-2310/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.atmosenv.2013.02.040

a b s t r a c t

Few studies have measured the N2O emission fluxes from vegetable fields. In order to identify thecharacteristics and the influencing factors of N2O emissions from different vegetable fields, we measuredN2O emissions for a full year from four typical fields, including an open-ground vegetable field that hasproduced vegetables for over 20 years (OV20), a recently developed open-ground vegetable field thatwas converted from a maize field three years earlier (OV3), a recently developed greenhouse vegetablefield that was converted from a maize field 3 years earlier (GV3) and a typical local maize field (Maize).Four different fertilization treatments were set additionally in the recently developed open-groundvegetable field. These were: no fertilizer or manure (OV3_CK), manure only (OV3_M) and the combi-nation of manure with different rates of chemical fertilizer application (OV3_MF1 and OV3_MF3). Theresults showed that N2O emission fluxes fluctuated between 0.3 � 0.1 and 912.4 � 80.0 mg N2OeNm�2 h�1 with the highest emission peak occurring after fertilization followed by irrigation. Nitrogen applica-tion explained 64.6e84.5% of the N2O emission in the vegetable fields. The magnitude of the emissionpeaks depended on the nitrogen application rate and the duration of the emission peaks was mainlyassociated with soil temperature when appropriate irrigation was given after fertilization. The N2Oemission peaks occurred later and lasted for a longer period when the soil temperature was <24 �C inMay. However, emission peaks occurred earlier and lasted for a shorter period when the soil temperaturewas around 25e33 �C from June to August. The annual N2O emissions from the fertilized vegetable fieldswere 1.68e2.38 times higher than that from the maize field, which had an emission value of2.88 � 0.10 kg N ha�1 a�1. The N2O emission factor (EF) of manure nitrogen was 0.07% over the wholeyear, but was 0.11% and 0.02% in the spring cucumber season and the autumn cabbage season, respec-tively. The EF of chemical nitrogen was 1.10e1.78% for the vegetable field over the whole year and it washigher in recently developed vegetable field than in the established field (1.54e1.78% vs. 1.10%). Vege-table fields may contribute greatly to the national greenhouse gas inventories due to the high fertilizerapplication rates, frequent irrigation and an increased number of tillageeplanting cycles.

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T. Diao et al. / Atmospheric Environment 72 (2013) 70e76 71

1. Introduction different fields were selected and one of the fields had fourdifferent treatments:

Table 1

Nitrous oxide (N2O) is one of the most important greenhousegases. It has a 298 times higher global warming potential (GWP)than CO2 (IPCC, 2007) and is produced mainly by anthropogenicactivities. N2O produced by agricultural activities accounted for 92%of China’s total N2O emissions in 1994 and the direct and indirectemissions from agricultural fields contributed 94.4% to the totalagriculture related emissions (PRC, 2004). The harvest area ofvegetable cropswas 18 414.31 kha in 2010 in China and it accountedfor 11.6% of the total cropped area according to the data of ChinaStatistical Bureau (http://www.stats.gov.cn). The nitrogen (N) fer-tilizer rates in vegetable fields and in greenhouse vegetable fields inthe rapidly developing area are 3e4 times and 4e5 times higherthan that in cereal fields in China, respectively (Chen et al., 2004; Juet al., 2006; Li et al., 2010; Qiao et al., 2009; Yu et al., 2010; Gaoet al., 2011). In the 2006 IPCC Guidelines for National GreenhouseGas Inventories, soil was just classified into two types: upland soiland paddy soil, with regards to the N2O default soil emission factor(EF) for cropland soil. However, the characteristics and EF of N2Oemitted from vegetable soils are different with that from cerealfields because of the higher N fertilization rate, frequent irrigationand more frequent tillageeplanting cycles in vegetable fields.Therefore, measurements of N2O fluxes and specific EFs that areappropriate to vegetable soils are needed for an accurate estimationof the national N2O emission inventory.

N2Oemitted fromsoil is primarily produced throughnitrificationand de-nitrification processes (Bremner, 1997; Stevens et al., 1997;Wrage et al., 2001). However, practices adopted in vegetable pro-duction, such as higher application rates for N fertilizer, frequentirrigation and multiple planting-harvest cycles, can stimulate orpromote the emission of N2O (Rudaz et al., 1991; Dobbie et al., 1999;Maggiotto et al., 2000; Guo et al., 2010). Many researchers havereported that N application rate for individual vegetable crops ishigher than 600 kg N ha�1 in rapidly developing regions such as inBeijing (Li et al., 2007), Shandong (Tang et al., 2004; He et al., 2009),Jiangsu (Xiong et al., 2006; Min et al., 2012), and Shanghai (Yu et al.,2001). However, N recovery was only 8e12% in the intensivelycultivated vegetable fields (Liang, 2011) compared with 28.2% and26.1% for wheat and maize, respectively (Zhu, 1998; Zhang et al.,2008). It means the loss rates of N including the loss by the formof N2O are potentially higher in vegetable fields.

Although Zheng et al. (2004) and Wang et al. (2011) classifiedvegetable fields as a specific ecosystem type to estimate the na-tional N2O greenhouse gas emission inventory, there is a lack ofavailable EF values for different types of vegetable fields. It isnecessary to make detailed measurements of N2O emissions underdifferent N rates from different kinds of vegetable fields (open-ground or protected fields) in various climate zones. The EF ofmanure N may differ from that of chemical N, because the avail-ability and recovery efficiency of different type’s N are varied.

The purpose of this study was to investigate the characteristicsand emission factors of N2O emissions from open-ground vegetablefields with varied planting histories and a greenhouse vegetablefield. And these were then compared with a typical maize field insuburban Beijing, North China. Our study also tried to identify theEFs of chemical and manure N applied to vegetable fields.

Soil properties of the experimental fields.

Field Clay(<0.002mm)%

Soil organicmatter(g kg�1)

Totalnitrogen(g kg�1)

Olsenphosphorus(mg kg�1)

Availablepotassium(mg kg�1)

pH

Maize 23.6 16.9 0.71 11.3 103.2 8.20OV3 21.6 19.53 1.24 26.8 109.3 7.80GV3 23.6 19.12 1.17 21.1 128.8 7.78OV20 20.0 24.52 1.41 28.0 140.0 8.00

2. Material and methods

2.1. Treatments and soils

This study was conducted at Shanzhuang village in the northernsuburban area of Beijing, located on the North China Plain. Four

a) A typical spring maize field under local fertilization and man-agement practices (maize);

b) A typical open-ground vegetable field converted from a maizefield three years earlier (OV3) with four different fertilizationtreatments:

b1) No fertilization control (OV3_CK);b2) Manure amendment only (OV3_M);b3) Combination of manure and chemical fertilizers (OV3_MF2)

andb4) Combination of manure and 70% chemical N of treatment b3

(OV3_MF1)c) A typical open-ground vegetable field converted from a maize

field twenty years earlier (OV20) andd) A sunlight greenhouse vegetable field converted from a maize

field three years earlier (GV3).

The plot areas were 8.5 m2 for the OV3, GV3 and OV20 fields and1000 m2 for the maize field. The four fields were adjacent and notfurther than 2000 m from each other. Four different treatments inthe OV3 field were assigned using a randomized block design withthree replicates. The soils were sampled before the study began inMarch 2011. The physical and chemical properties of the study soilsfrom different fields are listed in Table 1.

2.2. Field management

Over the past three years, the OV3 field has been planted with a“spring eggplanteautumn cabbage” pattern and then left fallowfrom late November to the next April. The GV3 field has usuallybeen planted with different seasonal vegetable crops in the year-round without any fallow period. The GV3 field was a 10 m wideand 50 m long sunlight greenhouse oriented in an eastewest di-rection. The greenhouse was covered with polyethylene plastic filmalong the southern face, and the other three sides were brick walls.The plastic film was present all year round and a 20 cmwide slit atapproximately 1m from the groundwas created in order to provideventilation in the summer (from late June to early September), butthis silt was covered with overlapped film in the winter. Thegreenhouse was covered at night with a mat between Novemberand the next March. At the spring cucumber and autumn cabbagegrowing periods, the greenhouse was irrigated on a schedulesimilar to that of the OV3 field (i.e. once per week after trans-planting spring cucumber, once every three days after cucumberharvesting began and once per month after planting autumn cab-bage). In winter and early spring, the greenhouse was irrigatedevery 25 days. The irrigation rate was usually 50 mm each time.

The spring cucumber was transplanted on May 10, 2011 andharvesting finished on August 06, 2011 (89 d in total) with theseedling space of 70*30 cm. They were irrigated once a week aftertransplantation and then irrigated every three days until harvest-ing, with an irrigation rate of 50 mm each time. Autumn cabbagewas sown on August 09, 2011 and harvested on November 11, 2011

T. Diao et al. / Atmospheric Environment 72 (2013) 70e7672

(95 d in total), with the seedling space of 70*40 cm. Autumn cab-bage seedlings were irrigated four times on August 08, September05, September 23 and October 25, with an irrigation rate of 50 mmeach time respectively. Non-cavolo cabbage was sown in thegreenhouse, whereas cavolo cabbagewas sown in the open-groundfield. This was because the higher temperatures in the greenhousemay impact the head-forming process of cavolo-cabbages. In GV3,after the cabbages were harvested, garlic was sown on December09, 2011 and harvested in the following June. Spring maize wassown on May 12 and harvested on October 6 (148 d in total)without additional irrigation.

All the vegetable fields were tilled before each vegetable cropwas sown or transplanted and all were treated with a basal appli-cation of fertilizer or commercial chicken manure. The types, ratesand dates of each fertilization for all the treatments are listed inTable 2.

2.3. Sampling and measurements

Gas samples were collected using the closed-chamber method.The sampling chambers were made of opaque PVC material andwere 0.25 m in diameter and 0.30 m in height. The bottom frame ofeach static chamber was inserted into the soil to a depth of 10 cm.Water was poured into the bottom grooves of the chambers tomake the covers gas tight. Sampling chambers were put betweenthe seedlings at each plot and one chamber for each plot. Only thesoil other than seedlings was covered by the chambers at eachsampling time. Gas samples were collected for a full year fromMarch 04, 2011, to March 03, 2012. They were collected between9:00 am and 10:00 am every seven days, but the sampling fre-quency increased after the application of fertilizer or manure (twosampling dates were missed in late July because of continuousrain). The air samples in the chambers were mixed by moving thesyringe pistons in and out and then about 250 ml of gas wasinjected into the aluminium bags (Bought from GuangmingResearch & Design Institute of Chemical Industry, Dalian, China)using a three-way valve linked to the top of the syringe. The airsamples in the aluminium bags were carried to the laboratory tomeasure the concentration of N2O using a gas chromatograph (GC)(7890A GC System, Agilent Technologies, USA) as soon as possible(usually within two days). As our preliminary experiments andother studies found that the concentration of N2O increased line-arly within 25 min (Wan et al., 2005), we controlled the coveringtime of closed chambers to between 20 min and 25 min. The airtemperature inside each sampling chamber and the soil tempera-ture at 5 cm depth were measured using thermally sensitive probesat each sampling date. Soil water content was also measured using

Table 2Types, rates and dates of N input for the different treatments (kg N ha�1).

Treatment Spring cucumber

Basal (May 05, 2011) Topdressing 1 (Jun 21, 2011) Topdressing

Type Rate Type Rate Type

Maize ComF 225.0 e e e

OV3_M M 636.0 e e e

OV3_MF1 M 636.0 ComF 47.3 ComFDAP 81.0

OV3_MF2 M 636.0 ComF 67.5 ComFDAP 81.0

GV3 M 636.0 ComF 47.3 ComFDAP 81.0

OV20 M 636.0 ComF 47.3 ComFDAP 81.0

Notes: Abbreviation: M e Chicken Manure, ComF e Complex synthetic fertilizer containa Topdressing of autumn cabbage was done using liquid calcium and potassium fertili

a portable water detector (Mode TZS-1K, Zhejiang Top InstrumentCorporation Ltd., China) by the Frequency Domain Reflectometer(FDR) method at 5 cm depth. Measured soil water content (v/v) wasconverted to gravimetric content, which was calibrated under aseries of water content gradients using the experimental soil inadvance. Soils were also sampled each month to measure themineral N and gravimetric moisture content (data not shown).

The N2O concentration was measured using a HP7890A GCequipped with an electron capture detector. The detailed GC con-dition and calibration for measuring the N2O were described by Xieand Li (2005) and Liu et al. (2011). N2 was used as the carrier gasand 10% CO2 was used as the make-up gas to decrease the bias andenhance the accuracy of the N2O measurements, as described byWang et al. (2010). The flux of N2O emissions were calculated usingthe equations described by Xie and Li (2005), Wan et al. (2005) andLiu et al. (2011). The cumulative N2O emissions were calculatedusing the individual fluxes and the time duration between the twoadjacent sampling periods. EF was calculated as the percentage ofcumulative N2OeN emissions compared with the applied N duringa given period (either a specific plant growing period or over awhole year).

2.4. Data analysis

Data analyses, such as one-way ANOVA and shortest significantranges (SSR) at the 5% level were conducted using SPSS 13.0 soft-ware (SPSS Inc., USA).

3. Results and analyses

3.1. The seasonal dynamics and impact factors affecting N2Oemissions from different fields

The variation of N2O emission fluxes over a whole year (March04, 2011eMarch 03, 2012) is shown in Fig. 1a and b. N2O emissionfluxes ranged between 0.3 � 0.1 and 912.4 � 80.0 mg N2OeN m�2 h�1. The highest emission flux, 912.4 � 80.0 mg N2OeN m�2 h�1, was observed on the OV3_MF2 treatment after thebasal application followed by irrigation in the spring cucumberseason. The N2O emission peaks caused by the following two top-dressings in the spring cucumber season and by the basal appli-cation in the autumn cabbage season were only 32.5%, 39.4%, and43.8%, respectively, of the peak that followed the basal applicationin the OV3_MF2 treatment, although the temperature was higherduring those periods than it was in May. This was mostly becausethat the chemical N rates for the topdressings at all the season andthe following basal application for autumn cabbage were only

Autumn cabbagea Winter garlic Total N

2 (Jul 07, 2011) Basal (Aug 08, 2011) Basal (Dec 09, 2011)

Rate Type Rate Type Rate

e e e e e 225.0e M 636.0 e e 1272.047.3 M 636.0 1494.9

ComF 47.367.5 M 636.0 1555.5

ComF 67.547.3 M 636.0 M 636.0 2130.9

ComF 47.347.3 M 636.0 1494.9

ComF 47.3

ing 15% N, 15% P2O5 and 15% K2O, DAP e DiAmmonium Phosphorus.zer and was a complementary application for all the treatments.

1000M i

800

900

-1)

Maize

OV3_MF1

GV3

500

600

700

ug N

2O-N

m-2

h- GV3

OV20

300

400

500

N2O

Flu

x (u

100

200

0

4-M

ar

25-M

ar

15-A

pr

6-M

ay

27-M

ay

17-J

un

8-Ju

l

29-J

ul

19-A

ug

9-Se

p

30-S

ep

21-O

ct

11-N

ov

2-D

ec

23-D

ec

13-J

an

3-Fe

b

24-F

eb

DateDate

1000

800

900

1000OV3__CK

OV3_M

OV3 MF1

600

700

2O-N

m-2

h-1)

_

OV3_MF2

300

400

500

N2O

Flu

x (u

g N

2

100

200

300N

0

4-M

ar

25-M

ar

15-A

pr

6-M

ay

27-M

ay

17-J

un

8-Ju

l

29-J

ul

19-A

ug

9-Se

p

30-S

ep

21-O

ct

11-N

ov

2-D

ec

23-D

ec

13-J

an

3-Fe

b

24-F

eb

Date

a

b

Fig. 1. a) N2O fluxes from the different fields between Mar 04, 2011 and Mar 03, 2012 in the northern suburb of Beijing, China. b) N2O fluxes for different fertilizer applicationtreatments from the open-ground vegetable field between Mar 04, 2011 and Mar 03, 2012 in the northern suburb of Beijing, China. Arrows indicate the specific dates ofN applications.

T. Diao et al. / Atmospheric Environment 72 (2013) 70e76 73

58.4e83.3% of that for the basal application of spring cucumber. Itimplied that higher N2O emissions were mainly caused by higherrates of applied N under appropriate temperature and water re-gimes in the growing period of vegetable crops for open-groundfields.

The vegetable fields were always irrigated timely in our study,so soil moisture was not a limitation factor here. The measured soilmoisture values were between 30% and 70% water-filled pore space(WFPS) and mostly fell between 50% and 70% WFPS in the wholeyear (data shot shown here). Therefore, N2O emissions were likelyto be associated with fertilization and soil temperature under theseappropriate soil moisture regimes in this study. The magnitude ofthe N2O emission peaks depended on the N application rate,whereas the occurrence time and duration of emission peaks wereassociated with soil temperature (Fig. 2). For instance, the N2Oemission peak occurred at the 8th day after the basal fertilizerapplication to the spring cucumber and lasted approximately 20days when the soil temperature was approximately 22 �C at 5 cm

soil depth in May. However, the N2O emission peak occurred earlierand the peak duration was only 5 days after the first topdressingwhen the soil temperature at 5 cm depth was around 25 �C (Fig. 2).This indicated that N2O emission peaks lasted longer under mildtemperature condition (mostly <24 �C in May) than under highertemperature condition (mostly >25 �C from June to August).

The OV3_M treatment, which received only manure N but nochemical N, did not result in substantial N2O emission peakscompared with the OV3_MF treatment, that received both chemicalN and manure N. This result implies that chemical N application isthe main contributor to N2O emissions from arable soils and thatappropriate manure applications do not result in high N2O emis-sions when the soil temperature is mild (mostly<24 �C in May). Noobvious emission peak after the first topdressing in the OV20treatment was observed because of a missed irrigation caused by atemporary malfunction in the irrigation equipment at this field.This further indicated that N-fertilizer application followed byirrigation produced higher N2O emissions.

35

25

30

35Maize

OV3_CK

OV3_M

20

25

erat

ure

(OC

) OV3_MF1

OV3_MF2

GV3

10

15

Soil

Tem

pe

OV20

0

5

Mar

Mar

Apr

May

May Jun

-Jul

-Jul

Aug Sep

Sep

Oct

Nov

Dec

Dec Jan

Feb

Feb

-10

-5

4-M

25-M

15-A

6-M

27-M

17-J 8- 29-

19-A 9-

S

30-S

21-O

11-N 2-D

23-D

13-

3-F

24-F

Date

Fig. 2. Soil temperature at 5 cm depth for the different treatments during the experimental period (Mar 04, 2011eMar 03, 2012).

T. Diao et al. / Atmospheric Environment 72 (2013) 70e7674

The cumulative annual N2O emissions were varied for thedifferent vegetable fields. They were as low as 1.8 � 0.2 kg N2OeN ha�1 for the only manure N addition treatment and as high as6.9 � 0.5 kg N2OeN ha�1 for the OV3_MF2 treatment where acombination of chemical and manure N had been applied (Table 3).If the cumulative annual N2O of the OV3_MF1 treatment is set to100%, then the annual N2O emissions for the OV3_CK, OV3_M andmaize fields were 17.5%, 33.9% and 55.3%, respectively, and 81.1%,114.5% and 131.6% for the OV20, GV3 and OV2_MF2 treatments,respectively. The four primary N2O application periods were: (i) thebasal application to spring cucumber, (ii) first and (iii) secondarytopdressings to spring cucumber and (iv) the basal application toautumn cabbage. The average N2O emissions during the four pe-riods were 42.9% (ranging from 27.7% to 50.1%), 3.2% (ranging from0.2% to 7.3%), 13.5% (ranging from 4.9% to 17.4%) and 19.2% (rangingfrom 14.8% to 26.0%) of the total annual N2O production, respec-tively. The average N2O emissions from the application treatmentsduring these four periods contributed 78.8% (ranging from 64.6% to84.5%) of the annual N2O production, but their duration onlyaccounted for 17.5% of the 366-day experimental period.

For the maize field, N2O emissions during the higher tempera-ture (25e33 �C) period, started on July 12 and lasted to August 20,accounted for 56.0% of the total annual N2O production, mainlybecause of the boost in emissions under higher temperature con-ditions. Cumulative N2O emissions from the vegetable field of non-application treatment (CK) during this high temperature periodalso accounted for 46.5% of the total annual N2O production(Table 3). The concurrence of tillage and irrigation immediately

Table 3Cumulative annual N2O emissions (kg N2OeN ha�1 a�1) and the contributions (%) made

Field Mar 04eMay 07

May 08eMay 31

Jun 01eJun 20

Jun 21eJun 27

Jun 28eJul 11

JuAu

Maize 7.2 8.0 15.6 0.6 2.0 33OV3_CK 14.3 7.8 6.5 0.5 3.5 15OV3_M 7.4 27.7 8.5 2.1 2.8 15OV3_MF1 2.5 44.3 7.4 5.8 1.4 12OV3_MF2 1.9 42.9 5.7 7.3 1.4 17GV3 1.3 49.7 6.6 0.2 1.0 4OV20 2.2 50.1 5.7 0.5 1.6 17Averagea 3.1 42.9 6.8 3.2 1.7 13

Notes: Different letters indicated in the parentheses in last column mean that there area The average of the treatments that received either manure or chemical fertilizer.

before sowing the autumn cabbage (Aug 08e20) also promotedsubstantial N2O emissions, which accounted for 31.5% of the annualN2O production, even though there was no application of fertilizerin the OV3_CK treatment. These also indicated that temperature isanother important factor for N2O emission fluxes. However, theannual N2O emissions from the fertilized vegetable fields were1.68e2.38 times higher than they were from the maize field.

3.2. Emission factors for the nitrogen applied to the differentvegetable fields

Most researchers consider chemical and manure N togetherinstead of distinguishing them by types when the EF of applied Nwas calculated. In this study, we calculated the EFs of applied N bytypes: either chemical or manure N. When the emissions due tomanure N were deducted as background emission, the actual EF ofchemical N can be obtained (Table 4). If we consider manure andchemical N together, the EF of N was 0.22%e0.40% over the entireyear, ranging from 0.11 to 0.57% for the spring cucumber season andfrom 0.02% to 0.17% for the autumn cabbage season (Table 4).However, the calculated EF of chemical N was 1.10e1.78% over thewhole year and 1.22e1.91% and 0.65e2.84% for the spring andautumn seasons, respectively, and it was higher in the greenhousefield than in open-ground fields, especially during the autumncabbage season (2.84% vs. 0.65e1.43%). Our results indicate that it isimportant to identify the types or forms of the applied N when theEF of applied Nwas calculate, because the availability and loss pathsof different types of N vary.

by the different periods from Mar 04, 2011 to Mar 03, 2012.

l 12eg 07

Aug 08eAug 20

Aug 21eSep 03

Sep 04eDec 27

Dec 28eMar 03

Annualemission

.9 22.1 2.5 4.2 3.8 2.88 � 0.20 (c)

.0 31.5 3.6 10.0 7.5 0.91 � 0.10 (a)

.4 19.5 5.7 6.3 4.6 1.77 � 0.20 (b)

.8 18.7 2.5 2.3 2.1 5.22 � 0.40 (e)

.4 17.0 2.5 2.2 1.8 6.87 � 0.50 (f)

.9 26.0 2.8 4.3 3.1 5.97 � 0.50 (e)

.1 14.8 1.8 3.8 2.4 4.23 � 0.30 (d)

.5 19.2 3.1 3.8 2.8

significant differences in the annual N2O emissions at p < 0.05 after SSR testing.

Table 4Emission factors for the nitrogen applied to the vegetable fields.

Treatment EFs of all forms of nitrogena EF of chemical nitrogenb

Cucumber Cabbage þgarlic

Entireyear

Cucumber Cabbage Entireyear

OV3_M 0.11 0.02 0.07 e e e

OV3_MF1 0.42 0.12 0.29 1.57 1.43 1.54OV3_MF2 0.57 0.17 0.40 1.91 1.38 1.78GV3 0.42 0.11 0.24 1.53 2.84 ec

OV20 0.35 0.07 0.22 1.22 0.65 1.10

a The emissions by the no fertilizer treatment were deducted as the backgroundemission caused by the null control.

b The emissions of manure nitrogenwere deducted as the background emission ofthe manure control.

c There was no non-manure control during the winter-garlic season in the GV3field, therefore we did not calculate the whole year EF of chemical N for GV3 field.

T. Diao et al. / Atmospheric Environment 72 (2013) 70e76 75

The EF of chemical N was different for the different vegetablefields. It was higher in the recently developed field than it was inthe established field (1.54e1.78% vs. 1.10%). The EF for the green-house field was higher than in the open-ground fields during theautumn cabbage season (2.84% vs. 0.65e1.43%). This may have beendue to the significantly higher temperature in the greenhouseduring the cabbage growing period compared to that in the open-ground fields (Fig. 2). As the soil did not receive any N applicationsbetween March and early May, the relatively higher temperature ingreenhouse did not show a higher EF during this period.

The total N applied to open-ground fields in our experiment was1272.0e1555.5 kg N ha�1 a�1, in which chemical N contributed10.7e14.9% of the total N. However, the chemical N usually accountsfor 50e80% of the total N supply in most vegetable fields in China(Chen et al., 2004; Li et al., 2007; He et al., 2009). Therefore, theactual annual N2O production and EFs for vegetable soils, whichmostly receive chemical N, would be higher than our results.

4. Discussion

4.1. Emission factors for nitrogen applied in the different vegetablefields

Our results indicated that N2O EFs were varied for the differentvegetable fields. The EFs of applied chemical N were higher inrecently developed vegetable field than it was in established field(1.54e1.78% vs. 1.10%). This may be due to the development ofunfavourable conditions in the long-term vegetable fields and inthe greenhouse fields, which then affected the N2O emissionprocesses. It has been reported that mono-culture of vegetablecrops over a number of years usually results in changes in the soilproperties, such as soil structure compaction (Chen et al., 2004;

Table 5Literature summary on N2O EFs in soils used to grow vegetables in China based on whol

Site Measurementperiod (d)

N rate of individualvegetable (kg ha�1)

Yearly N rate(kg ha�1 a�1)

Proportion ofmanure N (%)

Jiangsu 431 129e332 1636 37

Sichuan 378 200 600 0Shandong 687 127e870 574e1590 28Hubei 731 40e360 200e600 0

Jiangsu 1497 118e548 1074e1294 w50

Jiangsu 427 162e300 870e948 0

a This estimation did not have a zero N background treatment.

Cai et al., 2011), secondary salinity (Grattan and Grieve, 1999; Shiet al., 2009) and an increase in soil-borne diseases (Sun et al.,2010). All these factors can limit the nutrient turnover rate (Jinet al., 2004; Shi et al., 2009; Wo et al., 2010) in the vegetablefields and then impact the nitrification and de-nitrification pro-cesses and their related N2O emissions (Li et al., 2001; Zhang et al.,2002; Jin et al., 2004). Moreover, the imbalance in the soilmicroorganism communities caused by long-term mono-culturingof vegetable crops may also impact the N2O emission processesmediated by the microorganisms (Richard et al., 1996; Li et al.,2006).

4.2. Implications for estimating the N2O EF: year-round calculationsand distinguishment by the different types of nitrogen

Because vegetable fields are usually planted with more than onevegetable crop in a year, especially in protected fields, whole yearmeasurements and estimations of EFs are necessary in order tocalculate the accurate N2O emissions. As measurements in the in-dividual or incomplete vegetable seasons cannot reflect the year-round conditions, including N application rates, temperature, wa-ter regimes and plant growing status, EF calculations based on theindividual vegetable growth period usually deviate from the actualyear round EF. Our calculation (Table 3) showed that EF estimationsbased on either the spring cucumber or the autumn cabbage seasonwere inconsistent with that calculated based on the year round.

There are still very little data available for N2O emissions fromvegetable soils. To date, our literature search found 14 publishedpapers reporting the N2O emissions from vegetable soils in China,of which only six studies (as listed in Table 5) measured the N2Oemissions over a whole year or for a longer period. Data show thatthe year round EFs of applied N from vegetable fields ranged be-tween 0.29% and 4.98% and our re-calculations also indicated thatmanure N had relative lower EFs than chemical N. One of the higherEF values (4.98%) in Table 5 possibly over-estimated the EF becauseit did not distinguish the chemical N from manure N. This furtherindicates that it is important to calculate the N2O EF by dis-tinguishing the N types.

In contrast to our experiment, unnecessarily high levels ofchemical N fertilizers are applied in the current vegetable pro-duction system. As the area planted with vegetables is stillincreasing in China, we can deduce that vegetable fields contributegreatly to the national greenhouse gas inventories due to theirhigher fertilization rates and other intensive management prac-tices, such as frequent irrigation and tillage caused by frequentplantingeharvesting cycles. Further studies on year-round N2Oemission from different vegetable fields and the estimation of theEFs based on the N types under widely different climate zones arestill needed in order to decrease uncertainty in the national N2OInventory.

e year measurements.

EFtotal N(%)

EF Manure N

(%)Yearly rotation Reference

0.73a No data RadisheBaby bok choy (BBC)eLettuce-BBC

Xiong et al., 2006

0.38 N/A LettuceeBrussel sprouteCabbage Yu et al., 20080.29e0.30 0.27 TomatoeTomato He et al., 20090.33e1.13 N/A PeppereRaddisheSpinache

PeppereCabbageeCabbageQiu et al., 2010

0.59e4.98 No data RaddisheCeleryeRaddish etc. Yao et al., 2006;Mei, 2009

0.70e0.73 0.38e0.54 TomatoeCucumbereCelery Min et al., 2012

T. Diao et al. / Atmospheric Environment 72 (2013) 70e7676

5. Conclusions

In summary, N applications contributed about 78.8% (rangingfrom 64.6% to 84.5%) of the year-round cumulative N2O emissionsfrom vegetable fields. The magnitude of the emission peaksdepended on the nitrogen application rate and the durations of theemission peaks were mainly associated with soil temperature un-der appropriate irrigation. The annual N2O EF of manure N was0.07% in our experiment, much lower than the IPCC default of 1.0%(IPCC, 2006). The year-round EF of chemical N was higher inrecently developed field than in established vegetable field (1.54e1.78% vs. 1.10%). Due to the high N application rates, frequent irri-gation and tillageeplanting cycles in the vegetable productionsystem, vegetable fields may contribute considerably to nationalgreenhouse gas emissions.

Acknowledgements

This study was financially supported by the National NaturalScience Foundation of China (Approval No. 31071865 and31272249). The 12th five-year National Technologies R&D program(2013BAD11B03) and the Basic Scientific Research Foundation(BSRF) of National Non-Profit Scientific Institute of China also partlysupported this research.

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