REGULAR ARTICLE

Ozone pollution influences soil carbon and nitrogensequestration and aggregate composition in paddy soils

Taiji Kou & Lirui Wang & Jianguo Zhu & Zubin Xie &

Yanling Wang

Received: 14 December 2013 /Accepted: 18 March 2014# Springer International Publishing Switzerland 2014

AbstractBackground and aims Much attention has focused onthe effects of tropospheric ozone (O3) on terrestrialecosystems and plant growth. Since O3 pollution iscurrently an issue in China and many parts of the world,understanding the effects of elevated O3 on soil carbon(C) and nitrogen (N) sequestration is essential for effortsto predict C and N cycles in terrestrial ecosystems underpredicted increases in O3. Thus the main objective ofthis study was to determine whether an increases in

atmospheric O3 concentration influenced soil organicC (SOC) and N sequestration.Methods A free-air O3 enrichment (O3-FACE) experi-ment was started in 2007 and used continuous O3 expo-sure from March to November each year during cropgrowth stage in a rice (Oryza sativa L.)—wheat(Triticum aestivum L.) rotation field in the JiangsuProvince, China. We investigated differences in SOCand N and soil aggregate composition in both elevatedand ambient O3 conditions.Results Elevated atmospheric O3 (18–80 nmol mol−1 or50 % above the ambient) decreased the SOC and Nconcentration in the 0–20 cm soil layer after 5 years.Elevated O3 significantly decreased the SOC concentra-tion by 17% and 5.6 % in the 0–3 cm and the 10–20 cmlayers, respectively. Elevated O3 significantly decreasedthe N concentration by 8.2–27.8 % in three layers at the20 cm depth. In addition, elevated O3 influenced theformation and transformation of soil aggregates and thedistribution of SOC and N in the aggregates across soillayer classes. Elevated O3 significantly decreased themacro-sized aggregate fraction (16.8 %) and associatedC and N (0.5 g kg−1 and 0.32 g kg−1, respectively), andsignificantly increased the silt+ clay-sized aggregatefraction (61 %) and associated C (1.7 g kg−1) in the 0–3 cm layer. Elevated O3 significantly decreased themacro-sized aggregate fraction (9.6 %) and associatedC and N (1.4 g kg−1 and 0.35 g kg−1, respectively), andsignificantly increased the silt+ clay-sized aggregatefraction (41.8 %) and decreased the corresponding as-sociated N (0.14 g kg−1) in the 3–10 cm layer. ElevatedO3 did not significantly effect the formation and

Plant SoilDOI 10.1007/s11104-014-2096-7

Responsible Editor: Eric Paterson.

Taiji Kou and Jianguo Zhu contributed equally to this publication.

T. Kou (*) : L. WangCollege of Agriculture, Henan University of Science andTechnology,70# Tianjin Road, Luoyang, Henan Province 471003,People’s Republic of Chinae-mail: tjkou@aliyun.com

T. Kou : J. Zhu : Z. XieState Key Laboratory of Soil and Sustainable Agriculture,Institute of Soil Science, Chinese Academy of Sciences,Nanjing 210008, People’s Republic of China

L. WangPuyang Vocational and Technical College,Puyang 457000, People’s Republic of China

Y. WangInternational Center for Ecology, Meteorology &Environment, School of Applied Meteorology, NanjingUniversity of Information Science and Technology,Nanjing 210044, People’s Republic of China

transformation of aggregates in the 10–20 cm layer, yetit did significantly increase the C concentration in themacro-sized fraction (1 g kg−1) and decrease the Nconcentration in the macro- and micro-sized fractions(0.24 g kg−1 and 0.16 g kg−1, respectively).Conclusion Long-term exposure to elevated atmospher-ic O3 negatively affected the physical structure of thesoil and impaired soil C and N sequestration.

Keywords Ozone pollution . Soil aggregate . Free-airozone enrichment (O3-FACE) . Carbon cycle .

Environment change

Introduction

Ozone is one of the best-documented air pollutants inEast Asia and in many parts of the world (Oltmans andLevy 1994; Ashmore et al. 2006). The global averagetropospheric O3 concentration is estimated to increase atan annual rate of 0.3–2 % (Thompson 1992; Vingarzan2004; Ashmore et al. 2006) and is expected to increaseby 50 % by 2020 relative to the 1980s (Hough andDerwent 1990). The effect of tropospheric O3 on terres-trial ecosystems has received considerable attention(Thompson 1992; Ashmore et al. 2006). Many O3 ex-posure experiments (using closed chambers, open topchambers, or O3-FACE systems) have elucidated re-sponses of various ecosystems to elevated troposphericO3 (De Temmerman et al. 1992; Manning 2005; Wanget al. 2009; Morgan et al. 2006; Kou et al. 2012a). Manyof these studies have focused on the effects of O3 onplants, most agreeing that O3 inhibits plant growth andaccelerates plant senescence (Rounsevell et al. 1999;Kou et al. 2009). Elevated O3 has also been demonstrat-ed to reduce photosynthetic rates (Nie et al. 1993; Wanget al. 2009) and crop and forest productivity (Wang et al.2004; Feng et al. 2008; Shi et al. 2009), and to alter Cand N metabolism, and subsequently allocation of re-sources (e.g. C, N) belowground (McCrady andAndersen 2000; Kanerva et al. 2006; Jones et al. 2009;Kou et al. 2012b). This response of plants to O3 couldresult in changes in soil properties in which these plantsgrow (Rounsevell et al. 1999; Kou et al. 2012a). Forexample, numerous studies have demonstrated that ele-vated O3 influences the soil microbial community (Samiet al. 2008; Chen et al. 2010), and alters the concentra-tion of DTPA-extractable micronutrients (Wang et al.2010). This is important as it is well known that soils are

important C sinks within the biosphere (Buyanovskyand Wagner 1998; Lal 2004). However, less is knownabout the effect of elevated O3 on soil C and N seques-tration and their stability. Thus it is important to betterunderstand C and N cycles in the context of predictedincreases in atmospheric O3.

The role of aggregation in regulating C and N dy-namics has been extensively investigated in the last fewdecades (e.g. Elliott 1986; Beare et al. 1994; Six et al.1998, 2000b; Christensen 2001; Haile et al. 2008; Chenet al. 2010). Soil aggregation exerts some level of phys-ical protection for soil organic matter (SOM) (Beareet al. 1994). Generally, the most Stable C is stored inthe smallest silt+ clay-sized aggregate fraction(<0.053 mm), and SOC protected by macro-sized ag-gregates is in relatively short-term storage (Six et al.2002). However, there is little information availableabout the effects of elevated O3 on the distributions ofSOC and N in soil aggregates and their stability. Thusour understanding of the mechanisms underlying SOCand N sequestration is limited with regard to elevatedO3.

The paddy fields of the Yangtze River Delta region ofSoutheastern China are one of most heavily O3-pollutedregions in China. In light of the larger amount of Cdeposition to paddy soils than other agricultural soils(Pan et al. 2004), our understanding of the responses ofSOC and N sequestration and their stability under ele-vated O3 is essential. Using a O3-FACE rice- wheatrotation experiment (initiated in 2007) in SoutheastChina (Tang et al. 2010), we assessed the effects ofelevated atmospheric O3 on SOC and N stability andsoil aggregation. Specifically we determined if differ-ences existed in: (1) SOC and N concentration, (2) soilaggregate composition and transformation, and (3) SOCand N concentration in aggregate size fractions follow-ing 5 years of rotational rice and wheat cropping in soilsexposed to different atmospheric O3 concentrations.

Materials and methods

Experimental site

The study site was located in Jiangdu, Jiangsu Province,China (N 32°35′5″, E 119°42′0″) in which summerrice—winter wheat rotation has long been practiced.The region’s mean annual precipitation is 980 mm, witha mean annual temperature of 14.9 °C. The total annual

Plant Soil

sunshine and frost-free period were more than 2,100 hand 220 days, respectively. The soil is classified as aShajiang Aquic Cambiosol in the Chinese classification(Cooperative Research Group on Chinese SoilTaxonomy 2001), and has a sandy-loamy texture ac-cording to the US classification (Soil Survey Staff2003). The soil properties are as follows: silt (0.05–0.002 mm) 28.5 %, clay (<0.002 mm) 13.6 %; bulkdensity 1.16 g cm-3; SOC 18.4 g kg−1; total N1.45 g kg−1; total P (as P2O5) 0.63 g kg−1, and pH 7.2.

O3-FACE system

The O3-FACE system was set up in 2007 and is de-scribed in detail in Tang et al. (2010). Briefly, itconsisted of: (1) four octagonal rings (14 m in diameter)with target O3 concentrations of about 50 % higher thanthe ambient atmosphere (hereinafter referred to as theO3-FACE ring; one of four rings normally operated until2008), and (2) four comparison rings without O3 enrich-ment (hereinafter referred to as ambient ring). The O3-FACE rings were installed more than 70 m from theambient rings. The O3 exposure was continuous fromearly March to harvest in the wheat growth season, andfrom 1 week following rice transplanting to harvest inthe rice growth season. Pure O3 at high pressure wasreleased towards the center of O3-FACE rings 50 cmabove the crop canopy from 09:00 to sunset each day.No O3 was released on rainy days to limit acute damageto leaves. The atmospheric O3 concentration variedbetween 18 and 120 nmol mol−1 in the O3-FACE ringsand between 18 and 80 nmol mol−1 in the ambient rings.

Crop cultivation

Rice (cv. Wuyunjing No. 21) was transplanted at aspacing of 2 seedlings hill−1 and 24 hills m−2 in earlyJune of each year and harvested in early October.Winter wheat (cv. Yangmai No. 16) was sown in earlyNovember of each year at a density of 180 seedlingsm−2 and harvested in late May of the next year. Theaboveground biomass was removed following harvest.Fertilizers (NPK) was applied as: (1) urea (46 % N),(2) superphosphate (12 % P2O5), and (3) potassiumchloride (60 % K2O). Approximately 435 kg ha−1 and478 kg ha−1 urea were applied to the rice crop and thewinter wheat crop, respectively. Approximately625 kg ha−1 superphosphate and 125 kg ha−1 potassi-um chloride were equally applied during the rice and

winter wheat growth periods. Field management (pes-ticide, herbicide, and fungicide application, and irriga-tion and drainage) closely followed local agronomicpractices.

Soil sampling and analysis

Soil samples from six rings (three rings for each O3

treatment) operated since 2007 were collected with asoil auger (3 cm in diameter) after winter wheat harvestin lateMay 2012. The harvest was carried out by hand toreduce potential affects of mechanized harvesting on thesoil. The samples were taken from three depths (0–3, 3–10 and 10–20 cm) in eight randomly selected samplingpoints with no obvious surface trampling for each plot.The eight sub-samples at each sampling point and depthclass were blended to obtain a composite sample foreach depth class per plot. Field-moist soil samples werepassed through an 8 mm sieve, air-dried, and mixedthoroughly. A portion of each sample was passedthrough 0.149 mm and 0.841 mm sieves for SOC andtotal N determinations, respectively; the remainder wasused for aggregate analysis.

Three aggregate size classes (>0.25, 0.25–0.053, and<0.053 mm) were separated according to Six et al.(2002) by wet-sieving soil through a series of two sieves(0.25 and 0.053 mm), as adapted by Kou et al. (2012c).The overall procedure yielded water-stable, macro-(>0.25 mm), micro- (0.25–0.053 mm), and silt+ clay-sized fraction (<0.053 mm) aggregates. All fractionswere backwashed in pre-weighed aluminum pans, driedovernight in an oven at 50 °C and weighed. For furtheranalysis of aggregate associated C and N, some oven-dried soil of each fraction was used to determine theSOC and total N. The C and N concentration of all soilsamples were determined using a CHNS/O Analyzer(PERKIN ELMER 2400, Series II).

Statistical analysis

Statistically significant differences were identifiedusing analysis of variance (ANOVA) using SPSS11.5 software (Windows version 11.5; SPSS inc,Chicago, IL) and Least Significant Difference (LSD)calculations at P=0.05. Differences were consideredsignificant at P<0.05.

Plant Soil

Results

Response of soil organic carbon and nitrogenconcentrations to O3 treatments

The SOC and N distribution differed with soil depth inthe 20 cm soil profile under the different O3 treatments(Fig. 1). Elevated O3 decreased the SOC and N concen-tration after 5 years. Elevated O3 significantly decreasedthe SOC concentration by 17% and 5.6 % in the 0–3 cmand 10–20 cm layers, respectively. Elevated O3 signifi-cantly decreased the total N concentration by 27.8 %,8.2 % and 13.6 % in the 0–3 cm, the 3–10 cm and the10–20 cm layers, respectively.

Aggregate size distribution

The distribution of soil aggregate fraction-size classeswas dissimilar between the first two layers and the 10–20 cm layer, and similar at same layer in the 0–20 cmsoil depth between the two O3 treatments (Table 1). The>0.25 mm fraction (macro-aggregate) in depths of 0–3 cm and 3–10 cm accounted for 48.2–58 % and 49.3–54.5 %, respectively of total aggregates under elevatedand ambient O3 concentrations. The 0.25–0.053 mmfraction (micro-aggregate) was greatest in the 10–20 cm layer (46.4–47.8 %) under the two treatments.In the 0–3 cm and 3–10 cm layers, elevated O3 signif-icantly decreased macro-aggregates by 16.8 % and9.6 %, and significantly increased silt+ clay aggregatefractions (<0.053 mm) by 61 % and 41.8 %, respective-ly; but did not increase in micro-aggregates. The

distribution of aggregate size classes was not signifi-cantly affected by O3 treatment in the 10–20 cm layer.

Aggregates associated carbon and nitrogen

The O3 exposure and soil depth influenced SOC and Nconcentration in different aggregate size classes (Figs. 2and 3). The associated C and N between the two O3

treatments was mainly concentrated in the macro-aggregates within the three soil layers. Elevated O3 sig-nificantly decreased the C concentration in the macro-aggregate class and significantly increased it in the silt+clay-sized aggregate fraction (by 14.8 %) in the 0–3 cmlayer. In the 3–10 cm layer, elevated O3 significantlydecreased the C concentration in the macro-aggregatesby 9.3 % but had not in the micro- and silt+ clay -sizedaggregate fractions. Elevated O3 significantly increasedmacro-aggregate associated C by 7.6% at a depth of 10–20 cm, but did not influence the C concentration in the<0.25 mm fraction (e.g. <0.053 mm and 0.053–0.25 mm). The N concentration in the macro-aggregates in the 0–3 cm layer significantly decreased(by 17.3 %) under elevated O3; but no significant differ-ences were found in the <0.25 mm fractions. At a depthof 3–10 cm, elevated O3 significantly decreased the Nconcentration in macro-aggregate by 17.2 %, yet de-creased the same in the silt+ clay-sized aggregate frac-tion by 9.3 %. Elevated O3 significantly decreased the Nconcentration in both macro- and micro-aggregates by14.6 % and 14.4 % in the 10–20 cm layer, and increasedsilt+ clay-sized aggregate fraction by 10.2 %.

a

bcc c b

0

5

10

15

20

25

0-3 3-10 10-20Depth (cm)

SOC

con

cent

ratio

n (g

kg-1

)

Ambient O3-FACEO3-FACE

bca

a

d

bcd

0.0

0.4

0.8

1.2

1.6

2.0

2.4

0-3 3-10 10-20Depth (cm)

N c

once

ntra

tion

(g k

g-1) Ambient O3-FACEO3-FACEa

Fig. 1 Soil organic carbon (SOC) concentration (left) and nitro-gen (N) concentration (right) in 0–20 cm paddy soil across soildepth classes as affected by an elevated ozone (O3). Lower caseletters indicate differences (at the 0.05 level) in the SOC and Nconcentrations among all treatments within all depth class, respec-tively. O3-FACE is the abbreviation of free-air O3 concentration

enrichment and refers elevated O3 concentration with a range of18–80 nmol mol−1 during the 5 year experiment. Ambient refers toambient O3 concentration with a range of 18–80 nmol mol−1

during the 5 year experiment. Values are means ± 1SE (n=3)

Plant Soil

Discussion

Carbon and nitrogen concentration in different layersto 0–20 cm soil depth

In this present study, elevated O3 significantly decreasedthe SOC concentration in the 0–3 cm and 10–20 cmlayers, suggesting that O3 impeded C sequestration inthe topsoil (0–20 cm). Felzer et al. (2004) also reportedless SOC sequestration under elevated O3 and reducedSOC storage due to O3 pollution since the 1950s.Generally, the differences in the quantity and quality ofprecursor substances influence formation and storage ofSOC. In this study region, root residue was the primaryprecursor substance for SOC since the abovegroundbiomass of both wheat and rice is removed followingharvest, and no manure is applied. Previous evidencefrom the O3-FACE system suggested that elevated O3

significantly decreases the residue C quantity and the C/N ratio of wheat (Yangmai No.16) roots (Kou et al.2012b), and decreases photosynthetic rates of rice(Wuyunjing No. 21) (Wang et al. 2009). In addition, areduced total and root biomass was observed in wheat(Kou et al. 2012b) and in rice (975 and 145 g m−2 underelevated O3 and 1,065 and 159 g m

−2 under ambient O3,respectively) in response to elevated O3. Other studieshave demonstrated that higher O3 decreases allocationof photosynthate below ground (Cooley and Manning1987; Nouch et al. 1991; Jones et al. 2009), thus reduc-ing root biomass in wheat (McCrady and Andersen2000) and rice (Jin et al. 2001; Ariyaphanphitak et al.2005). Decreasing C input from root residue in a rota-tional rice-wheat cropping reduces SOC accumulation.Other than C inputs via crop residue or organic

amendments (Rasmussen and Parton 1994; Rudrappaet al. 2005), SOC accumulation in farmlands is theoret-ically controlled by SOM decomposition via heterotro-phic respiration (Kou et al. 2007). Some evidence sug-gests that elevated O3 slightly decreased heterotrophicrespiration during the plant growth period (Nelson andEdwards 1991; Islam et al. 2000; Kasurinen et al. 2004),possibly preventing SOM decomposition. However,Felzer et al. (2004) found the O3 exposure resulted in a27 % C loss by tillage, promoting a loss from SOMdecomposition. In this study, elevated O3 significantlydecreased the SOC concentration in depths of 0–3 cmand 10–20 cm, and had no significant effect in the 3–10 cm layer (Fig. 1). The differences observed werelikely due to elevated O3 decreasing the quantity of rootresidue and its distribution in the corresponding soillayer, and possibly stimulating SOC decomposition inthe 0–3 cm layer. Rice and wheat roots are localized inthe middle of the 0–15 cm plough layer (Shi et al. 2012),and are fewer in number in the 0–3 cm layer because ofseed planting at around 3–5 cm depth and even less inthe 10–20 cm layer due to the compacted plough panlayer at 15–20 cm. It appears that a more aerobic envi-ronment in the 0–3 cm layer (relative to the 3–20 cmlayer) after tillage operations and under unsaturatedconditions enhances organic C mineralization by micro-organisms. Thus it may be important to further study theeffects of elevated O3 on microbial C emissions.

In this study, total N concentration decreased at thethree different layers in the 0–20 cm soil depth underelevated O3. Similar data were reported by Li et al.(2010) and by Kanerva et al. (2006) in a meadowecosystem. Generally, the total N concentration willdecline with a decrease in SOC under elevated O3 due

Table 1 Soil aggregate distribution in topsoil (0–20 cm) under elevated and ambient ozone concentrations across soil depth classes

Soil depth (cm) Treatment Aggregate size class (mm)

>0.25 0.25–0.053 <0.053

0–3 O3-FACE 48.2±0.1 d 41.7±2.1 b 10.1±0.9 a

Ambient 58±0.1 a 35.8±2.1 c 6.2±1 c

3–10 O3-FACE 49.3±0.1 c 39.3±2.5 bc 11.5±1.1 a

Ambient 54.5±0.1 b 37.4±2.7 bc 8.1±0.5 b

10–20 O3-FACE 42.2±0.1 e 47.8±2 a 9.9±0.8 a

Ambient 42.9±0.1 e 46.4±2.5 a 10.7±1.1 a

Data are expressed as means ± 1 SE (n=3). Different letters in the same column mean significant differences at the 0.05 level. O3-FACE isthe abbreviation of free-air ozone (O3) concentration enrichment and refers elevated O3 concentration with a range of 18–80 nmol mol−1

during the 5 year experiment. Ambient refers to ambient O3 concentration with a range of 18–80 nmol mol−1 during the 5 year experiment

Plant Soil

to the positive relationship between SOC and total N.Several studies have demonstrated that soil N transfor-mation under elevated O3 is accelerated due to an in-creases in crop N-uptake, soil urease activity, and thepopulation of soil ammonia-oxidizing bacteria anddenitrifying bacteria (Chen et al. 2010; Shi et al. 2012;Li et al. 2010). Thus, the common action between

reducing SOC concentration and N transformation leadsto a decrease in total N concentration in the topsoil (0–20 cm). Total N concentration under elevated O3 signif-icantly declined in the three layers. This was likely dueto the greater decrease in SOC and a loss of N transfor-mation (e.g. N-uptake, gaseous, and leaching), resultingin a significant decrease in topsoil N under elevated O3.

a

c

d

b

cc

0

5

10

15

20

>0.25 0.25-0.053 <0.053

0-3 cm

Ambient O3-FACEO3-FACE

d

c

a

cd

b

0

5

10

15

20

>0.25 0.25-0.053 <0.053

SO

C c

on

cen

trat

ion

in

ag

gre

gat

es (

g kg

-1)

3-10 cm

b

cc cc

a

0

5

10

15

20

>0.25 0.25-0.053 <0.053

Aggregate size class (mm)

10-20 cm

Fig. 2 Distributions of aggregate associated carbon in topsoil (0–20 cm) under elevated and ambient ozone (O3) concentrations(which vary within 18–120 nmol mol−1 and 18–80 nmol mol−1

during the 5 year experiment, respectively) across soil depthclasses. Lower case letters indicate differences (at the 0.05 level)in the soil organic carbon (SOC) concentration among all treat-ments within the same depth class, respectively. O3-FACE is theabbreviation of free-air O3 concentration enrichment and referselevated O3 concentration. Ambient refers to ambient O3 concen-tration. Values are means ± 1SE (n=3)

a

bc

b bbc

0.0

0.5

1.0

1.5

2.0

2.5

>0.25 0.25-0.053 <0.053

0-3 cm

Ambient O3-FACEO3-FACE

a

bc

b

cc

0.0

0.5

1.0

1.5

2.0

2.5

>0.25 0.25-0.053 <0.053N c

on

cen

trat

ion

in

ag

gre

gat

e (g

kg-

1)

3-10 cm

a

cdd

b bc

e

0.0

0.5

1.0

1.5

2.0

2.5

>0.25 0.25-0.053 <0.053

Aggregate size class (mm)

10-20 cm

Fig. 3 Distributions of aggregate associated nitrogen in topsoil(0–20 cm) under elevated and ambient O3 concentrations (whichvary within 18–120 nmol mol−1 and 18–80 nmol mol−1 during the5 year experiment, respectively) across soil depth classes. Lowercase letters indicate differences (at the 0.05 level) in the nitrogen(N) concentration among all treatments within the same depthclass, respectively. O3-FACE is the abbreviation of free-air O3

concentration enrichment and refers elevated O3 concentration.Ambient refers to ambient O3 concentration. Values are means ±1SE (n=3)

Plant Soil

Aggregate size distribution and stability

Soil aggregates are the basis of good soil structure (Chenet al. 2010; Kou et al. 2012c). In this study, macro-aggregates in the 0–10 cm depth (i.e., 0–3 cm and 3–10 cm) and micro-aggregates in the 10–20 cm layerdominated, regardless of O3 concentration. These dataindicate that soil structure was better in the 0–10 cmlayer than in the 10–20 cm layer (Six et al. 2000b; Kouet al. 2012a). Elevated O3 evidently influenced theformation and transformation of soil aggregates in the0–10 cm layer but not in the 10–20 cm layer, suggestingthat soil quality in the 0–10 cm layer is more susceptibleto elevated O3 (Kou et al. 2012a). Elevated O3 signifi-cantly reduced macro-aggregates and enhanced silt+clay-sized aggregate fractions in the 0–10 cm layer,and significantly increased micro-aggregates in the 0–3 cm layer and slightly increased such in the 3–10 cmlayer. This suggests that O3 pollution promoted macro-aggregate breakdown. Six et al. (1998) thought that thedisintegration of macro-aggregates is the primary mech-anism in reducing soil C. Plant roots are one of theimportant binding agents for macro-aggregates (Sixet al. 2004). Elevated O3 decreases photo-assimilatedistribution belowground (Cooley and Manning 1987;Nouch et al. 1991; Jones et al. 2009; Kou et al. 2012b)and thus reduces root residue incorporated into SOM,which reduces micro-aggregate binding into macro-aggregates (Tisdall and Oades 1982; Thomas et al.1993; Six et al. 2000b). Theoretically, a larger microbialpopulation will decompose more SOM, thus increasingmicrobial biomass under elevated O3 (Chen et al. 2010;Li et al. 2010). This should increase SOM mineraliza-tion, thereby facilitating the aggregate breakdown pro-cesses (Lichter et al. 2008). This subsequently promotestransformation of macro-aggregates to micro-aggregatesand silt+ clay-sized fractions. In addition, aggregatebreakdown is a good measure for soil erodibility as thebreakdown to finer, more transportable particles andmicro-aggregates increases erosion risk (Carter 2002;Le Bissonnais 2003). Consequently, surface soil (0–10 cm) in a rotational rice—wheat cropping practiceunder elevated O3 will be at a greater risk of erosion.

Carbon and nitrogen distributions in aggregates

Soil organic C decomposition can be slowed by physicalprotection of soil aggregates (Elliott 1986; Six et al.2000a). Our study showed a higher C and N

concentration in macro-aggregates than in the smaller-sized aggregates in the 0–20 cm soil profile among alltreatments. These data indicate that rotational rice-wheat cultivation leads to a gain of C and N-richmacro-aggregates in paddy soil. However, Lichteret al. (2008) observed that C and N were stored inmicro-aggregates in a Cumulic Haplustoll soil in awheat-maize system. These differing results may bedue in part to differences in soil type and land-usepattern. The present study also indicated that the influ-ence of elevated O3 on C and N in aggregates was notsimilar among either the same size fractions at differentlayers in the 0–20 soil depth or various size fractions inthe same layer. These data suggest that elevated O3 hascomplex effects on the distribution and transformationof C and N in aggregates in topsoil. Elevated O3 signif-icantly decreased the C concentration in macro-aggregates in the 0–3 cm and 3–10 cm layers, andsignificantly increased the C concentration in silt+clay- sized aggregate fractions in the 0–3 cm layer andin macro- aggregates in the 10–20 cm layer. These datasuggest that elevated O3 leads to a loss of C-rich macro-aggregates in the 0–10 cm layer, and a gain in C-richmacro-aggregates in the 10–20 cm layer and C-rich silt+clay-sized aggregates in the 0–3 cm layer. Thus, itappears that elevated O3 decreases the easilydecomposed SOC in the 0–10 cm layer and increasesthat in the 10–20 cm layer and a stable SOC storage inthe 0–3 cm layer. The SOC contained in smaller sizeaggregates has a slower turnover rate than in larger sizeaggregates (Jastrow et al. 1996; Six et al. 2000a;Christensen 2001). Nevertheless, this present study in-dicated that elevated O3 significantly decreased the Nconcentration in the macro-aggregates in the three soillayers and in the micro-aggregate in the 10–20 cm layer,and significantly increased the N concentration in silt+clay- sized aggregate in the 0–3 cm layer. These datasuggest that elevated O3 leads to losses in N-rich macro-aggregates in the 0–20 cm layer and N-rich micro-aggregates in the 10–20 cm layer. Carbon and N miner-alization rates increase when the aggregate structure isdisrupted (Lichter et al. 2008). Therefore, the decreasein macro-aggregate N may be a result of the disintegra-tion of macro-aggregates under elevated O3, resulting ina loss of microbial organic N mineralization. However,the N decrease in micro-aggregates in the 10–20 cmlayer under elevated O3 is presumably a result of in-creasing deep root N uptake. It is apparent that betterunderstanding of the stability of organic C and N formed

Plant Soil

in different O3 concentration is required. For example,further distinguishing the chemical- structural character-ization of organic C forms in soils (Six et al. 2002, 2004)and the interaction of C and N in aggregate processeswould be of significant value.

Conclusion

Tropospheric O3 pollution negatively influences plantgrowth and terrestrial ecosystems. In this study, elevatedatmospheric O3 decreased SOC and N concentration intopsoil. The effects of elevated O3 on SOC and N wererelated to soil depth and aggregate size classes.Decreases in SOC and N, macro-sized fractions, andcorresponding associated C and N under elevated O3

were greater in the upper (>3 cm) soil layer than in the3–20 cm soil layer. These data demonstrate that long-term exposure to elevated atmospheric O3 may alter thephysical structure of soil and sequestration of C and N inpaddy topsoil.

Acknowledgments This work was supported by the NationalNatural Science Foundation of China (grant no. 41003030 and40901146), Open Research Fund Program of State Key Laborato-ry of Soil and Sustainable Agriculture (grant no. Y052010030),the International S & T Cooperation Program of China (Grant no.2009DFA31110), the Knowledge Innovation Program of ChineseAcademy of Sciences (grant no. KZCX2-EW-414), and the GlobalEnvironment Research Fund by the Ministry of the Environment,Japan (Grant no. C-062).

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