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Ecological Engineering 60 (2013) 1– 9

Contents lists available at ScienceDirect

Ecological Engineering

journa l h om epage: www.elsev ier .com/ locate /eco leng

esponses of plant–soil properties to increasing N deposition andmplications for large-scale eco-restoration in the semiarid grasslandf the northern Loess Plateau, China

iangwei Hana, Atsushi Tsunekawab, Mitsuru Tsubob, Hongbo Shaoc,d,∗

Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy ofciences (CAS), Beijing 100101, ChinaArid Land Research Center, Tottori University, Hamasaka 1390, Tottori 680-0001, JapanKey Laboratory of Coastal Biology & Bioresources Utilization, Yantai Institute of Coastal Zone Research (YIC), Chinese Academy of Sciences (CAS), Yantai64003, ChinaInstitute of Life Sciences, Qingdao University of Science & Technology, Qingdao 266042, China

r t i c l e i n f o

rticle history:eceived 20 June 2013eceived in revised form 5 July 2013ccepted 6 July 2013

eywords:rassland ecosystemtipa bungeanaoil organic carbon

cycleco-restoration

a b s t r a c t

Increasing nitrogen deposition influences natural and semi-natural ecosystems, especially nutrient-poorecosystems. Grassland on the northern areas of Loess Plateau, China, suffers from both wind and watererosion resulting in a nutrient-poor ecosystem. However, experimental investigation of the effects ofnitrogen deposition on grassland in this region is scarce. In the current study, an in situ experiment wasinitiated at the northern part of Loess Plateau, which is also a coalmine base, to investigate the responsesof Stipa bungeana dominant grassland to nitrogen deposition. To indicate the ravine character of LoessPlateau, China, experiments were conducted on two slopes with opposite slope aspects. On each site,3 m × 5 m plots were exposed to either ambient N deposition (control) or ambient +2.5 g m−2 yr−1 (lowN), +5 g m−2 yr−1 (medium N), and +10 g m−2 yr−1 (high N) added as NH4NO3. After 1 year of N addition, theplots exposed to the added N had significantly higher concentration of mineral N (NH4

+-N + NO3−-N) in the

0–20 cm soil layer compared to plots exposed to ambient N deposition. Soil organic carbon and soil totalN were not altered by the N addition. S. bungeana exposed to the added N exhibited a significant increasein aboveground tissue N concentration on shady and sunny sites (p > 0.05), as well as an increase in N/Pratio. N concentrations of S. bungeana and simulated N deposition levels had a clearly linear relationship(R2 > 0.9). The N recovered in S. bungeana aboveground tissue accounted for 16.4–27.2% of the added Nat the shady site, and 22–35% at the sunny site. However, the tissue N or P concentration of Lespedeza

davurica, a legume plant, was not altered by the added N. The effects of simulated N addition at the shadyand sunny sites on the soil and the plants were very similar; however, they differed in the extent of soiland plant N concentration changes. Observations after 1 year of N addition suggest that N addition cansignificantly and rapidly affect N availability (mineral N) and gramineous plant tissue chemistry in thenorthern Loess Plateau, China, which provides important implications for large-scale eco-restoration of

, Chi

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the northern Loess Plateau

. Introduction

Human activities such as increasing combustion of fossil fuelsn industry and motor vehicles as well as the massive fertilization

n agriculture have dramatically driven up the nitrogen depositionGalloway et al., 2004; Reay et al., 2008). Because nitrogen, an ele-

ent required by plants from soil in large quantities, frequently

∗ Corresponding author at: Institute of Life Sciences, Qingdao University of Sci-nce & Technology, Qingdao 266042, China. Tel.: +86 532 84023984.

E-mail address: shaohongbochu@126.com (H. Shao).

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925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2013.07.053

na.© 2013 Elsevier B.V. All rights reserved.

imits growth in natural and semi-natural ecosystems (Bowmannd Steltzer, 1998; Vitousek and Howarth, 1991), increased N avail-bility from N deposition influences terrestrial ecosystems (Abert al., 2003; Hornung and Sutton, 1995; Stevens et al., 2004; Wolit al., 2010).

N deposition has been shown to profoundly cause changes inhe grassland plant and soil (Cabezas and Comín, 2010). When

acroscopically subjected to chronic low-level nitrogen addition,

he number of grassland plant species is significantly reducedClark and Tilman, 2008; Lu et al., 2011), and the composition ofhe species changes (Brooks, 2003; Ouyang et al., 2011). Tissue Noncentrations are positively correlated with N addition in some

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pecies of calcicolous grassland, such as Briza media, Hieraciumilosella, and Thymus praecox, and higher rates of N addition resultn greater N concentration (Morecroft et al., 1994). Urban soilseceiving high N deposition often have higher NO3

−-N concentra-ions compared with rural soils receiving less N deposition (Sirulnikt al., 2007b). At the same time, N deposition could also resultn either N mineralization or nitrification; however, the effectsary with soil conditions, especially moisture and/or temperatureCarroll et al., 2003; Morecroft et al., 1994; Sirulnik et al., 2007b;ourlitis et al., 2007c).

Anthropogenic nitrogen deposition poses a major threat tohe long-term stability of semi-natural ecosystems, particularlyhose with low fertility (Bobbink et al., 1998). The northern Loesslateau, China, falls under such region because long-term soil ero-ion results in soil infertility and a fragile ecological environmentWang et al., 2011). Furthermore, its unique landform characteris-ics and the severe soil erosion worldwide have made the northernoess Plateau region a research hotspot (Zhang and Chen, 2007;ing et al., 2013). The stability of the local natural and semi-naturalcosystem, which could be influenced by N deposition, is of greatignificance in the study of soil erosion (Wang et al., 2010a). Thus,scertaining the influence of increased N deposition in the natu-al and semi-natural ecosystems within the Loess Plateau is notnly necessary to improve understanding of N deposition in var-ed ecosystems, but also directly connects to local environmentalafety and ecological stability (Mander and Mitsch, 2011). How-ver, currently, limited information is available on this topic.

In the current study, a field experiment was carried out to eval-ate the influence of N deposition on grassland in the northernlateau for conducting large-scale eco-restoration in the region.he study site was in a coalmine base on the northern part of theoess Plateau, China. The current rate of N deposition was measuredo be 2.2 g N m−2 yr−1 (Wei et al., 2010), and expected to increasedn the future (Galloway et al., 2004). However, the northern Plateauelongs to a semi-arid area where water deficiency is consideredhe primary plant growth limitation factor (Wang et al., 2010b;ia and Shao, 2008); the effect of atmospheric N deposition is thusmbiguous (Webb et al., 1978). In the present study, the changes oflant and soil chemistry of the local natural grassland in responseo simulated N deposition were examined. The objectives were toetermine: (1) how do different plants of northern Loess Plateaurassland ecosystem response to the first year simulated N depo-ition in the main plant nutrient concentration; (2) whether therst year simulated N deposition changes soil organic carbon and

concentration.

. Materials and methods

.1. Site description

The study was conducted in the Liudaogou Catchment, located4 km west of Shenmu County, Shaanxi Province. Average annualrecipitation is 437.4 mm, 77% of which falls from June toeptember in a normal year. The mean annual temperature is 8.4 ◦C,ith a mean minimum of −9.7 ◦C in January and a mean maximum

f 23.7 ◦C in July. The annual cumulative temperature above 10 ◦C is228 ◦C. The mean frost-free season is 169 d. The annual potentialvaporation average is 785.4 mm, and the mean degree of desicca-ion (i.e., the ratio of potential evaporation to annual precipitation)s 1.8.

The natural vegetation type at the study site is shrubby grass-and. The major shrub species is Caragana korshinskii Kom., aeguminous shrub that can self-correct its N. The present studyocuses on grassland, in which the dominant vegetation is the Stipa

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gineering 60 (2013) 1– 9

ungeana. On average, the S. bungeana accounts for approximately5% and 75% of the total aboveground biomass at the shady andunny slopes, respectively. The second most-abundant plant onoth sites is the Lespedeza davurica Schindl.

The topography of Loess Plateau is characterized by gullies andavines. Slopes are the main land component. Some plant growthlements (e.g., solar radiation, soil temperature, and soil moisture)ary according to slope aspect. Two grassland sites were thus estab-ished: one on a shady slope facing north-northeast (38◦47′25.6′′

, 110◦22′14′′ E), and the other on a sunny slope facing south-outheast (38◦47′42.4′′ N, 110◦21′28′′ E). Approximately 60 sheepraze in the area once a month during the summer or fall; however,he grass is not cut. Grazing was excluded for nearly 1 year prioro the experiments and during the study by setting up net fencesround the study sites to prevent differential grazing betweenhem. The slope angle of the shady site was 6◦, and the slope aspectas 330◦ (with a north azimuth of 0◦). The slope angle of the sunny

ite was 10◦, and the slope aspect was 150◦.

.2. Plots and treatments

On each of the two slopes, 12 plots (3 m × 5 m) were establishedn August 2007, with the long side of each plot running down thelope. The plots were arranged parallel to one another, with 0.5 muffer strips between them. To reduce nutrient uptake by plantsurrounding them, the plots were separated from one anothery installation of concrete slabs buried to a depth of 35 cm andxtending 15 cm above the ground, thereby forming a randomizedxperimental design with three replicates of each treatment.

Four treatments were established comprising three N deposi-ion levels and a control: (1) control, which received only distilledater; (2) low N (2.5 g N m−2 yr−1); (3) medium N (5 g N m−2 yr−1);

nd (4) high N (10 g N m−2 yr−1). The low N treatment correspondedo the current N deposition level (2.2 g N m−2 yr−1), as measuredt approximately 90 km the south of the study site by Wei et al.2010), the value was higher considering that the study site wasloser to the coal factories and mine base. According to Gallowayt al. (2004), N deposition increases dramatically on Loess Plateau;he high N treatment level in the present study was based from theirrediction result of 2050. Nitrogen was applied as NH4NO3 solu-ion. The spray applications were scheduled in advance, roughlyalf month apart. The total amount of nutrients was divided amonghe seven application times spread over the growing season. Thereatments were initiated on 22 and 23 May 2008 at the shadynd sunny sites, respectively and were terminated in late August.t each application, the required amount of chemical fertilizer forach plot was weighed, dissolved in 1.5 L of distilled water, andpplied to each plot with a handheld high-pressure sprayer. Theontrol plots received only 1.5 L of distilled water. The total amountf water added to the plots by these treatments corresponded to.7 mm of extra precipitation per year. The experimental plots wereot protected from natural deposition.

.3. Mineralization and nitrification

Net mineralization and nitrification were measured using fieldncubation methods modified from Robertson et al. (1999). In eachlot, a pipe (25 cm long and 5 cm diameter) was inserted 20 cm intohe soil, which was cleared of vegetative matter, in mid-August. Athe same time, two soil samples were collected near each pipe, onerom the 0–10 cm layer, and the other from the 10–20 cm layer.

he collected samples were transported to the laboratory for ini-ial mineral N concentration measurement. The pipe remained inhe soil for 1 month before being removed and transported to theaboratory for mineral N measurement. During incubation, the pipe

X. Han et al. / Ecological Engineering 60 (2013) 1– 9 3

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ig. 1. Soil NO3−-N concentration (�g NO3

−-N g−1 soil) at the beginning (August) anet nitrification rates (�g NO3

−-N g−1 soil d−1). The upper figures are for sunny site

as covered with plastic film to minimize rapid moisture changesnd to prevent N leaching by precipitation. The soil in the pipes waslso divided into two samples: one from the 0–10 cm layer, and thether from the 10–20 cm layer. The samples were transported tohe laboratory for chemical analysis. The soil samples were siftedhrough a 2 mm sieve before measurement.

.4. Field sampling

Soil samples for measuring the total N and SOC were col-ected from a depth of 1 m divided into six layers (0–10, 10–20,0–40, 40–60, 60–80, and 80–100 cm) using a 2.5 cm-diameteroil auger. Three soil cores were collected from each plot. Thehree samples for each layer were bulked for a single repre-entative sample of that layer for each plot. The samples werearefully examined, and roots, leaves, and other unwanted mate-ial, such as large stones, were removed. All samples were air-driednd ground to pass through a 0.25 mm screen before chemicalnalysis.

Plant samples of the first and the second dominant species wereollected, namely, S. bungeana and L. davurica, respectively. Theboveground L. davurica and S. bungeana fresh tissue samples wereollected in mid-September. The fresh plant tissue samples wererought to the laboratory, washed with drinking water and distilledater, oven-dried at 105 ◦C for 30 min for deactivation of enzymes,

hen oven-dried again at 65 ◦C for approximately 72 h to achieveonstant weight. The dry samples were mashed, passed through a

mm screen, sealed in plastic bags for moisture protection, and setside for the chemical analysis.

setm

end (September) of the net mineralization field incubation period and the deducedthe lower are for shady site.

.5. Chemical analysis

SOC concentration is measured using the dichromate oxidationethod with the application of external heat (Nelson and Sommers,

982). Soil total N (STN) was measured using the Kjeldahl digestionrocedure (Bremner and Tabatabai, 1972), where 0.5000–1.0000 gf soil samples were digested in 5 mL concentrated H2SO4 and aatalyst containing K2SO4, CuSO4, and Se at a ratio of 100:10:1.oil mineral N was measured using the colorimetric method withutomatic flow injection (Page, 1982); 2.5 g of soil was mixed with5 mL 1 M KCl for a 1:1 solution and shaken for 60 min at roomemperature. The extracts were filtered through qualitative filteraper and analyzed colorimetrically using automatic flow injec-ion (FIAstar 5000 system, FOSS TECATOR co.). The total N of thelant tissue was determined using the Kjeldahl method (Bremnernd Tabatabai, 1972), in which 0.2000–0.5000 g of dry grass tis-ue sample was digested 5 mL concentrated H2SO4 until it becameimpid (H2O2 was added during the digestion). The digests werelso measured colorimetrically for P using the molybdenum blueethod in an auto-analyzer (Quikchem 3000, Lachat Instruments,ilwaukee, WI, USA).

.6. Statistical analysis

Data were statistically analyzed using Version 13.0 of the SPSS

oftware (SPSS Institute Inc., 2006). One-way ANOVA was used toxamine the main effects of N deposition levels for each site. Whenhe ANOVA test showed significant differences between treat-

ents, the least-significant-difference test with a significant level

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f 0.05 was then used to test for significant differences betweenreatments.

. Results

.1. NO3−-N and net nitrification

During the month of August, the soil NO3−-N concentration was

ignificantly higher in the N-added plots compared with the controlFig. 1), with the higher N addition resulting in a higher soil NO3

−-Noncentration in the 0–10 cm and 10–20 cm soil layers. This effectas more obvious on the shady site compared with the sunny site.fter incubation, the samples were collected in mid-September, theO3

−-N concentration of soils in the PVC pipe continued to show anncreasing trend due to the effect of the N addition; however, theifference between the medium- and high-N treatments became

ess, especially in the 0–10 cm layer, where the soil NO3−-N con-

entration was even lower than in the high-N treatment. NO3−-N

oncentration was significantly greater in the surface soil (0–10 cm)ompared with the subsurface soil (10–20 cm) (p < 0.05).

The effect of N addition on the net nitrification rates was notbvious (Fig. 1). Some relatively low-N, graphically low N, andedium-N treatments resulted in positive net nitrification rates,hereas some high-N treatments had negative net nitrification

ates.

.2. Mineral N and net mineralization

The soil mineral N concentration (NH4+-N + NO3

−-N) pattern inesponse to N addition was similar to that of the NO3

−-N con-entration. In mid-August, N addition significantly increased theineral N in the surface (0–10 cm) and subsurface (10–20 cm)

oil layers (Fig. 2). The high-N treatment plots had the greatestoil mineral N concentration at both sites (p < 0.05). The plots thateceived medium- and low-N treatments usually had greater min-ral N concentrations in comparison with the control plots, but noto a significant level (p > 0.05). In surface and subsurface soil lay-rs, the mineral N concentrations of the soil samples collected ineptember did not differ among the medium- and high-N treat-ents. The mineral N concentration decreased with the increase of

oil depth. Higher N treatments tended to have lower net mineral-zation rate; however, the effect was not significant.

.3. Profile distribution of soil organic carbon and total nitrogen

The added N failed to significantly alter SOC and STN throughouthe soil profile at either site (Fig. 3). The mean SOC and STN valuesecreased significantly with increasing soil depth (p < 0.001), andaried more widely in the upper soil layers, particularly at the shadyite.

SOC was higher in the top 20 cm soil of the shady site comparedo the sunny site; however, below 20 cm, it was generally lower.TN was linearly correlated with SOC (R2 = 0.944, p < 0.01); thus,he distribution pattern of STN was consistent with that of SOC.TN was higher in the top 20 cm soil of the shady site (Fig. 3c and). In the 20–100 cm layer, no significant difference between sitesas found.

.4. N, P, and N/P ration of S. bungeana and L. davurica

The N concentration of the aboveground plant samples (see

ig. 4a and b) showed a very clear dose response to the increasing Nnputs for the S. bungeana at both sites. The tissue N concentrationf S. bungeana (y) versus N addition level (x) followed a nice lin-ar relationship: y = 0.4159, x + 12.058 (R2 = 0.9536) for the shady

rnde

gineering 60 (2013) 1– 9

ite, and y = 0.6055, x + 12.869 (R2 = 0.9914) for the sunny site. Theffect of N was most pronounced in the sunny site, with N con-entration increments of 9.9%, 19.2%, and 46.3% for 2.5, 5, and 10 g

m−2 yr−1 treatment, respectively, in comparison with the control.he increases of tissue N concentration in the shady plots weremaller than in the sunny site in two of the three levels N treat-ents; the increments were 6.7%, 22.6%, and 33.4% for the three

reatments.The N concentration of L. davurica was approximately

7.5 mg g−1 dry weight, which is generally higher than S. bungeanan the control plots. Contrary to the results of S. bungeana, the Noncentration of L. davurica was not markedly altered in responseo the N treatments at both sites.

The treatment did not have an effect on the tissue P concen-ration (Fig. 4c and d). P concentration of L. davurica was generallyigher and had bigger variability compared with that of S. bungeana.ean ratio of nitrogen to phosphorus (N/P) at 14.6 and 14.1 were

btained for S. bungeana in the control plots on the shady and sunnylots, respectively (Fig. 4e and f). N treatments increased the N/P of. bungeana, especially for the sunny plots. However, the N/P ratiosf L. davurica varied highly, and showed no clear trend due to theffect of the increasing levels of N addition.

Combined with the results of the aboveground biomass previ-usly reported (see Han et al., 2011), the N pool in S. bungeanaas calculated. The results are listed in Table 1. The N pools in S.

ungeana were greater in the higher N treatment than in the lower treatment and control for both sites (Table 1). In shady plots, theigh N treatment resulted in significantly higher aboveground Nool. Compared to the control, the increment of the N pool in thehady plots was 0.41, 1.36, and 1.71 g N m−2, corresponding to anccumulation of 16.4%, 27.2%, and 17.2% of the added N, for 2.5,, and 10 g N m−2, respectively. In sunny plots, the effect of plant

absorption was more pronounced, with the N pool in the rangerom 2.75 g N m−2 to 2.20 g N m−2, and the accumulation rate in theange from 22.0% to 35.1%.

. Discussion

.1. Reponses of soil chemistry

The results of the soil and plant response in the first year of simu-ated N deposition could reflect the intrinsic characteristic (nutrienteficit condition) of grasslands on Loess Plateau, China, and enablehe analysis of the ability of plant and soil to absorb the additional.

Soil mineral N concentrations before and after incubation couldargely reveal the effect of the simulated N deposition on the soilhemical properties. The increased soil NO3

−-N concentration inesponse to the simulated N deposition, especially in the medium-nd high-N treatment, at both sites in the present study is not sur-rising (Aber et al., 1989). This phenomenon has been observed

n other simulated N deposition experiments, as well as in areasubject to high natural N deposition. For example, in an experi-ental study of N deposition influences, Vourlitis et al. (2007a)

eported that the soil-extractable mineral N (NO3−-N + NH4

+-N)as significantly increased in chaparral and coastal sage scrub in

urface (0–10 cm) and subsurface (30–40 cm) soil. Sirulnik et al.2007b) found that urban soils with chronic elevated levels of

deposition often had higher NO3−-N concentrations than soils

rom a rural site located in a protected habitat that has historically

eceived relatively low levels of urban pollution. Increased nitrateitrogen could affect water in despoil since it is easily leachedown (Emmett et al., 1993). Besides, The results of the soil min-ral N concentrations are of great influence in the present study

X. Han et al. / Ecological Engineering 60 (2013) 1– 9 5

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ig. 2. Soil mineral N concentrations (�g mineral N g−1 soil) at the beginning (Augeduced net mineralization rates (�g mineral N g−1 soil d−1). *Significant differencere for sunny site; and the lower are for shady site.

rea, Loess Plateau, China, where the serious soil erosion couldash off vast soils (Shi and Shao, 2000) and cause nonpoint pol-

ution (Brookshire et al., 2007). The increased concentrations ofineral N in the soil perhaps maintain soil N in an area where ero-

ion leads to N depletion and limitation, but will exacerbate theollution.

No significant difference in the SOC after the addition of N wasound in the present study. Although different results (increasingr decreasing) were also reasonable (Dijkstra et al., 2004; Hagedornt al., 2001; Hyvönen et al., 2008; Tu et al., 2011), the null effectsf the short-time N deposition in the SOC have frequently beeneported in studies on grassland and other ecosystems (Brady and

eil, 1996). In semi-arid shrublands, the N addition did not sig-ificantly alter the carbon pools during the one-year N deposition

xperiment (Vourlitis et al., 2007b). Neff et al. (2002) studied thetability and turnover of the soil carbon under long-term N depo-ition in dry meadow communities at an alpine site (Niwot Ridge)

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able 1 pool (SE in parentheses) in the aboveground tissue of S. bungeana.

Treatments Shady

N pool (g N m−2) Difference fromcontrol (g N m−2)

Percentage addedN accumulated (%

Control 2.17 (0.38) – –

Low N 2.68 (0.52) 0.41 16.4

Medium N 3.53 (0.74) 1.36 27.2

High N 3.89 (1.34)* 1.71 17.2

* Significant vs. control at p < 0.05 level.

nd the end (September) of the net mineralization field incubation period and thes control, p < 0.05; error bars are the standard error of the mean. The upper figures

n Colorado and found no significant changes in bulk soil carbon.n northern temperate forests, Nadelhoffer et al. (1999) estimatedhe changes in biomass and soil carbon caused by N depositionsing the 15N-tracer method, and concluded that N deposition didot significantly increase the carbon pools. The lack of response ofhe bulked SOC samples to N addition does not necessarily meanhat the N deposition had no effect on the processes involved inhe SOC cycle (Neff et al., 2002; Scheuner and Makeschin, 2005)ecause the SOC level is regulated by plant productivity (plant lit-er) (Han et al., 2011), decomposition processes (Dijkstra et al.,004), and potential soil erosion influenced by the vegetation cov-rage in the study area. The nitrogen addition could regulate thesehree processes, possibly in different directions, without detectableet changes. SOC in the observed increased aboveground biomass

Han et al., 2011) in the present study may change when theitter decomposes; therefore, further investigation should beonducted.

Sunny

)N pool (g N m−2) Additional N

accumulatedPercentage addedN accumulated (%)

1.87 (0.51) – –2.75 (0.81) 0.88 35.13.17 (0.48)* 1.30 27.24.07 (0.56)* 2.20 22.0

6 X. Han et al. / Ecological Engineering 60 (2013) 1– 9

Shady

0

20

40

60

80

100

0.1 0.2 0.3 0.4 0.5 0.6

STN (g kg-1)

Soil

dep

th (

cm)

control

low N

medium N

high N

Sunny

0

20

40

60

80

100

0.1 0.2 0.3 0.4 0.5 0.6

STN (g kg-1

)

Soil

dep

th (

cm)

Shady

0

20

40

60

80

100

1 2 3 4 5 6

SOC (g kg-1 )

Soil

dep

th (

cm)

Sunny

0

20

40

60

80

100

1 2 3 4 5 6

SOC (g kg-1 )

Soil

dep

th (

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(c) (d)

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Fig. 3. Distribution of SOC (a and b) and STN (c an

Soil total nitrogen was primarily in the form of organic N (morehan 99% in the present study), and stored in the soil organic mat-er, which explains the approximately parallel distribution of STNo that of SOC. On the contrary, the mineral N comprised a verymall fraction of the total N. The changes of STN in response tohort time N addition may be covered by other factors and are notignificant.

N deposition has been reposted to increase the net N mineral-zation and nitrification in other studies (Fenn et al., 2005; Lovettnd Rueth, 1999). For example, Sirulnik et al. (2007a) found thatertilization with nitrite ammonium elevated the net N mineral-zation. Morecroft et al. (1994) found that nitrite ammonium couldtimulate nitrification in the acidic soil. However, the relation-hip between N deposition and N mineralization or nitrificationeasured from late August to September was not clear in the

resent study. In most cases, the net N mineralization or nitrifi-ation was very low or negative in the present study. N addition

ad some positive effect on N mineralization and nitrificationhen deposition rate was less than 5 g N m−2 in the present

tudy, 5 g N m−2, while 10 g N m−2 decreased N mineralization anditrification.

sc

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ithin the soil profile at the shady and sunny sites.

.2. Responses of gramineous plant S. bungeana and leguminouslant L. davurica

When subjected to simulated N deposition, the gramineouslant S. bungeana and leguminous plant L. davurica had differentesponses in N and P concentrations. The aboveground tissue N con-entration of S. bungeana significantly increased, suggesting thathe added N stimulated the plant N uptake and increased the above-round N storage, a result consistent with several other gramineouslants (Morecroft et al., 1994). As the N deposition increased from.5 g m−2 yr−1 to 10 g m−2 yr−1, the N concentration of S. bungeanaontinued to grow, implying that S. bungeana could be a denotativelant of N deposition level in the study area. The clear linear rela-ionship between the tissue N concentration and the N depositionevel might be used to estimate the N deposition level from the Noncentration in S. bungeana. Because the direct measurement of

deposition is difficult, indirect estimation of N deposition using

uch relationship could have some advantage; therefore, more spe-ific work needs to be conducted.

The P concentration of S. bungeana did not change significantly;owever, the high N deposition treatment increased N/P. N/P in

X. Han et al. / Ecological Engineering 60 (2013) 1– 9 7

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ig. 4. N–P concentration, and N/P ratio of S. bungeana and L. davurica on the shad m−2 yr−1). *Significant difference vs. control, p < 0.05; error bars are ±1 SE.

lant foliage is useful in assessing the nutrient limitation in terres-rial ecosystems and to indicate the nitrogen saturation (Drenovskynd Richards, 2004; Tessier and Raynal, 2003). The increase of/P ration with high N treatment in the present study may indi-ate a P limitation under the high level of N addition, which islso an ecological problem caused by N deposition (Arroniz-Crespot al., 2008; Carfrae et al., 2007; Finzi, 2009; Kritzler and Johnson,010).

Unlike S. bungeana, the tissue N concentration of L. davurica wasot changed by the N addition. This result could not be surprising,

iven that the L. davurica is a leguminous plant. Leguminous plantsould self-feed N nutrient through bionitrogen fixation when soil

is not sufficient; usually, their growth is not N-limited and notensitive to N addition (Cao et al., 2011).

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t) and sunny (right) slopes exposed to simulated N treatments (0, 2.5, 5, and 10 g

.3. Soil and plant N pool

Soil is a direct acceptor of deposited N. In the present study, theoil active N (i.e., mineral N) content (N pool, calculated as N con-entration × bulk density × soil depth) in the 0–20 cm surface layeras greater in the N-added plots. If the complex N chemical and

iological reversion processes (mainly N mineralization and immo-ilization) were ignored and the added N as a nitrogen source wasssumed to be dispersed to different pools, the present data sug-est that only a fraction of the added N was recovered in the surface

oil. For example, compared with the control plots, 1.39 g m−2 and.61 g m−2 more N was accumulated in the surface soil layers of–20 cm in 5 g m−2 N treatment in shady and sunny sites, whichmount to 27.9% and 12.2% of the added N, respectively.

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N accumulation in the aboveground tissue of S. bungeana waslso significantly increased. The recovery rate of N in the above-round tissue of S. bungeana was in range of 16.4–35.1%. This valueas greater than the Scots pine stands in Sweden with recover-

ng rates of around 9%, 5%, and 3% of the total N application forhe 4.5 g m−2 N, 9 g m−2 N, and 18 g m−2 N regimes (Ring et al.,011). As the N deposition increased, the N recovery amount in theoil showed an increasing trend in the present study. However, theelationship between N deposition rate and tissue N accumulationraction of S. bungeana was not obvious, which is understandableecause the aboveground N pool size is sensitive to the N uptake,llocation, and aboveground biomass production, and because theoverage of S. bungeana is not constant. The sum of S. bungeana and. davurica biomass generally accounts for more than 95% of theotal aboveground biomass (Han et al., 2011). Furthermore, N con-entration, as well as the aboveground biomass of L. davurica wasot significantly affected by the N addition (Han et al., 2011). Thus,

absorbed by S. bungeana represented the most N accumulated inhe aboveground plant tissue of the whole plot.

The recovery rates of added N in the soil and aboveground tis-ue were in the range of 34–55%. The rest of the added N wento other pools, such as stored in belowground biomass, absorbedy the minority plants other than S. bungeana, lost in the runoffZheng, 2005), leached down in deep soil (Singleton et al., 2001;

illiamson et al., 1998), and/or volatilized to the air (Phoenix et al.,003). Melin and Nômmik (1988) found that nearly 50% of the Ndded as NH4NO3 at 150 kg N ha−1 yr−1 was present as organic N inhe litter layer down to a depth of 85 cm in the mineral soil a yearfter application.

.4. Comparison between shady and sunny sites

The present study was conducted in shady and sunny slopes.he effects at the two sites on the soil and plant were very similar,lthough they differed between sites in some ways. For instance,he increase in surface soil mineral N concentration was greater onhe shady site than the sunny site, especially when the simulated Neposition rate was high (Figs. 1 and 2). On the contrary, when sub-

ected to N addition, N concentration of S. bungeana abovegroundissue was greater on the sunny site (Fig. 4a and b). The differ-nt increment of the surface soil mineral N concentration could bettributed to a large extent to the soil conditions. As indicated by theOC concentration, which results from greater productivity (bet-er soil moisture condition) and lower decomposition rate (loweremperature), the shady site was more fertile. Furthermore, the soilexture was finer, which enabled the soil to retain more nutrientsi.e., mineral N in the present study). In contrast, the coarse soil onhe sunny site had stronger permeability resulting in greater leach-ng. The surface runoff was found to be greater on the sunny site.oth conditions could decrease the soil mineral N concentration.

The higher tissue N concentration on the sunny site was unpre-ictable, but could be explained. The tissue N concentration wasetermined by the plant N uptake, as well as biomass productionLu et al., 2011). Assuming that the uptake ability and allocationf S. bungeana was similar on the two sites, the excessive plant Nptake, and limited plant growth on the sunny site likely becausef the soil water shortage (Han et al., 2011) resulted in the greater

concentration in the sunny site.

. Conclusions

The present study has reported the changes of soil and domi-ant plant species after the first year of the simulated N deposition.he plant was found to be very sensitive to N addition. After 1

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ear of simulated N deposition, the N concentration in the above-round tissue of the dominant plant S. bungeana was significantlyncreased, and showed a clear linear relationship with the simu-ated N addition levels. N/P ratio of S. bungeana was also increased;owever, the L. davurica chemistry was not significantly influenced.he soil organic carbon and the soil total N were not susceptible tohe simulated N deposition; however, the mineral N concentrationf the soil was significantly increased. The results of the presenttudy show that only a small fraction of the added N was recoveredn the 0–20 cm soil layer and the aboveground tissue. More thanalf of the added N went to other pools. On the shady and sunnyites, the soil and plant response to the N addition showed a simi-ar trend, but differed in the extent of N concentration changes. Thencrease in soil mineral N was generally greater in the shady site,

hereas in the sunny site, the increase of plant N concentrationboveground was greater.

cknowledgements

This research was supported by the Japan Society for the Pro-otion of Science and Core University Program, China Postdoctoral

cience Foundation (No. 2012T50130), and the Global Center ofxcellence Program of the Ministry of Education, Culture, Sports,cience and Technology of Japan.

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