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Impact of land use change on pro

Impact of land use change on pro ?le distributions of soil organic carbon fractions in the Yanqi Basin

Juan Zhang a ,b ,Xiujun Wang a ,c ,?,Jiaping Wang a ,b

a State Key Laboratory of Desert and Oasis Ecology,Xinjiang Institute of Ecology and Geography,Chinese Academy of Sciences,Urumqi,Xinjiang 830011,China

b Graduate University of Chinese Academy of Sciences,Beijing 100049,China

Earth System Science Interdisciplinary Center,University of Maryland,College Park,MD 20740,USA

a b s t r a c t

a r t i c l e i n f o Article history:

Received 8May 2013

Received in revised form 6November 2013Accepted 27November 2013Keywords:

Land use change SOC

SOC fraction Native land Cropland

Land use change is recognized as one important driving force for soil organic carbon (SOC)dynamics.The arid regions in China have experienced signi ?cant land use changes over the past decades.A study was carried out to evaluate the impacts of land use change on SOC fractions in the Yanqi Basin,northwest China.Soil samples were collected from 24pro ?les in cropland and native land,and labile,semi-labile,and recalcitrant organic carbon were measured.All SOC fractions showed a gradual decrease with depth over the 0–100cm in the native land.However,SOC fractions in the cropland revealed uniform distributions over the 0–30cm and 30–100cm.On average,labile,semi-labile,and recalcitrant carbon contents in the cropland were 2.2±0.3(1.3±0.4),1.5±0.4(0.7±0.3),and 8.5±2.0(3.1±1.8)g kg ?1over the 0–30cm (30–100cm),respec-tively.Converting native land to cropland resulted in signi ?cant increases of recalcitrant (2.0kg m ?2),semi-labile (0.3kg m ?2),and labile carbon (0.3kg m ?2)over the 0–30cm.The proportion of recalcitrant SOC stock increased from 59.9%in the native land to 64.8%in the cropland.This study suggests that converting native land to cropland in arid region not only enhances SOC stocks but also leads to longer-term SOC storage.

1.Introduction

Soil organic carbon (SOC),the largest carbon pool on land,plays an important role in the global carbon cycle.The global SOC pool is proxi-mately 1500Pg C in the top 1m,which is two times of the global terrestrial biomass (Amundson,2001;Jobbágy and Jackson,2000).Thus,small changes in the SOC stock may have large impacts on the atmospheric CO 2concentration.Therefore,the stability of SOC is critical to the global carbon cycle and climate change (Belay-Tedla et al.,2009).

Soil organic carbon dynamics is determined by the balance between inputs (e.g.,addition of plant residues)and outputs (e.g.,SOC decompo-sition),which is in ?uenced by many factors,such as climate conditions,soil properties,and land use management (Jobbágy and Jackson,2000;Wang et al.,2001).Temperature has a large effect on both carbon ?xation and SOC decomposition in humid climate zones whereas precipitation constrains plant growth (thus carbon inputs)and SOC decomposition in arid regions (Jobbágy and Jackson,2000).Soil properties,especially texture can affect SOC decomposition because clay may act as aggregates by binding particles together,which provides physical protection

(Bronick and Lal,2005).On the other hand,land use change may impact SOC dynamics by changing the rates of carbon inputs and decomposition of SOC in soil (Li et al.,2010;Poeplau et al.,2011).

In addition to these external factors,SOC stability is also in ?uenced by the chemical structure of SOC,which is a heterogeneous mixture of compounds with various turnover times (Krull et al.,2003;Parton et al.,1987).Generally,SOC pool can be chemically divided into labile,semi-labile,and recalcitrant pools that have different sensitivities to changes of environmental conditions (Parton et al.,1987;Rovira and Vallejo,2002).For example,labile pool is more active and sensitive to physical and chemical disturbances than other fractions (Purakayastha et al.,2007;Zhang et al.,2012).Changes in SOC fractions may provide an early indicator of changes in total SOC (Banger et al.,2009).

There has been evidence of land use change impacts on SOC dynamics.For instance,a few studies indicated that converting lands with native vegetation (i.e.forest and pasture)to cropland resulted in loss of SOC in tropical and temperate humid regions (Del Grosso et al.,2009;Dinesh et al.,2003;Post and Kwon,2000;Wang et al.,2001).However,some other studies in arid regions showed different results.For example,Fallahzade and Hajabbasi (2012)reported that SOC content increased 3–7times for the upper 30cm after converting desert land to cropland in arid land of the Central Iran.Cochran et al.(2007)suggested that converting shrub land to cropland increased labile and semi-labile SOC fractions in the 0–20cm in a semi-arid region of the Columbia Basin.Zhang et al.(2012)also showed that conversion from desert steppe to

Catena 115(2014)79–84

?Corresponding author at:5825University Research Court,College Park,MD 20740-3823,USA.Tel.:+13014051532.

E-mail address:wwang@https://www.wendangku.net/doc/738742675.html, (X.

https://www.wendangku.net/doc/738742675.html,/10.1016/j.catena.2013.11.019

Contents lists available at ScienceDirect

Catena

j ou r n a l h o m e pa ge :ww w.e l s e v i e r.c o m /l oc a t e /c a t e n a

arable land led to an increase in total SOC stock and labile SOC stock after 50years cultivation in the Longzhong region of Loess Plateau,China.

Here,we present a study carried out in a typical arid area,the Yanqi Basin that is located in Xinjiang,northwest China.There have been land use changes since1950,i.e.,converting native land to cropland.We collected soil samples from1m soil pro?les in both native land and cropland,and determined labile,semi-labile,and recalcitrant SOC frac-tions.The objective of our study is to examine the vertical distributions of SOC fractions,and to evaluate the impacts of land use change.

2.Material and methods

2.1.Experimental site

The Yanqi Basin(41°53′–42°51′N,86°46′–85°08′E,1037–1339m in altitude)is in a transition region between the northern and the southern part of Xinjiang,with the continental desert climate condition.Average annual precipitation is less than80mm,with60%of the rainfall during summer.Annual evaporation varies from2000to2449mm.Annual mean temperature is8.5°C,annual cumulative temperature above 10°C is3414–3694°C,and sunshine time from3074to3163h yr?1. Brown Desert soil and Grey-brown Desert soil,developed from limestone parent material,are the main soil types,and classi?ed as a Haplic Calcisol (FAO-UNESCO-ISRIC,1988).Sampling sites span both sides of the Kaidu River(Fig.1).The typical native vegetations are Phragmites australis (Cav.)Trin.ex Steud.,Alhagi sparsifolia Shap.,and Tamarix ramosissima Ledeb.Main crops are hot pepper(Capsicum annuum Linn),tomato (Solanum lycopersicum),and corn(Zea mays)et al.

2.2.Soil sampling and analyses

Soil samples were collected in August and November,2010from the native land(12pits)and cropland(12pits).We collected120soil samples from?ve layers,i.e.,0–5cm,5–15cm,15–30cm,30–50cm, and50–100cm.These samples were air-dried,thoroughly mixed,and passed through a2mm sieve for pH and electrical conductivity(EC). Representative sub-samples were crushed to0.25mm for total SOC,SOC fractionation,and total nitrogen(TN)measurement.Soil pH and EC were measured at1:5soil-to-water ratio using pH and conductivity meters.Total SOC was measured by the Walkley–Black method (Walkley and Black,1934).Soil TN was determined by a KJELTEC2300 type fully automatic azotometer(Shiyomi et al.,2011).Soil bulk density (BD)was also measured in this study,by the core method(Blake and Hartge,1986).

We used the two-step acid hydrolysis procedure with H2SO4as the extractant to determine labile and semi-labile carbon,which was re-ported by Rovira and Vallejo(2002).Brie?y,20mL of5N H2SO4was added to0.5–1.0g soil,and hydrolyzed for30min at105oC in sealed Pyrex tubes.The hydrolysate was recovered by centrifugation and decantation,and prepared for labile carbon analysis.The remaining res-idue was hydrolyzed with2mL of26N H2SO4overnight at room tem-perature under continuous shaking.The concentration of the acid was then brought down to2N by dilution with de-ionized water and the sample was hydrolyzed for3h at105oC with occasional shaking.The hydrolysate was recovered and prepared for semi-labile carbon analysis. The remaining residue was dried at60oC,then prepared for recalcitrant carbon analysis by the Walkley–Black method(Walkley and Black, 1934).

2.3.Statistical analysis

We use independent sample t-test to determine the signi?cance for the differences in SOC fractions for each layer among land use types.All the statistical analyses are carried out using SPSS18.0(Statistical Package for Social Science)and all the?gures are produced using Origin8.5 software.

3.Results

3.1.Soil properties

Soil properties in surface layer are shown in Table1.Generally,soil pH is higher than8in this region,with no signi?cant difference between the native land and cropland.On average,soil BD is1.5g cm?3for

the

Fig.1.Map of Xinjiang and locations of soil sampling in the Yanqi Basin.

80J.Zhang et al./Catena115(2014)79–84

native land and 1.3g cm ?3for the cropland.Soil water content is higher (18.4%)in the cropland than in the native land (12.1%).Soil EC is much lower in the cropland (0.7ms cm ?1)than in the native land (6.1ms cm ?1),which may be a result of irrigation on cropland.A similar result was reported by Li et al (2007a)who showed a decrease in salt content following conversion of native land to cropland.Average surface SOC content is higher (9.4g kg ?1)in the cropland than in the native land (6.6g kg ?1).Soil TN content shows a larger variation in the native land (0.4–1.0g kg ?1)than in the cropland (0.9–1.2g kg ?1).On average,the C/N ratio shows no signi ?cant difference between the cropland (11.5)and native land (10.1).3.2.Vertical distribution of total SOC

Fig.2presents vertical distributions of total SOC in the native land and cropland.In general,total SOC content is lower in the native land than in the cropland,particularly in the topsoil.The native land reveals small vertical variation in total SOC over depth whereas the cropland shows a pronounced decreasing trend with depth except at the I4site.There is little vertical change in total SOC below 40cm at most sites in both the native land and cropland.

3.3.Vertical distributions of SOC fractions on native land

Vertical distributions of the SOC fractions in the native land are shown in Fig.3.There is a considerable variability among the sampling sites,particularly in the labile carbon that ranges from b 1g kg ?1to N 2g kg ?1.Both labile and semi-labile fractions show small vertical variation over depth when semi-labile carbon is less than 1g kg ?1in the surface layer (see sites A1,B2,E and H).However,vertical distribu-tions of the labile and semi-labile carbon show great difference when semi-labile carbon is N 1g kg ?1in the surface layer.Semi-labile carbon is signi ?cantly higher in the 0–30cm (N 0.85g kg ?1)than below 50cm

Table 1

Plant species and soil properties in surface layer of the sampling sites.Sites Plant species

pH BD

(g cm ?3)SWC (%)EC

(ms cm ?1)SOC

(g kg ?1)TN

(g kg ?1)C/N

A1Phragmites australis (Cav.)Trin.ex Steud.7.7 1.6 4.7 2.4 4.90.411.6A2Phragmites australis (Cav.)Trin.ex Steud.8.4 1.716.311.47.20.79.7A3Phragmites australis (Cav.)Trin.ex Steud.8.2 1.325.1 3.810.6 1.110.1B1Alhagi sparsifolia Shap.8.7 1.417.50.3 4.70.68.3B2Alhagi sparsifolia Shap.

8.2 1.6 4.70.8 6.50.610.2C1Tamarix ramosissima Ledeb.8.6 1.616.514.19.50.910.1C2Tamarix ramosissima Ledeb.

8.6 1.617.112.6 3.90.49.2D Halostachys caspica C.A.Mey.ex Schrenk 8.4 1.518.311.5 6.10.512.6E Populus tomentosa Carr 8.4 1.6 6.90.1 6.80.513.8F Glycyrrhiza uralensis Fisch.7.9 1.38.8 5.28.40.810.3G Sophora alopecuroides Linn 8.1 1.5 5.6 1.88.10.98.9H Acroption repens DC.Prodr.

8.0 1.7 3.89.1 3.00.4 6.7Mean 8.3 1.512.1 6.1 6.60.710.1S.D.0.30.17.1 5.3 2.30.2 1.9I1Capsicum annuum Linn 8.1 1.312.00.211.00.911.7I2Capsicum annuum Linn 8.0 1.318.10.412.6 1.013.2I3Capsicum annuum Linn 7.7 1.315.3 1.610.60.812.6I4Capsicum annuum Linn 8.1 1.619.00.28.6 1.17.6J1Solanum lycopersicum 8.3 1.320.90.38.7 1.08.8J2Solanum lycopersicum 8.4 1.320.50.413.3 1.49.5K1Zea mays 8.5 1.419.30.213.2 1.211.5K2Zea mays 8.0 1.223.2 1.214.1 1.211.7L Beta vulgaris 8.4 1.110.50.412.70.913.6M Gossypium spp.8.1 1.320.3 1.416.6 1.213.8N Helianthus annuus 8.2 1.222.50.413.8 1.112.1O Brassica campestris L.

8.5 1.419.3 2.214.9 1.311.3Mean 8.2 1.318.40.79.5 1.111.5S.D.

0.2

0.1

3.9

0.7

2.4

0.2

1.9

Note:Native land,sites A1–H;Cropland,sites I1–O;S.D.,standard deviations;BD,bulk density;SWC,soil water content;EC,Electrical

conductivity.

Fig.2.Pro ?le distribution of total SOC in the native land (a)and cropland (b).

J.Zhang et al./Catena 115(2014)79–84

(~0.50g kg?1)(also see Table2).The exception is found at the sites A3 and F that show a small increase in the bottom layer.Recalcitrant carbon in the surface soil is N4g kg?1in~75%of the pro?les,particularly at the site A3where the carbon content reaches9.30g kg?1.Overall,there is a sharp decrease in recalcitrant carbon over depth,from4.5g kg?1near the surface to1.9g kg?1below50cm.

3.4.Vertical distribution of SOC fractions on cropland

Vertical distributions of the SOC fractions reveal relatively uniform distributions over the0–30cm and30–100cm in most pro?les in the cropland,but a sharp decrease around the30cm depth(see Fig.4).In general,SOC fractions tend to be the highest in the5–15cm layer, particularly for the semi-labile and recalcitrant carbon.On average, labile carbon is N2g kg?1above30cm,but b1.5g kg?1below30cm. Semi-labile carbon is around1.5g kg?1in the0–30cm,but less than 1g kg?1for two thirds of pro?les below30cm.Recalcitrant carbon changes from7.8to9g kg?1above30cm to b3.8g kg?1below 30cm(see Table2).On average,the variations of labile,semi-labile, and recalcitrant carbon in the cropland are26.2%,34.3%,and36.5% respectively(Table2).4.Discussion

Native arid land is characterized by sparse vegetation coverage and low SOC storage(Li et al.,2010).Our results show that total SOC stock in the upper1m is6.1kg m?2for the native land,and9.8kg m?2 for the cropland in the Yanqi Basin.While those values are relatively lower than those reported by Wang et al.(2003),i.e.,12.1kg m?2in native shrub land and10.9kg m?2in cropland in northwest China, the SOC stock is higher than the mean value of5.4kg m?2in Xinjiang reported by Li et al.(2007b).

A modeling study was conducted for a farmland in northern Xinjiang,which showed0.1,1.5,and0.7kg m?2for labile,semi-labile, and recalcitrant carbon,respectively,over the0–20cm(Xu et al., 2011).Our data show that the stocks(over0–30cm)of labile,semi-labile,and recalcitrant carbon are0.8,0.65,and3.5kg m?2,respectively, in the cropland.It appears that there are some differences in the de?ni-tions of these SOC pools.

The percentages of labile,semi-labile,and recalcitrant SOC are 22–25%,13–15%,and60–65%,respectively,in the Yanqi basin (Table3).These results are in line with those on Mediterranean native lands at La Vall de Gallinera,Alacant province(E Spain),with

the Fig.3.Pro?le distributions of labile,semi-labile and recalcitrant carbon(g kg?1)in the native land.

Table2

Means and coef?cients of variation(CV)for labile,semi-labile and recalcitrant carbon(g kg?1)in the native land and cropland.

Land use types Depths

(cm)Labile carbon Semi-labile carbon Recalcitrant carbon Total SOC

Mean

(g kg?1)

(%)

Mean

(g kg?1)

(%)

Mean

(g kg?1)

(%)

Mean

(g kg?1)

(%)

Native land0–5 1.3439.050.8331.96 4.4745.90 6.6434.25 5–15 1.1849.240.6836.18 3.4368.85 5.2956.69

15–30 1.0852.180.7944.78 2.6659.77 4.5444.86

30–500.9755.700.5645.88 2.2075.42 3.7461.94

50–1000.9280.610.4929.34 1.9279.05 3.3369.95 Cropland0–5 2.2518.20 1.4526.908.7522.5012.4519.18 5–15 2.2216.20 1.6734.109.0222.0012.9119.41

15–30 2.0929.70 1.4119.107.8125.2011.3121.04

30–50 1.3629.400.7842.30 3.7365.10 5.8751.65

50–100 1.2037.500.6349.20 2.4347.70 4.2641.46 82J.Zhang et al./Catena115(2014)79–84

percentages of labile,semi-labile,and recalcitrant carbon being20–30%, 10–15%,and55–70%,respectively(Rovira et al.,2012).A similar proportion of recalcitrant carbon(54%)was also reported in semi-arid shrub-steppe ecosystem at Grant County,Washington(Cochran et al., 2007).There has been evidence that carbon sequestration ef?ciency is much higher in the arid and semi-arid regions than in the humid region (Bolinder et al.,2007;Yan et al.,2007;Zhang et al.,2010).

Converting native land to irrigated cropland in the Yanqi Basin results in an increase of SOC stock(0–1m)by3.7kg m?2,with0.6, 0.4,and2.8kg m?2,for the labile,semi-labile,and recalcitrant carbon, respectively.Zhang et al.(2012)reported that in the Longzhong region of the Loess Plateau,conversion from desert steppe to cropland increased SOC stock from3.9to5.1kg m?2in the top1m as a result of20years of cropping.These results imply that the longer the cropping,the greater the SOC increase.

The increase of SOC stock is mainly found in the0–30cm following conversion of native land to cropland.The labile,semi-labile,and recalci-trant carbon pools show an increase of0.3,0.3,and2.0kg m?2,respec-tively,which are statistically signi?cant(see Table4).The recalcitrant carbon stock shows~40%increase over the0–30cm as a result of cropping,which may re?ect different properties between crops and native plants.For example,crops have relatively higher C:N ratio(data not shown).

The increase of SOC stock in cropland may attribute to fertilization and irrigation which can increase plant production and the rate of plant residue return into soil(Fallahzade and Hajabbasi,2012).In addi-tion,agricultural practices,such as incorporation of plant residue and addition of manure are also responsible for the SOC accumulation (Rasmussen and Collins,1991).

It is known that soil respiration is largely related to the small,but bio-reactive labile carbon pool whereas long-term carbon storage is associated with the recalcitrant carbon fraction(Trumbore et al., 1990).Our study shows that the proportion of recalcitrant SOC increases from59.9%to64.8%following the land use change,implying that converting native land to cropland may lead to long-term carbon storage.

5.Conclusion

This study demonstrates a signi?cant increase in both total SOC and SOC fractions as a result of land use change from native land to irrigated cropland in the Yanqi Basin.The labile,semi-labile,and recalcitrant carbon stocks increase from1.5,0.9,and3.6kg m?2in the native land to2.1,1.3,and6.4kg m?2in the cropland,respectively.The increase is mainly found in the top30cm,following the order:recalci-trant pool(2.0kg m?2)N semi-labile pool(0.3kg m?2)≈labile pool (0.3kg m?2).The proportion of recalcitrant SOC stock increases

from Fig.4.Pro?le distributions of labile,semi-labile and recalcitrant carbon(g kg?1)in the cropland.

Table3

Total SOC stock(0–100cm)and percentage of each fraction.

Land use types Labile carbon Semi-labile carbon Recalcitrant carbon

Native land(kg m?2) 1.530.90 3.63

Cropland(kg m?2) 2.12 1.34 6.38

Native land(%)25.214.959.9

Cropland(%)21.513.664.8Table4

Means and standard deviations(S.D.)of SOC fractions(kg m?2).

Land use types Depths

(cm)

Labile carbon Semi-labile

carbon

Recalcitrant

carbon

Mean S.D.Mean S.D.Mean S.D. Native land0–300.530.260.350.14 1.490.89?

30–100 1.000.730.550.19 2.14 1.67??Cropland0–300.850.140.650.14 3.460.85

30–100 1.270.450.690.33 2.92 1.57 Difference0–300.32???0.30??? 1.97???

30–1000.270.140.78

Note:Signi?cance of the difference was determined by t test.

?P b0.05.

??P b0.01.

???P b0.001.

J.Zhang et al./Catena115(2014)79–84

59.9%to64.8%following long-term cropping.Thus,we conclude that converting native land to cropland in arid region may also lead to long-term SOC storage.

Acknowledgment

This study is?nancially supported by the Hundred Talented Program of the Chinese Academy of Sciences.We would like to acknowledge Dongmei Peng for her technical work on the map creation.We are grateful for the reviewer's constructive comments.

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