Catena 115 (2014) 79–84

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Impact of land use change on profile distributions of soil organic carbonfractions 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, Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, Chinac Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740, USA

⁎ Corresponding author at: 5825University Research CouUSA. Tel.: +1 301 405 1532.

E-mail address: wwang@essic.umd.edu (X. Wang).

0341-8162/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.catena.2013.11.019

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 May 2013Received in revised form 6 November 2013Accepted 27 November 2013

Keywords:Land use changeSOCSOC fractionNative landCropland

Land use change is recognized as one important driving force for soil organic carbon (SOC) dynamics. The aridregions in China have experienced significant land use changes over the past decades. A study was carried outto evaluate the impacts of land use change on SOC fractions in the Yanqi Basin, northwest China. Soil sampleswere collected from 24 profiles in cropland and native land, and labile, semi-labile, and recalcitrant organiccarbon were measured. All SOC fractions showed a gradual decrease with depth over the 0–100 cm in thenative land. However, SOC fractions in the cropland revealed uniform distributions over the 0–30 cm and30–100 cm. 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−1 over the 0–30 cm (30–100 cm), respec-tively. Converting native land to cropland resulted in significant increases of recalcitrant (2.0 kg m−2), semi-labile (0.3 kg m−2), and labile carbon (0.3 kg m−2) over the 0–30 cm. The proportion of recalcitrant SOCstock increased from 59.9% in the native land to 64.8% in the cropland. This study suggests that converting nativeland to cropland in arid region not only enhances SOC stocks but also leads to longer-term SOC storage.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Soil organic carbon (SOC), the largest carbon pool on land, plays animportant role in the global carbon cycle. The global SOC pool is proxi-mately 1500 Pg C in the top 1 m, which is two times of the globalterrestrial biomass (Amundson, 2001; Jobbágy and Jackson, 2000).Thus, small changes in the SOC stock may have large impacts on theatmospheric CO2 concentration. Therefore, the stability of SOC is criticalto the global carbon cycle and climate change (Belay-Tedla et al., 2009).

Soil organic carbon dynamics is determined by the balance betweeninputs (e.g., addition of plant residues) and outputs (e.g., SOC decompo-sition), which is influenced 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 carbonfixationand SOC decomposition in humid climate zones whereas precipitationconstrains plant growth (thus carbon inputs) and SOC decomposition inarid regions (Jobbágy and Jackson, 2000). Soil properties, especiallytexture can affect SOC decomposition because clay may act as aggregatesby binding particles together, which provides physical protection

rt, College Park,MD20740-3823,

ghts reserved.

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

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

There has been evidence of landuse change impacts on SOCdynamics.For instance, a few studies indicated that converting lands with nativevegetation (i.e. forest and pasture) to cropland resulted in loss of SOC intropical and temperate humid regions (Del Grosso et al., 2009; Dineshet al., 2003; Post and Kwon, 2000; Wang et al., 2001). However, someother studies in arid regions showed different results. For example,Fallahzade and Hajabbasi (2012) reported that SOC content increased3–7 times for the upper 30 cm after converting desert land to croplandin arid land of the Central Iran. Cochran et al. (2007) suggested thatconverting shrub land to cropland increased labile and semi-labile SOCfractions in the 0–20 cm in a semi-arid region of the Columbia Basin.Zhang et al. (2012) also showed that conversion from desert steppe to

80 J. Zhang et al. / Catena 115 (2014) 79–84

arable land led to an increase in total SOC stock and labile SOC stock after50 years cultivation in the Longzhong region of Loess Plateau, China.

Here, we present a study carried out in a typical arid area, the YanqiBasin that is located in Xinjiang, northwest China. There have been landuse changes since 1950, i.e., converting native land to cropland. Wecollected soil samples from 1 m soil profiles in both native land andcropland, and determined labile, semi-labile, and recalcitrant SOC frac-tions. The objective of our study is to examine the vertical distributionsof 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, 1037–1339 m inaltitude) is in a transition region between the northern and the southernpart of Xinjiang, with the continental desert climate condition. Averageannual precipitation is less than 80 mm, with 60% of the rainfall duringsummer. Annual evaporation varies from 2000 to 2449 mm. Annualmean temperature is 8.5 °C, annual cumulative temperature above10 °C is 3414–3694 °C, and sunshine time from 3074 to 3163 h yr−1.BrownDesert soil andGrey-brownDesert soil, developed from limestoneparentmaterial, are themain soil types, and classified as a Haplic Calcisol(FAO-UNESCO-ISRIC, 1988). Sampling sites span both sides of the KaiduRiver (Fig. 1). The typical native vegetations are Phragmites australis(Cav.) Trin. ex Steud., Alhagi sparsifolia Shap., and Tamarix ramosissimaLedeb. Main crops are hot pepper (Capsicum annuum Linn), tomato(Solanum lycopersicum), and corn (Zea mays) et al.

2.2. Soil sampling and analyses

Soil sampleswere collected in August and November, 2010 from thenative land (12 pits) and cropland (12 pits). We collected 120 soilsamples from five layers, i.e., 0–5 cm, 5–15 cm, 15–30 cm, 30–50 cm,and 50–100 cm. These samples were air-dried, thoroughly mixed, andpassed through a 2 mm sieve for pH and electrical conductivity (EC).Representative sub-samples were crushed to 0.25 mm for total SOC,

Fig. 1.Map of Xinjiang and locations o

SOC fractionation, and total nitrogen (TN) measurement. Soil pH andEC were measured at 1:5 soil-to-water ratio using pH and conductivitymeters. Total SOC was measured by the Walkley–Black method(Walkley and Black, 1934). Soil TN was determined by a KJELTEC 2300type fully automatic azotometer (Shiyomi et al., 2011). Soil bulk density(BD) was also measured in this study, by the core method (Blake andHartge, 1986).

We used the two-step acid hydrolysis procedure with H2SO4 as theextractant to determine labile and semi-labile carbon, which was re-ported by Rovira and Vallejo (2002). Briefly, 20 mL of 5 N H2SO4 wasadded to 0.5–1.0 g soil, and hydrolyzed for 30 min at 105 ºC in sealedPyrex tubes. The hydrolysate was recovered by centrifugation anddecantation, and prepared for labile carbon analysis. The remaining res-idue was hydrolyzed with 2 mL of 26 N H2SO4 overnight at room tem-perature under continuous shaking. The concentration of the acid wasthen brought down to 2 N by dilution with de-ionized water and thesample was hydrolyzed for 3 h at 105 ºC with occasional shaking. Thehydrolysate was recovered and prepared for semi-labile carbon analysis.The remaining residue was dried at 60 ºC, then prepared for recalcitrantcarbon analysis by the Walkley–Black method (Walkley and Black,1934).

2.3. Statistical analysis

We use independent sample t-test to determine the significance forthe differences in SOC fractions for each layer among land use types. Allthe statistical analyses are carried out using SPSS 18.0 (Statistical Packagefor Social Science) and all the figures are produced using Origin 8.5software.

3. Results

3.1. Soil properties

Soil properties in surface layer are shown in Table 1. Generally, soilpH is higher than 8 in this region,with no significant difference betweenthe native land and cropland. On average, soil BD is 1.5 g cm−3 for the

f soil sampling in the Yanqi Basin.

Table 1Plant 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

A1 Phragmites australis (Cav.) Trin. ex Steud. 7.7 1.6 4.7 2.4 4.9 0.4 11.6A2 Phragmites australis (Cav.) Trin. ex Steud. 8.4 1.7 16.3 11.4 7.2 0.7 9.7A3 Phragmites australis (Cav.) Trin. ex Steud. 8.2 1.3 25.1 3.8 10.6 1.1 10.1B1 Alhagi sparsifolia Shap. 8.7 1.4 17.5 0.3 4.7 0.6 8.3B2 Alhagi sparsifolia Shap. 8.2 1.6 4.7 0.8 6.5 0.6 10.2C1 Tamarix ramosissima Ledeb. 8.6 1.6 16.5 14.1 9.5 0.9 10.1C2 Tamarix ramosissima Ledeb. 8.6 1.6 17.1 12.6 3.9 0.4 9.2D Halostachys caspica C. A. Mey. ex Schrenk 8.4 1.5 18.3 11.5 6.1 0.5 12.6E Populus tomentosa Carr 8.4 1.6 6.9 0.1 6.8 0.5 13.8F Glycyrrhiza uralensis Fisch. 7.9 1.3 8.8 5.2 8.4 0.8 10.3G Sophora alopecuroides Linn 8.1 1.5 5.6 1.8 8.1 0.9 8.9H Acroption repens DC. Prodr. 8.0 1.7 3.8 9.1 3.0 0.4 6.7Mean 8.3 1.5 12.1 6.1 6.6 0.7 10.1S.D. 0.3 0.1 7.1 5.3 2.3 0.2 1.9I1 Capsicum annuum Linn 8.1 1.3 12.0 0.2 11.0 0.9 11.7I2 Capsicum annuum Linn 8.0 1.3 18.1 0.4 12.6 1.0 13.2I3 Capsicum annuum Linn 7.7 1.3 15.3 1.6 10.6 0.8 12.6I4 Capsicum annuum Linn 8.1 1.6 19.0 0.2 8.6 1.1 7.6J1 Solanum lycopersicum 8.3 1.3 20.9 0.3 8.7 1.0 8.8J2 Solanum lycopersicum 8.4 1.3 20.5 0.4 13.3 1.4 9.5K1 Zea mays 8.5 1.4 19.3 0.2 13.2 1.2 11.5K2 Zea mays 8.0 1.2 23.2 1.2 14.1 1.2 11.7L Beta vulgaris 8.4 1.1 10.5 0.4 12.7 0.9 13.6M Gossypium spp. 8.1 1.3 20.3 1.4 16.6 1.2 13.8N Helianthus annuus 8.2 1.2 22.5 0.4 13.8 1.1 12.1O Brassica campestris L. 8.5 1.4 19.3 2.2 14.9 1.3 11.3Mean 8.2 1.3 18.4 0.7 9.5 1.1 11.5S.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. Profile distribution of total SOC in the native land (a) and cropland (b).

81J. Zhang et al. / Catena 115 (2014) 79–84

native land and 1.3 g cm−3 for the cropland. Soil water content ishigher (18.4%) in the cropland than in the native land (12.1%). Soil ECis much lower in the cropland (0.7 ms cm−1) than in the native land(6.1 ms cm−1), whichmay be a result of irrigation on cropland. A similarresult was reported by Li et al (2007a) who showed a decrease in saltcontent following conversion of native land to cropland. Average surfaceSOC content is higher (9.4 g kg−1) in the cropland than in the native land(6.6 g kg−1). Soil TN content shows a larger variation in the native land(0.4–1.0 g kg−1) than in the cropland (0.9–1.2 g kg−1). On average,the C/N ratio shows no significant difference between the cropland(11.5) and native land (10.1).

3.2. Vertical distribution of total SOC

Fig. 2 presents vertical distributions of total SOC in the native landand cropland. In general, total SOC content is lower in the native landthan in the cropland, particularly in the topsoil. The native land revealssmall vertical variation in total SOC over depth whereas the croplandshows a pronounced decreasing trend with depth except at the I4 site.There is little vertical change in total SOC below 40 cm at most sites inboth 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 areshown in Fig. 3. There is a considerable variability among the samplingsites, particularly in the labile carbon that ranges from b1 g kg−1

to N2 g kg−1. Both labile and semi-labile fractions show small verticalvariation over depth when semi-labile carbon is less than 1 g kg−1 inthe surface layer (see sites A1, B2, E and H). However, vertical distribu-tions of the labile and semi-labile carbon show great difference whensemi-labile carbon is N1 g kg−1 in the surface layer. Semi-labile carbonis significantly higher in the 0–30 cm(N0.85 g kg−1) than below50 cm

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

82 J. Zhang et al. / Catena 115 (2014) 79–84

(~0.50 g kg−1) (also see Table 2). The exception is found at the sites A3and F that show a small increase in the bottom layer. Recalcitrant carbonin the surface soil is N4 g kg−1 in ~75% of the profiles, particularly at thesite A3 where the carbon content reaches 9.30 g kg−1. Overall, there is asharp decrease in recalcitrant carbon over depth, from 4.5 g kg−1 nearthe surface to 1.9 g kg−1 below 50 cm.

3.4. Vertical distribution of SOC fractions on cropland

Vertical distributions of the SOC fractions reveal relatively uniformdistributions over the 0–30 cm and 30–100 cm in most profiles in thecropland, but a sharp decrease around the 30 cm depth (see Fig. 4). Ingeneral, SOC fractions tend to be the highest in the 5–15 cm layer,particularly for the semi-labile and recalcitrant carbon. On average,labile carbon is N2 g kg−1 above 30 cm, but b1.5 g kg−1 below 30 cm.Semi-labile carbon is around 1.5 g kg−1 in the 0–30 cm, but less than1 g kg−1 for two thirds of profiles below 30 cm. Recalcitrant carbonchanges from 7.8 to 9 g kg−1 above 30 cm to b3.8 g kg−1 below30 cm (see Table 2). On average, the variations of labile, semi-labile,and recalcitrant carbon in the cropland are 26.2%, 34.3%, and 36.5%respectively (Table 2).

Table 2Means and coefficients of variation (CV) for labile, semi-labile and recalcitrant carbon (g kg−1

Land use types Depths(cm)

Labile carbon Semi-labile

Mean(g kg−1)

CV(%)

Mean(g kg−1)

Native land 0–5 1.34 39.05 0.835–15 1.18 49.24 0.68

15–30 1.08 52.18 0.7930–50 0.97 55.70 0.5650–100 0.92 80.61 0.49

Cropland 0–5 2.25 18.20 1.455–15 2.22 16.20 1.67

15–30 2.09 29.70 1.4130–50 1.36 29.40 0.7850–100 1.20 37.50 0.63

4. Discussion

Native arid land is characterized by sparse vegetation coverage andlow SOC storage (Li et al., 2010). Our results show that total SOC stockin the upper 1 m is 6.1 kg m−2 for the native land, and 9.8 kg m−2

for the cropland in the Yanqi Basin. While those values are relativelylower than those reported by Wang et al. (2003), i.e., 12.1 kg m−2 innative shrub land and 10.9 kg m−2 in cropland in northwest China,the SOC stock is higher than the mean value of 5.4 kg m−2 in Xinjiangreported by Li et al. (2007b).

A modeling study was conducted for a farmland in northernXinjiang, which showed 0.1, 1.5, and 0.7 kg m−2 for labile, semi-labile,and recalcitrant carbon, respectively, over the 0–20 cm (Xu et al.,2011). Our data show that the stocks (over 0–30 cm) of labile, semi-labile, and recalcitrant carbon are 0.8, 0.65, and 3.5 kg m−2, respectively,in the cropland. It appears that there are some differences in the defini-tions of these SOC pools.

The percentages of labile, semi-labile, and recalcitrant SOC are22–25%, 13–15%, and 60–65%, respectively, in the Yanqi basin(Table 3). These results are in line with those on Mediterranean nativelands at La Vall de Gallinera, Alacant province (E Spain), with the

) in the native land and cropland.

carbon Recalcitrant carbon Total SOC

CV(%)

Mean(g kg−1)

CV(%)

Mean(g kg−1)

CV(%)

31.96 4.47 45.90 6.64 34.2536.18 3.43 68.85 5.29 56.6944.78 2.66 59.77 4.54 44.8645.88 2.20 75.42 3.74 61.9429.34 1.92 79.05 3.33 69.9526.90 8.75 22.50 12.45 19.1834.10 9.02 22.00 12.91 19.4119.10 7.81 25.20 11.31 21.0442.30 3.73 65.10 5.87 51.6549.20 2.43 47.70 4.26 41.46

Fig. 4. Profile distributions of labile, semi-labile and recalcitrant carbon (g kg−1) in the cropland.

Table 4Means and standard deviations (S.D.) of SOC fractions (kg m−2).

Land use types Depths(cm)

Labile carbon Semi-labilecarbon

Recalcitrantcarbon

Mean S.D. Mean S.D. Mean S.D.

83J. Zhang et al. / Catena 115 (2014) 79–84

percentages of labile, semi-labile, and recalcitrant carbon being 20–30%,10–15%, and 55–70%, respectively (Rovira et al., 2012). A similarproportion of recalcitrant carbon (54%) was also reported in semi-aridshrub-steppe ecosystem at Grant County, Washington (Cochran et al.,2007). There has been evidence that carbon sequestration efficiency ismuch 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 Basinresults in an increase of SOC stock (0–1 m) by 3.7 kg m−2, with 0.6,0.4, and 2.8 kg m−2, for the labile, semi-labile, and recalcitrant carbon,respectively. Zhang et al. (2012) reported that in the Longzhong regionof the Loess Plateau, conversion from desert steppe to croplandincreased SOC stock from 3.9 to 5.1 kg m−2 in the top 1 m as a resultof 20 years of cropping. These results imply that the longer thecropping, the greater the SOC increase.

The increase of SOC stock is mainly found in the 0–30 cm followingconversion of native land to cropland. The labile, semi-labile, and recalci-trant carbon pools show an increase of 0.3, 0.3, and 2.0 kg m−2, respec-tively, which are statistically significant (see Table 4). The recalcitrantcarbon stock shows ~40% increase over the 0–30 cm as a result ofcropping, which may reflect different properties between crops andnative plants. For example, crops have relatively higher C:N ratio (datanot shown).

The increase of SOC stock in cropland may attribute to fertilizationand irrigation which can increase plant production and the rate ofplant residue return into soil (Fallahzade and Hajabbasi, 2012). In addi-tion, agricultural practices, such as incorporation of plant residue and

Table 3Total SOC stock (0–100 cm) and percentage of each fraction.

Land use types Labile carbon Semi-labile carbon Recalcitrant carbon

Native land (kg m−2) 1.53 0.90 3.63Cropland (kg m−2) 2.12 1.34 6.38Native land (%) 25.2 14.9 59.9Cropland (%) 21.5 13.6 64.8

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, butbio-reactive labile carbon pool whereas long-term carbon storage isassociated with the recalcitrant carbon fraction (Trumbore et al.,1990). Our study shows that theproportion of recalcitrant SOC increasesfrom 59.9% to 64.8% following the land use change, implying thatconverting native land to cropland may lead to long-term carbonstorage.

5. Conclusion

This study demonstrates a significant increase in both total SOC andSOC fractions as a result of land use change from native land to irrigatedcropland in the Yanqi Basin. The labile, semi-labile, and recalcitrantcarbon stocks increase from 1.5, 0.9, and 3.6 kg m−2 in the nativeland to 2.1, 1.3, and 6.4 kg m−2 in the cropland, respectively. Theincrease is mainly found in the top 30 cm, following the order: recalci-trant pool (2.0 kg m−2) N semi-labile pool (0.3 kg m−2) ≈ labile pool(0.3 kg m−2). The proportion of recalcitrant SOC stock increases from

Native land 0–30 0.53 0.26 0.35 0.14 1.49 0.89⁎

30–100 1.00 0.73 0.55 0.19 2.14 1.67⁎⁎

Cropland 0–30 0.85 0.14 0.65 0.14 3.46 0.8530–100 1.27 0.45 0.69 0.33 2.92 1.57

Difference 0–30 0.32⁎⁎⁎ 0.30⁎⁎⁎ 1.97⁎⁎⁎

30–100 0.27 0.14 0.78

Note: Significance of the difference was determined by t test.⁎ P b 0.05.⁎⁎ P b 0.01.⁎⁎⁎ P b 0.001.

84 J. Zhang et al. / Catena 115 (2014) 79–84

59.9% to 64.8% following long-term cropping. Thus, we conclude thatconverting native land to cropland in arid region may also lead tolong-term SOC storage.

Acknowledgment

This study isfinancially supported by theHundred Talented Programof the Chinese Academy of Sciences. We would like to acknowledgeDongmei Peng for her technical work on the map creation. We aregrateful for the reviewer's constructive comments.

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