Soil & Tillage Research 104 (2009) 263–269

Changes in soil organic carbon, nutrients and aggregation after conversionof native desert soil into irrigated arable land

Xiao Gang Li a,*, Yin Ke Li b, Feng Min Li a, Qifu Ma c, Ping Liang Zhang b, Ping Yin b

a MOE Key Laboratory of Arid and Grassland Ecology, School of Life Science, Lanzhou University, Lanzhou 730000, PR Chinab Department of Resources and Environmental Sciences, Gansu Agricultural University, Lanzhou 730070, PR Chinac School of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA 6009, Australia

A R T I C L E I N F O

Article history:

Received 16 May 2008

Received in revised form 26 February 2009

Accepted 22 March 2009

Keywords:

Cropping

Carbon sequestration

Clay stability

Nitrogen

Phosphorus

A B S T R A C T

This study aimed at investigating the effects of agricultural exploitation on desert soil organic C, N and P,

and soil aggregation. Four land uses were assessed: (1) 5-year wheat (Triticum aestivum L.)/barley

(Hordeum vulgare L.) + 5-year maize (Zea mays L.); (2) 5-year wheat/barley + 5-year alfalfa (Medicago

sativa L.); (3) 6-year wheat/barley + 4-year acacia (Robinia pseudoacacia L.) and (4) uncultivated desert

soil. The desert soil contained total organic C (TOC) of 3.1, 3.7 and 4.2 g kg�1 and particulate organic C

(POC) of 0.6, 0.7 and 0.8 g kg�1 at 0–10, 10–20 and 20–30 cm depths, respectively. The soil TOC

concentration was increased by 32–68% under wheat–maize rotation and by 27–136% under wheat–

acacia at 0–20 cm depth, and by 48% under wheat–alfalfa only at 0–10 cm depth. This contrasted with an

increase in the soil POC concentration by 143–167% at depth 0–20 cm under wheat–maize and by 217%,

550% at depth 0–10 cm under wheat–alfalfa and wheat–acacia, respectively. The desert soil had

13 Mg ha�1 TOC stock and 2 Mg ha�1 POC stock at depth 0–30 cm, whereas crop rotations increased the

soil TOC stock by 30–65% and POC stock by 200–350%. Over the 10-year period, the rates of TOC

accumulation were 0.6, 0.3, 0.8 Mg ha�1 year�1 and the rates of POC accumulation were 0.4, 0.4 and

0.7 Mg ha�1 year�1 under wheat–maize, wheat–alfalfa and wheat–acacia rotations, respectively. At 0–

30 cm depth, total soil N was increased by 61–64% under wheat–maize and wheat–acacia, but total soil P

was reduced by 38% under wheat–alfalfa. A significant improvement in clay stability but not in aggregate

water-stability was observed in cultivated soils. The results showed a significant increase in soil organic

C pool but unimproved macro-aggregation of the desert soil after 10 years of cultivation.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

In China, desert soil accounts for about 20% of its total land and iswidely distributed in the northwestern and northern regions(Institute of Soil Science, Academia Sinica, 1978a). Desert soils inthese regions have been cultivated for cropping since the HanDynasty (206 B. C.–220 A. D.) where water supply is less limited. Aspopulation and food demand continuously increase, conversion ofdesert soils into arable lands has been largely expanded in thenorthwestern China over last several decades. However, theagriculturalexploitation has threatened the long-term sustainabilityof desert soils due to scarce water resources and increased winderosion (e.g. generation of air-born dusts from the surface of newly-cultivated soils during windy seasons). In addition, because of verydry climate, the desert soils in China are highly saline and have lowclay and organic matter contents (Institute of Soil Science, Academia

* Corresponding author. Tel.: +86 0931 8150788; fax: +86 0931 8912848.

E-mail addresses: lixg2002@hotmail.com, lixiaogang@lzu.edu.cn (X.G. Li).

0167-1987/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2009.03.002

Sinica, 1978a; Zhao et al., 2006). As a result, these soils are naturallyloosely-aggregated (Li et al., 2006), and are susceptible to winderosion, especially under conventional tillage. The growing threat offood insecurity and under-privileged population in China and acrossthe globe necessitates a critical appraisal of agronomic strategiesneededtoenhance and sustain productivitywhile mitigatingclimatechange, improving biodiversity, restoring quality of soil and waterresources, and improving the environment (Lal, 2009).

Soil organic matter serves as an important storehouse ofnutrients, drives nutrient cycle, maintains soil structural stability,aids the infiltration of air and water, promotes water retention, andreduces erosion (Gregorich et al., 1994). Under crop cultivation,changes in soil organic matter status would determine thedynamics of desert soil quality. The size-fractionation of soilorganic C is an important technique in assessing short-termchanges in soil organic C induced by land use. The coarse (2–0.05 mm) organic C represents the recently incorporated residues(litters and dead roots), and is a sensitive indictor of soil organic C(Quiroga et al., 1996; Conant et al., 2003). Mineralized C over short-term in laboratory incubation is frequently used to evaluate the

X.G. Li et al. / Soil & Tillage Research 104 (2009) 263–269264

influence of land management on soil organic matter status andbio-availability (Saggar et al., 2001). Total soil N concentrationestimates total N pools that are mostly in organic form in soil andtransformed into inorganic form during decomposition of soilorganic matter. Soil P is often low and poorly mobile, and criticallylimits plant growth (Sharpley, 2000). The measurement of Olsen-Pcan estimate the level of soil P supply. Soil aggregation is closelyrelated to soil organic matter content (Tisdall and Oades, 1982) andaffect many soil biological, chemical and physical processesincluding erodibility (Kay, 2000). Thus, changes in soil aggregationmay play an important role in the soil quality of desert soils undercrop cultivation.

The impacts of converting native grasslands and bushlands toarable lands on soil organic C, nutrients and physical properties arewidely reported in the literature. However, changes in soilproperties caused by agricultural exploitation of desert soils arehardly documented. In this study we investigated the effects ofcrop cultivation on temporal changes in soil organic C, N and P andsoil aggregation of desert soils in the northwestern China. Suchknowledge would assist the farmers to conduct proper agriculturalpractices for the long-term sustainability of desert soils.

2. Materials and methods

2.1. Description of experimental site

The experimental site (398140N, 998840E) was located in GaotaiCounty in Hexi Corridor of northwest China, with a flat topographyand an altitude of 1350 m. The climate is of typically dry continent,with an annual precipitation of 79 mm, a pan evaporation amountof 1967 mm and an annual average temperature of 7.6 8C. Thevegetative lands naturally form on semi-locomotive ridge dunesand wind-eroded billabongs with a relative height of 1–3 m, andthe depth of water table varies from 6 to 9 m around the year. Thedominant plant species are some xerophil and ultra-xerophil typessuch as Nitraria sibirica Pall., Calligonum L., Alhagi pheud alhagi

Desv., Artemisia. The soil was classified as grey brown desert soil(Institute of Soil Sciences, Chinese Academia of Sciences, 1978a),similar to the Aridisols.

The site was surrounded by man-made oasis (croplands,cultivated from desert soil decades ago) to the north, west andeast, and adjacent to a vast desert to the south. In 1996–1997, atotal of 2900 ha desert soil was converted into agricultural landwith irrigation using the water from Heihe River with its origin asmelted glaciers in the Qilian Mountains. The salt concentration ofthe river water was 575 mg l�1 and pH 7.63 (Zhou and Dong, 2002).

2.2. Experimental design

Our investigation was conducted on a farm, which started togrow spring wheat (Triticum aestivum L.) and barley (Hordeum

vulgare L.) in a field of 56 ha (800 m � 700 m) in 1996 andcontinued the same cropping for 5 years under conventionaltillage. In April 2001, an area of 20 ha was converted intocontinuous maize (Zea mays L.) and 33 ha into continuous alfalfa(Medicago sativa L.). In April 2002, the remaining 3 ha were seededas acacia (Robinia pseudoacacia L.) nursery to provide saplings forbuilding a windbreak.

Maize was sown in early April and harvested in mid Septemberwith a grain yield of about 6–7.5 t ha�1. Over the growing season,516 kg N and 86 kg P2O5 ha�1 were applied, including one basefertilization of urea and di-ammonium phosphate, and threetopdressings of urea later. Irrigation was applied 4–5 timesthrough the growing season, totaling 11250 m3 ha�1. Maize strawswere removed for fodder each year. During the maize growingperiod, the soil was mulched with plastic film. This practice is

common for maize and other crops in the region mainly forincreasing soil temperature and reducing evaporation. Alfalfa wassown in April 2001 and grew for 5 years, and was irrigated at11250 m3 ha�1 annually (allotted at 3–4 times). Nil fertilizer wasapplied throughout 5 years of growth, except for base fertilizer atsowing with urea at 69 kg N ha�1. The annual hay production wasabout 15 t ha�1. Acacia was irrigated at an annual rate of about7500 m3 ha�1 without fertilization. Leaf biomass was not collectedfor fodder or any other usage. At the time of soil sampling, acaciaplants had grown to a height of about 3 m.

The experimental design was applied in one piece of land foreach treatment, a common practice with these types of studies (cf.Ashagrie et al., 2007; Noellemeyer et al., 2008; He et al., 2008).With the provision of a big area and random sampling, theobtained information would certainly reflect the changes in soilorganic C, nutrients and aggregation induced by cropping, becausethe topography, vegetation and cultivation were comparablebetween the treatments.

2.3. Soil sampling

In April 2006, three blocks (each >667 m2) were randomlyselected from each of the three treatments for soil sampling toproduce three pseudo replications. At each block, five sub-samplesat each depth of 0–10, 10–20 or 20–30 cm were taken using a scoopand mixed as a composite sample. Adjacent to sampling areas of thethree treatments, three uncultivated blocks were randomly selectedand sampled in the same way. The three land uses (5-year wheat/barley + 5-year maize; 5-year wheat/barley + 5-year alfalfa; 6-yearwheat/barley + 4-year acacia) were compared with the native desertsoil. Overall, 36 composite soil samples were collected, comprisingfour land uses, three depths and three replicates.

Soil bulk density was determined at depths of 2.5–7.5, 12.5–17.5 and 22.5–27.5 cm using a cutting ring (inner diameter of5.03 cm, and volume of 100 cm3) (Institute of Soil Science,Academia Sinica, 1978b), to represent the bulk density of soildepths 0–10, 10–20 and 20–30 cm, respectively.

After air-drying, each composite sample was split into twoparts. One part was sieved at <2 mm for the analysis of soil basicproperties, clay dispersion, and organic C and nutrient contents.The other part was passed through a 5-mm sieve for measuring soilaggregate stability.

2.4. Measurements of basic soil properties

Soil pH was measured at the ratio of 1 soil:2.5 water, andelectrical conductivity (EC) in saturated extracts. Soil carbonatecontents, expressed as CaCO3, were determined by CO2 volumetricmethod (Qin, 2002). Soil particle distribution was determined bythe pipette method, following the procedure described by theInstitute of Soil Science, Academia Sinica (1978b), with theexception of CaCO3 washing by acid. This minor change made itpossible that particle composition data matched dispersible claycontents in the soils (see below).

2.5. Measurements of total, particulate and mineralizable organic

carbons

Thirty grams of air-dried soil (2-mm fraction) were shaken for2 h in 60 ml of 0.5% (w/v) hexametaphosphate solution to dispersethe soil. The dispersed soil was wet-sieved through a 0.05-mmsieve, and the fractions of sand and sand-sized organic materials(particulate fraction) retained on the sieve were oven-dried at60 8C and weighed. Total and particulate (2–0.05 mm) organic Cwere analyzed with the Walkley and Black’s dichromate oxidationmethod (Nelson and Sommers, 1982). Mineral-associated

X.G. Li et al. / Soil & Tillage Research 104 (2009) 263–269 265

(<0.05 mm) C was estimated as the C difference between the totaland particulate fractions (Conant et al., 2003).

Air-dried soil sample (2 mm, equivalent to 70 g oven-dried) wasadjusted to 60% water-holding capacity and incubated in a sealed500-ml jar (containing a vial with 10 ml of 0.6 M NaOH solution fortrapping CO2 and a vial of distilled water to maintain humidity inthe jar) at 25 8C for 50 days. The CO2 trapping vials were changed at5, 10, 20, 30 and 50 days of incubation, and the amount of CO2–Ctrapped was determined by titrating unused NaOH with 0.2 M HClin the presence of excess BaCl2 using phenolphthalein as anindicator. Three blank samples (no soil-packed vials in the jar)were also included in the incubation. Total amount of CO2–Cproduced from each soil over the 50-day incubation wasdesignated as readily mineralizable C.

2.6. Analyses of total nitrogen, total and available phosphorus

Total N was determined via the semi-micro Kjeldahl digestionprocedure; total P was determined after digestion of soil withHClO4–H2SO4 and available P (Olsen-P) in soil was extracted by0.5 M NaHCO3 (pH 8.5) (Bao, 2000).

2.7. Determinations of aggregate stability and clay dispersibility

Thirty grams of air-dried soil aggregates (5–2 mm) weretransferred onto the 0.25-mm sieve of a wet sieving apparatus.The water level was adjusted such that aggregates on the sievewere just submerged at the highest point of oscillation. Afterstanding aggregates in the water for 5 min, the apparatus wasoscillated at 30 cycles min�1 for 5 min. Following drying at 60 8Cand weighing, the stable aggregates on 0.25-mm sieve were soakedin distilled water, and then crushed using a forefinger in a bowl.The subsequent sieving through the 0.25-mm sieve retained>0.25-mm mineral particles, of which the dry mass was thensubtracted to obtain the dry mass of >0.25 mm stable aggregates(Li et al., 2006). The result was expressed as water-stable aggregatepercentage (WSAP, %) calculated as follows:

WSAP ¼ Mwsa

Ma

� �� 100

Table 1Some physico-chemical properties of the desert soils under different cropping rotation

Sampling depth

and land use

Particle size distribution (%)

Sand

(2–0.05 mm)

Silt

(0.05–0.002 mm)

Clay

(<0.002 mm)

Clay + s

(<0.05

0–10 cm

Uncultivated 36 58 6 64

Wheat–maize 43 49 8 57

Wheat–alfalfa 64 29 7 36

Wheat–acacia 31 58 11 69

10–20 cm

Uncultivated 28 66 6 72

Wheat–maize 39 53 8 61

Wheat–alfalfa 67 28 5 33

Wheat–acacia 29 65 6 71

20–30 cm

Uncultivated 31 62 7 69

Wheat–maize 39 52 9 61

Wheat–alfalfa 65 31 4 35

Wheat–acacia 31 64 5 69

Summary of ANOVAa

Land use (LSD0.05)b (11)*** (10)*** NS (11)***

Depth NS NS NS NS

Land use � depth NS NS NS NS

a Statistical significance: NS – not significant, ***P � 0.001.b The numbers in parentheses stand for values of least significance differences (LSD

where Mwsa is the corrected dry mass of >0.25 mm water-stableaggregates and Ma is the corrected dry mass of 5–2 mm soilaggregates.

Dispersible clay was determined by the pipette method. Air-dried 2-mm sieved soils (20 g) were placed in a 1000-ml cylinder,wetted with distilled water and left to stand for 30 min. Water wasslowly added and made up to 1000 ml, and the cylinder was theninversed 10 times in 1 min by hand. After settling for the requiredtime (calculated according to the Stokes law), 25 ml of suspensioncontaining dispersible clay was pipetted from the depth of 10 cmbelow the liquid surface and transferred to an aluminum tray fordrying at 105 8C. Dispersible clay (DC) content (%) was calculatedon oven-dry weight basis of soil. Clay dispersibility was expressedas clay dispersion percentage (CDP):

CDP ¼ DC

CC

� �� 100

where CC is the clay content in soil.

2.8. Statistics

The land use and soil depth were two main factors. The physicaland chemical parameters, organic C and nutrient concentrationswere analyzed by two-way analysis of variance (ANOVA) usingSPSS 10.0. Least significant difference (LSD) test was used tocompare the means at P � 0.05. The significance of linear relationsbetween various parameters was expressed by the Pearson’sproduct moment correlation coefficient.

3. Results

3.1. Basic soil properties

Soil clay content was similar between different land uses,whereas sand content was higher and silt content lower underwheat–alfalfa than those under wheat–maize, wheat–acacia ornative desert soil (Table 1). Soil bulk density was the highest underalfalfa, followed by maize or acacia rotation, and the lowest in thenative soil. Salinity was significantly lower in the cultivated soils

s.

Bulk density

(Mg m�3)

Electrical conductivity in

saturated extracts (dS m�1)

pH

(H2O)

Carbonate as

CaCO3 (g kg�1)ilt

mm)

1.10 11.13 8.6 120

1.27 3.21 8.2 141

1.60 1.14 9.0 125

1.32 3.10 8.2 144

1.21 11.24 8.6 126

1.25 3.14 8.2 141

1.56 0.95 9.1 124

1.33 2.92 8.0 145

1.13 11.33 8.6 147

1.36 3.15 8.5 156

1.50 0.94 8.9 120

1.32 2.87 8.1 139

(0.07)*** (0.53)*** (0.1)*** NS

NS NS NS NS

NS NS NS NS

) at 0.05 level.

Table 2Effects of cropping rotations on soil total organic C (TOC), particulate and mineral-associated organic C and mineralizable C, organic N and available P at different soil depths.

Sampling depth

and land use

TOC

(g kg�1)

Particulate

organic C

Mineral-associated

organic C

Mineralizable C Total N

(g kg�1)

C/N of soil

organic matter

Total P

(g kg�1)

Olsen-P

(mg kg�1)

g kg�1 % in TOC g kg�1 % in TOC g kg�1 % in TOC

0–10 cm

Uncultivated 3.1 0.6 19 2.5 81 0.26 8 0.26 12 0.77 16

Wheat–maize 5.2 1.6 30 3.6 70 0.61 12 0.46 12 0.81 21

Wheat–alfalfa 4.6 1.9 41 2.7 59 0.86 19 0.41 11 0.53 4

Wheat–acacia 7.3 3.9 52 3.4 48 1.03 14 0.58 13 0.81 13

10–20 cm

Uncultivated 3.7 0.7 18 3.0 82 0.24 7 0.25 15 0.78 14

Wheat–maize 4.9 1.7 33 3.3 67 0.64 13 0.48 10 0.89 21

Wheat–alfalfa 3.2 1.1 34 2.1 66 0.53 17 0.33 10 0.48 2

Wheat–acacia 4.7 1.6 34 3.1 66 0.55 12 0.41 12 0.89 9

20–30 cm

Uncultivated 4.2 0.8 20 3.4 80 0.28 7 0.33 13 0.84 14

Wheat–maize 4.4 1.5 36 2.9 64 0.66 17 0.41 11 0.82 17

Wheat–alfalfa 2.7 0.9 33 1.8 67 0.51 19 0.24 11 0.48 2

Wheat–acacia 4.1 1.2 31 2.9 69 0.47 11 0.39 11 0.82 9

Summary of ANOVAa

Land use (LSD0.05)b ** *** *** (0.6)** *** *** (3)*** (0.10)** (2)* (0.11)*** (9)**

Depth * *** NS NS NS ** NS NS NS NS NS

Land use � depth (LSD0.05)b (1.6)* (1.0)** (11)* NS (11)* (0.24)** NS NS NS NS NS

a Statistical significance: NS – not significant, *P � 0.05, **P � 0.01., ***P � 0.001.b The numbers in parentheses stand for values of least significance differences (LSD) at 0.05 level.

X.G. Li et al. / Soil & Tillage Research 104 (2009) 263–269266

than in the native soil after 10 years of cultivation. Across the 0–30 cm depth, soil pH in the native desert soil was lower than underwheat–alfalfa but higher than under wheat–maize or wheat–acacia (Table 1). All soils were highly calcareous.

3.2. Organic C pools

Compared with the desert soil, total soil organic C concentrationwas 32-68% higher under wheat–maize and 27–136% higher underwheat–acacia at depths 0–20 cm, and 48% higher under wheat–alfalfa only at depth 0–10 cm (Table 2). Wheat–maize croppingincreased particulate organic C concentration by 143% at 0–10 cmdepth and by 167% at 10–20 cm depth, compared with increments of217% and 550% at 0–10 cm depth by wheat–alfalfa and wheat–acacia cultivation, respectively. Across soil depths, mineral-asso-ciated organic C was similar between desert soil and the soilscultivated with wheat–maize or wheat–acacia rotation, butsignificantly lower in the wheat–alfalfa soil (Table 2). Soil cultivationincreased the proportion of particulate organic C and consequentlydecreased the proportion of mineral-associated organic C in total soilorganic C at 0–30 cm depth (Table 2). Among the three cultivatedsoils, the proportion of particulate organic C was highest underwheat–acacia, followed by wheat–alfalfa, and then by wheat–maizeat 0–10 cm depth, but was similar between the cultivationtreatments at depths 10–30 cm (Table 2).

Total soil organic C stock in the layer of 0–30 cm was increasedby 52% under wheat–maize, by 29% under wheat–alfalfa and by

Table 3Stocks of total soil organic carbon C and its size-fractions in the 0–30 cm layer of

soils under different cropping rotations.

Land use Total organic

Ca (Mg ha�1)

Particulate

organic Ca (Mg ha�1)

Mineral-associated

Ca (Mg ha�1)

Uncultivated 13b 2c 11

Wheat–maize 19a 6b 13

Wheat–alfalfa 16ab 6b 10

Wheat–acacia 21a 9a 12

a Within the same column, means with different letters are different and those

without letter are not different at P = 0.05.

65% under wheat–acacia, compared with the desert soil (Table 3).This was mainly due to an increase in particulate organic C stock by200% under either wheat–maize or wheat–alfalfa, and by 350%under wheat–acacia (Table 3). It was estimated that the soils underwheat–maize, wheat–alfalfa or wheat–acacia rotation had totalorganic C sequestration rates of 0.6, 0.3 and 0.8 Mg ha�1 year�1 andparticulate organic C sequestration rates of 0.4, 0.4 and0.7 Mg ha�1 year�1, respectively, over the 10-year period ofcultivation.

At 0–20 cm depth, mineralizable C increased in all thecultivated soils, whereas at 20–30 cm depth it increased only inthe wheat–maize soil (Table 2). Mineralizable C was significantlyrelated to particulate organic C concentration in all sampled soils(Fig. 1), confirming that microbial respiration during soil organic Cdecomposition was mainly localized in the particulate fraction.The proportion of mineralizable C in total soil organic C was thehighest under wheat–alfalfa, followed by wheat–maize or wheat–acacia, and the lowest in the desert soil (Table 2).

Fig. 1. A positive linear relationship between soil mineralizable C and particulate

organic C in the 0–30 cm layer of native and cultivated desert soils.

Fig. 2. A reverse linear relationship between clay dispersion percentage and total

soil organic C in the 0–30 cm layer of native and cultivated desert soils.

X.G. Li et al. / Soil & Tillage Research 104 (2009) 263–269 267

3.3. Total N, P and Olsen-P

Compared with the desert soil, soil total N across depth 0–30 cm was increased by 61% and 64% under wheat–maize andwheat–acacia rotations, respectively, but not in the soil of wheat–alfalfa rotation (Table 2). There was a significant correlationbetween soil total N and total organic C (r = 0.89, P < 0.001). The C/N value for total organic matter in the desert soil was significantlyhigher than those in the soils under wheat–maize or wheat–alfalfabut similar to that in the wheat–acacia soil (Table 2). Comparedwith the desert soil, a substantial depletion (38%) of total P wasfound in the wheat–alfalfa soil but not in the soils under wheat–maize or wheat–acacia at 0–30 cm depth (Table 2). Soil Olsen-Pconcentration under wheat–alfalfa rotation was lower than thosein the wheat–maize or desert soils, but similar to that in thewheat–acacia soil (Table 2).

3.4. Structural stability

All measurements showed that soil aggregate water-stabilitywas very low, and nearly all the aggregates (5–2 mm) were elapsedwhen subjected to wet sieving (Table 4). However, soil water-stability was significantly higher under wheat–acacia rotation thanthe other land uses (Table 4). There was no significant correlationbetween water-stable aggregate percentage and total soil organicC (r = 0.26, P > 0.05).

Despite similar soil clay contents between the land uses,dispersible clay was lower in all the cultivated soils than in thedesert soil. The most significant effect of land uses on soildispersible clay was found at the depth of 0–10 cm, followed by at10–20 cm and the least at 20–30 cm (Table 4). Across the depths,the rate of clay dispersion was significantly higher in the desert soilthan those in the cultivated soils, but similar between thecultivated soils (Table 4). Correlation analysis showed a negativecorrelation between total soil organic C and clay dispersion (Fig. 2),indicating that increase in soil organic C under crop cultivationcontributed to wrapping clay particles in micro-aggregates (0.25–0.002 mm).

Table 4Effects of cropping treatments on soil structural stability.

Sampling depth

and land use

Dispersible

clay (%)

Clay

dispersion (%)

Water-stable

>0.25 mm

aggregate (%)

0–10 cm

Uncultivated 5 80 0.3

Wheat–maize 2 37 1.3

Wheat–alfalfa 2 20 1.1

Wheat–acacia 1 13 2.4

10–20 cm

Uncultivated 4 69 0.2

Wheat–maize 2 28 1.6

Wheat–alfalfa 1 30 1.3

Wheat–acacia 2 29 2.0

20–30 cm

Uncultivated 3 56 2.5

Wheat–maize 2 22 1.2

Wheat–alfalfa 2 40 1.5

Wheat–acacia 2 49 4.0

Summary of ANOVAa

Land use (LSD0.05)b *** (17)*** (1.3)*

Depth effect NS NS NS

Land use � depth

effect (LSD0.05)b

(1)* NS NS

a Statistical significance: NS – not significant, *P � 0.05, ***P � 0.001.b The numbers in parentheses stand for values of least significance differences

(LSD) at 0.05 level.

4. Discussion

In the desert, soil water evaporation is normally much higherthan precipitation. Due to the dominating upward (i.e. toward soilsurface) direction of water movement in the soil profile, solublesalts in the groundwater move upward and accumulate in topsoil.The desert soil in northwestern China is naturally saline (Instituteof Soil Science, Academia Sinica, 1978a), as found in the presentwork. During cultivations, irrigation with low salt (575 mg l�1)water melted from glaciers in Qilian Mountains washed solublesalts down the soil profile.

The present study demonstrated that 10-year cultivation ofdesert soil increased soil organic C pool mainly due to a significantaugment in particulate organic C fraction but not in mineral-associated C. Because of scarce precipitation (<100 mm), thedesert soils in the region naturally possess sparse vegetation andthereby have limited organic matter input into the soil (Institute ofSoil Sciences, Chinese Academia of Sciences, 1978a). However, inthe cultivated desert soils of this study, irrigation largely increasedsoil moisture and plant biomass input to the soil. We estimatedthat annual root biomass input at the 0–30 cm layer was3375 kg ha�1 for maize and 2769 kg ha�1 for wheat, and annualaboveground biomass (i.e. leaves, stems) was about 6000 kg ha�1

for maize and 5250 kg ha�1 for wheat. Senesced leaves, dead rootsand stem remainders after harvest were the primary source ofcoarse organic C (Quiroga et al., 1996; Conant et al., 2003). As a

Fig. 3. A positive linear relationship between mineral-associated organic C and

clay + silt content in the 0–30 cm layer of native and cultivated desert soils.

X.G. Li et al. / Soil & Tillage Research 104 (2009) 263–269268

result, cultivated vegetation often returns more C than virgin sites,despite the removal of harvested C (Balesdent et al., 1998).

Hassink (1997) defined the capacity of soil to reserve C by itsassociation with silt and clay particles. Six et al. (2002) found thatthe capacity of soils to sequester C varied with the type of land usesand the level of silt/clay content in soil. In this study the absence ofincrease in mineral-associated C under cultivation might berelated to low clay contents of the desert soil, and a lowermineral-associated organic C in the wheat–alfalfa soil was possiblydue to its lower clay + silt content than under the other land uses(Tables 1 and 2). A positive correlation between mineral-associated organic C concentration and clay + silt content (Fig. 3)indicated that recalcitrant mineral-associated organic C wasmainly controlled by fine soil mineral fraction after 10 years ofcultivation.

Our results showed that in the desert region the extent to whichsoil organic C was increased by cultivation varied with the landuses. The significant increase in particulate organic C fractionoccurred at 0–20 cm depth under wheat–maize and only at 0–10 cm depth under wheat–alfalfa or wheat–acacia. This wasprobably because particulate organic C strongly relates to above-ground and belowground (root derived) litter inputs (Gale andCambardella, 2000; Wander and Yang, 2000). Dead roots andleaves, and stem remainders were annually transferred to the deepsoil by ploughing under annual maize. In contrast, under perennialalfalfa or acacia the aboveground litters were mainly added intothe topsoil while the main roots were still alive and notincorporated into soil organic matter after 4–5 years of growth.In addition, rates of irrigation and fertilization influenced plantbiomass production and consequently the accumulation patternsof particulate organic C fraction in soils under different crops.

The significant increase in mineralizable C content in cultivatedsoils versus the desert soil in this study might mean thatcultivation enhanced desert soil microbial activity, an importantcomponent of soil quality (Saviozzi et al., 2001). Furthermore,changes in potentially mineralizable substrate are often associatedwith changes in the quantity and quality of soil organic matter(Gregorich et al., 1994). We suggest that significant increases inmineralizable C fraction in the present study can be attributed tothe increases in particulate organic C fraction in cultivated soils ascompared with the desert soil (Fig. 1). The higher proportion ofmineralizable C in total organic C in soil under wheat–alfalfa thanthat under wheat–maize or wheat–acacia could be due to anaccelerated mineralization of soil organic C during incubation assoil pH was higher under wheat–alfalfa than under wheat–maizeor wheat–acacia. Li et al. (2007) have reported that an increase insoil pH stimulates soil organic C mineralization in the salt-rich soilsbecause increasing alkalinity (5.9–10.0) can favor the survival ofbacteria-dominated microbial community with low assimilationefficiency of organic C. In addition, plant biomass compositionvaries widely among species (Wedin and Tilman, 1990), whichmay result in variable chemical characteristics of soil organicmatter (Guggenberger et al., 1994) and colonization by micro-organisms (Ormeno et al., 2006) and consequently variabledecomposability under different croppings. All these factors wouldcontribute to the stimulated C mineralization of soil C observed inthe wheat–alfalfa rotation.

In this study, annual application of N and P fertilizers increasedtotal soil N and maintained soil P available to maize, and most ofthe aboveground biomass in maize was harvested. Acacia fixed N2,and also had annual input of leaf litters to increase total soil N asthe plants were not harvested. However, there was no significantincrease in total N but lower Olsen-P and total P in the soil underalfalfa compared with the desert soil. The N2-fixation by alfalfa wascounteracted by N removal due to hay production, and a lack of Pfertilizer to alfalfa (high P-uptake capacity, Andrew and Phillip,

1989) might have lowered not only labile pool but also recalcitrantpool of soil P.

Although soil organic C in the desert soil was increased by cropcultivation, the increment was still not sufficient to improvewater-stability of soil aggregates in the present study. Aggregatewater-stability is not directly related to soil susceptibility to winderosion (Chepil, 1953). However, low aggregate water-stability inthe cultivated desert soils would have unfavorable effects on othersoil physical conditions such as soil aeration, infiltration andpenetration. A small increase in aggregate water-stability underacacia was possibly due to the absence of tillage disruption(Bronick and Lal, 2005), which might mean that reduction in tillagedisturbance on desert soil increases aggregate water-stability. Ithas been well established that soil organic matter helps stabilizeclay in salt-affected soils (Loveland et al., 1987; Hodgkinson andThorburn, 1995) and in arid soils (Goldberg et al., 1988). In thisstudy, aggregate water-stability of the desert soil was notimproved after 10 years of wheat–maize or wheat–alfalfa rotation,but a substantial increase in clay stability occurred in all thecultivated soils. Clay stabilization serves a primary step for soilaggregation (Tisdall and Oades, 1982). It is likely that a prolongcrop cultivation of the desert soil may eventually improve soilaggregate water-stability.

5. Conclusions

Organic C pool in desert soils increased with an increase inparticulate organic C fraction but not in mineral-associated organicC after 10 years of cultivation. The magnitude of increase in organicC pool and the extent of changes in nutrient concentrations weredependent on the type of land uses and associated managements,e.g. irrigation and fertilization. There was an improvement in claystability but no increase in water-stable aggregates in thecultivated desert soil. The results indicate that loose soil macro-aggregation of the desert soil was not alleviated after 10 years ofcultivation.

Acknowledgements

This work was financed by ‘‘973’’ program (2007CB106804) andinnovation group project of China Ministry of Education. We wouldlike to thank Mr. Hongbin Li for his assistance with fieldinvestigation and soil sampling.

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