YuQiang Li ,YinPing Chen ,ShaoKun Wang ,WenDa Huang ,JianPeng Zhang

1.Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.School of Environmental and Municipal Engineering,Lanzhou Jiaotong University,Lanzhou,Gansu 730070,China

1 Introduction

The biogeochemical cycles of carbon(C)in terrestrial ecosystems have received increasing research attention over the last decade because the emission of carbon dioxide(CO2),the most important greenhouse gas,into the atmosphere contributes greatly to global warming(IPCC,2007).Human practices of land use,land-use change and forestry are changing the natural rate of exchange of C between the atmosphere and the terrestrial biosphere.Therefore,it is essential to investigate how carbon stocks change in response to land-use activities(IPCC,2000).In view of climate change,soil organic matter(SOM)has become increasingly important as the largest terrestrial C pool.Precise estimations of soil organic carbon(SOC)storage are thus of decided importance to detect the potential for C sequestration or emission induced by land-use changes(Wiesmeieret al.,2012).

Natural lands that are converted to cropland or suffer from degradation often lead to a depletion of the SOC stock,primarily due to reduced input of biomass,enhanced decomposition by physical disturbance,and reduced vegetation coverage and the resultant increased soil erosion(Post and Kwon,2000;Lal,2009;Poeplau and Don,2013).However,land-use change can provide positive impacts on atmospheric CO2concentration by decreasing emissions that would occur without intervention or sequestering CO2from the atmosphere into vegetation and soil(Pearsonet al.,2005;Cantarelloet al.,2011).

The use of physical fractionation,according to density and size of soil particles,has increased steadily in studies of organic matter turnover in soil over the recent decades,because it emphasizes the importance of interactions between organic and inorganic soil components in the turnover of organic matter(Christensen,2001).Density fractionation,typically using a solution at a density of 1.6 to 2.0 g/cm3,separates soil into low-and high-density fractions that are referred to as light fraction(LF)and heavy fraction(HF),respectively(Christensen,1992;Swanstonet al.,2002).The LF has been suggested as an early indicator of land useor management-induced changes in soil quality(Soonet al.,2007;Sequeiraet al.,2011).Particle-size fractions(sand,silt,and clay)of C pools have also been widely used to determine the dynamics and turnover of SOM and underlying mechanisms under various land-use and climate changes(Christensen,2001;Grandy and Robertson,2007;Magidet al.,2010;Heet al.,2012).

The Horqin Sandy Grassland of northern China(42°41'N to 45°15'N,118°35'E to 123°30'E,Figure 1)was covered by lush vegetation and an important pastoral region of Inner Mongolia before 1950.However,this region has seriously undergone desertification during recent decades,primarily due to a combination of the region's fragile ecology with inappropriate anthropogenic activities,such as overgrazing,extensive cultivation,firewood harvesting,and excessive groundwater withdrawal(Liet al.,2013).At present,the Horqin Sandy Grassland's landscape is dominated by sand dunes with different vegetation cover and by irrigated croplands,that is,the region has become the most important part of the semiarid agro-pastoral ecotone in northern China.Due to this special geographical features,C loss or sequestration induced by land-use and cover type changes have attracted considerable scientific attention in this area(Zhaoet al.,2009;Liet al.,2013,2014).However,little research is available on the variation of C in soil particle-size and density fractions over time in response to land-use and cover type changes in this semiarid degraded area.

The objectives of the present study are(1)to investigate the variation of organic C in bulk soil and its physical fractions(size and density),and(2)to demonstrate the allocation pattern of C fractions,in response to land-use and cover type changes.

Figure 1 Location of the study area(the Horqin Sandy Land of Inner Mongolia,China)

2 Materials and methods

2.1 Study area

The study sites are located in the southern part of the Horqin Sandy Grassland,Inner Mongolia,China,near the Naiman Desertification Research Station of the Chinese Academy of Sciences(42°55'52"N,120°41'56"E,377 m a.s.l.;Figure 1).The region has a continental semiarid monsoon temperate climate regime.Mean annual precipitation is 366 mm,of which 70% falls from June to August,and mean annual potential evaporation is 1,935 mm.Mean air temperature is 6.8 °C,with a minimum monthly mean of-13.2 °C in January and a maximum monthly mean of 23.5 °C in July.The frost-free period ranges from 130 to 150 days.Mean wind speed is 4.3 m/s,with occasional occurrence of gales with a wind speed ≥20 m/s in winter and spring(Zhaoet al.,2005).According to the Food and Agriculture Organization of the United Nations soil classification system(FAO,2006),the zonal soils are classified as Kastanozems,but as a result of desertification,the current dominant soils are Arenosols with a coarse texture and a loose structure.

In the present study,we selected five types of land-use and cover for sampling as follows:(1)Non-desertification grassland(NDG)which was open and flat,and had a vegetation cover of more than 75%.The dominant species were perennial grasses and forbs such asPennisetum centrasiaticum,Phragmites communis,Leymus secalinus,andMelissitus ruthenicus,and the perennial semishrubLespedeza davurica.(2)The 30-year-old farmland(30F).This type of site was established from the region's native grassland and has been cultivated for long periods.(3)The 30-year-old plantation(30P)which was established in the area with native grassland using seedlings of poplar(Populus simonii).The spacing within and between the rows was 2m×3m.(4)Continuous grazing site(CG).This area with native grassland had been subjected to long-term continuous grazing,with a moderate degree of desertification.Vegetation cover was less than 10% and the dominant plant was the annual speciesAgriophyllum squarrosum,with very sparse distribution ofArtemisia halodendronandCaragana microphylla.(5)The 16-year exclosure(16EX).Livestock grazing had been excluded from the area with continuous grazing for 16 years at the time of the study.The vegetation cover is currently 40%.The dominant plant isA.halodendron,with small numbers ofC.microphyllaand annual forbs such asEuphorbia humifusa,Salsola collina,andSetaria viridis.Images of the landscape of the study area are shown in Figure 2.

In summary,the non-desertification grassland represent an optimal natural situation,as a baseline to evaluate variation of SOC due to the land-use change(i.e.,the conversion of grassland to farmland and plantation)and land-cover change(i.e.,the grassland degradation as a result of continuous grazing),whereas the continuous grazing site as another baseline to assess the effectiveness of desertification control measures(i.e.,exclosure).

Figure 2 Photographs of a typical area of non-desertified grassland(NDG)(a),30-year-old farmland(30F)(b),30-year-old plantation(30P)planted with poplar(Populus simonii)(c),and continuous grazing(CG)and 16-year-old grazing exclosure(16EX)(d)

2.2 Experimental design and soil sampling

We established three plots(each 30m×30m)in each land-use and cover types.Soil samples were collected from six 1m×1m subplots randomly established within each plot,using a stainless-steel cylinder(5 cm in height and 100 cm3in volume)to a depth of 5 cm.After carefully removing large surface plant debris by hand,a composite soil sample was prepared using the soil collected at 10 locations within each subplot.Therefore,within each plot,we obtained six composite soil samples,yielding a total of 18 composite samples for each land-use and cover types and 90 samples across all plot types.To determine soil bulk density,we collected three additional intact soil cores at each subplot using the same stainless-steel cylinder.

2.3 Laboratory analyses

Soil samples were air-dried and hand-sieved through a 2-mm mesh to remove roots and other large debris.No gravel(more than 2 mm)was found for any soils.Our preliminary experiment did not detect water-stable aggregate in each soil sample.Therefore,soil particle-size fractions were analyzed by the dry sieving method.Each soil sample was separated into four fractions using an electronic shaker equipped with a nest of sieves with openings of 2.00,0.25,0.10,and 0.05 mm:coarse sand+non-water-stable aggregate(NSA)(2.00 to 0.25 mm),fine sand(0.25 to 0.10 mm),very fine sand(0.10 to 0.05 mm),and silt+clay(<0.05 mm).

The LF organic matter was isolated using a modification of the methods described by Janzenet al.(1992)and Grandy and Robertson(2007).Approximately 20 g of air-dried bulk soil was weighed into a 100-mL beaker,followed by the addition of 80 mL aqueous NaI solution at a density of 1.6 g/cm3.The solution was swirled by hand for 30 s,and the content was dispersed using a probe-type sonic disrupter for 1 min.The beakers were then covered and the suspension was allowed to equilibrate for 48 hours at room temperature.The suspended material(LF)was suctioned onto a Whatman No.1 filter paper,and washed thoroughly with 5 aliquots of 0.01 M CaCl2and 10 aliquots of distilled deionized water.After drying at 55 °C approximately for 16 hours,the LF was scraped from the filter paper and weighed to the nearest 0.0001 g.The LF dry matter content was expressed as a percentage of the total soil mass.To obtain enough of the LF to carry out the required analyses for C concentration,it was necessary to extract two to six 20-g portions of each soil.

A subsample of the air-dried bulk soil and each particle-size fraction were weighed and dried at 105 °C for 24 hours for determination of the gravimetric water content.A portion of each bulk soil sample and all of the collected LF and coarser soil particle(2.00 to 0.25 mm)was ground to pass through a 0.25-mm mesh before determination of the carbon concentration.The organic C concentrations for bulk soil,LF,and each particle-size fraction were determined using the Walkley-Black dichromate oxidation procedure(Nelson and Sommers,1982).

2.4 Data analyses

The organic C storage(g/m2)per unit area by volume to a depth of 5 cm in bulk soil(SOC storage)and in LF and particle-size fractions(FOC storage)were calculated using the following equations:

whereCSis the bulk SOC(g/kg),BDis the soil bulk density(g/cm3),His the soil layer's thickness(=5 cm),PFis the proportion of LF dry matter content or each particle-size fraction to the total soil mass(%),andCFis the carbon concentration in LF or each particle-size fraction(g/kg dry fraction).

Data set were submitted to normality tests(Shapiro-Wilk 5%)and error variance homogeneity tests(Levene 5%)before further testing.No transformation was performed since data met the assumption of homogeneity of variances.The measured variables and the resultant C storage were analyzed by land-use and cover type,using one-way ANOVA.When the ANOVA results were significant(P<0.05),we compared means using the least-significant-difference(LSD)test.The statistical analysis was performed using version 13.5 of the SPSS software(SPSS,Chicago,IL,USA).

3 Results

3.1 Soil particle-size distributions and bulk density

Except for very fine sand(0.10 to 0.05 mm),the proportions of the other particle-size fractions increased after the conversion of grassland into farmland and plantation(Table 1).The proportions of coarse sand+NSA(2.00 to 0.25 mm;hereafter,the "coarse fraction")and fine sand(0.25 to 0.10 mm)were significantly higher at the CG site compared to the NDG site,whereas the very fine sand and silt+clay(<0.05 mm)were significantly lower.The establishment of an exclosure in the area with continuous grazing led to decrease in the coarse fraction and fine sand contents,whereas increase in very fine sand and silt+clay contents.

The soil bulk density decreased significantly as a result of the conversion of grassland into farmland and plantation(Table 1).There were no significant differences among CG,16EX,and NDG,although the values are slightly higher in CG than that at the 16EX and NDG sites.

Table 1 Soil particle-size distributions and bulk density to a depth of 5 cm at the sites with continuous grazing(CG),16-year-old grazing exclosure(16EX),30-year-old plantation(30P),30-year-old farmland(30F),and non-desertified grassland(NDG)

3.2 Carbon concentrations in bulk soil,particle-size and LF fractions

Compared with values at the NDG site,total SOC concentration increased by 74.0% at the 30P site,whereas decreased by 16.9% and 39.2% at the 30F and CG sites,respectively(Figure 3).Total SOC increased by 44.6% at the 16EX site compared to the value at the CG site(4.22 g/kg).The level of SOC to a depth of 5 cm in the bulk soil at the 16EX site(6.10 g/kg)was close to that at the NDG site(6.94 g/kg).

For each particle-size fraction,organic C concentration was higher at the 30P site than at the NDG site,but no significant difference was found for coarse fraction(Figure 3).The C concentration for coarse fraction and fine sand were lower at the 30F site than at the NDG site,whereas the values were slightly higher for very fine sand and silt+clay;the significant difference was only found for coarse fraction between these two sites.Comparing the values between CG and NDG sites,the C concentration at the NDG site was significantly higher for coarse fraction and fine sand,but significantly lower for silt+clay.Following the practice of grazing exclosure,the C concentration for coarse fraction and fine sand increased significantly at the 16EX site compared with values at the CG site.

The LF dry matter content was highest at the 30P site and lowest at the CG site.However,there was no significant difference among NDG,30F,and 16EX sites and no significant difference between 16EX and CG sites(Figure 4a).The LFOC concentration(Figure 4b)ranged from 198 g/kg at the NDG site to 226 g/kg at the 30P site,but the value did not differ significantly among all the sites.

Figure 3 Organic carbon concentrations in the bulk soil and in four particle-size fractions(NSA,non-water-stable aggregates)at the sites with continuous grazing(CG),16-year-old grazing exclosure(16EX),30-year-old plantation(30P),30-year-old farmland(30F),and non-desertified grassland(NDG).Values represent means ± SD.Bars labeled with different letters differed significantly(P<0.05)between land-use and cover types for each fraction

Figure 4 The values of(a)the light fraction(LF)dry matter content(LF as a % of the total soil mass)and(b)the light fraction organic carbon(LFOC)concentration at the sites with continuous grazing(CG),16-year-old grazing exclosure(16EX),30-year-old plantation(30P),30-year-old farmland(30F),and non-desertified grassland(NDG).Values represent means ± SD.Bars labeled with different letters differed significantly(P<0.05)between land-use and cover types

3.3 Changes in carbon storage

Compared with the value at the NDG site,total SOC storage(Table 2)increased by 261 g/m2at the 30P site,but decreased by 121 and 157 g/m2at the 30F and CG sites,respectively.The establishment of exclosures increased the total SOC storage with a value of 111 g/m2(compared between 16EX and CG sites).

The proportion of total SOC storage accounted for by the LFOC(Figure 5)ranged from 19.7% at the CG site to 35.5% at the 30P site.The proportion of total SOC storage accounted for by C in each particle-size fraction(Figure 5)decreased in the following order:at the NDG site,coarse fraction = very fine sand >fine sand >silt+clay;at the 30P and 30F sites,coarse fraction >very fine sand >fine sand >silt+clay;at the CG and 16EX sites,very fine sand >coarse fraction >fine sand >silt+clay.

Table 2 The organic carbon storage(C,g/m2)in the bulk soil(total SOC),light fraction(LFOC),and the four particle-size fractions at the sites with continuous grazing(CG),16-year-old grazing exclosure(16EX),30-year-old plantation(30P),30-year-old farmland(30F),and non-desertified grassland(NDG)

Figure 5 The proportion of total SOC storage accounted for by C storage in the light fraction(LF)and for each of the particle-size fractions(NSA,non-water-stable aggregates)at the sites with continuous grazing(CG),16-year-old grazing exclosure(16EX),30-year-old plantation(30P),30-year-old farmland(30F),and non-desertified grassland(NDG)

4 Discussion

4.1 Soil carbon is influenced by land-use and cover type

Numerous studies have documented that soil C is strongly controlled by land use(Wilsonet al.,2011;Heet al.,2012;Wiesmeieret al.,2012;Twongyirweet al.,2013;Zhanget al.,2013).Guo and Gifford(2002)summarized that land-use changes resulted in a decrease of soil C stock from pasture to plantation,native forest to plantation,native forest to crop,and pasture to crop,whereas an increase from native forest to pasture,crop to pasture,crop to plantation and crop to secondary forest.Cultivation practices generally lead to a decrease in these storage and an increase in bulk density,primarily because cultivation leads to soil compaction,reduction in litter input,and increase in exposure of physically-protected particulate organic C to microbial attack and rapid oxidation(Karcheganiet al.,2012;Wiesmeieret al.,2012).Changes in land use released about 25 Pg C to the atmosphere over the period 1700–1990 in the United States,largely from the conversion of forests to agricultural lands and from cultivation of prairie soils(Houghtonet al.,1999).At a site with temperate climate in Australia,soil C was in the order of woodland >pasture >cropland while bulk density broadly showed the reverse pattern;total C storage was 22.5 kg/m2in woodland and 5.7 kg/m2in cropland to a depth of 30 cm(Wilsonet al.,2011).In southeast Germany,Wiesmeieret al.(2012)reported that grassland stored the highest amount of SOC(11.8 kg/m2)and the cropland stored the lowest(9.0 kg/m2)down to 100 cm depth.In China's loess plateau,Zhanget al.(2013)found that grassland had the highest SOC storage(6.89 kg/m2),whereas terraced cropland had the lowest SOC storage(3.06 kg/m2),to a depth of 100 cm.

In our present study,the results indicated that the conversion from grassland into farmland showed SOC loss,whereas from grassland into plantation showed SOC accumulation.Desertification caused by long-term livestock grazing in the study area resulted in considerable SOC decrease.Afforestation and grassland restoration through grazing exclusion are two potentially important strategies for C sequestration in drylands(Nosettoet al.,2006;Malagnoux,2007).These practices can increase ecosystem C pools through the accumulation of plant biomass,increase of biodiversity,and reduction of soil erosion due to increased coverage of the ground by vegetation(Wofsy,2001;Nosettoet al.,2006;Lal,2008).The low rates of decomposition and soil respiration that occur in a dry environment may favor C sequestration by soils of re-vegetated areas in drylands(Grünzweiget al.,2003;Perez-Quezadaet al.,2011).

Our present study supports the practice of grazing exclusion in degraded grasslands with positive effects on SOC accumulation.In this case,SOC almost reached the level of normal grassland since the livestock had been excluded for 16 years in an area with moderate desertification.However,there have been inconsistent results in research literature for the effect of grazing exclusion on SOC pool.This is primarily due to complicated factors such as regional climate,practice duration,initial soil structure and type,sampling soil depth,plant community composition,degradation threshold,and the degree of degradation prior to grazing exclusion and plantation(Shrestha and Stahl,2008;Chenet al.,2010;Sasakiet al.,2011).

4.2 Effects of land-use and management on carbon in soil physical fractions

Many studies have reported that LF material,comprised largely of incompletely decomposed organic residue,can provide a sensitive indicator of the effects of land-use and management practices on SOM.For example,in the time frame of 2 to 5 years(Haynes,2000),4 years(Soonet al.,2007),and 6 years(Robles and Burke,1998),the LFOC pool changed significantly due to land-use change and management,but total SOC did not.Our results indicate that the LFOC represented a relatively large proportion of total SOC storage(ranging from 19.7% to 35.5%)in different plot types in the Horqin Sandy Grassland,compared to the small proportion of LF dry matter content to total soil mass(ranging from 0.39% to 1.89%).The variation pattern in LFOC storage induced by land-use and cover type changes is similar to that in total SOC storage.However,the extent of increase or decrease in LFOC in response to land-use and type changes is higher than that in SOC.That is,LFOC is more sensitive than SOC to changes in land-use and cover type.

Previous studies indicated that the majority of SOM is associated with silt-and clay-size fractions;values normally ranged from 10% to 30% of total soil C in the sand-size fraction(>0.05 mm),20% to 40% in the silt-size fraction(0.05 to 0.002 mm)and 35% to 70% in the clay-size(<0.002 mm)fraction(Feller and Beare,1997).Similar results by Christensen(2001)reported that in temperate arable soils,50% to 75% of SOM is present in clay-size fraction,while silt accounts for another 20% to 40% and sand-size fraction for <10%.In China's Black soil(Lianget al.,2009),the C pool in clay+silt fraction accounted for 75% and 82% of total SOC pool,respectively,for non-cultivated and cultivated soils.In contrast,the sand-size fraction of tropical soils retained a much higher proportion of the whole soil C than temperate soils,which ranged from 29% to 64%(Feller and Beare,1997).In the present study,the sand-size fraction(>0.05 mm)contained from 85% to 98% of the total SOC storage in different land-use and cover types.The highest value occurred in the 16EX site and the lowest in the 30F site.

Many studies have investigated the effects of land-use on SOM associated with particle-size fractions.Under vegetated fallow,the increase in SOC content was mainly due to an accumulation of particulate organic matter in the sand-size fraction in sandy soils and to an accumulation of C in the sand-and clay-size fractions of the clayey soil(Feller and Beare,1997).In typical steppe ecosystems of Inner Mongolia in China,Heet al.(2012)found that increases in C storage mainly occurred in sand and silt fractions in the topsoil(0 to 10 cm)with grazing exclusion and mowing.In China's Horqin Sandy Grassland,Chenet al.(2010)reported that C in all particle-size fractions(except clay)initially decreased following afforestation of grassland and subsequently increased as the forest matured.In the present study,C storage in all particle-size fractions increased rapidly following the practice of grazing exclusion in desertified areas.The most C accumulation occurred in fine sand and the lowest in very fine sand for the conversion of grassland into plantation.The C appeared to decrease in the coarse fraction and very fine sand,whereas an increase occurred in the fine sand and silt+clay,when the grassland was converted into farmland for 30 years.

In some studies(e.g.,Chenet al.,2010;Heet al.,2012),C concentrations were significantly higher in the silt+clay fractions than that in the sand fractions.However,in the present study,there was a significantly higher C concentration in the sand fraction than in the silt+clay at the 16EX,30P,and NDG sites.This pattern probably ascribed to the determination method for particle-size fractionation.The dry sieving method instead of wet sieving method was used in the present study.Increased non-water-stable aggregate,dominated by plant debris,was retained in sand fractions by dry sieving method.Unfortunately,we did not separate the non-water-stable aggregates from the coarse sand,and cannot confirm this hypothesis.

5 Conclusions

In the semiarid Horqin Sandy Grassland of northern China,changes in land-use and cover type significantly influenced C in the bulk soil and different soil particle-size and SOM density fractions.Total SOC storage decreased with the conversion of grassland into farmland,whereas increased with the conversion of grassland into plantation.Long-term continuous livestock grazing of grassland led to considerable C loss from the soil,but the SOC can increase rapidly following the practice of grazing exclusion.The LF organic matter played a major role in SOC dynamics and the allocation of organic C in soil particle-size fractions respond differently to different patterns of land-use and cover type changes.

This research was supported by the National Natural Science Foundation of China(41271007 and 31170413),the National Science and Technology Support Program of China(2011BAC07B02),and One Hundred Person Project of the Chinese Academy of Sciences.The authors are grateful to the anonymous reviewers for their critical review and comments on drafts of this manuscript.

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