Geoderma 230–231 (2014) 22–28

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Carbon sequestration in an intensively cultivated sandy loam soil in theNorth China Plain as affected by compost and inorganicfertilizer application

Jianling Fan a, Weixin Ding a,⁎, Jian Xiang a, Shenwu Qin a, Jiabao Zhang a, Noura Ziadi b

a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, Chinab Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Boulevard Hochelaga, Quebec City, Quebec G1V 2J3, Canada

⁎ Corresponding author. Tel.: +86 25 8688 1527; fax: +E-mail address: wxding@issas.ac.cn (W. Ding).

http://dx.doi.org/10.1016/j.geoderma.2014.03.0270016-7061/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 January 2014Received in revised form 28 March 2014Accepted 31 March 2014Available online xxxx

Keywords:Carbon sequestrationSoil carbon saturationCompostCrop residuesWheat–maize system

Understanding the balance between soil organic carbon (SOC) accumulation and depletion under different fertil-ization regimes is important for improving soil quality and crop productivity and formitigating climate change. Along-term field experiment established in 1989 was used to monitor the influence of organic and inorganicfertilizers on the SOC stock in a soil depth of 0–60 cm under an intensive wheat–maize cropping system in theNorth China Plain. The study involved seven treatments with four replicates: CM, compost; HCM, half compostnitrogen (N) plus half fertilizer N; NPK, fertilizer N, phosphorus (P), and potassium (K); NP, fertilizer N and P;NK, fertilizer N and K; PK, fertilizer P and K; and CK, control without fertilization. Soil samples were collectedand analyzed for SOC content in the 0–20 cm layer each year and in the 20–40 cm and 40–60 cm layers everyfive years. The SOC stock in the 0–60 cm depth displayed a net decrease over 20 years under treatments withoutfertilizer P or N, and in contrast, increased by proportions ranging from 3.7% to 31.1% under the addition ofcompost and fertilizer N and P. The stabilization rate of exogenous organic carbon (C) into SOC was only 1.5%in NPK-treated soil but amounted to 8.7% to 14.1% in compost-amended soils (CM andHCM). The total quantitiesof sequestered SOCwere linearly related (P b 0.01) to cumulative C inputs to the soil, and a critical input amountof 2.04 Mg C ha−1 yr−1 was found to be required tomaintain the SOC stock level (zero change due to cropping).However, the organic C sequestration rate in the 0–60 cm depth decreased from 0.41 to 0.29 Mg C ha−1 yr−1 forHCM and from 0.90 to 0.29 Mg C ha−1 yr−1 for CM from the period of 1989–1994 to the period of 2004–2009,indicating that the SOC stock was getting to saturation after the long-term application of compost. The estimatedSOC saturation level in the 0–60 cm depth for CMwas 61.31Mg C ha−1, whichwas 1.52 and 1.14 times the levelsfor NPK and HCM, respectively. These results show that SOC sequestration in the North China Plain may mainlydepend on the application of organic fertilizer. Furthermore, the SOC sequestration potential in the 0–20 cm layeraccounted for 40.3% to 44.6% of the total amount in the 0–60 cm depth for NPK, HCM, and CM, indicating that theSOC sequestration potential would be underestimated using topsoil only and that improving the depth distribu-tion may be a practical way to achieve C sequestration.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The dynamics of soil organic carbon (SOC) stocks and the role thatthe soil may play in the long-term accumulation and sequestration ofatmospheric CO2 are of great concern because of their impacts on themitigation of climate change, the sustainability of crop productivity,and soil fertility (Kirchmann et al., 2004; Srinivasarao et al., 2012). Incropland, a high SOC level can be achieved through appropriatecrop rotations, proper application rates for inorganic fertilizers andorganic manures, conservation tillage methods, and integrated soilfertility management (Bhattacharyya et al., 2011; Srinivasarao et al.,

86 25 8688 1000.

2012; Wright and Hons, 2005). Such enrichment of the SOC stock willhelp in maintaining good soil health for sustainable crop productionas well as in managing global climate change (Majumder et al., 2008;Mandal et al., 2008).

The dynamics of the SOC pool depends on the balance betweencarbon (C) input and output through different pathways and is stronglyinfluenced by soil management practices (Ding et al., 2012). A numberof studies showed that balanced fertilization with nitrogen (N), phos-phorus (P), and potassium (K) in combination with manures increasedSOC concentrations andmaintained high crop yields (Cai andQin, 2006;Marriott and Wander, 2006; Purakayastha et al., 2008). After a meta-analysis of published data from 74 studies, Gattinger et al. (2012)concluded that SOC contents in the topsoil layer were 0.18% higher,SOC stocks were 3.5 Mg C ha−1 higher, and SOC sequestration rates

23J. Fan et al. / Geoderma 230–231 (2014) 22–28

were 0.45 Mg C ha−1 yr−1 higher in organically farmed soils than innon-organically managed soils. Triberti et al. (2008) reported that ma-nure application could efficiently increase SOC content and that cattlema-nure had a higher SOC sequestration rate (0.26Mg C ha−1 yr−1) than didslurry (0.18 Mg C ha−1 yr−1) and crop residues (0.16 Mg C ha−1 yr−1)during a 28-year maize–wheat rotation in Italy southeastern Po Valley.In contrast, other researchers found that the application of inorganic fer-tilizers destroyed soil structure and caused the loss of organic C (Mikhaand Rice, 2004; Wu et al., 2004). However, Cai and Qin (2006) foundthat the application of N–P–K, N–P, and P–K fertilizer combinationsover 14 years increased the SOC stock by 3.7, 2.6, and 0.6 Mg C ha−1, re-spectively. This uncertainty among studies is attributed partly to the spe-cific processes governing C sequestration under management practices,as those processes vary with soil type, climate, and crop rotation (Lianget al., 2012). Therefore, it is important to assess C sequestration for spe-cific climates, soils, and crop systems to draw site-specific conclusions.

The North China Plain, the largest and most important agriculturalregion in China, accounts for 18.6% of the country farmland and sup-ports a population of 203 million. This region produces more than 75%and 32% of the national wheat and maize, respectively (China StatisticsBureau, 2011). A long-term field experiment was established in an in-tensively cultivated sandy loam soil in the North China Plain, where arotation of winter wheat (Triticum aestivum L.) and summer maize(Zeamays L.) is practiced, tomonitor changes in soil fertility under com-post and inorganic fertilizer application. Previous studies showed thatlong-term compost application and fertilization significantly increasedSOC concentrations (Ding et al., 2007) and SOC stocks (Cai and Qin,2006) in the 0–20 cm soil. However, limited information is availableon SOC sequestration in soil profiles as affected by crop managementand fertilization. The objectives of the present study were (i) to assessthe effect of 20-year of compost and inorganic fertilizer application onthe dynamics of the SOC stock in the 0–60 cm soil profile, (ii) to evaluatethe influence of compost and inorganic fertilizer on the SOC saturationlevel and sequestration potential, and (iii) to understand the relation-ship between SOC sequestration and organic C inputs.

2. Materials and methods

2.1. Site description

A long-termfield experimentwas established in September 1989 on awell-drained field at the Fengqiu State Key Agro-Ecological ExperimentalStation, in Fengqiu County, Henan Province, China (35°00′ N, 114°24′ E).Two crops per year, winter wheat (T. aestivum L.) and summer maize

Table 1Application rates for nitrogen (N), phosphorus (P), potassium (K), and compost for the differe2009.

Crop Treatmenta Inorganic fertilizer

N(kg N ha−1)

P(kg P ha−1)

Wheat CK 0 0NP 150 32.7NK 150 0PK 0 32.7NPK 150 32.7HCM 75 21.5CM 0 10.4

Maize CK 0 0NP 150 26.2NK 150 0PK 0 26.2NPK 150 26.2HCM 75 15CM 0 3.9

a CK, control without fertilization; NP, fertilizer N and P; NK, fertilizer N and K; PK, fertilizecompost.

b Values in the same column followed by different lowercase letters are significantly differe

(Z. mays L.), were cultivated. The 30-year mean annual temperaturewas 13.9 °C, with the lowest mean monthly value, −1.0 °C, in Januaryand the highestmeanmonthly value, 27.2 °C, in July. Themean precipita-tionwas 615mm. The soil, derived from alluvial sediments of the YellowRiver, is classified as an Aquic Inceptisol and contained 52% sand, 33%silt, and 15% clay. Before the experiment started in 1989, the pre-soilcontained 4.48 g kg−1 organic C, 0.43 g kg−1 total N, 0.50 g kg−1 totalP, 18.6 g kg−1 total K, 1.93 mg kg−1 available P, and 78.8 mg kg−1

available K.

2.2. Experimental design

The field experiment included seven treatments: compost (CM);half compost N plus half fertilizer N (HCM); fertilizers N, P, and K(NPK); fertilizers N and P (NP); fertilizers N and K (NK); fertilizers Pand K (PK); and control without fertilization (CK). The treatmentswere arranged in a randomized block design with four replicates. Eachplot measured 9.5 m by 5 m. The detailed experimental design andthe application amounts for the inorganic fertilizers and compost aresummarized in Table 1. The basal fertilizers (all the compost and partof the inorganic fertilizers) were broadcast evenly onto the plowedsoil (0–20 cm in depth) by tillage before sowing. The supplementaryfertilizer urea was surface-applied by hand and then incorporated intothe plowed layer with irrigation water or precipitation. For the experi-ments, the compost was prepared by mixing wheat straw, rapeseedcake, and cottonseed cake in a ratio of 100:40:45 and fermenting themixture for two months at the experimental farm. This proportionwas calculated on the basis of the component C and N contents, withthe goal of applying a total amount of organic C in compost (per hectareper season) equal to the amount in harvested wheat straw and anamount of organic N equivalent to 150 kg N ha−1. The wheat straw,rapeseed cake, and cottonseed cake were machine-ground to about5 mm in length before composting. The rapeseed and cottonseedcakesweremachine-dried residues from the extraction of oil from rape-seed and cottonseed and were obtained from a commercial cooking oilbusiness. The amounts of P and K were generally lower than the pre-scribed doses, so the compost was supplemented with calcium super-phosphate and potassium sulfate prior to application. The compostcontained 422 g C kg−1, 54.4 g N kg−1, 8.1 g P kg−1 and 19.5 g K kg−1.

Maize was sown directly into each plot in early June, and the dis-tances between rows and between hills were 70 and 25 cm, respec-tively. After two weeks, the seedlings were thinned to about 50,000per hectare, and the mature maize was harvested in late September.Wheat was sown directly in early October (distance of 15 cm) and

nt treatments as well as average grain yields and amounts of shoot biomass from 1989 to

K(kg K ha−1)

Compost(kg ha−1)

Grain yield(kg ha−1)

Shoot biomass(kg ha−1)

0 0 545.3 db 794.1 c0 0 4443.1 a 4747.2 a

124.5 0 567.9 d 860.7 c124.5 0 1088.8 c 1368.0 c124.5 0 4681.5 a 5169.5 a97.6 1379 4608.7 a 5060.3 a70.7 2758 3627.6 b 4090.1 b0 0 821.0 b 2501.5 c0 0 6688.8 a 6511.9 a

124.5 0 909.2 b 2577.9 c124.5 0 1595.2 b 3425.3 b124.5 0 7161.2 a 7070.7 a97.6 1379 7049.2 a 6846.4 a70.7 2758 6365.5 a 6672.8 a

r P and K; NPK, fertilizer N, P, and K; HCM, half compost N plus half fertilizer N; and CM,

nt among treatments at P b 0.05.

24 J. Fan et al. / Geoderma 230–231 (2014) 22–28

harvested in early June. During the experimental period (1989–2009),herbicide was sprayed generally about 20 days after sowing, and visibleweeds were pulled.

2.3. Crop biomass measurements and carbon input estimates

At harvest, the wheat and maize plants were removed completelyfrom the field, except for the stubble and roots, which were left in thesoil. The grains and shoots were separated manually and weighed.Approximately 200 g of grains and six stalks from each plot werewashed with tap water, rinsed with distilled water, dried at 70 °C for72 h and weighed.

Plant biomass inputs were estimated from shoot biomass and theratio of shoots to stubble and roots. The amounts of roots and stubbleleft in the field were calculated by the ratio of roots to shoot biomass(23% for maize and 22% for wheat) and the ratio of stubble to shootbiomass (26% for both wheat and maize), respectively (Kong et al.,2005; Rasse et al., 2006). The rhizodeposition C of wheat and maizewere assumed to be equal to the root biomass C (Bolinder et al., 1999).Therefore, the amounts of annual C inputs into the soil under the differenttreatments, including crop residues (stubble, roots, and rhizodeposits)and compost, were estimated.

2.4. Soil sampling and analysis

Soil samples were collected in the 0–20 cm layer each year aftermaize harvest and in the 20–40 cm and 40–60 cm layers every fiveyears. From each replicate plot, 10 soil cores in different layers were col-lected with a 2.5 cm diameter auger and then mixed to form one com-posite sample. Fresh samples were stored at 4 °C in the field and thentransported to the laboratory for subsequent analysis. The SOC contentswere determined using a wet oxidation procedure with K2Cr2O7. Thebulk density was measured by the core method in the field every fiveyears at a different depth (every 20 cm from 0 to 60 cm). The SOC stocksin the 0–20 cm layer were calculated every year, and the SOC stocks inthe 20–40 cm and 40–60 cm layers and the 0–60 cm profile were esti-mated every five years.

2.5. Data calculation and statistical analysis

The SOC stocks (TSOC, Mg C ha−1) were calculated as follows:

TSOC ¼ SOC � BD� D� 10

where SOC is the SOC concentration (g C kg−1), BD is the bulk density(Mg m−3), D is the soil depth (0.2 m), and 10 is a factor to adjust units.

The amount of SOC sequestration (ΔTSOC, Mg C ha−1)was calculatedas follows:

ΔTSOCi ¼ TSOCi−TSOCpre‐soil

where TSOCi and TSOCpre-soil represent the SOC stocks in the year i and in1989, respectively.

The SOC sequestration rate (Rseq, Mg C ha−1 yr−1) was calculated asfollows:

Rseq ¼ ΔTSOCi=t

where t denotes the interval (years) between two soil samples collectedin the same plot.

The stabilization rate of exogenous organic C into SOC in the entire0–60 cm depth was calculated as follows:

Rst ¼

X60

d¼0

ΔTSOCi

CCIi

where Rst is the stabilization rate of exogenous organic C (%), ΔTSOCirepresents the amount of SOC sequestration in the year i, and CCIi indi-cates the total amount of input organic C during the interval betweentwo soil samples collected.

One-way ANOVA analysis was used to analyze the differences incrop yield, shoot biomass, and SOC stock among the treatments andwas followed by LSD method at P = 0.05. All statistical analyses wereperformedwith SPSS 13.0 software forWindows (SPSS Inc., Chicago, IL).

3. Results

3.1. Input of organic carbon

The inputs of organic C ranged from 0.93 Mg C ha−1 yr−1 in the CKtreatment as stubble, roots, and rhizodeposits to 5.40 Mg C ha−1 yr−1

in the CM treatment as stubble, roots, rhizodeposits, and compost(Table 2). The HCM, NPK, and NP treatments had higher yields andamounts of aboveground biomass (Table 1) and resulted in higher Cinputs from crop residues as stubble, roots, and rhizodeposits, namely3.40, 3.50, and 3.22 Mg C ha−1 yr−1 for HCM, NPK, and NP, respectively.

3.2. Change in soil organic carbon stocks

The SOC stock in the 0–60 cm soil profile was influenced by theapplication of compost and mineral fertilizers (Table 3). The SOC stockin the 0–20 cm layer was highest in the CM treatment, followed byHCM and NPK, and increased significantly (P b 0.05) over 20 years, by70.4%, 36.4%, and 7.5% for CM, HCM, and NPK, respectively, comparedwith the stock in the pre-soil. In contrast, the SOC stock decreased byproportions ranging from7.0% to 27.7% in the CK, PK, andNK treatments.In the 20–40 cm layer, however, the SOC stock did not significantlychange in the CM, HCM, NPK, NP, and PK treatments but did decreasesignificantly in the NK and CK treatments. In the 40–60 cm layer, alltreatments increased the SOC stock significantly (P b 0.05), by propor-tions ranging from 9.0% to 21.2%. For the 0–60 cm soil profile, CM andHCM increased the SOC stock significantly, by 31.1% and 19.0%, respec-tively, and NPK and NP also increased the SOC stock significantly but toa lesser extent, by 3.7% and 5.2%, respectively. In contrast, PK did notshow any effect, and CK and NK displayed a significant negative effect.

The distribution of the SOC stock in the profile, as indicated by theSOC stock and their proportions at different depths, was altered primar-ily by fertilization (Table 3). In the CK treatment, the SOC stock in the40–60 cm layer was higher than the stocks in the 0–20 cm and 20–40cm layers and the value in the corresponding layer of the pre-soil. Incontrast, the SOC stock was highest in the 0–20 cm layer and lowestin the 20–40 cm layer in all fertilized soils. Moreover, the increase inthe SOC stock over 20 years in the NPK, HCM, and CM treatments accu-mulated primarily in the 0–20 cm and 40–60 cm layers rather than inthe 20–40 cm layer.

3.3. Organic carbon sequestration rate

The SOC sequestration rate differed significantly depending on thesoil layer and the treatment (Table 3). The rate in the 0–20 cm soillayer ranged from 0.01 to 0.51 Mg C ha−1 yr−1 in the CM, HCM, NPK,and NP treatments, but from −0.05 to −0.20 Mg C ha−1 yr−1 in theCK, NK, and PK treatments. In the 40–60 cm layer, a positive seques-tration rate was also observed in all the treatments, even CK, albeitmuch lower (0.05 to 0.12 Mg C ha−1 yr−1) than in the 0–20 cmlayer. However, this was not the case for the 20–40 cm layer (from−0.02 to −0.06 Mg C ha−1 yr−1), indicating that the 20–40 cmsoil lost SOC rather than sequestered any organic C.

For the 0–60 cm soil profile, the highest sequestration rate wasfound in soil treated with compost, namely 0.58 Mg ha−1 yr−1 forCM and 0.35 Mg C ha−1 yr−1 for HCM, and the rate was muchlower in soil treated with NPK and NP, namely only 0.07 and 0.10 Mg

Table 2Estimates of annual carbon (C) inputs into soil from compost and crop residues.

Treatmenta Root biomassb Stubble biomassc C input from rootsd C input from stubble C input from compost Total

Wheat Maize Wheat Maize Wheat Maize Wheat Maize

(kg ha−1) (kg ha−1) Mg C ha−1 yr−1

CK 182.6 551.5 206.5 519.5 0.07 0.22 0.08 0.26 0 0.93NP 1091.9 1455.9 1234.3 651.8 0.44 0.58 0.49 0.69 0 3.22NK 198.0 565.9 223.8 1720.6 0.08 0.23 0.09 0.27 0 0.97PK 314.6 750.4 355.7 668.7 0.13 0.30 0.14 0.35 0 1.35NPK 1189.0 1577.0 1344.1 886.8 0.48 0.63 0.54 0.75 0 3.50HCM 1163.9 1526.2 1315.7 1863.7 0.47 0.61 0.53 0.72 1.16 4.56CM 940.7 1488.1 1063.4 1803.7 0.38 0.60 0.43 0.70 2.33 5.40

a CK, control without fertilization; NP, fertilizer N and P; NK, fertilizer N and K; PK, fertilizer P and K; NPK, fertilizer N, P, and K; HCM, half compost N plus half fertilizer N; and CM, com-post.

b Maize and wheat roots represented 23% and 22% of shoot biomass, respectively (Kong et al., 2005).c The ratio of stubble to shoot biomass was assumed to be 26% for maize and wheat (Rasse et al., 2006).d Carbon content was assumed to be 0.40 forwheat andmaize (Johnson et al., 2006). The C input from rootswas considered twice because the amount of rhizodepositionwas assumed

to equal to the root biomass C at harvest (Bolinder et al., 1999).

25J. Fan et al. / Geoderma 230–231 (2014) 22–28

C ha−1 yr−1, respectively. In contrast, negative values (−0.20 to−0.03 Mg C ha−1 yr−1) were observed in the control soil and in thesoil with the unbalanced fertilizers NK and PK (Table 3).

4. Discussion

4.1. Soil organic carbon sequestration

Continuous cultivation without the addition of N or P fertilizer (CK,NK, and PK treatments) over 20 years caused a significant decrease inthe SOC stock in the 0–60 cm soil profile (Table 3). This decreasewas at-tributedmainly to low inputs of exogenous organic C from crop residues(0.93–1.35 Mg C ha−1 yr−1), which were lower than the magnitude ofmineralized SOC (1.99–2.19 Mg C ha−1 yr−1) measured in the field in2002–2003 (Ding et al., 2007). Such SOC losses during continuous culti-vation are a global phenomenon (Davidson and Ackerman, 1993;Mandal et al., 2008). However, the SOC depletion rate in the presentstudy (1.4%–10.7%) was much lower than the 24% to 43% measured byDavidson and Ackerman (1993) and the 30% to 60% measured by Lal(2004), probably because of the low SOC stock in the present study'spre-soil. In contrast, the application of compost (HCM and CM treat-ments) as well as NPK and NP fertilizers significantly caused a net in-crease in the SOC stock. This increase was due primarily to the greaterinput of exogenous organic C as crop residues and compost.

The significant linear relationship between SOC sequestration rateand input organic C (Fig. 1) indicated that even after 20-year of inputsof crop residues, compost, or both, the studied soil was still unsaturated.Furthermore, the results revealed that in order to maintain SOC stocklevels (zero change), the critical amount of C input to the soil was2.04 Mg C ha−1 yr−1 (Fig. 1), which was much lower than amounts re-ported by Kong et al. (2005) (3.1 Mg C ha−1 yr−1) after 10 years of

Table 3Soil organic carbon (SOC) stock and sequestration rate at different soil depths after 20-year of

Treatmenta SOC stock (Mg C ha−1)

0–20 cm 20–40 cm 40–60 cm

Pre-soil 14.5 d (38.9)b 11.8 a (31.6) 11.0 d (29.5)CK 10.5 f (31.5) 10.5 b (31.6) 12.3 bc (37.0)NP 14.6 cd (37.2) 11.3 ab (28.8) 13.4 a (34.0)NK 12.9 e (36.0) 10.6 b (29.7) 12.3 bc (34.3)PK 13.5 de (36.6) 11.2 ab (30.3) 12.2 bc (33.1)NPK 15.6 c (40.3) 11.1 ab (28.7) 12.0 c (31.0)HCM 19.8 b (44.6) 11.5 ab (25.8) 13.2 a (29.7)CM 24.7 a (50.5) 11.2 ab (23.0) 13.0 ab (26.5)

a CK, control without fertilization; NP, fertilizer N and P; NK, fertilizer N and K; PK, fertilizer Ppost.

b Values in the same column followed by different lowercase letters are significantly differenthe total SOC stock.

cropping in Davis, California, under a Mediterranean-like climate, byMandal et al. (2008) (3.41 Mg C ha−1 yr−1) for a 36 year double-cropped rice system under a subtropical climate, and by Majumderet al. (2008) (3.56 Mg C ha−1 yr−1) for a 19 year rice–wheat systemin subtropical India. The lower critical amount of C input required tomaintain a constant SOC stock level in the present study might be dueto a lower initial SOC level (4.48 g kg−1) and a higher C sequestrationrate (maximum of 0.58 Mg C ha−1 yr−1). The three abovementionedstudies reported initial SOC concentrations that were much higher (9.6,11.4, and 14.2 g kg−1, respectively) and SOC sequestration rates thatwere much lower (maximum of 0.56, 0.41, and 0.15 Mg C ha−1 yr−1,respectively) than those in the present study.

The stabilization rate of exogenous organic C into SOCwas a negativevalue for the CK, NK, and PK treatments; 0.7% for NP and 1.5% for NPK;and as high as 8.7% for HCM and 14.1% for CM (Fig. 2). Bertora et al.(2009) reported that only 54% of manure C was decomposed duringone year, a proportion lower than that for maize roots (72%). The presentstudy clearly indicated that compost efficiently and effectively increasedthe SOC stock, whereas crop roots and rhizodeposits displayed a verylimited effect in the studied soil. The application of compost, especiallycompost high in phenolic and lignin residues, was found to acceleratethe accumulation of lignocelluloses and hemicelluloses (Mandal et al.,2008) and to significantly reduce the specific activities of soil polyphenoloxidase and invertase in comparison with inorganic fertilizer (Yu et al.,2012a), resulting in a decrease in the mineralization rate per unit of SOC(Yu et al., 2012c). Previous studies demonstrated that compost applica-tion accelerated the formation of aggregates (Yu et al., 2012b) and alteredthe microhabitats of microorganisms, inducing a shift in the microbialcommunity towards higher populations of denitrifiers and facultativeanaerobes in aggregates (Doran, 1980; Zhang et al., 2013), possibly be-cause of an increase in pore-filling organic matter (mainly as particulate

application of inorganic fertilizers and compost.

SOC sequestration rate (Mg C ha−1 yr−1)

0–60 cm 0–20 cm 20–40 cm 40–60 cm 0–60 cm

37.3 d – – – –

33.3 f −0.20 −0.06 0.07 −0.2039.3 c 0.01 −0.02 0.12 0.1035.8 e −0.08 −0.06 0.06 −0.0836.8 de −0.05 −0.03 0.06 −0.0338.7 c 0.05 −0.04 0.05 0.0744.4 b 0.26 −0.02 0.11 0.3549.0 a 0.51 −0.03 0.10 0.58

and K; NPK, fertilizer N, P, and K; HCM, half compost N plus half fertilizer N; and CM, com-

t at P b 0.05. Data in parentheses indicate the proportions (%) of SOC stock in each layer to

Fig. 1.Relationship between sequestered soil organic carbon (SOC) and cumulative carbon(C) inputs after 20-year of application of inorganic fertilizers and compost.

Fig. 3. Dynamics of the soil organic carbon (SOC) sequestration rate from 1989 to 2009 forthe treatments with half compost nitrogen plus half fertilizer nitrogen (HCM) andcompost (CM).

26 J. Fan et al. / Geoderma 230–231 (2014) 22–28

organic matter or amorphous organic materials) in microaggregates(Zhuang et al., 2008). It is known that an increase in Gram-positive bacte-ria in comparisonwithGram-negative bacteria cannot only producemoreprecursors for the formation of recalcitrant organic C but also generatemore assimilation products for the growth of fungi and actinobacteria(Hill et al., 2008; Schneckenberger et al., 2008). Fungal productsare more chemically resistant to decay (Simpson et al., 2004), andactinobacteria are known to produce chitinases (Rinnan and Bååth,2009), resulting in more recalcitrant organic C accumulation. Therefore,it can be concluded that an efficient increase in the SOC stock woulddepend primarily on the application of organic fertilizer to soil poor inorganic C in the North China Plain, which induced a shift in the microbialcommunity towards higher populations of denitrifiers and facultativeanaerobes and produced more recalcitrant organic C.

4.2. Soil organic carbon saturation

The application of compost and NPK fertilizer significantly (P b 0.05)increased the SOC stock by 19.0% and 31.1% and by 3.7%, respectively(Table 3). However, the mean SOC sequestration rate decreased gradu-ally, from 0.41 to 0.90 Mg C ha−1 yr−1 in the period of 1989–1994 to0.29 Mg C ha−1 yr−1 in the period of 2004–2009, for the HCM and CM

Fig. 2. Stabilization of input organic carbon (C) (crop residue, compost, or both) into soilorganic carbon (SOC) (%) under different treatments (1989–2009). CK, control withoutfertilization;NP, fertilizer nitrogen (N) and phosphorus (P); NK, fertilizer N and potassium(K); PK, fertilizer P and K; NPK, fertilizer N, P, and K; HCM, half compost N plus half fertil-izer N; and CM, compost.

treatments (Fig. 3). These results suggest that the SOC stock wasreaching saturation after the long-term application of compost. Therelationship between SOC stock and experimental year fitted similar re-ciprocal models for treatments of NPK, HCM and CM (Fig. 4). The resultsshowed that the studied soil would be saturated at 61.31 Mg C ha−1 inthe CM treatment, 53.68 Mg C ha−1 in the HCM treatment, and as littleas 40.24Mg C ha−1 in theNPK treatment (Fig. 4). Therefore, the SOC se-questration potential for the NPK treatmentwas 1.54Mg C ha−1, whichwas only 17% and 13% of the potential for the HCM and CM treatments,respectively. These results suggest that the studied soil would be satu-rated at a quite low level with long-term inorganic fertilizer applicationcompared with long-term compost application. This was attributedmainly to the fact that compost application could improve soil aggrega-tion and aggregate-associated organic Cwhereas inorganic fertilizer hadno obvious effect (Yu et al., 2012b). Therefore, to elevate the SOC con-tent in soil poor in organic C in the North China Plain, organic fertilizerswould be needed to supply exogenous organic C other than crop resi-dues to the soil.

In this study, the SOC sequestration potential (saturation level) in the0–20 cm layer was 40.3% to 44.6% of the total amount in the 0–60 cm soilprofile for the NPK, HCM, and CM treatments (Fig. 4). These figures areclose to the ratios of the 0–20 cm layer to the 0–100 cm layer reportedin grasslands (42%) and forestlands (50%) (Jobbágy and Jackson, 2000).Furthermore, all treatments showed negative SOC sequestration rates inthe 20–40 cm soil layer but positive values in the 40–60 cm layer(Table 3), indicating that the 20–40 cm soil layer lost rather than seques-tered organic C and the 40–60 cm soil layer had higher C sequestrationpotential than the 20–40 cm layer did. This observation was attributedmainly to the presence of a sandy soil layer in the 20–40 cm soil layerin the study site and to the much lower SOC sequestration capacity ofsandy soil (Bayer et al., 2006). Although inorganic fertilizer and compostapplication was restricted to the surface layer, applied C from the surfacesoil layer couldmove downward and accumulate in the deeper soil layersalong with root biomass C inputs in these layers. Bhattacharyya et al.(2011) found that the 15–30 cm soil layer was themost efficient in stabi-lizing applied organic C and that the proportion of applied manure C sta-bilized in this layer was 1.37 times the proportion in the 0–15 cm layerand 6.14 times the proportion in the 30–45 cm layer. However, with re-spect to the low C-retention capacity of the 20–40 cm soil layer (sandysoil) at the study site, organic C may have moved downward from the0–20 cm layer, passed through the 20–40 cm layer, and been sequesteredin the 40–60 cm soil layer. The results of the present study suggest thatthe SOC sequestration potential would be underestimated using topsoilonly and that improving the depth distribution may be a practical wayto achieve C sequestration.

Fig. 4.Dynamics and simulations of soil organic carbon (SOC) stocks by experimental yearfor the treatments with fertilizer nitrogen, phosphorus, and potassium (NPK), half com-post nitrogen plus half fertilizer nitrogen (HCM), and compost (CM).

27J. Fan et al. / Geoderma 230–231 (2014) 22–28

5. Conclusions

The SOC stock in the 0–60 cm depth increased by proportions rang-ing from 3.7% to 31.1% over 20 years under the addition of compost andfertilizer N and P, but decreased by proportions ranging from 1.4% to10.7% under treatments without fertilizer P or N. The total quantitiesof sequestered SOCwere significantly (P b 0.01) linearly related to cumu-lative C inputs to the soil. Moreover, the estimated SOC saturation level inthe 0–60 cm depth for the CM treatment was 61.31 Mg C ha−1, whichwas much higher than the estimated levels for NPK (40.24 Mg C ha−1)andHCM(53.68Mg Cha−1). These results suggest that organic fertilizerswould be needed to supply exogenous organic C other than crop residues

to the soil in order to elevate the SOC content in the soil poor in organic Con theNorth China Plain. However, the organic C sequestration rate in the0–60 cmdepth decreased from0.41 to 0.29Mg C ha−1 yr−1 for HCMandfrom 0.90 to 0.29 Mg C ha−1 yr−1 for CM from the period of 1989–1994to the period of 2004–2009, indicating that the SOC stock was reachingsaturation after the long-term application of compost. Furthermore, theSOC sequestration potential (saturation level) in the 0–20 cm layerwas 40.3% to 44.6% of the total amount in the 0–60 cm soil profile forthe NPK, HCM, and CM treatments, indicating that the SOC sequestra-tion potential would be underestimated using topsoil only and thatimproving the depth distribution may be a practical way to achieve Csequestration.

Acknowledgments

This study was supported by the Strategic Priority ResearchProgram — Climate Change: Carbon Budget and Relevant Issues ofthe Chinese Academy of Sciences (Grant No. XDA05050507), the Na-tional Basic Research Program of China (Grant No. 2011CB100503),and the Natural Science Foundation of China (Grant Nos. 41001134and 41271243).

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