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Soil organic carbon sequestration in upland soilsof northern China under variable fertilizer

management and climate change scenarios

ARTICLE · MARCH 2014

DOI: 10.1002/2013GB004746

CITATIONS

2

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86

12 AUTHORS, INCLUDING:

Yasuhito Shirato

National Institute for Agro-Environmental Sci…51 PUBLICATIONS 614 CITATIONS

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Toshichika Iizumi

National Institute for Agro-Environmental Sci…61 PUBLICATIONS 353 CITATIONS

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Jinzhou Wang

University of Maryland, College Park

4 PUBLICATIONS 2 CITATIONS

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Daniel Vaughan Murphy

University of Western Australia

134 PUBLICATIONS 3,795 CITATIONS

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Soil organic carbon sequestration in upland soils

of northern China under variable fertilizer

management and climate change scenarios

Guiying Jiang1, Minggang Xu1, Xinhua He1,2, Wenju Zhang1, Shaomin Huang3, Xueyun Yang4,

Hua Liu5, Chang Peng6, Yasuhito Shirato7, Toshichika Iizumi7, Jinzhou Wang1, and Daniel V. Murphy1,8

1Ministry of Agriculture Key Laboratory of Crop Nutrition and Fertilization, Institute of Agricultural Resources and Regiona

Planning, Chinese Academy of Agricultural Sciences, Beijing, China, 2School of Plant Biology, University of Western

Australia, Crawley, Western Australia, Australia, 3Institute of Plant Nutrition and Resources Environment, Henan Academy o

Agricultural Sciences, Zhengzhou, China, 4College of Natural Resources and Environment, Northwest A & F University,

Yangling, China, 5Institute of Soil and Fertilizer, Xinjiang Academy of Agricultural Sciences, Urumqi, China, 6Northeast

Agricultural Research Center of China, Jilin Academy of Agricultural Sciences, Changchun, China, 7National Institute for

Agro-Environmental Sciences, Tsukuba, Japan, 8Soil Biology and Molecular Ecology Group, School of Earth and

Environment, Institute of Agriculture, University of Western Australia, Crawley, Western Australia, Australia

Abstract We determined the historical change in soil organic carbon (SOC) stocks from long-term eldtrials that represent major soil types and climatic conditions of northern China. Soil carbon and general

circulation models were validated using these eld trial data sets. We then applied these models to predict

future change in SOC stocks to 2100 using two net primary production (NPP) scenarios (i.e., current NPP or 1%

year1 NPP increase). The conversion rate of plant residues to SOC was higher in single-cropping sites than in

double-cropping sites. The prediction of future SOC sequestration potential indicatedthat these soils will be a

net source of carbon dioxide (CO2) under no fertilizer inputs. Even when inorganic nutrients were applied, the

additional carbon input from increased plant residues could not meet the depletion of SOC in parts of

northern China. Manure or straw application could however improve the SOC sequestration potential at all

sites. The SOC sequestration potential in northern China was estimated to be4.3 to 18.2t C ha1 by 2100. The

effectof projected climate change on the annual rate of SOC change didnot differsignicantly between climate

scenarios. The average annual rate of SOC change under current and increased NPP scenarios (at 850 ppm CO2

was approximately 0.136 t C ha1 yr1 in northern China. These ndings highlight the need to maintain, andwhere possible increase, organic carbon inputs into these farming systems which are rapidly becoming

inorganic fertilizer intensive.

1. Introduction

Soil organic carbon (SOC) sequestration in agricultural soil is directly affected by anthropogenic activities

and climate change; both can alter net primary production (NPP) and organic matter decomposition

[Yan et al., 2010]. Carbon inputs to soil can be increased in arable farming systems where (i) crop

production has not yet achieved maximum water use ef ciency and/or where irrigation is available,

(ii) nutrient limitations are overcome with fertilizers, and (iii) where additional organic sources are applied;

potentially converting agricultural soil into a net carbon store. The capacity for further SOC sequestration

in agricultural soils is estimated at 140 to 170 Pg C [Lal , 2004], which is more than 10% of the existingglobal terrestrial SOC store. As such the Intergovernmental Panel on Climate Change [ IPCC, 2007a] has

identied SOC sequestration as a cost-effective and environmentally friendly option to mitigate increasing

atmospheric carbon dioxide (CO2).

China has more than 20% of the world population and 8% of the total world arable land [Food and Agriculture

Organization, 2010]. Agriculture was responsible for 15 to 18% of the total greenhouse gas emissions in China

during 2007; with contributions from agricultural land being 43 to 47% from methane (CH4), 33 to 34% from

nitrous oxide (N2O) and 19 to 23% from CO2 [Guo and Zhou, 2007; Lin et al., 2012]. Crop production is the

major land use occupying an area of 122 million ha [National Bureau of Statistics of China, 2012] and accounts

for 7 to 12% of the SOC stock under arable production systems worldwide [Schlesinger , 1999]. Furthermore,

additions of organic waste to agricultural soil have occurred for thousands of years (e.g., Loessial soil) in China

JIANG ET AL. ©2014. American Geophysical Union. All Rights Reserved. 319

PUBLICATIONS

Global Biogeochemical Cycles

RESEARCH ARTICLE10.1002/2013GB004746

Key Points:

• The RothC model is suitable for

SOC simulation in upland soil in

Northern China

• The climate change did not signi-

cantly affect annual rateof SOC change

• Inorganic fertilizer intensive farming

needorganic carbon inputs for SOCkept

Supporting Information:

• Readme

• Table S1

• Table S2

• Table S3

• Figure S1

• Figure S2

• Figure S3

Correspondence to:

M. Xu and D. V. Murphy,

[email protected];

[email protected]

Citation:

Jiang, G., et al. (2014), Soil organic

carbon sequestration in upland soils of

northern China under variable fertilizer

management and climate change

scenarios, Global Biogeochem. Cycles, 28,

319–333, doi:10.1002/2013GB004746.

Received 25 SEP 2013Accepted 25 FEB 2014

Accepted article online 3 MAR 2014

Published online 26 MAR 2014

https://www.researchgate.net/publication/229917944_Direct_measurement_of_soil_organic_carbon_content_change_in_the_croplands_of_China?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/229917944_Direct_measurement_of_soil_organic_carbon_content_change_in_the_croplands_of_China?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/229917944_Direct_measurement_of_soil_organic_carbon_content_change_in_the_croplands_of_China?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/8515631_Soil_Carbon_Sequestration_Impacts_on_Global_Climate_Change_and_Food_Security?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/8515631_Soil_Carbon_Sequestration_Impacts_on_Global_Climate_Change_and_Food_Security?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/8515631_Soil_Carbon_Sequestration_Impacts_on_Global_Climate_Change_and_Food_Security?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/235238641_Carbon_Sequestration_in_Soils?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/235238641_Carbon_Sequestration_in_Soils?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/235238641_Carbon_Sequestration_in_Soils?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==http://publications.agu.org/journals/http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1944-9224http://dx.doi.org/10.1002/2013GB004746https://www.researchgate.net/publication/229917944_Direct_measurement_of_soil_organic_carbon_content_change_in_the_croplands_of_China?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/8515631_Soil_Carbon_Sequestration_Impacts_on_Global_Climate_Change_and_Food_Security?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/235238641_Carbon_Sequestration_in_Soils?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/223947357_Greenhouse_gas_emissions_and_mitigation_measures_in_Chinese_agroecosystems?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==http://dx.doi.org/10.1002/2013GB004746http://dx.doi.org/10.1002/2013GB004746http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1944-9224http://publications.agu.org/journals/


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aiding the stabilization of SOC [Liang et al., 2012; Zhao et al., 2008]. However, China is currently the largest

consumer of inorganic fertilizer in the world, accounting for 90% of the global increase in use [Liu and

Diamond, 2005]. Through increased inorganic fertilizer use, adoption of modern plant cultivars and increased

areas of irrigation, crop grain yields in China have approximately doubled between 1980 (wheat 1.9 t ha1,

maize 3.1 t ha1) and 2010 (wheat 4.7 t ha1, maize 5.4 t ha1) [National Bureau of Statistics of China, 2012].

However, during this 30 year period SOC stocks in agricultural systems employing common management

practices (i.e., tillage, inorganic fertilizers, straw removal, and no animal manure application) have only

changed slightly; with a general decrease in arid/semiarid regions and increase in humid/semihumid

regions [Sun et al., 2010; Yan et al., 2010; Yu et al., 2009]. Reported SOC changes in agricultural soils vary

(2.0 to 0.6% yr1) [Yan et al., 2010] with an average SOCsequestration rate of 21.9 Tg C yr1 between 1980

and 2000 [Sun et al., 2010]; equivalent to 0.21% of the estimated 10,070 Tg C stored in upland soils in China

[ Xie et al., 2007].

A change in crop growth will alter the carbon input to soil from plant residues, which is typically the main

source of new SOC in arable land (unless manure is applied). Net primary production (NPP) is affected by

climatic variables such as temperature, precipitation, atmospheric CO2, and the length of crop growth period

[Ye et al., 2013]. It is reported that the yields of wheat and maize have responded negatively to warming at the

global scale, although the impact on other crops (e.g., rice) is less certain [Lobell and Field , 2008]. Wan et al.

[2011] modeled future changes in SOC stocks for upland soils in China based on historical plant carbon input

rates without consideration of manure or straw application. They predicted that SOC would decrease in most

upland soils, especially in northern China. No consideration was given in their future predictions to increases

in NPP based upon improved plant breeding and/or adoption of “best practice” agronomic management. It isexpected that the rate of straw retention in China could increase from 40% [Gao et al., 2002] to90%[Sun et al.

2010], and that no-tillage practices could be extended to 50% of the nations cropland by 2050; with organic

manure inputs likely to remain the same (110 Tg C yr1) [Li et al ., 2003]. Based on a historical crop NPP

increase of approximately 12 Tg C yr1 from 1960 to 1999 [Huang et al., 2007] and a future increase in NPP o

approximately 6 Tg C yr1, Sun et al. [2010] calculated a further 55% increase in NPP by 2050 for China

(equivalent to a 1% annual NPP increase from 2000 to 2050).

In our study, we measured the historical change in SOC stocks from long-term eld trials for the major soil

types and climatic conditions of northern China and then modeled future changes in SOC stocks using

different climate and carbon input scenarios. We wanted to quantify the difference in current SOC

sequestration rates when organic residues are added to soils compared to common management practice

Figure 1. Location of the eight long-term experimental sites in upland soils of northern China.

Global Biogeochemical Cycles 10.1002/2013GB004746


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ization_and_properties_of_water-extractable_organic_carbon_in_soils_of_the_south_Loess_Plateau_in_China?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==https://www.researchgate.net/publication/257638657_Long-term_combined_application_of_manure_and_NPK_fertilizers_influenced_nitrogen_retention_and_stabilization_of_organic_C_in_Loess_soil?el=1_x_8&enrichId=rgreq-0b8422f1-adaa-4f1a-9c4e-ea5dba3ca39f&enrichSource=Y292ZXJQYWdlOzI2MDUzMTUxMztBUzoxOTkyMTU3NDY2ODY5NzZAMTQyNDUwODE0MzYxMQ==


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We also wanted to determine how climate change would alter SOC stocks and if future SOC stocks could be

increased with inorganic fertilizer alone or if organic residue/manure inputs will be required. Our specicaims were to (i) measure the historical change in SOC stocks from eight long-term fertilizer trials (15–28 years

treatments of inorganic fertilizers and/or manure/straw application) that represent the major soil types and

climatic conditions of northern China, (ii) use the historical climate and soil data to validate global climate

models and the RothC carbon model for northern China, and (iii) model future changes in SOC stocks to 2100

under two plant carbon input scenarios (no change to NPP or an annual 1% NPP increase).

2. Methods and Materials

2.1. Field Research Sites

Our study consisted of eight long-term (i.e., 15–28 years) experimental sites on upland soils in the northern

regions of China (Figure 1). The climate at these sites ranged from arid to semihumid and from mild to

warm temperate. Annual mean temperature ranged from 4.5°C in the northeast to 14.5°C in the western

central region, annual precipitation ranged from 127 mm in the northwest to 832 mm in the central easternregion, and evaporation was 1 to 18 times greater than precipitation (Table 1). The annual cropping

rotation was either single or double crops and consisted of various crop sequences of predominately

wheat or maize (Table 1). The four single-cropping trial locations were signicantly cooler (4.5–8.0°C) and

drier (127–540 mm) compared to the four double-cropping sites (11.0–14.5°C and 575–832 mm) (P


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soil, Zhangye site), Calcic Kastanozem (Dark Lossial soil, Pingliang site), Haplic Luvisol (Brown Fluvo-aquic

soil, Changping site), Calcaric Cambisol (Fluvo-aquic soil, Zhengzhou and Xuzhou sites), and Cumulic

Anthrosol (Lossial soil, Yangling site).

Long-term eld plots (n = 8for eld trial location, n = 1–3 for within trial plot replicates) varied in size: 33 m2 at

Zhangye and Xuzhou; approximately 200 m2

at Changping, Yangling, and Pingliang and approximately400m2 at Urumqi, Gongzhuling, and Zhengzhou. Inorganic nitrogen (N), phosphorus (P), and potassium (K)

fertilizers were applied as urea, calcium superphosphate, and potassium chloride, respectively. There were

three fertilizer treatments common to each eld trial: no fertilizer (Control), inorganic fertilizer only (NP or

NPK), and inorganic fertilizer plus manure (M) addition (NP + M or NPK + M). In addition, one additional

fertilizer treatment was sampled depending on the site: manure only (M) at Zhangye and Xuzhou, and

inorganic fertilizer plus straw (S) return (NP+ S or NPK + S) at the other six sites. The total N applied

(inorganic+ organic) was equal for the NPK and NPK + M treatments at ve sites but was higher in the

NPK+ M treatment at Zhangye, Pingliang, and Xuzhou due to an additional manure N application (Table S1 in

the supporting information). For the NP + M and NPK + M treatments 30% of the total N was from the

inorganic fertilizer, while the remainder was organic manure N.

Organic carbon input into soil included plant residues (plant roots + stubble) plus any treatment addition o

organic manure or crop straw return. The average annual carbon inputs from manure, straw, and plantresidues at each site are reported in Table S2. All aboveground biomass (not including stubble) was removed

from the plots at harvest; the straw was returned to plots in the NP + S and NPK + S treatments. The average

C/N ratio of straw was 67/1 for wheat and 50/1 for maize. The carbon input from roots was estimated by the

ratio of belowground biomass to aboveground biomass. Total plant biomass carbon was proportioned to

roots as 30% for wheat and 26% for maize [ Li et al., 1994], and we assumed that 75.3% and 85.1% of the tota

root biomass were in the surface 20 cm of soil for wheat [Fang et al., 2011; Lu and Xiong, 1991; Ma, 1987; Miao

et al., 1989], and maize [Li et al., 1992; Liu and Song, 2007], respectively. The contribution of carbon input from

stubble was estimated using the ratio of stubble biomass to straw biomass. For wheat we used the average

for fertilized plots of 13.1% and for control plots 18.3%, while for maize we used 3.0% for all plots [ Xu et al.

2006]. To convert plant dry matter into the equivalent amount of carbon, we used the national average

carbon concentrations for wheat (399g C kg1) and maize (444 g C kg1) residues on an oven-dried basis

[NCATS , 1994].

The source of manure changed with local availability (pig, goat, horse, or cattle) and varied with a C/N ratio

between 11/1 and 26/1 (Table S2). Annual carbon inputs from manure ranged from 0.43 to 8.69 t C ha1 yr1

depending on the site and application year [Fan et al., 2008; NCATS , 1994; Xu et al., 2006]

Soil tillage was by a mouldboard plow. For single-cropping or double-cropping eld sites tillage occurred

before seeding once or twice a year, respectively. The tillage depth was 25 cm at Gongzhuling, Urumqi, and

Zhengzhou where prior measurements determined that 80% of the straw/manure inputs would remain

within the surface 20 cm of soil, while tillage was to 20 cm at other eld sites where 100% of the straw/

manure inputs remained within this soil depth [ Xu et al., 2006].

2.2. Soil Analysis

Composite soil samples (0–20 cm depth) were randomly collected from each plot at each eld site (n = 5–10

cores per composite sample; 5 cm in diameter) after harvest but before tillage (i.e., September–

October). Soisamples were air dried before being sieved (


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2.3. Carbon Model

The climatic data used in the RothC model (RothC-26.3) [ Jenkinson and Coleman, 1999] consisted of monthly

mean air temperature (°C), precipitation (mm), and open pan evaporation (OPE; mm). Temperature and

precipitation data for eachsite were collected fromthe nearest meteorological station of the China Meteorologica

Administration. Because the OPE data were unavailable, we calculated potential evapotranspiration (PET)according to the Food and Agriculture Organization (FAO) Penman-Monteith method [ Allen et al., 1998] and

converted the PET to OPE by OPE = PET/0.75 [ Jenkinson and Coleman, 1999]. Since the land was irrigated at

Urumqi, Zhangye, Yangling, Changping, Zhengzhou, and Xuzhou, we added summed irrigation water

with precipitation.

Soil input data for modeling were based upon the clay content (%) and the initial SOC content (t C ha1) fo

each trial site. To determine the inert organic matter (IOM) pool for the RothC model we used the equation

IOM = 0.049 × SOC1.139 [Falloon et al ., 1998]. Management data on monthly soil cover (bare or vegetated)

were obtained from Xu et al. [2006]. In the RothC model the added straw was treated as crop residue and

animal manure as farm yard manure.

In modeling each set of eld trial data, we set the initial SOC value in the RothC model to the measured SOC

content from the initial value of each eld trial treatment plot (Table 2) and then simulated the change in SOC

during the trial period for each set of fertilizer treatments. To run the model, it is necessary to specify theinitial amount of SOC in each of ve dened organic matter pools: Decomposable Plant Material (DPM),

Resistant Plant Material (RPM), Microbial Biomass (BIO), Humied Organic Matter (HUM), and Inert Organic

Matter (IOM). The allocation of SOC among the different pools was unknown for these eld sites. However, as

described by Jenkinson and Coleman [1999] if we assume that the SOC content has reached equilibrium, then

RothC can be run inversely to calculate the amount of carbon that is needed to enter the soil annually to

maintain a specic level of SOC; the allocation of SOC into each of the four organic matter pools is dened at

the same time. This is a standard means by which to parameterize this model to equilibrium; at which point

the relative size of the carbon pools can be dened [see Jenkinson and Coleman, 1999; RRes, 2007]. For plant

residue C inputs we used a DPM: RPM ratio of 1.44 as this is a typical value for most agricultural crops and

grasses [ Jenkinson and Coleman, 1999]. The average weather data (monthly mean air temperature (°C),

precipitation (mm), and open pan evaporation (OPE; mm)) for each trial site from the start year to the end of

the simulation was used in this equilibrium model run.

Once the starting SOCcontent andits initial allocation among the organic matter pools hadbeen established

the model was run using carbon inputs according to the different carbon inputs scenarios: (A) from the initia

year to 2010, the carbon inputs were the measured data for each year (Figure 2); (B) after 2010, there are two

scenarios (i) for the current NPP carbon inputs scenario, the carbon inputs were the average values for each

site during the experimental period which are listed in Table S2; (ii) for the 1% annual increase in NPP carbon

inputs scenario, the carbon inputs are based on the values and justication provided in Sun et al. [2010] for a

1% annual increase by the current NPP carbon inputs (see carbon inputs scenarios in detail in paragraph 18)

2.4. General Circulation Models

Two general circulation models (GCMs) were selected: BCCR, the Bjerknes Centre for Climate Research,

University of Bergen, Norway, http://www.ipcc-data.org/ar4/model-BCCR-BCM2.html and IPSL, the Institute

Pierre Simon Laplace, France, http://www.ipcc-data.org/ar4/model-IPSL-CM4.html. These two global change

models represent a range of model characteristics and thus their climates scenarios. The future climate usingBCCR is cold and dry, while the IPSL is warm and wet when compared to historical observations (Table S3).

Both GCMs have been validated for use in China [Li et al., 2011], and we also found good agreement between

models and historical climatic data (1971–2000) when assessed for the trial sites used in this study (e.g.,

observed versus estimated total net radiation at Urumqi; Figure S3). Here we used extremes in CO 2concentration scenarios of 550 ppm (B1) and 850 ppm (A2) [IPCC, 2007b].

2.5. Climate Change Scenarios and Plant Residue Carbon Input Scenarios

We set ve climate scenarios until the year 2100 for the RothC modeling: no climate change, BCCR GCM

under two CO2 emission scenarios (B1, A2) and the IPSL GCM under two CO 2 emission scenarios (B1, A2).

Since the RothC model does not include the crop submodel routine, we set two carbon input scenarios:

(i) current NPP (the average of the eld trial experimental period) and (ii) 1% annual increase in NPP based on



http://www.ipcc-data.org/ar4/model-IPSL-CM4.htmlhttp://www.ipcc-data.org/ar4/model-IPSL-CM4.html


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the agricultural productivity improvements proposed by Sun et al. [2010]—we extrapolated this NPP scenario

to the year 2100 on the basis that similar further gains in NPP would be made as agricultural practices and

crop breeding continue to make improvements to plant productivity.

2.6. Evaluation of Model Performance and Statistical Analysis

We determined the coef cient of determination (R2) to represent the degree of association between the

modeled and measured data and the root-mean-square error (RMSE) to represent the magnitude of

differences between the modeled and observed values [Smith et al., 1997]. Analysis of variance (ANOVA) and

the least signicant difference methods (P < 0.05) were applied to compare treatment and climate effects on

crop yield, organic carbon input, and SOC dynamics. The t test was employed to assess differences in basic

site information, soil properties, crop yield, organic carbon input, and SOC dynamics between single-and

double-cropping sites.

Figure 2. The trend of annual plant carbon (C) input to soil at each upland eld site in northern China. Control = no fertilizer

NPK = inorganic nitrogen (N), phosphorus (P), and potassium (K); M = manure applied; S = straw returned.




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3. Results

3.1. Grain Yield and Carbon

Inputs Estimation

Average annual grain yield from the

eight long-term eld trials was4.0tha1 for wheat and 6.3 t ha1 for

maize. Grain yield in the control plots

was lowest (1.4 t ha1 yr1 for wheat

and 3.3t ha1 yr1 for maize) with a

decreasing trend during the 15 to

28 years of eld trials. Under the plots

with fertilization (i.e., NPK, NPK + M, and

NPK + S), the grain yield increased

during the experimental period in 5/8

of the eld trial sites with the

exceptions being at Pingliang,

Changping, and Xuzhou (Figure S1).

Generally, the grain yield under

NP + M/NPK + M plots was highest, but

there was no signicant difference between NP/NPK, NP + M/NPK + M, and NP + S/NPK + S plots. The additiona

manure or straw return had no signicant effect on grain yield compared to inorganic only fertilizer

application (Figure S1). The annual crop yield at double-cropping sites was almost two times that at single

cropping sites (Table S2). Total annual grain yield and total carbon inputs in plots receiving fertilizer (i.e., NPK

NPK+ M, and NPK+ S) were signicantly higher than those in the control treatment without fertilization for

both single-cropping and double-cropping sites; there was no signicant difference between fertilizer

treatments (P


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3.2. SOC Change in Northern China

The initial SOC content was higher for single-cropping sites (24 t C ha1) compared to double-cropping sites

(20 t C ha1; Table 2). After 15 to 28 years, in single-cropping sites, the SOC content in plots with inorganic

fertilizer was signicantly higher than in the control plot without fertilization. The annual carbon inputs was

higher in double-cropping sites (1.6 to 5.7 t C ha1 yr1) than single-cropping sites (0.9 to 3.9 t C ha1 yr1

Figure 4. Comparison between observed soil organic carbon (SOC) content (0–20 cm) and RothC modeled values for upland soils in northern China.




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Table S2), but this was not reected in the annual SOC change, which was greater in single-cropping

compared to double-cropping sites when considered for the same amount of carbon inputs (Figure 3).

The SOC sequestration potential was negative in the control plots, which were a net source of CO2 (Table 3)

However, even when NPK was applied the future SOC sequestration potential (until the year 2100) was

only 4.5 and 5.7 t C ha1 in double-cropping and single-cropping sites, respectively (Table 3). There was

signicantly higher SOC sequestration potential where inorganic fertilizer was combined with manure

(i.e., 20.2 t C ha1 in single-cropping sites and 16.1t C ha1 in double-cropping sites) or straw (i.e., 16.8 t C ha1

in single-cropping sites and 13.5t C ha1 in double-cropping sites) compared to NPK only plots. Averaged

across all eight sites, the SOC sequestration potential ranged from

4.3t C ha1

in the control plots to18.2 t C ha1 in NPK+ M plots.

3.3. Validation of RothC Model

The RothC model was able to adequately simulate SOC dynamics in all treatment plots (Figure 4) as modeled

SOC values tted well with the observed values (Figure 5). Both the modeled and observed SOC content

showeda declining trend in control plots at most sites. Modeled SOCvalues were at steady state, or increased

slightly, in plots that received inorganic fertilizer only (NP/NPK) but increased in plots with organic manure

(M, NP/NPK + M, NP/NPK+ S). The coef cient of determination (R2) between observed and modeled SOC

contents ranged from 59% to 94% (P


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3.4. Comparison of the Climate Condition Prediction by GCMs Until 2100

The two GCMs predicted two different climate conditions by the end of 21st century. Compared with the rs

20 years of the experiment, the average annual mean temperature (AMT) during the years 2080 to 2100 was

predicted to increase at all sites (Table S3). Annual mean temperature increments predicted by the IPSLmodel were signicantly higher than those predicted by the BCCR model (P


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inputs) annual SOC change among the ve climate conditions. Total annual SOC change was in the order:

NPKM>NPKS, NPK >Control (P


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under increased NPP scenario). At the Urumqi site the SOC conversion was 3.6% under the current NPP

scenario and 8.7% under an increased NPP scenario. For the average of these sites, the SOC conversion rate

was lower during 2050 to 2100 (where the SOC conversion was 1.3% under current NPP scenario and 3.1%

under increased NPP scenario) than during 2010 to 2050 when the SOC conversion was 5.4% under current

NPP scenario and 7.0% under the increased NPP scenario. This indicated that the SOC storage potential of

these soils was close to being reached in the later time period at all sites.

Under the IPSL-A2 climate scenario, representing the highest temperature scenario, even with an increased

NPP carbon input scenario, the SOC content decreased in the control plots at all sites except Changping

(Figure S2). Under this climate scenario for the NPK and NPK + S plots, the SOC increased at all sites except

Urumqi, where the SOC decreased gradually. However, for the NPK + M plot the SOC increased at all sites

under this climate scenario.

4. Discussion

The social and economic stability of China largely depends on agricultural development. Cropland in

northern China accounts for 65.8% of the 122 million hectares of total cropland in China [ National Bureau o

Statistics of China, 2012]. While inorganic fertilizers have played an important role in feeding the rapidly

growing world population, the application of organic amendments to agricultural elds has declined [ Ju

et al., 2005]. The decomposition rate of SOC was shown to be faster when inorganic fertilizer was applied

alone compared to with manure in Loessial soil in northwest China [Liang et al., 2012]. A future consequence

of inorganic fertilizer use without organic amendments in China will be declining SOC stores and increasing

CO2 release under current tillage practices. If China continues to maintain self-suf ciency in food production

[Solot , 2006], then arable lands will need to increase productivity without causing loss of soil fertility.

The RothC model was able to accurately predict the SOC dynamics in agricultural upland soil in northern

China. Yang et al. [2003] and Guo et al. [2007] applied the RothC model to upland soils (Black and Fluvo-aquic

soils) in northern China, and both reported that the SOC predicted agreed well with the experimental data

observed in unfertilized plots, in plots with inorganic fertilizers andwhere inorganic fertilizers were applied in

combination with manure. Our results were consistent with Yang et al. [2003] and Guo et al. [2007], and we

also found that the RothC model was suitable for use in predicting SOC stocks with straw application

(Figures 4 and 5).

Addition of animal manures and return of crop straw are well recognized as positive management options to

improve SOC as illustrated in this study. Farmers have used organic food waste and animal manures to

maintain crop production and soil fertility for thousands of years in China [Yang, 2006]. However, with

inorganic fertilizers currently being widely available, the application on manure to arable land has declined

from 99.9% in 1949 to 25% in 2003 [Huang et al., 2006; Yang et al., 2010]. Under the current NPP carbon inpu

scenario, the annual SOC change for the NPK + M treatment was 0.287t C ha1 yr1 with no climate change

0.252 t C ha1 yr1 assuming BCCR-A2 and 0.219 t C ha1 yr1 assuming IPSL-A2 climate conditions until

2100 in northern China. This would mean an additional 17.5 to 23.0 Tg C yr1 sequestered to the end of this

century if agricultural management practices were to apply NPK + M (without improvement in straw

retention or conservation tillage practices). Assuming the increased NPP carbon input scenario, the SOC

sequestered by 2100 would be 26.7 to 28.5 Tg C yr1 under BCCR-A2 and IPSL-A2 climate condition,

respectively. However, organic manure is now more commonly applied to vegetable crops than to graincrops—data from 200 agrometeorological stations conrmed this practice [Wan et al., 2011] which is unlikely

to change. It should also be recognized that a larger land area is required to “grow” manure than the input o

soil carbon [Schlesinger , 1999]. Increasing SOC stocks through manure application at one site may result in

depletion of SOC at other. Thus, manure is not likely to yield an environmentally sustainable net sink for

carbon across these large areas of arable land.

Throughout northern China, crop straw was historically used as fuel, animal feed and bedding, or burnt

directly within the eld. Since the 1980s, straw return was popularized by government policy as a practice to

improve soil fertility and decrease air pollution by not burning. The area of agricultural land where straw was

returned varied from 7% to 71% depending on the province; with an average 36.6% of all straw in China used

to improve soil fertility [Gao et al., 2002; Lu et al., 2009]. Under the current NPP carbon input scenario, the




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annual SOC change in northern China until the year 2100 within the NPK + S treatment was 0.162 t C ha1 yr1

assuming no climate change (12.6 Tg C yr1), 0.096 t C ha1 yr1 assuming BCCR-A2 (7.7 Tg C yr1), and

0.064 t C ha1 yr1 assuming IPSL-A2 climate conditions (5.1 Tg C yr1). Compared to NPK only, the return

of straw could sequester an additional 7.2, 3.6, and 2.2 Tg C yr1 SOC under these three climate conditions

(i.e., no climate change, BCCR-A2, and IPSL-A2, respectively) in northern China by 2100.In agricultural systems, tillage can be a major cause of SOC change; losses up to 50% of the starting SOC in

surface soils (20 cm) have been observed after cultivation for 30 to 50 years when natural vegetation is

converted to cultivated crops [Post and Kwon, 2000]. Based on the global database of 67 long-term

agricultural experiments, West and Post [2002] found on average that a change from conventional tillage to

no-tillage can sequester 0.57 ± 0.14 t C ha1 yr1. In China, Lu et al. [2009] determined that under the current

climate situation that no-tillage can sequester 0.800 Tg C yr1 (0.039 t C ha1 yr1). In our study, all the long

term experimental sites were plowed after harvest; as such we could not measure the effect of no-tillage on

carbon turnover. However, it is reported that the carbon conversion rate is 8% per year in plowed systems

and 10% per year in no-tillage systems [Duiker and Lal , 1999]. We attribute the lower SOC levels in the double

cropping sites in our study to the additional tillage each year associated with the planting of the second crop

along with the higher soil temperatures at these sites—both tillage and temperature are likely to have increased

organic matter decomposition rates [ Zhang et al ., 2010]. Assuming that organic manure inputs remain the same

[Li et al ., 2003] but that straw retention in China does increase [Gao et al., 2002; Sun et al., 2010], and that

no-tillage practices can be extended Sun et al. [2010] calculated a further 1% annual NPP increase from

2000 to 2050. Based on these improved agricultural management practices, this rate of increase in NPP

would result in an annual SOC changes of 0.002 under the control plot to 0.284 t C ha1 yr1 under the

NPK + M plot (a SOC conversion rate of 7.0%) until 2050 and a further 0.024 t C ha1 yr1 under the contro

to 0.209 t C ha1 yr1 under the NPK + M treatment (a SOC conversion rate of 3.1%) until 2100, assuming

the IPSL-A2 climate condition scenario. This is equivalent to an additional 0.33 Tg C yr1 until 2050 and

0.98 Tg C yr1 until 2100 of SOC sequestered compared to a no change in NPP scenario.

5. Conclusion

The prediction of future SOC sequestration potential demonstrated that under no fertilizer input, these soils

would be a net source of CO2 in most parts of northern China. Even when inorganic nutrients were applied, the

additional carbon input from increased plant residues could not meet the depletion of SOC in the northwest

sites. Manure or straw application could improve the carbon sequestration at all sites, with straw being a more

likely option into the future. The future SOC sequestration potential in northern China was 4.3 to 18.2t C ha1

by 2100 under current carbon input and existing climate conditions. The effect of climate change on the annua

rate of SOC change did notdiffersignicantly between theve climate scenarios; under the higher CO2 emission

scenario (i.e., A2) 8.1t C ha1 (0.062t C ha1yr1) and 10.7 t C ha1 (0.087t C ha1yr1) will be sequestered

under IPSL-A2 andBCCR-A2, respectively, with the current NPP C input scenario. Under the increased NPP C inpu

scenario, 20.5t C ha1 (0.182t C ha1yr1) and 23.8 t C ha1 (0.211t C ha1yr1) would be sequestered in

northern China. This doubling in the potential of future SOC sequestration under an increased NPP scenario

highlights the need to introduce both straw retention and no-tillage practices across the areas of northern China

where this is not commonly practiced.

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We acknowledge our colleagues fortheir unremitting efforts to the long-

term experiments, and we are also very

grateful to Wendy Wang, University of

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