The carbon count of 2000 years of rice cultivation

The carbon count of 2000 years of rice cultivationKARSTEN KALB I TZ * , KLAUS KA I SER † , SAB INE F I EDLER ‡ , ANGEL IKA K €OLBL § ,WULF AMELUNG ¶ , T INO BR €AUER * * , ZH IHONG CAO † † , AXEL DON ‡ ‡ , P I E T GROOTES * * ,

R E INHOLD JAHN † , LORENZ SCHWARK § § , VANESSA VOGELSANG ‡ , L I V IA WI S S ING § and

INGRID K €OGEL-KNABNER§

*Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, Universiteit van Amsterdam, Amsterdam 1090 GE,

The Netherlands, †Soil Sciences, Martin Luther University Halle-Wittenberg, Halle (Saale) 06120, Germany, ‡Institute for

Geography, Soil Science, Johannes Gutenberg-Universit€at Mainz, Mainz 55099, Germany, §Lehrstuhl f€ur Bodenkunde, Center ofLife and Food Sciences Weihenstephan, TU M€unchen, Freising-Weihenstephan 85350, Germany, ¶Institute of Crop Science and

Resource Conservation, Soil Science and Soil Ecology, University of Bonn, Bonn 53115, Germany, **Leibniz-Laboratory for

Radiometric Dating and Isotope Research, Christian Albrechts University Kiel, Kiel 24118, Germany, ††The Institute of Soil

Science Chinese Academy of Sciences, Nanjing 210008, China, ‡‡Institute of Agricultural Climate Research, Johann Heinrich von

Th€unen Institute, Braunschweig 38116, Germany, §§Institute for Geosciences, Christian Albrechts University Kiel, Kiel 24118,

Germany

Abstract

More than 50% of the world’s population feeds on rice. Soils used for rice production are mostly managed under

submerged conditions (paddy soils). This management, which favors carbon sequestration, potentially decouples

surface from subsurface carbon cycling. The objective of this study was to elucidate the long-term rates of carbon

accrual in surface and subsurface soil horizons relative to those of soils under nonpaddy management. We assessed

changes in total soil organic as well as of inorganic carbon stocks along a 2000-year chronosequence of soils under

paddy and adjacent nonpaddy management in the Yangtze delta, China. The initial organic carbon accumulation

phase lasts much longer and is more intensive than previously assumed, e.g., by the Intergovernmental Panel on

Climate Change (IPCC). Paddy topsoils accumulated 170–178 kg organic carbon ha�1 a�1 in the first 300 years;

subsoils lost 29–84 kg organic carbon ha�1 a�1 during this period of time. Subsoil carbon losses were largest during

the first 50 years after land embankment and again large beyond 700 years of cultivation, due to inorganic carbonate

weathering and the lack of organic carbon replenishment. Carbon losses in subsoils may therefore offset soil carbon

gains or losses in the surface soils. We strongly recommend including subsoils into global carbon accounting

schemes, particularly for paddy fields.

Keywords: carbon sequestration, inorganic carbon, land use, organic carbon, paddy, rice cultivation, soils, subsoils

Received 8 August 2012 and accepted 17 October 2012

Introduction

Nutrition of more than 50% of the world’s population

is based on rice, mostly grown under submerged condi-

tions on about 133 million ha of lowland fields (IRRI

database, 2011). Submerged rice cultivation results in

the formation of paddy soils. As these soils often accu-

mulate organic carbon in the top layer during the initial

phase of their development (Zhang & He, 2004; Liu

et al., 2006a,b; Shang et al., 2011; Wissing et al., 2011;

Wu, 2011), accrual of carbon is credited for during the

first 20 years of submerged rice cultivation (Eggleston

et al., 2006). The IPCC Guidelines for National Green-

house Gas Inventories assume an increase in organic

carbon stocks by 10% over 20 years, which adds up to

about 9 tons additional carbon in paddy soils per hect-

are, assuming clay soils at warm temperate moist cli-

mate (Eggleston et al., 2006). Wu (2011) estimated the

organic carbon accumulating in paddy topsoils in four

differ-ent subtropical Chinese landscapes to 8–25 tons

per hectare. The observed accumulation of organic car-

bon in paddy topsoils partly compensates for the large

methane emissions under submerged rice cultivation

(Denman & Brasseur, 2007; Bloom et al., 2010). In many

soils, however, more than 2/3 of total organic carbon

stocks are located in the subsoil (Jobb�agy & Jackson,

2000). The response of subsoil organic carbon to long-

term submerged rice cultivation remains largely

unknown, despite its potential importance for global

greenhouse gas balances. The coastal zones of the

tropics and subtropics including the large floodplains

are of particular relevance for effects of submerged rice

cultivation on soil carbon budgets. These areas

comprise more than 30 million ha of rice cultivation

under submerged conditions, as extrapolated for areasCorrespondence: Karsten Kalbitz, tel. 0031 20 525 7457, fax 0031 20

525 7432, e-mail: [email protected]

© 2012 Blackwell Publishing Ltd 1107

Global Change Biology (2013) 19, 1107–1113, doi: 10.1111/gcb.12080

between 0 and 25 m above sea level. In addition, these

soils often contain carbonates, which are not included

in the total carbon balance.

We assessed long-term changes in soil carbon stocks

upon submerged rice cultivation in the coastal region

of subtropical China (Fig. 1), comparing paddy and

adjacent nonpaddy soils along a 2000-year chronose-

quence (Figure S1; supporting information). The data

set on this unique chronosequence covers spatial

variability and analytical uncertainties on organic and

carbonate carbon in top- as well as in subsoils and

enables conclusions on changes in soil carbon stocks

beyond those of previous studies.

Materials and methods

Study area

The soils of the studied chronosequence are located on the

southern coast of the Hangzhou Bay, eastern China (Fig. 1),

which is one of the world’s regions known for the longest sub-

merged rice cultivation (Cao et al., 2006). The study sites are

located around Cixi (30°10′N, 121°14′E), Zhejiang province,

approximately 180 km south of Shanghai and 150 km east of

Hangzhou. The area is a marine deposit plain (Iost et al., 2007;

Cheng et al., 2009), with considerable portions of the depos-

ited sediments originating from the nearby Yangtze River

(Guo et al., 2000). The study area is representative for large

floodplains in the coastal zones of the tropics and subtropics,

which are intensively used for agriculture, in particular for

rice cultivation.

During the last 2000 years, nine different dikes were built

for land reclamation, resulting in a chronosequence of soils

(Cheng et al., 2009) (Fig. 1). We used the known dates of dike

construction and land-use history records to assign ages to

paddy soils and soils not used for submerged rice cultivation

(nonpaddy soils). Site selection aimed at minimum distances

between paddy and nonpaddy soils of same age. Accessibility

was an additional criterion of the site selection. It was not

possible to find any soil older than 700 years not used for rice

cultivation. Traditional paddy management in the region is a

rotation with rice grown during the wet season, followed by

wheat or another upland crop during the dry season (Fan

et al., 2005; Roth et al., 2011). We cannot assume a constant

management at all the sites during the 2000 years of paddy

soil development. However, changes in the general manage-

ment (e.g., introduction of mineral fertilizers, increasing addi-

tions of N fertilizer during the last 50 years, introduction of

high-yield varieties) and in general environmental conditions

(e.g., acid deposition) affected paddy as well as nonpaddy

soils at sampling sites along the chronosequence.

We considered the estuarine sediment in the tidal flat and

the marsh behind the most recent dike as time zero of soil

development. Soil texture, total content of elements and

mineral assemblage confirmed the comparability of parent

material of all soils.

The overall size of the study area is 433 km2 (in 1988), at an

elevation of 2.6–5.7 m above sea level (Zhang et al., 2004). The

climate is classified as a subtropical with periodical monsoon

rain. The mean annual temperature is 16.3 °C and the mean

annual precipitation is 1325 mm, with higher values in April

to October (Cheng et al., 2009). The total annual evaporation is

1000 mm. Irrigation is inevitable to maintain submerged

conditions during rice growing. Further details on ground

water table, geography, and geochemistry of the study area

are given by Cheng et al. (2009).

Soil sampling

We sampled profiles of paddy soils and control soils (non-

paddy soils, having the same age but were not exposed to sub-

merged rice cultivation) by horizon down to 1 m depth along

the chronosequence (Figure S1; supporting information) in

June 2008. Each three soil profiles were dug at each of the

different age states (soils under submerged rice cultivation for

50, 100, 300, 700, 1000, 2000 years; soils not under submerged

rice cultivation for 50, 100, 300, 700 years). Distances between

the three profiles were minimum 50 m, with most of them

located in different fields. We sampled about 10 kg soil per

Fig. 1 Location of the chronosequence and of sampling sites of soils subjected to submerged rice cultivation (paddy soils; P50–P2000),

under nonsubmerged cropping (nonpaddy soils; NP50–NP700), and of tidal flat/marsh profiles. Main roads are indicated.

© 2012 Blackwell Publishing Ltd, Global Change Biology, 19, 1107–1113

1108 K. KALBITZ et al.

horizon, using three walls of the pits, covering a total of about

2 m length of the profile walls. Therefore, the study accounts

for variability between different fields and small-scale (up to

2 m) spatial heterogeneity of soil properties. Undisturbed soil

cores (100 cm3, in triplicate) were used to determine the bulk

density of each horizon. Soils were classified according to

IUSS Working Group WRB (2006).

In addition, we sampled the tidal flat and two adjacent

marsh sites (behind the dike). The tidal flat was sampled to a

depth of 30 cm and a second sample was taken from 115 to

120 cm depth. The two marsh profiles were sampled to a

depth of 60 cm.

All samples were air dried and sieved to a size of <2.0 mm

prior to analyses.

Analyses and calculations

Total carbon concentrations (TC) of soil horizons were deter-

mined in duplicate by dry combustion at 950 °C, using a Vario

EL elemental analyzer (Elementar Analysensysteme, Hanau,

Germany) (limit of determination: 0.1 mg C g�1 soil). The

quality of the TC data was cross-checked by complementary

measurements of TC concentrations in two other laboratories,

with no significant differences.

Although TC analyses are well reproducible, determination

of inorganic carbon (IC) can be carried out using various

methods (Walthert et al., 2010). Effects of the different meth-

ods on IC contents are, unfortunately, not known, although

each method gives seemingly consistent results. However, IC

values are crucial for calculation of total organic carbon (TOC)

values, particularly in subsoils having relatively low-TOC and

high-IC concentrations. Therefore, we tested four soil samples

containing IC in the range expected for the soils of the test

chronosequence using five different methods:

(i) dissolution of carbonates with 42% phosphoric acid

and subsequent infrared detection of the evolving CO2

(C-MAT 550, Str€ohlein GmbH, Viersen, Germany; limit of

determination 0.1 mg C g�1 soil).

(ii) dissolution of carbonates with 15% HCl and determina-

tion of the volume of released CO2 by the Scheibler

apparatus (limit of determination 0.1 mg C g�1 soil).

(iii) treatment of soils with 0.3–0.8 ml 1 M HCl, drying at

105 °C, and determination of TC (Carlo Erba NA2000) in

treated and untreated samples. Sample weights were

corrected for changes by losses in carbonates and

accumulation of chloride. Carbonate C is calculated as

the difference in TC between treated and untreated

samples (limit of determination 0.1 mg C g�1 soil).

(iv) dissolution of carbonates with 10% HCl in a glass vessel

under He atmosphere where CO2 is trapped on an

adsorption column (Vario EL III with TIC module

SoliTIC; Elementar Analysensysteme). After completion

of the reaction, the CO2 is released from the column and

determined by thermal conductivity detection (limit of

determination 0.1 mg C g�1 soil).

(v) dissolution of carbonates with 5% HCl in an evacuated

glass ampoule overnight at 80 °C, freeze trapping of the

CO2 with liquid nitrogen and removal of other gases,

repetition of the procedure to purify the CO2, and volu-

metric quantification of CO2 (limit of determination ca.

0.05 mg C g�1 soil). The removal of carbonates with HCl

is routinely applied to obtain total organic carbon-derived

CO2 for 14C analysis by accelerator mass spectrometry

(Grootes et al., 2004).

All methods used certified reference material for calibra-

tion. The tested methods affected IC contents systematically,

with contents increasing from method (i) to method (v), with

the maximum difference [between methods (i) and (v)] aver-

aging 2.2 mg C g�1 soil. This difference equals 20 to >100% of

the IC contents of the soils. We decided to carry out the soil

analyses with the two methods covering best the range of IC

contents but also being suitable to process a large number of

samples. The selected methods (i) and (iv) differed systemati-

cally by 1.6 mg C g�1 soil. We measured IC in all samples

using method (i) and one-third of the samples (one profile

from each site) using method (iv), at least in duplicate. A

linear regression between the IC contents of both measure-

ments (r2 = 0.95, y = 1.3545x + 0.44) was used to calculate the

IC contents of method (iv).

Total organic carbon (OC) was calculated by subtracting IC

from TC. We calculated the bulk density by dividing the

mass of oven-dry soil (105 °C) by the sampling core volume.

Stocks of IC and OC were calculated for each horizon by mul-

tiplying carbon concentrations, thickness, and bulk density;

total soil carbon stocks were derived by summing up the

stocks held by the individual horizons of a profile to a depth

of 1 m. Two data sets of carbon stocks were produced due to

the use of two methods for determination of IC. In addition,

carbon stocks in topsoils (all A horizons including the plow

pan) and in subsoils (all B horizons including buried A

horizons) were calculated for each profile. Mean values and

standard errors were calculated using the three test profiles

per age (n = 3).

Bulk densities of the tidal flat and the marsh sites could

not be determined in the field. They were calculated based on

bulk densities of 28 profiles sampled in the region (Office of

Soil Investigation of Zhejiang province, 1994). According to

those data (and our own carbon data), carbon contents and

bulk densities were approximately constant with depth.

Therefore, we used the average bulk density of 1.32 g cm�3

to calculate carbon stocks to 1 m depth. We neither found

systematic depth gradients of TC, TOC, and TIC contents for

the tidal flat and marsh sites nor consistent differences in TC,

TIC, and TOC contents between the three profiles. We

assigned the first 10 cm as topsoils and the other parts as

subsoils, and calculated mean values and standard errors

based on the three profiles. Tidal flat and marshland profiles

were considered as time zero stage of soil development, thus

their carbon stocks served as references for changes with soil

development.

Soil texture was similar for all of the soils (silt loam, accord-

ing to IUSS Working Group WRB (2006)).

For radiocarbon dating, we used air-dried and sieved

(2-mm mesh) soil samples. Not soil-derived particles as well

as identifiable plant residues were manually removed.

Carbonates were destroyed with hydrochloric acid (pH < 1),


THE CARBON COUNT OF RICE CULTIVATION 1109

followed by freeze drying without washing. Samples were

combusted with CuO and silver wool in evacuated quartz

tubes at 900 °C for 4 h and the resulting CO2 was reduced to

graphite (Nadeau et al., 1998). 14C concentrations were mea-

sured with the 3 million Volt Tandetron AMS (accelerator

mass spectrometry) system at the Leibniz Laboratory at Kiel.

The average weighted 14C concentrations for topsoils (all A

horizons; Tables S1 and S2; supporting information) and sub-

soils (all B horizons underneath the topsoil, excluding buried

A horizons; Tables S1 and S2; supporting information) are

given in Fig. 3 in percent of modern carbon (pmC). We aimed

at a conservative estimate of carbon ages. Therefore, we

excluded buried A horizons from determination of mean 14C

concentrations. As the treatment with 1 M HCl gave consis-

tently the highest IC contents (cf. above), we assume removal

of carbonates prior 14C analysis to be complete.

We estimated the area of submerged rice cultivation (paddy

soils) in the coastal zones of the tropics and subtropics (cf. sec-

tion ‘Introduction’) using the database of the International

Rice Research Institute (IRRI) (2011). Coastal areas were

defined as any land between 0 and 25 m above sea level (IRRI

database).

Changes in carbon stocks over time were calculated by a

linear approach. We calculated IC and OC stocks using the

upper as well as the lower end of the range of IC and OC data,

thus covering the analytical uncertainty. Calculations were

carried out utilizing MS Excel, Sigma plot, and SPSS.

Results

Accumulation of organic carbon in topsoils

During the first 300 years of submerged rice cultiva-

tion, paddy topsoils accumulate organic carbon at an

average rate of 170–178 kg ha�1 a�1. The accumulation

was much larger than in nonpaddy control soils

(19–26 kg C ha�1 a�1) and lasted longer (Figs 2 and 3).

The contents of organic carbon in topsoils increased

even beyond 300 years and reached maximum after

2000 years (Fig. 3). The old, sediment-derived carbon

(49 pmC of 14C, suggesting an age of about 6000 years)

was quickly replaced by recently photosynthesized

carbon in topsoils, irrespective of land use, as indicated

by 14C concentrations of about 100 pmC already after

50 years of cultivation (Fig. 2).

Loss of organic carbon in subsoils

The studied subsoils lost organic carbon during the first

50 years after land embankment, then mostly inorganic

carbon until 300–700 years of paddy management, and

finally again organic carbon at later stages of soil devel-

opment when the organic carbon accrual in the surface

Fig. 2 Carbon stocks in soils (1 m depth) of the coastal area of southeast China, either used for submerged rice cultivation (paddy soils)

or not used for rice cultivation (nonpaddy soils) as dependent on period of cultivation. The reference of the chronosequence is the juve-

nile estuarine sediments, i.e., tidal flat/marsh (time = 0). IC, inorganic carbon; OC, organic carbon. Numbers in columns represent the

mean 14C content (pmC – percent modern carbon) of topsoils and subsoils. Analytical uncertainties determine ranges in inorganic

carbon contents (and therefore in organic carbon too); error bars represent the standard error of three replicated soil profiles. The 14C

concentrations in the tidal flat are consistent with a rapidly accumulating sediment of which the 14C concentration is half of that of the

atmosphere. The 61 pmC in the upper layer is about half of the elevated atmospheric 14C concentrations of the past 50 years, caused by

the atmospheric testing of nuclear weapons in the late 1950s/early 1960s. The concentration on the deeper layer is half of that of the

pre-1954 atmosphere. The change in C and 14C content in top- and subsoil along the chronosequence shows a replacement of original

by new carbon everywhere, even if the TOC concentration in the subsoil stays constant or decreases. Some irregularities in the change

over time may be the result of disturbances of the fields in the past.



soil approached a steady-state level, i.e., after 700 years

(Figs 2 and 3).

Despite similar carbon stocks and concentrations in

subsoils of paddy and nonpaddy soils, 14C concentra-

tions revealed large differences between the two types

of land use. Low 14C concentrations in paddy subsoils,

in particular after 50 years of rice cultivation, and steep

depth gradients in total organic carbon (Fig. 3, Tables

S1 and S2; supporting information) indicate limited

input of recent carbon (pmC > 100) into deeper soil

horizons, likely due to reduced water fluxes and less

deep rooting.

Paddy subsoils clearly lost organic carbon at later

stages of rice cultivation, i.e., after 1000 and 2000 years.

Taking the entire 2000 years into account, total losses of

organic carbon from subsoils equaled the accrual

of organic carbon in topsoils, with a carbon budget

between �3 and 12 kg C ha�1 a�1.

Loss of inorganic carbon

Carbon losses due to carbonate weathering were faster

in paddy soils (155–220 kg C ha�1 a�1) than in non-

paddy soils (80–116 kg C ha�1 a�1) in the first

300 years, with exceptional high loss rates of

395–545 kg C ha�1 a�1 during the first 50 years.

Discussion

The accumulation of organic carbon in paddy topsoils

was much larger than in nonpaddy control soils (differ-

ence in total organic carbon between paddy and non-

paddy topsoils after 300 years: 45 tons ha�1), lasted

longer, and therewith exceeded IPCC estimations

(Eggleston et al., 2006) as well as previous records

(Liu et al., 2006a,b; Shang et al., 2011; Wu, 2011) by far.

It seems the organic carbon accumulation potential of

paddy topsoil is not exhausted even after 2000 years

(Wissing et al., 2011), probably because of the combina-

tion of anaerobic conditions and high input of organic

matter (Sahrawat, 2004).

The conversion of tidal flats and marshlands into

arable land resulted in similar losses of sedimentary

carbon, despite its high age, by enhanced decomposi-

tion as the conversion of grasslands and forests into

arable land. Therefore, environmental conditions seem

more important for organic matter decomposition and

stabilization than its age or chemical structure (Schmidt

et al., 2011). The loss of old, sedimentary organic carbon

during cultivation should be of particular importance

in settings with organic-rich sediment, such as large

river floodplains and coastal zones we studied. How-

ever, carbon losses from materials low in carbon may

impact the global carbon balance to a great extent as

well. Subsoils, despite usually small organic carbon

concentrations, hold a large share of all terrestrial

carbon (Jobb�agy & Jackson, 2000). Unfortunately, we

neither do know the rates of decomposition of sedimen-

tary carbon during soil development and land-use

changes nor the contribution of sedimentary carbon to

total organic carbon in deep subsoils.

The IPCC carbon accounting scheme does not con-

sider changes in carbon stocks below 30 cm depth.

However, the organic carbon losses from paddy sub-

soils (�330 to �550 kg ha�1 a�1) during the first

50 years of cultivation exceeded those of nonpaddy

subsoils (�310 to �495 kg ha�1 a�1) slightly and offset

the topsoil carbon accumulation in that period

Fig. 3 Contents of organic carbon in soils (down to 1 m depth) of the coastal area of southeast China, either subjected to submerged

rice cultivation (paddy soils: P50–P2000) or other forms of arable use (nonpaddy soils: NP50–NP700), as dependent on period of culti-

vation. The reference of the chronosequence, the juvenile estuarine sediments in the tidal flat, is indicated with a gray area (dots repre-

sent sampling depths). The organic carbon content of the paddy soil after 2000 years is added in the uppermost right panel, indicating

the final state of soil development of the chronosequence.



(440–470 kg ha�1 a�1), thus questioning the positive

overall carbon balance during initial phases of paddy

cultivation.

Differences in subsoil carbon between paddy and

nonpaddy soils were most evident after 50 and

100 years of cultivation, as indicated by the different14C concentration (Fig. 2). Likely, the observed forma-

tion of the plow pan, a dense horizon at the transition

between top- and subsoil, prohibits efficient replace-

ment of carbon in deeper horizons, thus, decouples

surface from subsurface carbon cycling in paddy soils

(Wissing et al., 2011). The smaller input of fresh organic

carbon and the lesser replacement of old organic carbon

in paddy subsoils than in nonpaddy subsoils resulted

in similar concentrations and stocks of organic carbon

after 700 years of cultivation. The predominantly anaer-

obic conditions in paddy soils may have contributed to

the partial preservation of the old sedimentary organic

carbon (Sahrawat, 2004; Liu et al., 2006a,b).

The loss of organic carbon in subsoils at later stages

of rice cultivation, i.e., after 1000 and 2000 years was

probably because of prolonged limited carbon input

with roots and dissolved organic matter and progress-

ing mineralization of the old, sediment-derived organic

carbon. Nevertheless, the organic carbon balance of

paddy soils was more favorable than that of nonpaddy

soils at any time. However, rice cultivation under

submerged conditions produces large CH4 emissions of

14–>400 kg CH4 ha�1 a�1 (Wassmann et al., 2000; Cai

et al., 2003; Chen & Prinn, 2006; Denman & Brasseur,

2007; Zhang et al., 2009), with an IPCC default of

200 kg ha�1 season�1, which cannot be compensated

by accumulation of soil organic carbon.

The small net gain in organic carbon during paddy

soil development went along with large losses of inor-

ganic carbon from the entire profile due to rapid

carbonate weathering. Losses of inorganic carbon are

not restricted to paddy soils. According to the World

Reference Base for Soil Resources (IUSS Working

Group WRB, 2006) about 7% of all soils contain carbon-

ates. Depending on the drivers, carbonate weathering

can affect the atmospheric CO2 balance either positively

or negatively. Carbonate weathering may be CO2 neu-

tral, in the short term even beneficial (Liu et al., 2010),

when driven by respired CO2 because 1 mol CO2 will

be consumed during dissolution of 1 mol carbonate.

The consumed CO2 will be released again upon equili-

bration with the atmosphere, e.g., when reaching

surface water, resulting in carbonate precipitation. If

carbonate is dissolved due to acid rain, by the acetic

acid forming under reducing conditions, or by acids

forming upon application of ammonium-based nitro-

gen fertilizers, carbonate weathering can be a signifi-

cant CO2 source (Barnes & Raymond, 2009). Paddy

soils receive large amounts of N fertilizer, which might

be the reason for the accelerated decalcification of

paddy than of nonpaddy soils, especially during the

last 50 years.

We conclude that the commonly assumed carbon

sequestration of paddy fields needs revisiting. Topsoil

carbon accrual under paddy management is five times

larger and lasts about 15 times longer than expected.

During the first 50 years and beyond 700 years of rice

cultivation, losses of total inorganic and subsoil organic

carbon become increasingly important to paddy soils’

impact on the global carbon balance. On a long-term

perspective, rice cultivation under submerged condi-

tions does not have a positive impact on the soil carbon

budget, nevertheless, the organic carbon balance of

paddy soils was more favorable than that of nonpaddy

soils at any time. We recommend revision of the IPCC’s

greenhouse gas reporting scheme. It ought to include

losses of organic carbon from subsoils and the reap-

praisal of carbonate weathering. A stronger focus on

subsoils is urgently needed to fully understand the role

of soils in the global carbon cycle.

Acknowledgements

The study was funded by the German Research Foundation. Weare grateful to our Chinese colleagues from the Institute of SoilScience in Nanjing for identifying the chronosequence, initiatingthe project, and assistance during sampling. We thank Arnel B.Rala from the Geographic Information System Laboratory ofIRRI for calculating paddy soils areas.

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Supporting Information

Additional Supporting Information may be found in theonline version of this article:

Figure S1. Images of soils subjected to submerged rice culti-vation (paddy soils) for 50, 100, 300, 700, 1000, 2000 yearsand of reference soils under non-submerged cropping (non-paddy soils) for 50, 100, 300, 700 years. Soils were classifiedaccording IUSS Working Group WRB (2006).Table S1. Properties of soils developed under submergedrice cultivation (paddy soils) for 50–2000 years. Figuresgiven are means � standard errors calculated on resultsfrom samples of three soil profiles. The tidal flat and marshprofiles serve as references for the situation before cultiva-tion.Table S2. Properties of soils developed under non-sub-merged cropping systems (non-paddy soils) for 50–700 years. Figures given are means � standard errors calcu-lated on results from samples of three soil profiles.



The carbon count of 2000 years of rice cultivation

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