The carbon count of 2000 years of rice cultivation

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This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/gcb.12080 © 2012 Blackwell Publishing Ltd

Received Date : 08-Aug-2012 Accepted Date : 17-Oct-2012 Article type : Primary Research Articles

The carbon count of 2000 years of rice cultivation

Authors

Karsten Kalbitz1*, Klaus Kaiser2, Sabine Fiedler3, Angelika Kölbl4, Wulf Amelung5, Tino

Bräuer6, Zhihong Cao7, Axel Don8, Piet Grootes6, Reinhold Jahn2, Lorenz Schwark9, Vanessa

Vogelsang3, Livia Wissing4, Ingrid Kögel-Knabner4

* Corresponding author: email: [email protected], tel: 0031 20 525 7457, fax: 0031 20 525 7432

1 Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics, Universiteit van

Amsterdam, 1090 GE Amsterdam, The Netherlands

2 Soil Sciences, Martin Luther University Halle-Wittenberg, 06120 Halle (Saale), Germany

3 Institute for Geography, Soil Science, Johannes Gutenberg-Universität Mainz, 55099 Mainz

Germany

4 Lehrstuhl für Bodenkunde, Center of Life and Food Sciences Weihenstephan, TU München,

85350 Freising-Weihenstephan, Germany

5 Institute of Crop Science and Resource Conservation, Soil Science and Soil Ecology,

University of Bonn, 53115 Bonn, Germany

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6 Leibniz-Laboratory for Radiometric Dating and Isotope Research, Christian Albrechts

University Kiel, 24118 Kiel, Germany

7 The Institute of Soil Science Chinese Academy of Sciences, Nanjing 210008, PR China

8 Institute of Agricultural Climate Research, Johann Heinrich von Thünen Institute, 38116

Braunschweig, Germany

9 Institute for Geosciences, Christian Albrechts University Kiel, 24118 Kiel, Germany

Type of Paper: Primary Research Article

Abstract

More than 50% of the word’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 that of soils under non-paddy 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 non-paddy 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

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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.

Introduction

Nutrition of more than fifty percent of the world’s population is based on rice, mostly grown

under submerged conditions on about 133 million ha of lowland fields (IRRI database).

Submerged rice cultivation results in the formation of paddy soils. As these soils often

accumulate organic carbon in the top layer during the initial phase of their development (Zhang

& He, 2004; Liu et al., 2006; Wu, 2011; Shang et al., 2011; Wissing et al., 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 Greenhouse 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 hectare, assuming clay soils at warm temperate moist climate (Eggleston et al.,

2006). Wu (2011) estimated the organic carbon accumulating in paddy topsoils in four different

subtropical Chinese landscapes to 8–25 tons per hectare. The observed accumulation of organic

carbon in paddy topsoils partly compensates for the large methane emissions under submerged

rice cultivation (Bloom et al., 2010; Denman et al., 2007). In many soils, however, more than 2/3

of total organic carbon stocks are located in the subsoil (Jobbágy & Jackson, 2010). The

response of subsoil organic carbon to long-term submerged rice cultivation is 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 areas between 0 and 25 m

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above sea level. Additionally, 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 non-paddy soils

along a 2000-year chronosequence (Fig. 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 submerged 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 deposited 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

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construction and land use history records to assign ages to paddy soils and soils not used for

submerged rice cultivation (non-paddy soils). Site selection aimed at minimum distances

between paddy and non-paddy 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 management (e.g.,

introduction of mineral fertilizers, increasing additions 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 non-paddy 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 to 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

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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 submerged rice cultivation) by horizon down to 1 m depth along the

chronosequence (Fig. 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 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 115–120 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 determined in duplicate by dry

combustion at 950 °C, using a Vario EL elemental analyser (Elementar Analysensysteme,

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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.

While 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 methods 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, Ströhlein GmbH, Viersen, Germany; limit of determination 0.1 mg

C g–1 soil).

(ii) dissolution of carbonates with 15% HCl and determination 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).

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(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, Hanau, Germany). 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 volumetric 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 calibration. The tested methods affected IC

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

maximum difference (between method (i) and (v)) averaging 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 systematically 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 measurements (r2=0.95, y=1.3545x + 0.44) was used to calculate the IC contents

of method (iv).

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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 multiplying 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 plough 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.

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Soil texture was similar for all of the soils (silt loam, according 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), 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 measured

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; Table

S1, S2; supporting information) and subsoils (all B horizons underneath the topsoil, excluding

buried A horizons; Table S1, S2; supporting information) are given in Figure 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. Since the treatment

with 1 M HCl gave consistently 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. section ‘Introduction’) by using the database of the International Rice

Research Institute (IRRI). 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 done utilizing MS Excel, Sigma plot, and SPSS.

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Results

Accumulation of organic carbon in topsoils

During the first 300 years of submerged rice cultivation, paddy topsoils accumulate organic

carbon at an average rate of 170–178 kg ha–1 a–1. The accumulation was much larger than in non-

paddy control soils (19–26 kg C ha–1 a-1) and lasted longer (Figs. 2, 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 development when the organic carbon accrual in the surface soil

approached a steady-state level, i.e., after 700 years (Figs. 2, 3).

Despite similar carbon stocks and concentrations in subsoils of paddy and non-paddy soils, 14C

concentrations 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, 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.

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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 non-paddy

control soils (difference 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., 2006; Wu, 2011; Shang et al., 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 combination 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 decomposition 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

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(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. However, carbon losses from materials low in carbon may impact

the global carbon balance to a great extent as well. Subsoils, despite of usually small organic

carbon concentrations, hold a large share of all terrestrial carbon (Jobbágy & Jackson, 2010).

Unfortunately, we neither do know the rates of decomposition of sedimentary 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 consider changes in carbon stocks below 30 cm

depth. However, the organic carbon losses from paddy subsoils (–330 to –550 kg ha–1 a–1) during

the first 50 years of cultivation exceeded those of non-paddy subsoils (–310 to –495 kg ha–1 a–1)

slightly and offset the topsoil carbon accumulation in that period (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 non-paddy soils were most evident after 50 and

100 years of cultivation, as indicated by the different 14C concentration (Fig. 2). Likely, the

observed formation of the plough pan, a dense horizon at the transition between top- and subsoil,

prohibits efficient replacement 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 non-

paddy subsoils resulted in similar concentrations and stocks of organic carbon after 700 years of

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cultivation. The predominantly anaerobic conditions in paddy soils may have contributed to the

partial preservation of the old sedimentary organic carbon (Sahrawat, 2004; Liu et al., 2006).

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 progressing mineralization of the old, sediment-derived organic carbon. Nevertheless,

the organic carbon balance of paddy soils was more favorable than that of non-paddy 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

et al., 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 inorganic 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 (WRB 2006) about 7% of all soils contain carbonates. Depending on the drivers,

carbonate weathering can affect the atmospheric CO2 balance either positively or negatively.

Carbonate weathering may be CO2 neutral, 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 equilibration 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 nitrogen fertilizers, carbonate weathering can be a significant

CO2 source (Barnes & Raymond, 2009). Paddy soils receive large amounts of N fertilizer, which

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might be the reason for the accelerated decalcification of paddy than of non-paddy soils,

especially during the last 50 years.

We conclude the commonly assumed carbon sequestration of paddy fields needs revisiting.

Topsoil carbon accrual under paddy management is 5 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

conditions 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 non-paddy 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 reappraisal of carbonate weathering. A stronger focus on

subsoils is urgently needed in order to fully understand the role of soils in the global carbon

cycle.

Acknowledgments

The study was funded by the German Research Foundation. We are grateful to our Chinese

colleagues from the Institute of Soil Science in Nanjing for identifying the chronosequence,

initiating the project and assistance during sampling. We thank Arnel B. Rala from the

Geographic Information System Laboratory of IRRI for calculating paddy soils areas.

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Figure legends

Figure 1 Location of the chronosequence and of sampling sites of soils subjected to submerged

rice cultivation (paddy soils; P50–P2000), under non-submerged cropping (non-

paddy soils; NP50–NP700), and of tidal flat / marsh profiles. Main roads are

indicated.

Figure 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 (non-

paddy soils) as dependent on period of cultivation. The reference of the

chronosequence is the juvenile 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

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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 fifty years, caused by the atmospheric testing of nuclear weapons in the late

1950's/early 1960's. 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.

Figure 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 (non-paddy soils: NP 50–NP700), as dependent

on period of cultivation. The reference of the chronosequence, the juvenile estuarine

sediments in the tidal flat, is indicated with a grey area. 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.

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