Download - Effects of straw carbon input on carbon dynamics in agricultural soils: a meta-analysis

RE S EARCH REV I EW

Effects of straw carbon input on carbon dynamics inagricultural soils: a meta-analysisCHANG L IU * , MENG LU* , J UN CU I , BO L I and CHANGMING FANG

Coastal Ecosystems Research Station of Yangtze River Estuary, Ministry of Education Key Laboratory for Biodiversity and

Ecological Engineering, Institute of Biodiversity Science, Fudan University, Shanghai 200433, China

Abstract

Straw return has been widely recommended as an environmentally friendly practice to manage carbon (C) sequestra-

tion in agricultural ecosystems. However, the overall trend and magnitude of changes in soil C in response to straw

return remain uncertain. In this meta-analysis, we calculated the response ratios of soil organic C (SOC) concentra-

tions, greenhouse gases (GHGs) emission, nutrient contents and other important soil properties to straw addition in

176 published field studies. Our results indicated that straw return significantly increased SOC concentration by

12.8 � 0.4% on average, with a 27.4 � 1.4% to 56.6 � 1.8% increase in soil active C fraction. CO2 emission increased

in both upland (27.8 � 2.0%) and paddy systems (51.0 � 2.0%), while CH4 emission increased by 110.7 � 1.2% only

in rice paddies. N2O emission has declined by 15.2 � 1.1% in paddy soils but increased by 8.3 � 2.5% in upland soils.

Responses of macro-aggregates and crop yield to straw return showed positively linear with increasing SOC concen-

tration. Straw-C input rate and clay content significantly affected the response of SOC. A significant positive relation-

ship was found between annual SOC sequestered and duration, suggesting that soil C saturation would occur after

12 years under straw return. Overall, straw return was an effective means to improve SOC accumulation, soil quality,

and crop yield. Straw return-induced improvement of soil nutrient availability may favor crop growth, which can in

turn increase ecosystem C input. Meanwhile, the analysis on net global warming potential (GWP) balance suggested

that straw return increased C sink in upland soils but increased C source in paddy soils due to enhanced CH4 emis-

sion. Our meta-analysis suggested that future agro-ecosystem models and cropland management should differentiate

the effects of straw return on ecosystem C budget in upland and paddy soils.

Keywords: straw return, soil organic carbon, GHGs emission, carbon sequestration, meta-analysis

Received 10 May 2013 and accepted 6 December 2013

Introduction

Continuous increase in greenhouse gases (GHGs) in the

atmosphere is expected to contribute to global warming

(Lal & Kimble, 1997; Smith & Fang, 2010). How to

reduce and/or mitigate GHGs emission has become an

important global issue (Paustian et al., 2000). It has been

proposed that agricultural soils provide a potential sink

for atmospheric CO2 by sequestrating SOC (Smith et al.,

2000; Pan et al., 2003). By adoption of recommended

management practices (RMPs) in cropping systems, the

annual rate of SOC sequestration was estimated to be

0.4–0.8 Pg C globally, which accounted for 33.3–100%of the total potential of soil C sequestration in the world

(Lal, 2004a). Among all RMPs, straw return has been

suggested as a promising method to increase SOC

sequestration in croplands (Lugato et al., 2006; Lu et al.,

2009). To meet the challenges of managing agricultural

soils to capture atmospheric C, it is important to under-

stand the response of soil C cycle to straw input in

croplands.

Previous work has reported that SOC concentration

(Duiker & Lal, 1999; Li et al., 2010), active SOC fractions

(Chen et al., 2009; Malhi et al., 2011) and three major

biogenic GHGs (i.e., CO2, CH4, and N2O) emission

(Duiker & Lal, 2000; Zou et al., 2005; Ma et al., 2008) can

all be increased by straw incorporation. However,

many uncertainties still remain as to how cropland soils

respond to straw-C input (West & Marland, 2002). For

instance, some studies have shown that straw return

has negligible or negative effects on soil C stocks

(Campbell et al., 1998; Garcia-Orenes et al., 2009). Spe-

cifically, Wang et al. (2011) reported that straw incorpo-

ration to a depth of 0.23 m under conventional tillage

decreased SOC levels in surface layer (0–0.2 m depth).

Niu et al. (2011) also found that straw application stim-

ulated soil C loss at a depth of 0.2–0.4 m. Thus, differ-

ent results among experimental studies have indicated

that a comprehensive understanding of cropping sys-

tem response to straw input remains elusive.

Correspondence: Changming Fang, tel./fax + 86 21 5566 4990,

e-mail: [email protected]

*Contributed equally to this paper.

© 2014 John Wiley & Sons Ltd1366

Global Change Biology (2014) 20, 1366–1381, doi: 10.1111/gcb.12517

Global Change Biology

Straw return does not only directly increase C input

into the soil, but also influences soil physical and chemi-

cal properties and crop growth that determine whether

the benefits of agricultural SOC sequestration can be

realized or not (Lal, 2008). For example, straw return

influences soil nutrient contents. The total amount of

crop residues produced in 2001 was estimated to be

4 9 1010 Mg worldwide, containing about 8 9 107 Mg

of N, P, and K (Lal, 2008). Although the nutrients are

only a small portion of straw, they can be a favorable

complementarity for the soil and provide a long-term

supply of nutrients for crop growth. Returning straw

into soil can potentially improve the nutrient status of

agricultural soils (Lal, 2004b), which in turn stimulates

SOC sequestration due to increased crop rhizodeposition

(Kuzyakov & Schneckenberger, 2004). In addition, it is

extremely important to consider C fluxes (CO2 and CH4)

because C outputs may offset the benefits of C inputs

from straw return. The balance of C inputs and losses

determines soil C stocks. In general, there is an upper

limit to the capacity for soils to store organic C, namely

SOC content can become saturated once C inputs and

outputs stay at equilibrium (Stewart et al., 2007). How-

ever, C saturation of soil mineral fraction is not well

understood (West & Six, 2007). Addressing C saturation

in soils still faces some problems, e.g., why/when does

the saturation of SOC occur? What are the fundamental

factors that are responsible for SOC saturation?

The effects of straw return on soil C dynamics

depend on many factors (West & Marland, 2002), of

which straw-C input rate is obviously the primary one.

Most cases have shown that there is a strong linear rela-

tionship between straw-C input rate and SOC seques-

tration (Duiker & Lal, 1999; Kong et al., 2005; Li et al.,

2010). Other factors, such as experimental duration

(Shen et al., 2007), N application rate (Jacinthe et al.,

2002), methods of disposing straw (Jarecki & Lal, 2003)

and soil conditions (e.g., soil microorganism, texture,

and aggregates) (Lugato et al., 2006; Bandyopadhyay

et al., 2010), are also important in explaining the large

differences in the magnitude and direction of SOC

changes after straw return. Hence, there is a critical

need to assess the effects of straw incorporation on soil

C cycle under various soil and management conditions,

which greatly benefits the development of proper straw

management strategies and contributes to SOC seques-

tration in global agricultural ecosystems. In particular,

SOC storage in paddy soils is significantly higher than

that in upland soils (Pan et al., 2003). However, the

responses of organic C pools to straw return in these

soils remain uncertain.

Many agricultural practices have been advocated to

mitigate global climate warming, but their positive ben-

efits to C sequestration may be largely offset by high

emissions of GHGs (Six et al., 2004). For example, N fer-

tilizer application generally enhances soil C sequestra-

tion, but it also increases N2O emission (Liu & Greaver,

2009; Lu et al., 2011b). When appraising agricultural

mitigating impact on global climate change in terms of

changes in C amount in agro-ecosystems, we need to

fully take into account of GHGs budgets and the net

GWP determined by GHGs emission and SOC seques-

tration (Lal, 2004b; Lu et al., 2009; Shang et al., 2011).

Although straw return is considered as one of the most

sustainable and economical C sequestration methods

(Triberti et al., 2008), to our knowledge, there are no

quantitative analyses integrating its positive or negative

effects on current global warming.

Meta-analysis offers a formal statistical method to

compare and integrate the results of multiple studies

and to draw general patterns at regional and global

scales (Gurevitch & Hedges, 1999; Luo et al., 2006). In

this study, we conduct a meta-analysis to address the

following five main questions/hypotheses: (i) to what

extent is soil C content altered by straw return? We

hypothesize that soil C sequestration may increase

under straw C input; (ii) what are the main factors that

influence the response of soil C to straw return? Specifi-

cally, how do initial C and clay content, aggregates,

and/or climate factors influence the results? We

hypothesize that lower initial C concentration may lead

to higher response of soil C; (iii) considering that soil

has a limited gain in C stocks, our hypothesis is soil C

saturation may occur under long-term straw C input;

(iv) can the increase in straw return stimulate GHGs

emission due to the increase in substrate quantity? and

(v) how do soil nutrient availability, crop production,

and agro-ecosystems C balance respond to straw C

input?

Materials and methods

Data sources

We collected data from 176 peer-reviewed experimental stud-

ies that reported changes in soil C dynamic induced by straw

return (Appendix S1). The selected publications were collected

by searching Web of Science (1900–2012, http://apps.webof

knowledge.com/) and China Knowledge Resource Integrated

Database (http://www.cnki.net/) with the keywords ‘straw

return or crop residue’ and ‘soil organic carbon or greenhouse

gases.’ Compiled database included the responses to straw

addition of 23 variables related to C pools, fluxes, and other

associated parameters (Appendix S2). To avoid publication

bias, three criteria were set to select the proper observations:

(i) amounts of straw return, land uses, and experiment dura-

tions were clearly stated; (ii) experimental sites of treatments

and controls had similar soil types and managements (e.g.,

cropping system, fertilization, and irrigation); and (iii) the

© 2014 John Wiley & Sons Ltd, Global Change Biology, 20, 1366–1381

CARBON DYNAMIC AT STRAW RETURN 1367

means, standard deviations (or standard errors), and number

of replicates of chosen variables were directly reported or

could be calculated from reported data. Considering that

paddy rice and upland fields may respond differently to straw

return, we examined the effects of straw return on C dynamic

for these two types of land uses separately. In this study,

paddy fields/soils were referred to as arable land that was

submerged under water during one or more seasons of the

year for rice cultivation. Upland soils were all other irrigated

or nonirrigated agricultural systems where there was no regu-

lar submergence in whole year, except for temporary flooding

during irrigation or after heavy rain events. The effects of

straw incorporation (i.e., straw was ploughed/buried into the

soil) and cover (i.e., straw was mulched or covered on the sur-

face of soil) on SOC dynamics were tested in this analysis.

Experimental durations were classified into three categories:

short-term (1 ≤ duration ≤ 3 years), medium-term (3 < dura-

tion ≤15 years), and long-term (>15 years). Most compiled

data on CH4, CO2, and N2O fluxes reflected only short-term

effects (one or two growing seasons) in this analysis. For other

variables, studies with experimental durations less than 1 year

were excluded to avoid short-term noise.

Soil organic carbon concentration was converted from SOC

density with following equation (Pan et al., 2003):

SOCc ¼SOCp

H � q� 10ð1Þ

where SOCc is SOC concentration in g C kg�1 soil, SOCp is

SOC density in Mg C ha�1, q is soil bulk density in g cm�3,

and H is soil thickness in m. If the value of H was not clearly

pointed out, but soil-sampling depth was described by topsoil

or surface soil, we used 0.15 m to represent the value. In the

cases where data were missing, q was estimated following

Song et al. (2005)):

q ¼ 1:3770� e�0:0048�SOCc : ð2Þ

Data reported as soil organic matter concentrations were

converted to SOC concentration by multiplying with a conver-

sion factor of 0.58 (Pan et al., 2003). Given C concentration of

crop straw not available, straw-C input was estimated by

assuming that straw contained 40% C on average.

Data analysis

Meta-analysis was used to determine changes in soil C cycle

after straw return to soils in various cropping systems

(Hedges et al., 1999; Luo et al., 2006). Twenty-three variables

related to C pools, fluxes, important soil physicochemical

properties, and crop production were selected. Means (M),

standard deviations (SD), and sample sizes (n) of selected vari-

ables were extracted for each case. If only the standard errors

(SE) were given, SD was calculated by:

SD ¼ SEffiffiffin

p: ð3Þ

The natural log-transformed response ratio (lnRR) was

employed to reflect the effects of straw return on soil C pools,

gas fluxes, and soil properties in agricultural systems, and it

was calculated by following equation (Hedges et al., 1999):

lnRR ¼ lnðXt=XcÞ ¼ lnðXtÞ � lnðXcÞ ð4Þwhere Xt and Xc are means of the treatment and control

groups for variable X, respectively, and the variance (v) of X

is computed as:

v ¼ SD2t

ntX2t

þ SD2c

ncX2c

: ð5Þ

where SDc and SDt are standard deviation of control and treat-

ment groups, and nc and nt are sample sizes of control and

treatment, respectively.

In addition, the weighting factor (wij), weighted response

ratio (RR++), the standard error of RR++ (S), and 95% confi-

dence interval (CI) of RR++ were calculated as below (Curtis &

Wang, 1998; Luo et al., 2006):

wij ¼ 1

vð6Þ

RRþþ ¼

Pmi¼1

Pkij¼1

wijRRij

Pmi¼1

Pkij¼1

wij

ð7Þ

SðRRþþÞ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1

Pmi¼1

Pkij¼1

wij

vuuuutð8Þ

95%CI ¼ RRþþ � 1:96SðRRþþÞ: ð9ÞIf the 95% CI of RR++ for a given variable overlapped with

zero, the response of that variable to straw return represented

no significant differences between straw treatment and control

(Luo et al., 2006). The percentage change of a variable was cal-

culated by the equation of eRRþþ � 1� �� 100%.

Frequency distributions of lnRR were plotted to reflect the

variability of straw return effects among different studies by a

Gaussian function (i.e., normal distribution) (Luo et al., 2006):

y ¼ aeðx�lÞ22r2 ð10Þ

where y is the frequency of lnRR values within an interval, x

is the mean of lnRR for that interval, l and r2 are the mean

and variance of all lnRR values, respectively, and a is a coeffi-

cient indicating the expected number of lnRR at x = l.For assessing C budget caused by straw return, we esti-

mated responses of SOC, GHGs and DOC per unit straw-C

added by an equation adapted from Liu & Greaver (2009) and

Xie et al. (2010).

FX ¼ 1

n

Xni¼1

ðXti � XciÞCIRi

: ð11Þ

where FX represents contribution ratios of variable X (SOC,

GHGs or DOC) (Mg C or kg N ha�1 yr�1 per Mg

C ha�1 yr�1), Xti and Xci are values of variable X in the ith

sample of treatment and control, respectively, n is the sample

size, and CIR is straw-C input rate (Mg C ha�1 yr�1).

The net global warming potential (GWP) balance caused by

straw return, presented as CO2-C equivalent (CO2-Ceq), was

calculated based on the GWP of different GHGs as below:


1368 C. LIU et al.

netGWP ¼ �Fsoc � Fresidual�c þ FCH4� 16

44� 25þ FN2O

� 12

28� 1000� 298 ð12Þ

Where FSOC, FCO2, FCH

4, and FN

2O are contribution ratios of

SOC, CO2, CH4, and N2O emission. Fresidual-C is the mean

straw residual-C in soil and is set to 20% of originally incorpo-

rated straw-C following Jenkinson (1971) and Shields & Paul

(1973). The values of the GWPs of CH4 and N2O relative to

CO2 are 25 and 298 at a 100-year time horizon, respectively

(IPCC, 2007).

Sigma Plot 11.0 was used to fit data to normal distribution.

Regression analysis was applied to examine relationships of

lnRR of SOC concentration with lnRRs of other variables (soil

active C fraction, grain yield, and macro-aggregates) and

experimental factors (straw-C input rate, experimental dura-

tion, N fertilization rate, and soil depth). One-way ANOVA was

employed to examine whether the RR++ of variables differed

significantly between upland and paddy soils, straw disposal

methods of SOC (incorporation vs. cover), or among soil tex-

tures (clay, sand, and silt). We also calculated the total hetero-

geneity in lnRR among studies (QT); and in the case of

comparisons among groups, the differences in cumulative

effect size (QM) and residual error (QE) among groups (Rosen-

berg et al., 2000). All statistical analyses were performed using

SPSS 16.0.

Results

Responses of soil organic carbon to straw return

Straw-C input into the soil-enhanced SOC concentra-

tion of bulk soil and active SOC fractions (Fig. 1). The

RR++ of SOC across all the 343 pairs of comparisons

was 0.120 � 0.007 (RR++ � 95% CI, same as below;

Fig. 1a), meaning that straw return led to a 12.8%

increase in SOC concentration compared with the con-

trol. The lnRR of SOC exhibited a great variability

among different studies, with a range from �0.467 to

0.529. The frequency distribution of lnRRs could be

characterized by a Gaussian normal distribution

(R2 = 0.916 and P < 0.001). The RR++ (0.120) of meta-

analysis approach was consistent with that from the l

(a) (d)

(b) (e)

(c) (f)

Fig. 1 Frequency distributions of response ratios (lnRR) for soil organic C [SOC, (a)], microbial biomass C [MBC, (b)], particulate

organic C [(POC, (c)], dissolved organic C [(DOC, (d)], easily oxidizable C [(EOC, (e)], and light fraction organic C [(LFOC, (f)]

responses to straw-C input. The solid curve is a Gaussian distribution fitted to frequency data. The vertical dashed line is at lnRR = 0.



value (0.115) simulated by a Gaussian function model

(Fig. 1a, Appendix S2). The response was inconsistent

among studies, as indicated by a significant QT value

(QT = 1377.12 and P < 0.05; Appendix S3). The effects

of agricultural land use (paddy rice vs. upland) on the

accumulation of SOC under straw return were not sig-

nificant, with a RR++ of 0.117 � 0.010 in paddy soils

and 0.125 � 0.010 in upland soils. We also found that

clay loam soil generally had a lower effective size

(0.100 � 0.018) than that of sandy loam (0.157 � 0.016)

and silt soil (0.151 � 0.016). Meanwhile, RR++ values of

microbial biomass C (MBC), easily oxidizable C (EOC)

and dissolved organic C (DOC) were 0.267 � 0.011,

0.242 � 0.027 and 0.244 � 0.021, respectively, which

were all significantly different from zero (P < 0.001;

Fig. 1b, d, and e). The percentage increases were

similar in light fraction organic C (LFOC) and particu-

late organic C (POC), but higher than in other C fraction

(Fig. 1c and f). The increments of MBC, DOC, LFOC,

and POC in upland soils were larger than those in

paddy soils, while the increment of EOC in upland soils

was smaller than that in paddy soils (P > 0.05; Fig. 2).

Responses of CH4, CO2, and N2O to straw return

Responses of CH4, CO2, and N2O to straw return were

all significantly heterogeneous among studies (Appen-

dix S3). In comparison with the control group, straw

return significantly increased C fluxes, with a RR++ of

0.745 � 0.023 for CH4 and 0.328 � 0.027 for CO2 in

agricultural soils, both were significantly greater than

zero (P < 0.001; Fig. 3a and b). For soil CH4, CO2, and

Fig. 2 Weighted response ratios (RR++) of variables in agricultural system in response to straw return. Bars represent RR++ � 95% CI.

The sample size for each variable is shown next to the bar.


1370 C. LIU et al.

N2O, the frequency distribution of lnRRs could be char-

acterized by a Gaussian normal distribution (all

R2 > 0.900 and P < 0.001; Appendix S2). For the three

GHGs, the RR++ of meta-analysis was about consistent

with those from the l values from fitted Gaussian equa-

tions (Fig. 3, Appendix S2). An increase in CO2 fluxes

in paddy soils (0.412 � 0.039) was significantly higher

than that in upland soils (0.245 � 0.038) (P = 0.026;

Appendix S3). In this analysis, data were not sufficient

to estimate CH4 emission for upland cropping systems.

By contrast, N2O emission significantly decreased with

straw return by a RR++ of 0.123 � 0.020 (P < 0.001;

Fig. 3c). Fig. 2 also showed that N2O emission in the

two types of agricultural systems responded differently

to straw return. A significant increase in N2O

(RR++ � 95% CI = 0.079 � 0.048, P < 0.01) was found

in upland soils, while a drop was observed in paddy

soils (RR++ � 95% CI = �0.164 � 0.022, P < 0.001).

Responses of crop yield and soil properties to straw return

Crop yield greatly increased by straw return in both

upland and rice paddies (P < 0.001; Fig. 2), with a RR++

of 0.180 � 0.021 for upland crops and 0.084 � 0.015 for

paddy rice. Global average RR++ of crop yields was

0.116 � 0.012.

Overall, straw return improved soil nutrient levels.

The RR++ values of these soil nutrients were in the

range of 0.033 to 0.122, all being significantly greater

than zero (Fig. 4a–f). The increase in total N (TN) and

available N (AN) were similar, being higher in straw-

return treatment than in control. The total P (TP) and

available P (AP) were both increased due to straw

return. Increased percentage was lower in total K (TK)

than in available K (AK). Agricultural land use did not

significantly affect the changes in TN, TK, AN, and AK.

The changes in TP, however, showed significant differ-

ences between upland and paddy soils, with a RR++ of

0.088 � 0.020 in upland soils and �0.018 � 0.042 in

paddy fields (Fig. 2).

Straw addition slightly decreased soil bulk density,

with a RR++ of �0.020 � 0.012 (P < 0.01; Fig. 5a). Soil

porosity was increased due to straw input and the cor-

responding RR++ value was 0.079 � 0.036 (P < 0.001;

Fig. 5c). However, soil pH exhibited only a minor

change (RR++ � 95%CI = �0.003 � 0.020, P > 0.05;

Fig. 5b) in response to straw return. Overall, agricul-

tural land use has no significant impacts on soil bulk

density, porosity, and pH.

Straw input affected the distribution of soil aggre-

gates (Fig. 5d and e). Compared with those in controls,

soil macro-aggregates, and mean weight diameter

(MWD) increased in straw treatment, while soil micro-

aggregates decreased (all P < 0.001). The changes in

soil macro-aggregates, micro-aggregates, and WMD

showed no significant differences between upland and

paddy systems.

Factors affecting the responses of soil organic carbon tostraw return

Figure. 6 showed that straw-induced changes in SOC

significantly decreased with soil depth (b), and clay

content (d), while significantly increased with macro-

aggregates (h). Our results indicated a significantly

negative linear relationship between lnRR of SOC and

initial SOC concentration under straw addition

(R2 = 0.062, P = 0.001; Fig. 6c), whereas a significantly

positive linear for straw-C input rate (R2 = 0.025,

P = 0.003; Fig. 6a). The lnRR of SOC was also signifi-

cantly related to crop yields (Fig. 6g). A significantly

linear regression between the lnRR of soil MBC and the

lnRR of total SOC was shown in Fig. 6f, which was an

(a)

(b)

(c)

Fig. 3 Frequency distributions of response ratios (lnRR) for

CH4 (a), CO2, (b) and N2O (c) responses to straw return. The

solid curve is a Gaussian distribution fitted to frequency data.

The vertical dashed line is at lnRR = 0.



example for representing the relationship between

active SOC fractions and total SOC (P < 0.001). In addi-

tion, N fertilization rate, mean annual temperature

(MAT), and mean annual precipitation (MAP) were not

significantly linear with SOC changes (Appendix S4).

Soil organic carbon accumulation greatly differed

between methods of straw disposal (incorporation vs.

cover). The RR++ of SOC for straw incorporation

(0.123 � 0.009) was slightly higher than that for straw

cover (0.110 � 0.013; Appendix S3). In short-term or

long-term experiments, straw-induced changes in SOC

had no significant change with experimental duration

(Fig. 6e). Within medium-term experiments, however,

the RR++ of SOC significantly increased with the dura-

tion after straw return (P < 0.05).

Relationships between straw C input, duration, and SOCsequestered

The relationship between annually sequestered SOC (y:

Mg C ha�1 yr�1) and duration (x: year) was described

by the following equation: y = 4.64/x�0.41 (R2 = 0.372,

P < 0.0001; Fig. 7a). Soil C saturated at about 12 years

when C loss was equal to C input. Significant positive

regression relationship was found between cumulative

straw C input and SOC sequestered (R2 = 0.08,

P < 0.0001; Fig. 7b), which could be represented by the

fitted equation: y = 11.0 9 (1�e(�0.013x)).

Changes in agro-systems C balance at global scale

Table 1 showed that the response of SOC sequestration

to straw return was similar between in paddy

(0.160 � 0.034 Mg C ha�1 yr�1 per Mg C ha�1 yr�1)

and upland soils (0.166 � 0.024 Mg C ha�1 yr�1 per Mg

C ha�1 yr�1). Fluxes of CO2 increased by 0.518 � 0.101

and 0.409 � 0.068 Mg C ha�1 yr�1 per Mg C ha�1 yr�1

in paddy and upland soils, respectively. Methane (CH4)

was enhanced by 0.124 � 0.025 Mg C ha�1 yr�1 per Mg

C ha�1 yr�1 in paddy soils. N2O emission showed a

positive response to straw addition in upland soils, but

a negative response in rice paddies.

(a) (d)

(b) (e)

(c) (f)

Fig. 4 Frequency distributions of response ratios (lnRR) for Total N [(TN, (a)], Total P [(TP, (b)], Total K [(TK(c)], Available N [(AN,

(d)], Available P [(AP, (e)], and Available K [(AK, (f)] responses to straw addition. The solid curve is a Gaussian distribution fitted to

frequency data. The vertical dashed line is at lnRR = 0.


1372 C. LIU et al.

At global scale, 0.124 Pg C was sequestered annually

in world agricultural soils by straw return (Table 1).

Changes in net GWP due to straw return was estimated

to be �0.363 Mg C ha�1 yr�1 per Mg C ha�1 yr�1 in

upland soils, and 0.761 Mg C ha�1 yr�1 per Mg

C ha�1 yr�1 in rice paddies, meaning that a total of

0.137 Pg C yr�1 was increased by straw addition in

agricultural soils of the world.

Discussion

The effects of straw return on SOC sequestration

Soil organic carbon sequestration in cropping systems

has been considered as a cost-effective and environmen-

tally friendly strategy for sequestrating anthropogenic

CO2 emission (Lal, 2004a). Although numerous studies

have demonstrated that SOC sequestration was influ-

enced by extra organic C input, the magnitude and even

the direction of this response varied among studies.

Overall, we found that straw return significantly

increased SOC concentration by 12.8%, compared with

5.1% increase in SOC after adoption of no tillage

(Angers & Eriksen-Hamel, 2008), and to 3.5% increase in

SOC due to N fertilization (Lu et al., 2011b), suggesting

that straw-C input might further promote C accumula-

tion in arable soils. This could be explained by increased

crop-derived C input into the soil, as supported by the

positive linear-related between the lnRR of SOC and

straw-C input rate (Fig. 6a). Similar findings have been

reported from other field experiments at site scale (Dui-

ker & Lal, 1999; Kong et al., 2005; Li et al., 2010). Our

results also showed that crop yield was significantly

and positively linear with organic C input. High crop

productivity meant greater organic C input into the soil,

which consequently lead to additional C sequestration

in agricultural systems (Jarecki & Lal, 2003; Cai & Qin,

2006). Thus, our result supported our first hypothesis

and confirmed that application of straw was an effective

measure to enhance SOC level in croplands.

Straw return-induced SOC increase appeared to be

higher in upland than that in paddy soils, though the

(a) (d)

(b) (e)

(c) (f)

Fig. 5 Frequency distributions of response ratios (lnRR) for soil bulk density (a), pH (b), porosity (c), macro-aggregate (d), micro-

aggregate (e) and mean weight diameter (MWD, (f) responses to straw-C input. The solid curve is a Gaussian distribution fitted to

frequency data. The vertical dashed line is at lnR = 0.



difference was not statistically significant. This result

was not consistent with current knowledge that paddy

soils may have generally higher potential of SOC

sequestration than upland soils (Pan et al., 2003; Song

et al., 2005). In general, paddy soils have higher SOC

background (namely higher initial SOC concentration)

already than upland soils, and therefore, the same

amount of returned straw C would cause a less percent-

age change in SOC of paddy fields. Because the anaero-

bic condition during rice growing season in paddy soils

restricts SOC or straw decomposition, SOC in paddy

soils is more readily decomposable than that in upland

soils (Cui et al., 2012), leading to a significantly higher

SOC decomposition in paddy soils in consequent non

rice growing season. Interactions among various pro-

cesses or mechanisms are responsible for the complex

responses of paddy and upland soils to straw return.

Additionally, the effect of straw return on SOC

sequestration was influenced by the way how the

return was conducted. For instance, we found that

incorporating straws into soils tended to increase SOC

more evidently than covering straws on soil surface.

Although straw incorporation by tillage may strongly

disturb the soil and cause some losses of SOC due to

exposition to oxygen (Angers & Eriksen-Hamel, 2008;

Luo et al., 2010), the increase in contacting between

straw and soils can stimulate more straw C to trans-

form into organic C in the soil.

Our analysis suggested that changes in all active SOC

fractions, including MBC, POC, DOC, EOC, and LFOC,

were significantly associated with changes in SOC.

Haynes (2000) also reported that MBC and LFOC

increased linearly with SOC. However, the rates of

increases in active soil C fractions were higher than that

in total SOC, indicating that the active components of

SOC responded more sensitively to straw return.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 6 Relationships between the response ratio (lnRR) for the response to straw return of soil organic C (SOC) level and straw-C input

rate (a), soil depth (b), initial SOC concentration (c), clay content (d), experimental duration (e), lnRR of MBC (f), lnRR of yield (g) and

lnRR of macro-aggregates (h).

(a) (b)

Fig. 7 The relationship between straw C input, duration, and SOC sequestered in agricultural soils. (a) annual SOC sequestered vs.

duration (b) SOC sequestered vs. cumulative straw C input.


1374 C. LIU et al.

Similarly, Malhi et al. (2011) noticed that the change in

soil LFOC due to straw management was more remark-

able than total SOC. The reason was possibly that crop

residues mainly provided readily decomposable sub-

strates to the soil (Chen et al., 2009; Schulz et al., 2011).

All these findings suggested that active SOC fractions

could be proper indicators of the direction and magni-

tude of soil C cycling after straw return.

Macro-aggregates played an important role in the

accumulation of SOC (Six et al., 2002; Kong et al., 2005).

It was expectable that returning crop residues into the

soil could greatly affect the formation of soil macro-

aggregates (Duiker & Lal, 1999; Benbi & Senapati,

2010). Our synthesis revealed that straw addition

significantly increased the amounts of soil macro-aggre-

gates. There may be at least two explanations for the

straw-induced changes. First, straw decomposition

directly increased particles or colloids that were associ-

ated with mineral matter binding micro-aggregates into

macro-aggregates. Second, increased microbial biomass

(Fig. 1b) and activity (Pan et al., 2009) due to straw

addition might have led to a greater production of

microbial-derived binding agents and consequently

had a greater macro-aggregate formation rate. In addi-

tion, most of accumulated C occurred in mineral-associ-

ated fraction of macro-aggregates (Jastrow, 1996),

which slowed down SOC decomposition and promote

SOC stabilization (Bandyopadhyay et al., 2010).

Clay content has been considered as a crucial factor

to influence the capacity of soil to store C (Six et al.,

2002). Our data set covered a wide range of clay content

from 3% to 65%, and the result indicated that straw-

induced changes in SOC had a negative relationship

with clay content. A similar previous result was

observed by Huang et al. (2002), who reported that a

strong negative relationship existed between the

decomposition of wheat straw and clay content. One

possible reason was that biodegradation rates of straws

were probably slower in the soil with higher clay con-

tent (Balesdent et al., 2000), which may lead to lower

efficiency of C transferring from straws to soil minerals,

i.e., less humified SOC formed in these soils.

Our results showed that MAT and MAP were not

significantly linear-related with SOC changes in agri-

cultural soils (Appendix S4). Luo et al. (2010) also

pointed out that the pattern of change in soil C after the

adoption of no-tillage might not be related to climate

(i.e., MAT and MAP). These suggested that the agricul-

tural system was an intensively human-managed one,

and climate conditions only slightly influenced C

dynamics in agricultural soils.

May soil C saturation occur under straw return?

The effects of straw return were not evident in short-

term field experiments (1–3 years). In medium-term

Table 1 Estimates of net changes in the C balance of global agro-systems caused by straw return

Straw-induced changes per unit straw-C added

(Mean � SE; Mg C or kg N ha�1 yr�1 per Mg C ha�1 yr�1)

FSOC FCO2

FCH4

FDOC FN2O

Straw†

residual Sum‡

Upland 0.166 � 0.024 0.518 � 0.101 N/A 0.008 � 0.002 0.024 � 0.018 0.200 0.889

Paddy soils 0.160 � 0.034 0.409 � 0.068 0.124 � 0.025 0.0005 � 0.0003 �0.044 � 0.055 0.200 0.894

net GWP balance§ (CO2-Ceq) attributed to straw-C added (Mg C ha�1 yr�1 per Mg C ha�1 yr�1)

Upland �0.363

Paddy soils 0.761

Global scale

balance

under straw return

(Pg C yr�1)

net SOC†† net GWP

Agriculture

systems

�0.124 �0.137

†The percentage of straw-C residual was according to Jenkinson (1971) and Shields & Paul (1973).

‡Sum = FSOC + FCO2+ FCH

4+ FStraw residual + FDOC.

§net GWP balance (CO2-Ceq) attributed to straw-C added was estimated by using Eqn (12) (detail was provided in the Methods

section). The value of net SOC and GWP represent a net C source (positive) or sink (negative).

††net SOC = �ΣFSOC 9 S 9 CIRT. S is the area of arable land, and world areas of upland and paddy soils are 12.2 9 108 ha and

1.6 9 108 ha in 2009, respectively (data from http://www.fao.org). CIRT is the mean straw-C input rate (Mg C ha�1 yr�1) in the

world. If 50% of crop residue was returned to soil, the CIRT was estimated to be 0.517 and 0.755 (Mg C ha�1 yr�1) for upland and

paddy systems, respectively (Lal, 2008).



experiments (3–15 years), SOC contents significantly

increased with the duration of straw return. In long-

term experiments (>15 years), however, no obvious

changes in lnRR with time could be seen, indicating a

possible saturation of SOC levels after long-term appli-

cations of straw. These results could be envisaged as

the evidence of C saturation. Three possible mecha-

nisms might underpin organic C saturation in the soil:

(1) soil capacity to maintain organic C, (2) sequestration

duration, and (3) straw C input rate. The capacity of

soil to capture C mainly depend on clay content (i.e.,

chemical stabilization), aggregates (i.e., physical protec-

tion), and recalcitrant compounds in SOM (i.e., bio-

chemical stabilization; Six et al., 2002). Paddy soils

generally contain more recalcitrant compounds of SOM

than upland soils (Zhou et al., 2009), which means that

straw C may have more opportunity to transform resis-

tant components of SOC in rice paddies. Pre-experi-

mental SOC concentrations can also influence the

ability of straw inputs to result in soil C saturation

achieved. Our result indicated a significantly negative

linear-related between the response ratio of SOC and

initial C concentration under straw addition (Fig. 6c),

suggesting the influence of initial SOC concentration on

the effects of straw return (supported our second

hypothesis). A soil with a low initial soil C content has

a large saturation deficit that lead to a faster initial C

sequestration rate and a longer total duration to reach

the same C saturation (West & Six, 2007). Duration of C

sequestration has a significant impact on soil C satura-

tion, as longer duration favoring straw C to transform

into SOC. Our study revealed that 12 years on average

were required for the soil to reach C saturation (or a

steady state of C) under continuous straw return. A

similar study showed that with adoption of land restor-

ative measures, usually the rate of C sequestration

would be small beyond 15 year (Akala & Lal, 2000).

Whereas West & Six (2007) estimated that soil C satura-

tion might occur over a period of 26 years under inten-

sive rotation and 21 years under tillage cessation in

agronomic systems, both longer than that under straw

return in this study (12 year). High variability was seen

in the response of SOC to straw return at early stage,

and this variability declined along with time (Fig. 7a).

The change percentage of SOC was commonly small at

the early stage of straw return. It is still a technical chal-

lenge to detect a small change in SOC. A large number

of samples were required to obtain a reliable estimation

of SOC change (Conen et al., 2003). For most reported

long-term experiments, heavy sampling was practically

impossible. Other two possible reasons for the high var-

iability of SOC response at early stage of straw return

are disturbances of experimental implementation and

the quick release of unstable SOC pools. Cumulated

quantity of straw C input generally resulted in

increased supply of organic material and allowed SOC

to accumulate (Fig. 7b). However, changes in straw C

input rate may promote or retard soil C saturation but

do not alter soil C capacity. Luo and Weng (2011) pro-

posed that the terrestrial C cycle might present as a

dynamic disequilibrium, namely a disturbance caused

temporal changes in C source or sink at yearly and dec-

adal scales but had no impact on long-term C seques-

tration unless the disturbance regime was altered.

According to their framework, our results indicated

that addition of plant C could only result in a limited

gain in soil C stocks in a long term (Stewart et al., 2007;

Lu et al., 2009).

The response of GHGs fluxes to straw C input

Among GHGs, CO2 is one of the most significant con-

tributors to anthropogenic climate change. CO2 produc-

tion in arable soil could be mainly attributed to crop

residue decomposition, roots respiration, and minerali-

zation of SOC (Luo & Zhou, 2006). Our analysis indi-

cated that straw return had significant increased CO2

effluxes in both upland and paddy soils. This should be

partially due to increased plant productivity after straw

return, which often resulted in great root respiration

(Iqbal et al., 2009). Also, the increase in microbial bio-

mass under straw return may have positive conse-

quences in C fluxes. Lou et al. (2004) reported that

enhanced soil MBC could stimulate the decomposition

of SOC and straw, thereby stimulate C emission. Addi-

tionally, increases in soil moisture due to straw return

might accelerate CO2 emission, as suggested by Jabro

et al. (2008). Enhancement of soil porosity could also

improve soil diffusivity and contribute to CO2 emission

from the soil surface. Straw return induced increase in

soil moisture and soil porosity are both possible, but in

different circumstances. In sandy soils, straw return

induced increase in SOC could possibly increase water-

holding capacity of the soil due to the high water

absorption of organic matter and soil aggregation from

the banding function of SOC. In compact soil where

clay content is high, straw return induced increase in

SOC necessarily lead to an increase in soil porosity.

It was notable that CH4 emission in paddy soils was

increased by 110.7% under straw return (Fig. 3), which

was consistent with previous integrated analysis

reported by Xie et al. (2010). A number of studies have

demonstrated that CH4 emission was largely influenced

by the input of organic substrates under anaerobic con-

ditions (Cochran et al., 1997; Naser et al., 2007; Ma et al.,

2008). Addition of straw could selectively enhance the

growth of particular methanogenic populations and

therefore stimulated rates of CH4 production (Conrad


1376 C. LIU et al.

& Klose, 2006). In addition, anaerobic decomposition of

straw does not only provide substantial methanogenic

substrates, but also speed up a decrease in soil Eh,

favoring CH4 production (Cai et al., 1997; Ma et al.,

2008). Therefore, regulating soil moisture is an effective

means to reduce CH4 emission in paddy soils. Middle

season drainage of rice paddies often led to significant

drops in seasonal CH4 fluxes and has been recommend

as an important irrigation measure for mitigating global

warming (Zou et al., 2005; Xie et al., 2010).

Soil N2O produced naturally through microbial pro-

cesses of nitrification and/or denitrification, which

were profoundly influenced by soil N availability (e.g.,

NH4+), soil water content/oxygen and organic C sup-

ply (Chapin et al., 2002; Khalil et al., 2004). Our results

showed that straw return in upland soils increased

N2O emission, while a remarkable decline was seen in

rice paddies. In upland soils, straw return can provide

sufficient substrate for soil microbes and result in

enhanced soil N availability and hence an acceleration

of microbial processes. The increases in N2O flux might

be attributed to stimulated nitrification and denitrifica-

tion (Otte et al., 1996). A possible explanation, during

rice growth, is that soil water layer and low oxygen

could restrain nitrification and trap N2O in rice paddies

and thus inhibit its release to atmosphere (Cai et al.,

1997).

It should be noted that compared with CO2, the

responses of CH4 to straw return show largest variabil-

ity (Fig. 3). The lower concentration of trace gases and

considerably large difference in CH4 flux from different

sites (Appendix S2) may cause this variability.

Responses of soil quality and crop productivity to strawreturn

Understanding the relationships between agronomic

productivity, soil quality, soil organic matter, and

agronomic productivity is very important for the sus-

tainability of agricultural systems (Bhogal et al., 2009;

Sommer et al., 2011). The advantages of using straw to

improve soil quality and agricultural productivity

have been increasingly recognized (Jarecki & Lal,

2003). Our meta-analysis showed that crop yield could

increase by 12.3% after straw return, suggesting the

benefits of straw return to cropland productivity.

Crop straw is considered as a resource containing

readily available nutrients required for crop growth

(Pathak et al., 2006). Given that plant growth is usu-

ally limited by soil nutrients availability (Vitousek &

Howarth, 1991; Lu et al., 2011a), our results clearly

indicated that increased soil nutrient (particularly

available nutrients N, P, K) pools under straw return

may favor crop growth.

Straw return resulted in evident improvements in

soil physical conditions, as indicated by changes in soil

bulk density, total porosity, and water-holding capac-

ity, possibly due to enhanced SOC levels (Duiker & Lal,

1999; Zhu et al., 2010). Straw addition also improved

soil biological functions, which was supported by

increased microbial biomass (Bhogal et al., 2009). Over-

all, our synthesis suggested that straw return was an

effective measure to enhance agricultural soil quality

and crop productivity, which was especially important

for the sustainable development of agriculture.

Carbon balance in agro-ecosystems. Our analysis indi-

cated that straw return has promoted SOC accumula-

tion in agricultural soils. On average, 16.3% of organic

C in returned straws has been transferred into SOC,

which was slightly lower than reported transformed

ratio of 18.8% in China’s agricultural soils (Lu et al.,

2009). At global scale, the annual SOC sequestration by

straw return in agricultural ecosystems could be 0.124

Pg C, about 1.4% of global fossil-fuel emissions in 2008

(8.7 Pg C yr�1, Le Qu�er�e et al., 2009). This was close to

estimated C sequestration potential by adoption of con-

servation tillage and crop residue management (0.150–0.175 Pg C yr�1, Lal & Bruce, 1999).

After a certain time of straw return, a part of straws

will still exist in the soil in the form of plant residues.

Jenkinson (1971) and Shields & Paul (1973) reported

that after 4–5 years of decomposition in the field, the

residual straw was still 20% of the original straws

returned and subsequent decomposition of this residue

was very slow. In common practice of SOC determina-

tion, this part of C was not included. The potentially

significant contribution of this residual straw to the mit-

igation of global warming has been commonly ignored

and consequently GHGs mitigation capability by straw

return might be underestimated. Therefore, the overall

estimation of straw return in agricultural systems

should include the residual C in the estimates of global

net change. Our estimations indicated a net source of

GHGs at global scale in rice paddies, but a net sink in

upland systems. Increased CH4 emission largely offset

the C sink in paddy system. Overall, the global net

CO2-C mitigation capacity in agricultural soils was esti-

mated to be 0.137 Pg C yr�1. This indicated that

although CH4 and N2O emissions were taken into

account, straw return into agricultural soils could

reduce global climate warming. To achieve more bene-

fit of C sequestrating by straw return, agricultural

systems should be properly managed to reduce

straw-induced GHGs emission (especially CH4 emis-

sion in rice paddies). Two means have been proposed

to avoid high CH4 emission triggered by straw amend-

ment. One was to alter straw amendment from rice



growing season to non-rice growing season (Xie et al.,

2010; Shang et al., 2011). Another one was to regulate

water regime (e.g., midseason drainage and intermit-

tent irrigation) during rice growing season (Zhang

et al., 2011) to maximize the oxidation/minimize the

production of CH4. Midseason drainage could reduce

the total CH4 emission by ca. 50% (Yagi et al., 1996; Zou

et al., 2005). If this reduction could be achieved in all

paddy soils of the world, the global net sink of agricul-

tural soils was up to 0.205 Pg C per year.

Limitation of the study and improvement needed. Although

the meta-analysis provides a statistical approach to cal-

culate weighted response ratios across experiments

and sites, the overall effects of straw return on soil C

dynamics may be synthesized with uncertainties due

to the inherent limitations of methodologies used in

such analyses and/or of data quality deficiencies

among different studies. First, published data used

here may bias toward those studies with strong straw

return effects, because such studies might have been

more frequently reported (Gurevitch & Hedges, 1999).

According to soil map of the world (FAO, 2007) and

global distribution of straw return experiments

(Appendix S5), compiled database extracted in our

meta-analysis might have mostly come from multiyear

studies conducted on medium-fertility soils, while

those on fertile or infertile soils might have been

ignored because of insignificant results. The lack of

studies from infertile and fertile soils may have

influenced our conclusions. Second, the high heteroge-

neity of data sets from different studies, e.g., different

sampling depths, experimental durations and methods

of straw disposal, also influenced the overall effects of

straw return on soil C dynamics. GHGs fluxes usually

showed high temporal or spatial variability, especially

in highly disturbed agro-ecosystems. However, sample

sizes and experimental durations were often not suffi-

cient to represent spatial/temporal heterogeneity.

Also, most studies reporting CH4, CO2, and N2O emis-

sions lasted only one or two growing seasons. In this

work, the average annual flux was calculated from the

data of growing season. However, seasonal values

may not actually present the annuals (Johnson et al.,

2000). The ignorance of changes in nongrowing season

could underestimate/overestimate GHGs fluxes and

GWP because the responses of annual or growing sea-

son CO2, CH4, and N2O emission may be not consis-

tent. In addition, data of soil bulk density were not

available in some experimental sites. In this study,

bulk density was estimated by the relationship

between bulk density and SOC concentration using

Eqn (2), which might not actually represent true val-

ues. Uncertainties in estimating SOC stocks also might

influence the CO2-Ceq estimates. Nevertheless, together

with the modeling of Gaussian distribution and the

regression analysis, the method of meta-analysis

offered a powerful statistical analysis, and these

uncertainties and limitations might be unlikely to

change the response patterns of soil C dynamics in

Fig. 8 A simple mechanism for the responses of soil C dynamics to straw return in agro-ecosystems.


1378 C. LIU et al.

agricultural systems to straw return. Moreover,

according to our results, agricultural land uses, initial

soil C concentration, experimental duration all influ-

enced the overall pattern of soil C dynamics in rela-

tion to straw return. Thus, in future cropland

management, differentiated straw return strategies are

needed in upland and paddy rice soils to achieve the

goal of mitigating GHGs emission. The different

responses of land uses (upland vs. paddy soils) to

straw return need to be incorporated into agro-ecosys-

tem models to simulate soil C dynamics. In addition,

although our meta-analysis pointed out the potential

mechanism for how straw C input influence C dynam-

ics in croplands, given that the agricultural system

was an intensively human-managed system, further

studies focusing on interaction of straw return with

other cropland management would be highly needed

to reveal how long-term straw C input, tillage, fertil-

ization and irrigation interact to regulate cropland C

dynamics.

Conclusions

In summary, our results showed that straw return sig-

nificantly enhanced SOC accumulation in agro-ecosys-

tems. Straw-C input rate and soil clay content were

recognized as two major factors affecting the effects of

straw return on SOC. The relationship between

annual SOC sequestered and duration suggested that

soil C saturation would occur after 12 years of straw

C input. Straw return also improved overall soil qual-

ity, nutrient availability and crop yields. All these

changes suggested a net benefit of straw return to C

sequestration and crop productivity of agricultural

soils. However, it should be noted that straw return

also stimulated CH4 and CO2 emissions, which

deserves more attention by land managers in the con-

text of global warming. All these changes support a

conceptual framework in which there are positive

feedbacks between straw return, C cycles and soil

quality in agro-ecosystems (Fig. 8). Specifically, effec-

tive management strategies should be adopted to

reduce straw-induced GHGs emissions to realize

GHG mitigation potential of straw return in rice pad-

dies, e.g., optimized water regime and improved N

efficiency. Given that the total GWP increased due to

CO2, CH4, and N2O emissions, agricultural soils

amended with crop straws may play an important

role in mitigating future global climate warming. The

responses of SOC and GHGs emissions to straw

return as revealed by this synthesis can be potentially

useful for future cropland surface models for better

understanding and predicting the agro-ecosystem C

cycle feedbacks to straw C input.

Acknowledgements

We are grateful to the anonymous reviewers for their insightfulcomments, which greatly improved the manuscript. We thankthe authors whose data and work were included in this meta-analysis. This research was financially supported by the Minis-try of Science and Technology of China (Grant No.2010CB950604), and China National Natural Science Foundation(Grant No. 31070461; 31100352).

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Supporting Information

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

Appendix S1. List of studies included in this meta-analysis.Appendix S2. Percentage of change of 23 variables related to agricultural soil C cycles in response to straw-C input.Appendix S3. Statistical results of comparisons among groups.Appendix S4. Relationships between the lnRR of SOC for the response to straw addition of mean annual temperature (a), meanannual precipitation (b), and N application rate (c).Appendix S5. Global distribution of straw return experiments included in this meta-analysis.



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