Simulation of in situ Root Decomposition of Two Barley Cultivars

March 2014

ScienceDirect

Vol. 21 No. 1 16-24

Journal of Northeast Agricultural University (English Edition)

Available online at www.sciencedirect.com

Simulation of in situ Root Decomposition of Two Barley Cultivars Xu Jing-gang1, Duan Xue-jiao1, and Nooralla Juma2 1

College of Resources and Environment, Northeast Agricultural University, Harbin 150030, China

2

Faculty of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada T6G 2E3

Abstract: Root C and root-released C are closely related to soil organic matter content and mechanistic simulation modeling has proven to be useful for studying root and soil organic C dynamics in plant-soil ecosystems. A computer model was designed in this study to simulate the dynamics of root C and root released C decomposition in situ and the dynamics of different forms of C in soil under two barley cultivars (Abee and Samson). The results showed that on the 15th day, about 48% of the total 14C fixed in roots was respired for Abee and 42% for Samson. This indicated that the turnover rate of root 14C of Abee was higher than that of Samson. The percentage of water-soluble organic 14C, active microbial 14C and stable 14C over the total fixed 14C were not different between two barley cultivars. From the analysis of the model for two barley cultivars, the total 14C transformed into different soil pools (excluding CO2-C and root C pools) for the two barley cultivars was similar (26% for Abee and 25% for Samson), but the difference of 14C remaining in soil between the two barley cultivars was mainly because of the difference of 14C remaining in roots which have not been yet decomposed. Some of the information which could not be measured in the laboratory conditions was obtained in this study. Key words: simulation, root decomposition, two barley cultivars CLC number: S1

Document code: A

Article ID: 1006-8104(2014)-01-0016-09

decomposition can not be always possible (Paul and

Introduction

Van Veen, 1978). However, the intermediate processes

Root C and root-released C are the main sources of

the decomposition of the roots and plant debris in soil.

soil organic matter for plant-soil ecosystems. Through

　At present, most studies on root decomposition have

the process of the decomposition, energy is provided

been conducted with extracted or excised roots added

to microorganisms, nutrients are released for uptake

to disturbed soil samples that do not represent field

by both microorganisms and plants, and a proportion

conditions. Also, previous studies may be based on

of the photosynthetically fixed C is stabilized into soil.

the absence of fine roots which had higher turnover

However, the techniques used for the analyses of the

rate. Moreover, root sample preparation techniques,

decomposition of roots and root-released C in soil are

such as drying, grinding and mixing may also bias the

limited. The decomposition of the root materials in

estimation of root decomposition rates. Therefore, it

soil may be assessed only through the determination

is important to study the root decomposition in situ to

of the end products, such as CO 2, because simple

estimate the decomposition of roots in field conditions.

chemical analysis of the intermediate products of C

　 Mechanistic simulation models have proven to

are evenly important as the end products to understand

Received 17 September 2013 Xu Jing-gang (1959-), male, professor, supervisor of Ph. D student, engaged in the research of soil fertility and agricultural environmental protection. E-mail: [email protected] E-mail: [email protected]

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Xu Jing-gang et al. Simulation of in situ Root Decomposition of Two Barley Cultivars

be useful for the study of root and soil organic C

mics of different forms of C in soil under two barley

dynamics. They help to integrate the fragmentary

cultivars (Abee and Samson).

knowledge about the processes involved and therefore to develop a better understanding of the behavior of the soil ecosystem as a whole. They are also useful in

Model Descriptions

formulating and testing hypotheses, and establishing

The model was run on a Mac II si microcomputer

the relative importance of parameters (Verberne et al.,

using Stella II software. A simplified flow chart of the

1990).

simulation model is presented in Fig. 1. The model

　A computer model was designed to simulate the

consisted of two submodels: C submodel and

dynamics of root decomposition in situ and the dyna-

submodel.

Resistant root C

Active microbial C

Labile root C

14

C

CO2-C

Water soluble organic C

Active C

Protected microbial C

Stable C

Fig. 1 Flow chart of C in simulation model

on the root C. Without root C as substrate their bioC state variables

mass would decrease quickly. They accounted for

The C state variables in the model includes labile root

about 17%-19% (18% was used here) of the total

C, resistant root C, water-soluble organic C, active mi-

microbial C (Dinwoodie and Juma, 1988). Protected

crobial C and protected microbial C, active C, stable

microbial C represents the microorganisms which

C and CO2-C. Labile root C mainly represents the labile

mainly live in soil aggregates and feed on the soil

materials in the roots, such as sucrose, poly-sacchride

organic C (Verberne et al., 1990). They were much

and amino acids. Resistant root C is mainly composed

more stable than the active microorganisms. The

of structural components of the root tissues, such as

protected microbial C was calculated by subtracting

cellulose, semi-cellulose and lignin. Root C was mea-

the active microbial C from the total microbial C mea-

sured directly in the experiment and the initial pool

sured on 25-g soil samples by the chloroform fumiga-

sizes of the labile root C and resistant root C were esti-

tion technique (Jenkinson and Powlson, 1976). Active

mated by a double exponential equation. Water-soluble

C represents rapid cycling in soluble organic C pool

organic C represents a small, very rapid cycling pool

consisting of microbial metabolic pro-ducts and

consisting of labile organic C materials, such as meta-

recently stabilized materials (Paul and Juma, 1981).

bolites and cytoplasmic materials of dead organisms.

Campbell and Souster (1982) determined the active

Water-soluble organic C was measured with l0-g fresh

N fraction of a cultivated Black Chernozemic soil

soil using the method of McGill et al (1986). Active

from Saskatchewan to be 5.2% of the total soil N from

microbial C represents the microorganisms in the soil,

a clay loam. This percentage was used to calculate

which could easily access into the roots and grow fast

the initial pool size of active C in the present model. http: //publish.neau.edu.cn

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Journal of Northeast Agricultural University (English Edition)

Vol. 21　No. 1　2014

Fifty percent of soil organic C consists of chemically

simulation. The remainder of the soil organic C was

recalcitrant materials with a half life greater than 600

partitioned into microbial C, water-soluble organic C,

years (Campbell et al., 1967). This phase classified

active C and stable C. The initial pool size of the stable

as old C was not simulated in the model because it is

C was calculated by difference. The initial pool sizes of

very stable in the time period (80 days) of the model

different C pools in the model are presented in Table l.

Table 1　Initial pool sizes of total C (mg C • kg-1 soil) and 14C (kBq • kg-1 soil) in the model for two barley cultivars Abee

State

14 14

C

Samaon C14

C

C14

CO2-C

0

0

0

0

Labile root C

177

2 219

129

2 292

Resistant root C

192

1 195

203

1 875

Soluble C

75

4

83

5

Active microbial C

100

512

120

650

Protected microbial C

410

0

507

0

Active C

3 028

0

3 028

0

Stable C

28 777

0

28 777

0

　RCO =∑((1–Ymi)×Usi)+Mi×Mm

C state variables

2

14

C submodel was analogous to the C submodel. C 14

-l

pools consisted of C activity (kBq • kg soil). The 14

14

initial pool sizes of root C, water-soluble organic C 14

　 Where, Ymi is the maximum possible yield of microbial C for consumption of specific substrate i (unitless); Usi is the microbial uptake rate of specific

and microbial C were measured in the laboratory.

substrate i (mg • kg-1 soil • d-1); Mm is the microbial

The initial pool sizes of the rest pools were initialized

maintenance rate (d-1) and Mi is the microbial C (either

to O (Table 1).

active microbial C or protected microbial C) (mg C • kg-1 soil).

C flows between state variables

　Microbial death rate (Dm) was proportional to the

The flows within C submodel directly or indirectly control the flows within the C submodel. The uptake

amount of the microbial C present, respectively: 　Dm=dMi/dt=–Kd×Mi

of substrate by microbial organisms in the model was

-1 　Where, Dm is the microbial death rate (mg C • kg

calculated by the following equation:

soil • d-1); Kd is the microbial death rate constant (d-1);

　Usi=dSi/dt=Ki×Si

Mi is the microbial C. Forty-five percent (1–fr) of this

　Where, Usi is the uptake rate of the specific sub-

material was assumed to be water-soluble and entered

strate i by microbial C (either active microbial C or

the water-soluble organic C fraction. The remaining

14

-1

-1

protected microbial C) (mg C • kg soil • d ); Ki is the -1

55% (fr) was assumed to be insoluble or chemically

first order rate constant for the specific substrate i (d );

stable and entered the active C pool (Hunt et al.,

Si is the concentration of the specific substrate i (mg

1984).

-1

C • kg soil) and t is time (d).

　Active C was transferred to the stable C pool by a

　 Rate of microbial CO 2-C evolution (RCO 2) was

first order kinetic reaction that was dependent on the

calculated using growth and maintenance components

size of the active C pool. This simulates the chemical

(Hunt et al., 1984):

stabilization of the active organic C into more resistant

E-mail: [email protected]

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Xu Jing-gang et al. Simulation of in situ Root Decomposition of Two Barley Cultivars

forms (Juma and Paul, 1981).

C) of the pools from which the 14C was originating.

　Root respiration was not simulated in the model

　All the parameters used in the model were obtained

since it was negligible in the time period of the

from Literature (Hurst and Wagner, 1969; Sauerbeck

simulation.

and Gonzalez, 1977; Juma and Paul, 1981; Hunt et al., 1984; Verbern et al., 1990; Rutherford and Juma,

14

1992) and all the parameters were independent from

C flows between state variables 14

14

-1

the experiments for calibrating and validating the

soil • d ) was calculated by multiplying the C flow rate

model. The complete list of the parameters used in the

The flow of C between different C pools (kBq • kg -1

-1

-1

-1

(mg C • kg soil • d ) with the specific activity (kBq • mg

model is presented in Table 2.

Table 2　Parameter used in the model Symbol

Parameter description

Value

Unit

Krr-am

Uptake rate constant of labile root C by active microbial C

0.5

d-1

Rutherford and Juma,1992

Klr-am

0.01

d-1

Sauerbeck and Gonzalez, 1997

0.5*

d-1

Rutherford and Juma, 1992

Ka-pm

Uptake rate constant of resistant root C by active microbial C Uptake rate constant of soluble C by active and protected microbial C Uptake rate constant of active C by protected microbial C

0.003

d

Rutherford and Juma, 1992

Kst-pm

Uptake rate constant of stable C by protected microbial C

0.00016

d

Rutherford and Juma,1992

Ka-st

Transfer rate constant of active C to stable C

0.0005

d-1

Juma and Paul, 1981

Ksol-m

-1 -1

Reference

Verbern et al., 1990

Kamd

Death rate constant of active microbial C

0.5

d

Kpmd

Death rate constant of protected microbial C

0.02

d-1

Hurst and Wagner, 1969

Mm

Maintenance rate constant of microbial C

0.0025

d-1

Rutherford and Juma, 1992

Yr

Maximum possible yield of root C

0.4

Unitless

Hunt et al., 1984

Ysol

Maximum possible yield of soluble C

0.4

Unitless

Hunt et al., 1984

-1

Ya

Maximum possible yield of active C

0.4

Unitless

Hunt et al., 1984

Yst

Maximum possible yield of stable C

0.4

Unitless

Hunt et al., 1984

Mf

Microbial C to active C fraction coefficient

0.55

Unitless

Hunt et al., 1984

* If soluble C>50 (mg • kg-1 soil) then soluble C uptake rate =0.5 * (soluble C-50) else 0. Adsorbed soluble C is assumed to be 50 (mg • kg-1 soil).

microbial 14C were within the standard error bars of the

Results

experimental data over the incubation period. Water-

Model calibration with data of cultivar Abee

to Day 80 but over estimated from Day 0 to Day 25 by

The model was calibrated with the data of the root

the model.

soluble organic 14C was well simulated from Day 25

decomposition in situ of barley cultivar Abee in a Black Chernozem (Typic Cryoboroll). It was possible

Model validation with data of cultivar Samson

to produce model outputs that fitted the observed data.

The model was validated with the data of the root

The predicted CO2-C evolution and microbial C were

decomposition in situ of cultivar Samson in a Black

closely fitted to the experimental data (Fig. 2). The

Chernozem (Typic Cryoboroll). All the parameters in

model simulated the water-soluble organic C well

the model were not changed for the validation. The

from Day 40 to Day 80, but under estimated from Day

only change made for validation was the initial pool

14

0 to Day 25. The CO2- C recovery and the predicted

size (Fig. 3). http: //publish.neau.edu.cn

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Journal of Northeast Agricultural University (English Edition)

3 000

900  

300





Microbial C (μg C • g-1 soil)

Soluble C (μg C • g-1 soil)

0 0

20

40

60

140 120



100 



 

60 40

0

20



40

60

600

400







300 200

0

20





40

60

80











1 000 0 0

20

40

60

80

300 200 10









0

80

700

500 

2 000

80 Soluble 14C (Bq • g-1 soil)

600

CO2-14C (Bq • g-1 soil)



Microbial 14C (Bq • g-1 soil)

CO2-C (μg C • g-1 soil)

1 200

80

Vol. 21　No. 1　2014

0

20





40

60

80

60

80

700 500  

300



100



0

20



40



Incubation time (Day)

Fig. 2　Model outputs (lines) and experimental data (symbols and standard error bars ) for CO2-C, water-soluble organic C, microbial C, CO2-14C, water-soluble organic 14C and microbial 14C during incubation period for Abee

　The model simulated water-soluble organic C well

outputs and final pool sizes for the two time intervals

over the incubation period except on Day 5 when it

of two barley cultivars were determined using the

was underestimated. The model closely simulated

model (Tables 3 and 4). CO2-C evolved from Day 0

the microbial C on Day 0, Day 5, Day 15 and Day

to Day 15 accounted for about 32% and 31% of the

80 but overestimated on Day 25 and Day 40. The

total CO2 evolved during the incubation period for

14

model accurately simulated the evolution of CO2- C, 14

but overestimated the water-soluble organic C over 14

Abee and Samson, respectively. During this interval, 51% of CO2 evolved was due to the decomposition of

the incubation period. The microbial C was closely

the root C for Abee and 39% for Samson. The rests

simulated over the incubation period.

were due to the decomposition of soil organic matter. Active microbial C was reduced by 95% during the

Dynamics of C under two barley cultivars

first 15 days, as almost all the labile root C was decom-

The incubation period was divided into two periods: a

posed during this period. The protected microbial C

period of rapid changes (Day 0-Day 15) and a period

was relatively stable and the decrease was less than 5%

of slow changes (Day 15-Day 80). The inputs and

of its pool sizes for both cultivars during this period.

E-mail: [email protected]

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Xu Jing-gang et al. Simulation of in situ Root Decomposition of Two Barley Cultivars

The output of water-soluble organic C was greater

turnover rate of the water-soluble organic C was more

than its input; therefore, it decreased during the first

important than its pool size. Active C increased during

15 days for both barley cultivars. The input and output

the first 15 days as the input was greater than the

of water-soluble organic C were twice greater than its

output for both cultivars. Stable C decreased from Day

pool sizes for both barley cultivars, indicating that the

0 to Day 15 because the input was less than the output. 3 000

900  

300





Microbial C (μg C • g-1 soil)

Soluble C (μg C • g-1 soil)

0 0

20

40

60

140 

120 100 80

  

60 40

0

20



40



60

600

 

500 400

 

300 200



0

20

40

60

80







1 000 0 0

20

40

60

80

300 200 10

 

0

80

700





2 000

80 Soluble 14C (Bq • g-1 soil)

600

CO2-14C (Bq • g-1 soil)



Microbial 14C (Bq • g-1 soil)

CO2-C (μg C • g-1 soil)

1 200

700



0



20



40



60

80

60

80



500 

300 100

 

0

20



40



Incubation time (Day)

Fig. 3　Model outputs (lines) and experimental data (symbols and standard error bars ) for CO2-C, water-soluble organic C, microbial C, CO2-14C, water-soluble organic 14C and microbial 14C during incubation period for Samaon

　CO2-C evolved from Day 15 to Day 80 accounted

decrease during this period for two cultivars. The two

for about 68% to 69% of the total CO 2 evolved

microbial C pools followed the same trend as water-

during the incubation period for Abee and Samson,

soluble organic C and the outputs from these pools

respectively. The main source of CO2 evolved dur-

were still greater than the inputs. The input and output

ing this period was soil organic matter instead of

of the active microbial C were over 80 times greater

the root C. Forty-four percent of the root C was de-

than its pool size as estimated on Day 15 and the

composed between Day 15 and Day 80 for Abee and

input and output of protected microbial C was over

41% for Samson. The input of water-soluble organic

three times its pool size of Day 15 for the two barley

C was less than its output; therefore, it continued to

cultivars. In contrast to the first 15 days, active C http: //publish.neau.edu.cn

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Journal of Northeast Agricultural University (English Edition)

Vol. 21　No. 1　2014

decreased, during Day 15 and Day 80 as the output

continued to decrease during Day 15 and Day 80 for

was greater than the input for both cultivars. Stable C

two barley cultivars.

Table 3　Simulated inputs and outputs of C between Day 0 and Day 15 and between Day 15 and Day 80 for Abee Day 15 Variable

Input

Output

Day 80 Pool size Total C (mg

Input

Output

Pool size

C • kg-1 soil)

CO2-C

345

0

345

719

0

1064

Labile root C

0

177

0

0

0

0

Resistant root C

0

27

165

0

79

86

Soluble C

150

165

60

249

252

57

Active microbial C

284

371

4

205

207

2

Protected microbial C

289

300

407

984

1 066

325

Active C

179

161

3 046

305

654

2 697

Stable C

23

69

28 731

93

297

28 527

1 877

473

0

2 390

C (kBq • kg-1 soil)

14

CO2-14C

1 877

0

Labile root C

0

1 637

1

0

1

0

Resistant root 14C

0

167

1 028

0

492

536

14

Soluble 14C

767

755

16

176

188

4

Active microbial 14C

2 719

3 214

17

586

599

4

Protected microbial 14C

406

272

134

265

306

93

Active 14C

885

34

851

216

198

869

Stable 14C

5

0

5

28

0

33

Table 4　Simulated inputs and outputs of 14C between Day 0 and Day 15 and between Day 15 and Day 80 for Samson Day 15 Variable

Input

Output

Day 80 Pool size Total C (mg

Input

Output

Pool size 1076

C • kg-1 soil)

CO2-C

330

0

330

746

0

Labile root C

0

129

0

0

0

0

Resistant root C

0

29

174

0

83

91 57

Soluble C

162

184

61

279

283

Active microbial C

248

357

4

225

227

2

Protected microbial C

299

332

481

1 006

1 137

350

Active C

197

161

3 046

340

661

27 437

Stable C

23

69

28 731

93

297

28 527

2 040

739

0

2 779

C

14

CO2-14C

(kBq • kg-1 soil)

2 040

0

Labile root C

0

1 637

1

0

1

0

Resistant root 14C

0

262

1 613

0

771

842

14

Soluble 14C

847

833

19

245

258

6

Active microbial 14C

2 948

3 575

23

901

916

8

Protected microbial 14C

449

300

149

329

367

111

Active 14C

1 010

39

971

2 986

232

1 037

Stable 14C

6

0

6

33

0

39

E-mail: [email protected]

·23·

Xu Jing-gang et al. Simulation of in situ Root Decomposition of Two Barley Cultivars

Abee. The trend for Samson was the same: CO2-14C Dynamics of 14C under two bartey cultivars

(58%)>active 14C (22%)>root 14C (18%)>microbial

Seventy-eight percent of the total CO2-14C evolved

14

during the incubation period (80 days) was produced

(0.1%). From the analyses of the model for two barley

within the first 15 days for Abee and 73% for Samson.

cultivars, the total 14C transformed into different soil

The input was less than the output for active microbial

pools (excluding CO2-C and root C pools) for the two

14

C, therefore, active microbial C decreased during

barley cultivars was similar (26% for Abee and 25%

the first 15 days for both barley cultivars. In contrast,

for Samson). The difference of 14C remaining in soil

the protected microbial 14C increased during the same

between the two barley cultivars was mainly because

period since the input was greater than the output.

of the difference of 14C remaining in roots which had

Water-soluble organic 14C, active 14C and stable 14C

not been yet decomposed.

14

C (3%) > stable 14C (0.8%)>water-soluble organic 14C

all increased during the first 15 days. The two barley cultivars had the similar trends. 14

　The CO2- C evolved between Day 15 and Day 80

Conclusions

accounted for about 22% of the total CO2-14C evolved

The simulation model supplies more information on

during the incubation period for Abee and 27% for

root decomposition, especially on the intermediate

14

Samson. Water-soluble organic C, active microbial

processes which could not be measured experimentally

14

in the laboratory. Through this simulation study, the

14

C and protected microbial C decreased from Day

15 to Day 80 as the inputs were less than the outputs. 14

14

following conclusions could be drawn:

Active C and stable C continued to increase from

14 　1) About 48% of the total C fixed in roots was

Day 15 to Day 80 as the inputs were greater than the

respired for Abee and 42% for Samson, which indi-

outputs. The two barley cultivars had the similar trends

cated that the turnover rate of root 14C of Abee was

in 14C during Day 15 and Day 80.

higher than that of Samson. Different varieties of barely could fix different amount of C and had a

Discussion

different contribution to the soil organic matter pool. 　 2) The percentage of water-soluble organic 14

14

C,

14

The simulation model supplies more information on

active microbial C and stable C over the total fixed

root decomposition, especially on the intermediate

14

processes which can not be measured experimentally

The two barley cultivars had the same trend on di-

in the laboratory. By Day 15, about 48% of the total

stribution of 14C in different pools: CO2-14C>active

14

14

C fixed in roots was respired for Abee and 42% for

C were not different between the two barley cultivars.

C>root 14C>microbial 14C>stable 14C>water-soluble

Samson. This indicated that the turnover rate of root

organic 14C. This implied that most of the root C was

14

C of Abee was higher than that of Samson in the

respired during the growing season, but a small por-

first 15 days. The percentage of water-soluble organic

tion of it was converted into soil organic matter. Main-

14

taining soil organic matter content is important since it

14

14

C, active microbial C and stable C over the total 14

fixed C were relatively less and were not different 14

takes a long time to be accumulated.

between two barley cultivars. Active C accounted for

14 　3) The difference of C remaining in soil between

about 23% of the total fixed 14C on Day 15 for Abee

the two barley cultivars was mainly because of the

and 21% for Samson. On Day 80, the distribution of

difference of 14C remaining in roots which has not been

14

yet decomposed, showing that the more C was fixed

C in different pools was CO2-14C (61%)>active 14C

(22%)>root 14

14

C (14%)>microbial

14

C (0.8%)>water-soluble organic

C (3%)>stable

and transferred in roots, the more C was incorporated

14

into soil from the air.

C (0.l%) for

http: //publish.neau.edu.cn

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Journal of Northeast Agricultural University (English Edition)

Vol. 21　No. 1　2014

techniques to assess mineralization and immobilization of soil

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