simulating trends in soil organic carbon of an acrisol under no

www.elsevier.com/locate/geoderma

Geoderma 120 (2004) 283–295

Simulating trends in soil organic carbon of an Acrisol under

no-tillage and disc-plow systems using the Century model

Luiz Fernando Carvalho Leitea, Eduardo de Sa Mendonc�ab,*,Pedro Luiz Oliveirade de Almeida Machadoc, Elpıdio Inacio Fernandes Filhob,

Julio Cesar Lima Nevesb

aEmpresa Estadual de Pesquisa Agropecuaria da Paraıba-EMEPA, Rua Eurıpedes Tavares, 210, 58013-290 Joao Pessoa, Paraıba, BrazilbDepartamento de Solos, Universidade Federal de Vic�osa, 36571-000 Vic�osa, Minas Gerais, Brazil

cEmbrapa Solos-EMBRAPA, Rua Jardim Botanico 1024, 22460-000 Jardim Botanico, Rio de Janeiro, Brazil

Received 22 October 2002; received in revised form 22 July 2003; accepted 17 September 2003

Available online 10 December 2003

Abstract

Soil organic matter (SOM) and its different pools have key importance in nutrient availability, soil structure, in the flux of trace

gases between land surface and the atmosphere, and thus improving soil health. This is particularly critical for tropical soils. The

rates of accumulation and decomposition of carbon in SOM are influenced by several factors that are best embodied by simulation

models. However, little is known about the performance of SOM simulation model in an acid tropical soil under different tillage

systems including no-tillage (NT). Our objective was to simulate soil organic matter dynamics on an Acrisol under no-tillage and

different plowed systems using Century model. Tillage systems consisted of no-tillage, disc plow, heavy disc harrow followed by

disc plow, and heavy disc harrow. Soil C stocks simulated by Century model showed tendency to recovery only under no-tillage.

Also, simulated amounts of C stocks of slow and active pools were more sensitive to management impacts than total organic C.

The values estimated by Century of soil C stocks and organic carbon in the slow and passive pools fitted satisfactorily with the

measured data. Thus fitted, except for the active pool, Century showed acceptable performance in the prediction of SOMdynamics

in an acid tropical soil.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Acidic tropical soils; Soil carbon fractions; Long-term experiments; Century model

1. Introduction improved moisture and soil structure for plant growth

Soil organic matter (SOM) is an important compo-

nent of acid tropical soils and its significance can be

seen on the positive effects on Ferralsol cation ex-

change capacity, size and stability of aggregates,

0016-7061/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.geoderma.2003.09.010

* Corresponding author.

E-mail address: [email protected] (E. de Sa Mendonc�a).

(Goedert et al., 1997). Also, SOM has large influence

on the flux of the trace greenhouse gases between land

surface and the atmosphere (Batjes, 1996). Soil culti-

vation often leads to diminution of SOM content

(Castro Filho et al., 1991), but conservation tillage

such as no-tillage can improve soil conditions to those

found in forest soils (Machado and Silva, 2001).

Because of cost reductions and soil erosion control,

Table 1

Chemical and physical characteristics of an Acrisol (0–20 cm layer)

L.F. Carvalho Leite et al. / Geoderma 120 (2004) 283–295284

no tillage is widely used in Brazil over an area of 14

million ha (Pereira, 2002).

No tillage, combined with crop rotation involving

cover crops, favors the accumulation of plant residues

on the soil surface (Machado and Silva, 2001). In a

large study conducted in the USA, Kern and Johnson

(1993) reported that the widespread conversion of

major field crop production from conventional tillage

(mouldboard plow) to conservation tillage would

change the soil system from a source to a sink of

atmospheric carbon. Similar observations were also

reported later for European soils (Smith et al., 1997a).

These findings show the potential for agriculture to

contribute to global carbon mitigation, particularly

through no-tillage.

The ability to predict the effects of environment

(e.g. climate and atmospheric composition) and land-

use change on SOM dynamics is of utmost impor-

tance in formulating environmental and agricultural

policies (Smith et al., 1997b). Modeling is a powerful

means to simulate a range of intricate processes and

predict soil organic matter changes for long time

periods (Paustian et al., 1992).

Century is a model of terrestrial C, N, P, and S

dynamics that use a four pool SOM submodel. Cen-

tury has been successfully used in temperate ecosys-

tems (Parton and Rasmussen, 1994; Kelly et al., 1997;

Del Grosso et al., 2001), but in spite of some studies

about modeling tropical agroecosystems (Parton et al.,

1989; Motavalli et al., 1994), no information is

available on the use of mathematical models on the

dynamics of soil organic carbon in Acrisol under no-

tillage. Our objective was to simulate soil organic

matter dynamics on an Acrisol under no-tillage and

different plowed systems commonly used in Brazil

using Century model.

under different tillage systems from the State of Minas Gerais,

Brazil

Treatmenta pH

(H2O)

TOC

(dag kg� 1)

TN

(dag kg� 1)

Clay

(dag kg� 1)

Bulk

density

(mg m� 3)

NT 4.97 1.46 0.12 41 1.32

DP 5.05 1.28 0.10 38 1.22

HHPD 5.05 1.28 0.10 39 1.21

HH 5.00 1.26 0.11 37 1.24

AF 5.48 2.83 0.22 46 1.13

TOC: total organic carbon, TN: total nitrogen.a NT: no tillage; DP: disk plow; HHDP: heavy disk harrow+ -

disk plow; HH: heavy harrow; AF: secondary Atlantic Forest.

2. Material and methods

2.1. Experimental site

Simulations with the version 4 of the Century

model were carried out for an experimental site

established in 1985 at the Experimental Station of

the Federal University of Vic�osa, in Coimbra, State of

Minas Gerais, Brazil (20j45S and 42j51W; 700 m

asl). The mean annual temperature is 19 jC and

average rainfall is 1350 mm, and roughly two-thirds

of this rain falls in the warmer season of the year from

October to April.

The area was covered by Atlantic Forest until 1930

and was cultivated for 54 years with subsistence

crops, such as maize (Zea mays L.) and common

bean species. The soil in the experimental area is a

loamy Acrisol (Argissolo Vermelho-Amarelo, Brazil-

ian Classification System; Typic Kandiudult, US Tax-

onomy) and some chemical and physical character-

istics are shown in Table 1.

The experiment started at 1985 and consisted of

four soil management systems, arranged in a complete

randomized block design, with four replications. Plots

were 4� 11 m under maize/fallow/soybean succes-

sion. The tillage treatments were:

1. No tillage (NT)—no disturbance to the soil other

than sowing operation;

2. Disk plowing (DP)—plowing at 20 to 25 cm depth

with a three-fixed disk plow, in a single pass;

3. Heavy disk harrow + disk plowing (HHDP)—one

single pass at 0–15 cm using a heavy disk harrow

with 20 disks followed by disk plowing at 20–25

cm depth, with a three-fixed disk plow;

4. Heavy disk harrow (HH)—one harrowing at 10 to

15 cm depth with a heavy disk harrower of 20

disks weighing approximately 2 t;

In addition, as a reference, samples were taken

from an area under secondary Atlantic Forest (AF),

adjacent to the experiment, (100 m away) in the same

soil type. At the middleslope, four areas (4� 4 m)

L.F. Carvalho Leite et al. / Geoderma 120 (2004) 283–295 285

were placed along a 100-m transect in which soil

samples were collected.

2.2. Chemical analysis

Soil samples were collected, in April 2000, just

after the harvest period. In each plot, eight topsoil

samples (0–20 cm) were collected and combined

into one composite sample. In the area under At-

lantic Forest, 15 samples were collected from each

sampling area (n = 4) and bulked into one sample.

The samples were ground to pass a 2-mm sieve.

An aliquot of 100 g was separated and kept refri-

gerated at 4–8 jC before microbial biomass anal-

ysis. For all remaining analyses, the soil samples

were air-dried.

Total organic carbon (TOC) was obtained by wet

digestion with a mixture of potassium dichromate and

sulfuric acid, under heating (Yeomans and Bremner,

1988). Total N was measured in the soil samples with

a sulfuric digestion followed by determination in the

Kjedahl distillation (Bremner, 1996). Measurement of

microbial biomass was conducted by the irradiation–

extraction method—using microwave (Islam and

Weil, 1998), 0.5 mol l� 1 K2SO4 as extractant and

the biomass C was determined by wet combustion

(Yeomans and Bremner, 1988). The factor (KC) used

to convert the flow of C for the microbial biomass C

(CMB) was 0.33 (Sparling and West, 1988). The CMB

was used as an estimate of the active C pool (Paul,

1984; Motavalli et al., 1994). The free-light organic

carbon fraction (CLF) was determined by flotation in

NaI solution (d = 1.8 g cm� 3) as proposed by Sohi et

al. (2001). The isolated material was dried at 105 jCfor 72 h. The CLF, quantified by dry combustion

(Perkin Elmer 2400 CHNS/O elemental analyzer),

was further used as an estimate of the slow C pool.

Passive C pool was calculated using the following

equation:

Passive C ¼ TOC� ðCMB þ CLFÞ

Bulk soil density was determined on nondeformed

soil samples collected from a single field replicate.

The values of bulk soil density were used to calculate

SOM pools based on an equivalent soil mass (Angers

et al., 1997; Peterson et al., 1998).

2.3. Model parameterization

The Century SOM model was originally developed

and tested on data sets mainly from grassland and

wheat– fallow agriculture in the US Great Plains

(Parton et al., 1987, 1988). All parameter values

determined from these previous studies were initially

left unchanged to provide more criterious evaluations

of the simulations. As given by Paustian et al. (1992),

these general or non-site specific parameters include

the maximum specific decomposition rates for each

compartment, the constants that splits the flows of

decomposition products and the parameters that con-

trol the effects of soil texture, lignin/N ratios, temper-

ature, and moisture on decomposition rates.

Site-specific parameters and initial conditions, such

as soil texture (sand, silt and clay content), bulk

density, soil depth and total soil C and N content,

were given values obtained from the field experiment

at Coimbra. Monthly precipitation and mean maxi-

mum and minimum monthly temperatures from 1967

to 2000 were obtained from the weather station at the

Vic�osa Federal University. The parameter determining

potential crop productivity was based on the maxi-

mum production level observed during the course of

the field experiment in each treatment and temperature

curve, C/N ratios and lignin contents of biomass pools

were obtained through default crop parameterizations

distributed with Century model. The main input data

for the model are in Table 2.

Some model adjustments were made to improve

the tillage effects on SOM decomposition. First, the

plowing option was adjusted to increase its effect on

decomposition (Six et al., 1998). All clteff values

(cultivation’s effect on decomposition) for the DP,

HHDP and HH were changed from 1.6 to 5 (Table

2). The second change was added to increase the

length of time which plowing effects decomposition.

Since Century uses a monthly time-step each action

only affects SOM dynamics for that specific month

although some studies have shown that plowing

affects decomposition for several months (Metherell

et al., 1995). Thus, an option called ‘‘Additional

plowing effect’’ was used in the months following

plowing in order to keep the decomposition rates at

higher levels (Manies et al., 2000).

To initialize the percentage of total SOM in each of

the three pools (active, slow and passive) used indirect

Fig. 1. Modeled stocks of soil organic carbon (TOC) and organic

carbon pools of an Acrisol (0–20 cm) under Atlantic Forest.

Table 2

Model input for simulation of tillage systems using version 4 of the

Century model

Value

Soil variables

Texture (% sand,

% silt, % clay)

38, 16, 46

Bulk density (mg m� 3) 1, 13

Initial SOM

(g C m� 2; C/N)

Active surface 50, 12,5

Active soil 159, 9

Slow soil 1776, 22

Passive soil 4460, 12

Monthly weather variables

Mean total precipitation

(cm month� 1)

14

Mean maximum

temperature (jC)14.8

Mean minimum

temperature (jC)26.4

Cultivation variables

Multiplier for increased

decomposition

Active pool 1.0

(NT); 1,8 (DP);

2.0 (HHDP); 1.6 (HH)

Slow pool 1.0 (NT); 4.0 (DP);

5.0 (HHDP); 3.0 (HH)

Passive pool 1.0 (NT); 1.8 (DP);

2.0 (HHDP); 1.6 (HH)

Carbon pool values were obtained from direct method. NT: no

tillage; DP: disc plow; HHDP: heavy disk harrow+ disk plow; HH:

heavy harrow.


methods involving simulation of steady state organic

matter levels and direct methods using analytical

techniques. In the indirect method, Century model

parameterized all data including carbon pools for a

long term (6000 years) by equilibrium simulation.

Simulated values for carbon pools were used as input

variables for simulation of land use change. To each

treatment, the model simulated SOM dynamics for 54

years representing forest conversion into cropland

(e.g. maize and bean production) and later, for 66

years, representing different tillage systems. In the

direct method, the initial soil carbon pool sizes under

Atlantic Forest were estimated by laboratory analysis

and similarly used in the indirect method. All Century

estimates were based on a 20-cm depth. Both simu-

lated and measured values for TOC, active, slow and

passive pools in 2000 were subjected to linear regres-

sion and Pearson’s correlation. An average of these

data sets was taken from each treatment and subjected

to a Student’s t-test to determine the significance of

the coefficients at the 0.05 and 0.01 probability levels.

Simulations for nitrogen pools were also done for NT

and HHDP systems.

3. Results and discussion

3.1. Estimates of carbon pools by equilibrium values

Century model simulated the equilibrium values of

total organic carbon (TOC) and carbon pools (active,

slow and passive) for 6000 years. Compared to the

initial values, storage of both TOC and passive C pool

increased while active and slow carbon pools

remained generally constant (Fig. 1). The increase in

the passive carbon pool, reflected by TOC, is proba-

bly due to carbon pools that are highly recalcitrant,

physically protected against microbial attack and less

prone to oxidation. After reaching equilibrium, stocks

of TOC (64 mg ha� 1) and active carbon pool (1.60

mg ha� 1) were similar to those measured in the soil

under Atlantic Forest (Table 3). The stocks of TOC

are 3.9% higher than the value (61.5 mg ha� 1) found

by Silveira et al. (2000) in another soil under Atlantic

Forest in the Piracicaba river basin, Brazil. The

estimated value of the slow carbon pool (30.1 mg

ha� 1) was 42% higher than the measured value in the

forest soil. On the other hand, the estimated value of

Table 3

Measured values of total organic carbon (TOC) and carbon pools

(active, slow and passive) of an Acrisol under Atlantic Forest

(Brazil)

Pools Stocks (mg ha� 1)

Total organic carbon 63.95 (F 4.25)

Active 1.59 (F 0.02)

Slow 17.4 (F 1.23)

Passive 44.9 (F 4.01)

Values in parenthesis represent the standard error of the mean.


the passive carbon pool (32.3 mg ha� 1) was lower

than the measured data (Table 3). Hence, model fit to

measured slow and passive carbon pool was not good,

probably due to sizes of the passive pool higher in

Fig. 2. Time variation of the stocks of TOC (A) and active (B), slow (C) an

values obtained from the equilibrium simulation in the no-till (NT), disc p

harrow (HH).

tropical soils than the assumption of the Century

model.

Starting at initial values of carbon pools obtained

from the equilibrium simulation, Century estimates

for the stocks of TOC and carbon pools after 120

years decreased after changing forest into agriculture

(Fig. 2). Parfitt et al. (1997) studying the effects of

clay minerals and land use on organic matter pools

obtained similar results, i.e. a decrease of the TOC and

C pools after forest clearance and subsequently land

use under pasture and maize in an Inceptisol. In 1984,

before setting up the experiment, TOC stock was 28

mg ha� 1, which were 56% lower than the initial value

predicted by the model (64 mg ha� 1). In 2000, 15

years after the introduction of the tillage systems,

d passive (D) carbon pools simulated by Century based on the initial

low (DP), heavy harrow followed by disc plow (HHDP) and heavy


stocks of TOC in the soil under NT (29 mg ha� 1)

showed an increase compared to the stocks of TOC in

the NT soil at the beginning of the experiment. This

was in contrast with DP (24.5 mg ha� 1), HSPD (23.4

mg ha� 1), and HH (25.2 mg ha� 1) (Fig. 2). In 2050,

stocks of TOC in the soils under NT and plowed

systems were estimated as 26 and 15 mg ha� 1,

respectively, thus showing less soil organic matter

degradation under no tillage.

Also in both active and slow pools, carbon stocks

diminished with the change from forest to cropland.

After the introduction of tillage systems, it was

observed less significant changes in the carbon stock

of active and slow pools. However, the amount of

passive carbon stocks in the soils under NT was

significantly higher. In 2000, the amount of organic

carbon in the soil under NT was 5 mg ha� 1 higher

than the soils under DP, HHDP and HH systems. This

difference increased with time and in 2050 the stocks

of TOC in the soil under NT (23 mg ha� 1) were

estimated as two times higher than the amounts in the

soils under DP (12.8 mg ha� 1), HHDP (10.6 mg

ha� 1), and HH (13.2 mg ha� 1). These results show

the effect of soil disturbance by plowing, which favors

higher SOM mineralization rate and thus leads to an

increase in humification and in the passive carbon

pool.

3.2. Estimates of carbon pools by the measured

values

In the beginning of the field experiment, the stock

of TOC had declined to 40 mg ha� 1 (Fig. 3), a

decrease of 37%, compared to the 63.95 mg ha� 1,

initial value measured in the soil under Atlantic

Forest. The measured stocks of TOC were 20% lower

than the Century model equilibrium estimates of TOC

stocks. This difference is probably due to the higher

proportion of passive carbon pool in measured TOC.

As Century was originally developed for temperate

soils it is unlikely to be adjusted to tropical environ-

ments because organic–mineral associations in the

tropics are different from that observed in the temper-

ate grasslands.

Tillage systems did not change the trend to de-

creasing stocks of carbon. Fifteen years after setting

up the field experiment the stocks of TOC in the soils

under NT, DP, HHDP and HH were 38, 32, 31, and 34

mg ha� 1, respectively (Fig. 3). This tendency contin-

ued mainly due to conventional plowed systems and

in 2050 the projected stocks of TOC will be 34 mg

ha� 1 in the soil under NT, and approximately 20 mg

ha� 1 in the soils under DP, HHDP and HH. Despite

these results, soils under NT system were the only

ones to show a slight recovery in long term. Our

results are corroborated by Smith et al. (2001) that

observed at several soils groups and cultures rotation

that no tillage resulted, in long term, in the rate of

TOC gain rising as high as 0.15 mg ha� 1 C year� 1 in

Black Chernozem and Gleysolic soil groups whereas

the conventional tillage showed a loss of TOC.

Apparently, crop succession involving fallow, even

with no tillage, will demand too long time to reach

new equilibrium. This situation, however, may be

changed if a crop rotation involving a cover crop to

improve mulching (e.g. millet) is included or a ley

farming system is adopted. Ley farming involving

grass such as Brachiaria may greatly increase the

stock of soil organic carbon.

In 1984, carbon stocks of the active, slow and

passive pools were 0.2, 2 and 36 mg ha� 1. These

values, compared to those initial values in the soil

under forest, represented a decrease of 87%, 89%, and

19%, respectively (Fig. 3). This diminution trend was

still observed even after the adoption of NT system,

which shows that although the soil has been under NT

for 15 years, no soil disturbance without cover crop

management as pointed out by Machado and Silva

(2001) hardly help to improve an increase of TOC in

acidic tropical soils. As reported by Parton et al.

(1987), Metherell et al. (1993) and Del Grosso et al.

(2001) our results also indicate the high sensitivity of

the active and slow carbon pools to changes in soil

management parallel to a higher stability of the

passive carbon pool. In 2000, compared to the

amounts in the beginning of the experiment, the

carbon stocks of the active pool in the plowed soils

increased approximately 0.3 mg ha� 1. However,

Century simulation for 50 years showed a decrease

in the carbon stocks even in the soil under NT (Fig. 3),

indicating that, as reported by Machado and Silva

(2001), no tillage without cover crop management

(e.g. millet, sun hemp, black oat) hardly improve TOC

content of an acid tropical soil. In 2000, the modeled

carbon stocks of the slow pool in the soils under NT,

DP, HSDP and HH were 2.3, 2.8, 2.9 and 2.7 mg

Fig. 3. Time variation of organic carbon stocks (TOC) (A) and active (B), slow (C), and passive (D) pools simulated by Century based on the

initial values obtained from direct method in the no-till (NT), disc plow (DP), heavy harrow followed by disc plow (HHDP) and heavy harrow

(HH).


ha� 1, respectively (Fig. 3). The passive carbon pool

showed higher differentiation in carbon content

among tillage systems than the active and slow pools.

In 2000, the highest values of soil carbon stocks were

found in the soils under NT (36 mg ha� 1) and HH (31

mg ha� 1) and the lowest amounts were found in the

soils under DP (29.5 mg ha� 1) and HHDP (28 mg

ha� 1). This shows that, after setting up the field

experiment, only the carbon stocks of the soils under

NT did not decrease.

The highest proportion of soil TOC was found in

the passive pool (90%). Similar results were also

reported by Freixo et al. (2002) investigating tillage

and crop rotation interactions on organic carbon

fractions of a Ferralsol from southern Brazil. Increas-

ing proportion of passive carbon pool with simulta-

neous decrease of active and slow carbon pools may

indicate soil organic matter degradation because the

latter pools are highly associated to microbial activity

and decomposition and are the most relevant pools to

nutrient cycling.

3.3. Nitrogen pools by the measured values

The replacement of the Atlantic Forest by agricul-

ture also led to a decrease in the contents of total

nitrogen (TN). In 1984, TN stocks of the soil before

setting up the experiment were 3.34 mg ha� 1. This

corresponds to a 32% decrease relative to the soil

under forest (Fig. 4). Similar to what was observed for

TOC, the stocks of TN decreased in the soils under

NT and HHDP systems. However, this decrease was

Fig. 4. Dynamics of stocks of total nitrogen (TN) (A) and active (B), slow (C) and passive (D) nitrogen pools simulated by Century model at 0–

20 cm depth. NT: no-tillage; HHDP: heavy harrow followed by disc plow.


less pronounced in the soil under NT than under

HHDP. Although N losses in the soil under no-tillage

were lower than those observed under plowed sys-

tems, there is an apparent need to include legume

plants in the crop rotation as cover crops. This would

increase N inputs through biological N fixation in

addition to returning plant residues to the soil to

supplement soil fertilization. The use of cover crops

to improve mulching in the tropics is strongly recom-

mended in a crop rotation system to increase soil

carbon stocks (Machado and Silva, 2001).

From 1930 to 1984, N stocks in the active, slow

and passive pools followed similar trend of what was

observed for C stocks. The close relationship between

C and N can be observed in the nitrogen mineraliza-

tion. Nitrogen pools in the N submodel can only be

mineralized if CO2 is lost in the corresponding pool of

the C submodel, whose decomposition rate can be

regulated by N, P and S availability.

Soil tillage caused an increase of TN of the active

pool of both NT and HHDP systems. However, until

the end of the simulation period, TN stocks decreased

to the same levels found in the beginning of the

experiment. In the N submodel, the active pool

represents the microbial biomass N (Parton et al.,

1987) and it can be assumed that in a short run the

limited amount of available carbon substrates will not

support biomass in soil under NT and HHDP systems.

The slow pool N stocks were also decreased after

deforestation. In 1984, N stock estimated by Century

model was 0.1 mg ha� 1, which is approximately 85%

lower than the amount found in the soil under Atlantic

Forest. In 2000, similarly to what was observed in the

active pool, the N stocks in the slow pool were higher

in the soil under HHDP (0.22 mg ha� 1) than in the

soil under NT (0.1 mg ha� 1). From 2000 to 2050, N

stocks in the soil under NT were projected to tend to

increase up to 0.16 mg ha� 1 while in the soil under

HHDP, N stocks remained stable at 0.18 mg ha� 1

(Fig. 4). Hence, the tendency in the short term is that

the nitrogen stocks in the soil under NTwill be similar

or even higher than those found in the soil under HH.

In the passive pool, the decrease in the N stocks

after changing forest to agriculture was less pro-

nounced than that observed in the active and slow

pool. This is probably due to the high recalcitrance of

the passive pool (Romanya et al., 2000). For the

simulated period, contrary to the values observed in


the active and slow pools, N stocks in the passive pool

were higher in the soil under NT than under HHDP.

After 15 years, soil N stocks were 3.1 mg ha� 1 for

NT system and 2.4 mg ha� 1 for HHDP system.

Estimates for 2050 showed a significant decrease in

nitrogen stocks of soils under HHDP. In this year, N

stocks in the soil under NT were 2.7 mg ha� 1, which

is approximately 47% higher than the N stocks in the

soil under HHDP (Fig. 4).

3.4. Comparison between measured and simulated C

pools using Century model

In 2000, the Century model simulated TOC in

different tillage systems showed similar patterns to

those observed in the measured TOC data, especially

in the soil under NT (Fig. 5). In the soils under DP,

HHDP and HH the stocks of TOC estimated by

Century were higher than those shown by measure-

ments, but differences were 4%, 0.4% and 7%,

respectively. Similar trends were observed dividing

TOC into 70% passive carbon, 27% slow carbon and

3% active carbon. Falloon and Smith (2002), in their

modeling of the arable site at Martonvasar (Hungary)

divided TOC into similar proportions, but resulted in

an overestimation of measured TOC. A reasonable fit

to the measured data was found using 34% passive

carbon, 63% slow carbon and 2.6% active carbon. In

our study, the higher proportion of the passive carbon

pool than the other pools enabled an optimum fit

between measured and Century model values. Com-

pared to labile carbon fractions, in tropical soils the

largest TOC stocks are humified soil organic matter

probably due climatic conditions favoring microbial

decomposition all over the year and also chemical and

physical stability (Bayer et al., 2002; Leite et al.,

2003). It is well known that caolinitic soils, such as

the Acrisol of our study, are originally well structured

with adequate aggregate size distribution to promote

drainage (El-Swaify, 1980). Furthermore, differences

between tropical/subtropical and temperate soils sug-

gest the need for parameterization of the Century

model using regional data and measured carbon pools

thus contributing to more adequate modeling in the

tropics. However, reasonable Century model fit to the

measured TOC values were reported for both temper-

ate (Mikhailova et al., 2000; Alvarez, 2001) and

tropical (Parton et al., 1989; Motavalli et al., 1994)

soils. This shows the potential of Century model to

simulate soil organic carbon changes in soils under

different tillage systems and its sensitivity to distin-

guish organic carbon changes due to different tillage

systems.

Compared to the measured data obtained by mi-

crobial biomass carbon, in all tillage systems, the C

stocks of the active pool simulated by the Century

model were underestimated (Fig. 5). The differences

between simulated and measured data of C stocks

were 70%, 52%, 52% and 51% for NT, DP, HHDP

and HH, respectively. Similarly, Motavalli et al.

(1994) studying forest soils from Colombia, Peru

and Brazil with varying mineralogy reported that the

values of dissolved and biomass carbon stocks were

larger than Century simulated values. In oxidic soils,

Motavalli et al. (1994) reported that the simulated

values were 45%, 51% and 39% higher than the

measured values in Valenc� a, Ouro Preto e Una

(Brazil), respectively. In both studies, the results are

probably related to factors that control C to the active

pool. The Century model uses soil humidity, soil

temperature, soil texture and management as regula-

tors of the active pool. However, besides environmen-

tal aspects, microbial growth is affected by substrate

availability (organic matter) and soil chemical prop-

erties (e.g. soil pH, N content) that are not taken into

account by the model. Also, the mechanisms that

describe C exudation by roots and its microbial

metabolism are not clearly defined by the model and

the lack of these mechanisms in the model may

contribute to its inaccuracy. Additionally, the decom-

position rate of the active pool is likely to be over-

estimated or the kEC value needs to be calibrated for

acid tropical soils. Joergensen (1996) investigated the

effects of soil properties and different form of land use

on the calibration of the kEC value and found that a

kEC value of 0.38 can be recommended for C analysis

by dichromate consumption and a kEC value of 0.45

for that by UV-per sulfate or oven oxidation.

Compared to the measured data, apart from the soil

under HHDP, the C values of the slow pool simulated

by the Century model were underestimated (Fig. 5).

However, differences between measured and simulat-

ed values were small: 13%, 14%, and 16% for NT, DP

and HH systems, respectively (Fig. 5). This suggests

that also in tropical soils, the carbon of the free-light

fraction may represent the slow pool as proposed by

Fig. 5. Measured and simulated total organic carbon (TOC) (A), active (B), slow (C) and passive (D) carbon pool, and total nitrogen (TN) (E) in

different soil management systems. NT: no-tillage; DP: disk plow; HHDP: heavy disk harrow followed by disc plow and light harrowings; HH:

heavy disk harrowing (n= 4 for measured values).


Cambardella and Elliott (1992). On the other hand,

Motavalli et al. (1994) showed that measured stocks

of the free-light organic carbon fraction were lower

than Century simulated values. The discrepancies

between measured and simulated values varied in

soils showing 69% to 83% oxidic mineralogy. Mota-

valli et al. (1994) believed that the slow pool contains

substances other than free light organic carbon or the

extraction procedure is not efficacious to isolate all

free light organic carbon soils.

Underestimation of the Century active and slow

pool may be explained by the lack of important

chemical processes in acid tropical soils not considered

by the model. In tropical and subtropical acid soils, the

organic matter–Al complex is relevant in the control

of the Al toxicity and therefore in the soil organic


matter mineralization (Haynes and Mokolobate, 2001;

Meda et al., 2001). High acidity and high soil alumi-

num content are also responsible for the stabilization

of organic matter in acid tropical soils (Mendonc�a andRowell, 1994; Mendonc�a, 1995). Thus, parameteriza-

tion of simulation models should improve knowledge

about the effect of soil mineralogy, soil pH and soil

exchangeable aluminum on SOM formation and de-

composition in acid tropical soils.

Thus more detailed investigations are needed to

identify the underlying differences between the theo-

retical requirements of the Century pools and those

analytically quantified. Century simulated C stocks of

the passive pool and those estimated by difference

showed similar patterns, especially in the soils under

NT and HHDP. In the soils under DP and HH,

Fig. 6. Relationship between measured and simulated total organic carbon (

nitrogen (E). *, ** indicate significance at the 0.05 and 0.01 probability l

differences between measured and simulated C stocks

were 7% and 10%, respectively. Apart from the soil

under NT, N stock contents estimated by Century

were higher (7–10%) than measured values (Fig. 5).

These trends were also reported by Fernandes (2002)

in an Acrisol from southern Brazil in the no and

conventional tillage as in the native grass.

Regression analysis showed that the Century sim-

ulated TOC stocks correlated well (R2 = 0.91;

p< 0.05) with the measured values and a good 1:1

correspondence between simulated and measured val-

ues (Fig. 6). This indicates that Century model is able

to simulate the TOC dynamic from tropical soil under

different management systems. On the other hand,

correlations among simulated and measured C pools

indicated that only passive C pool correlated signifi-

A), C active pool (B), C slow pool (C), C passive pool (D) and total

evel, respectively; ns = not significant.


cantly (R2 = 0.89; p< 0.05). Thus, it is essential to

conduct more studies on acid tropical soils to optimize

the relationship between the theoretical concepts of

the Century model pools and the measured fractions.

The regression coefficient of measured TN versus

simulated TN has an R2 value of 0.97, but here is

not a good 1:1 correspondence between measured and

simulated TN values.

4. Conclusions

Both active and slow carbon pools were more

sensitive to soil management systems than total or-

ganic carbon and passive carbon pool. This indicates

that active and slow pools can be used as early

warning indicators of soil organic matter degradation.

The Century model simulated changes in the total

organic carbon content and obtained an excellent fit to

measured data (only 5% contrast) and this shows the

high potential of the model to simulate soil organic

matter dynamics in the tropics.

Similarly to the total organic carbon, the Century

model simulated values of passive carbon pool showed

similar patterns to those observed in the measured

data. However, the Century model underestimated

stocks of slow and especially active carbon pool and

thus there is a need to include some important chem-

ical process in the model in acid tropical soils.

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