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Soil & Tillage Research 93 (2007) 77–86

Soil compaction and forest litter amendment affect carbon

and net nitrogen mineralization in a boreal forest soil

Xiao Tan, Scott X. Chang *

Centre for Enhanced Forest Management, 442 Earth Sciences Building, Department of Renewable Resources,

University of Alberta, Edmonton, Alta., Canada T6G 2E3

Received 28 October 2005; received in revised form 15 March 2006; accepted 24 March 2006

Abstract

Mechanical site preparation in forests often results in soil compaction, mixing of forest litter with mineral soil, and/or

displacement of surface organic material in forest ecosystems. We used a 9-month laboratory incubation experiment to examine the

effects of soil compaction and forest litter amendment on microbial biomass C (MBC) and N (MBN), soluble organic C and N, and

C and net N mineralization rates in a boreal forest soil with a silt loam texture. Four treatments: forest litter unamended and

noncompacted (OM0C0, bulk density at 1.1 Mg m�3), forest litter unamended and compacted (OM0C1, 1.5 Mg m�3), forest litter

amended and noncompacted (OM1C0), and forest litter amended and compacted (OM1C1) were applied to the soil. Soil

compaction reduced MBC, MBN, soluble organic C and N on several sampling dates. Carbon mineralization and net nitrification

rates were reduced by soil compaction whether forest litter was amended or not. The total amount of C mineralized from OM0C0,

OM0C1, OM1C0, and OM1C1 in 9-month was 1.8, 1.6, 2.6 and 1.7 mg C g�1 soil, respectively. Forest litter amendment alone

increased MBC in the early stage of the incubation and soluble organic C and N on every sampling date except the initial one. Forest

litter amendment had a positive priming effect on C and net N mineralization and nitrification rates. The total amount of N

mineralized from OM0C0, OM0C1, OM1C0, and OM1C1 was 40, 39, 61, and 59 mg N kg�1 soil, respectively. We conclude that

soil compaction and forest litter amendment influenced microbial properties and processes in this boreal forest soil under controlled

conditions. Our results imply that forest management practices that alter soil porosity (through compaction) and organic matter

distribution in the soil profile can dramatically change soil C and N dynamics that may result in the eventual change in soil C and N

concentrations or availability.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Boreal forest; Carbon mineralization; Soluble organic C and N; Microbial biomass; Net N mineralization

1. Introduction

In boreal forest ecosystems where low soil tempera-

ture and low litter quality prevail, forest management

practices are often challenged by surface accumulation

of forest litter and slow rates of decomposition (Messier

et al., 1995). To enhance stand productivity, mechanical

* Corresponding author. Tel.: +1 780 492 6375;

fax: +1 780 492 1767.

E-mail address: Scott.Chang@ualberta.ca (S.X. Chang).

0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2006.03.017

site preparation after harvesting has been extensively

applied to improve environmental conditions for tree

growth, control weed competition, and stimulate

organic matter decomposition and nutrient release

(Salonius, 1983; McKinnon et al., 2002). Such practices

may incorporate forest litter into the mineral soil

through the use of plows, disc trenchers, choppers, and

crushers (McMinn and Hedin, 1990), and may at the

same time result in soil compaction from the use of

heavy forestry equipment (Greacen and Sands, 1980;

Kozlowski, 1999).

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–8678

Soil compaction increases soil bulk density and

decreases soil porosity and infiltration capacity (Grea-

cen and Sands, 1980; Huang et al., 1996). Changes in

soil physical, chemical, and biological properties are

correlated; for example, soil compaction can shift soil

conditions towards anaerobic state that is associated

with reduced aerobic microbial activities, increased

denitrification rates, and reduced uptake of nutrients as

a prelude to reducing plant growth (Greacen and Sands,

1980; Kozlowski, 1999). In an earlier field-based study,

we found that soil compaction reduced microbial N

immobilization and net nitrification rates in a mesic

aspen (Populus tremuloides Michx.) ecosystem in

northeastern British Columbia (Tan et al., 2005). Dick

et al. (1988) also found that soil compaction reduced

microbial activities, but they indicated that different soil

types might respond to soil compaction differently in

different climatic regimes with regard to alterations in

biological properties. Some boreal forests are fre-

quently subjected to water-logging, and under such

conditions soil compaction after clear-cut harvesting

may not affect microbial activities, since the resident

anaerobes would be well adapted to soil wetness

(Startsev et al., 1998). In general, the effects of soil

compaction on microbial properties and processes in

boreal forest soils are still poorly understood.

Soil organic compounds have different decomposi-

tion rates and form a continuum from labile to

recalcitrant fractions. Addition of labile C substrates

into the soil may increase microbial immobilization of

N (Vitousek et al., 1992) and cause lower net N

mineralization rate shortly after the incorporation of

organic matter (Frey et al., 2003). Messier et al. (1995)

found that adding and mixing the forest floor with

mineral soil had negative effects on nutrient availability

on low productivity sites, but had negligible effects on

high productivity sites when measured 2 and 5 years

after the treatments were applied. Such mixing may

control competing vegetation rather than stimulating

decomposition rates (Messier et al., 1995). Although it

has been hypothesized that adding and mixing forest

litter with mineral soil increases rates of organic matter

decomposition, there is no clear evidence that this is the

case in boreal ecosystems. Understanding N dynamics

in forest soils into which organic material has been

added is important for improving the management of

boreal forest soils.

Various aspects of the relationship between forest

litter manipulation and soil compaction have been

examined. For example, forest litter addition to soil has

been recommended as a means to prevent soil

compaction, to provide nutrients, to increase soil cation

exchange capacity, and to improve water retention in

badly degraded sandy soils (Greacen and Sands, 1980;

Soane, 1990; Kozlowski, 1999). However, we have

not found any work that focuses on the effects of

compaction on biological properties and processes in

boreal forest soils, where forest litter is incorporated

into the soil. Due to complexities such as spatial

variability, and interactions between soil compaction

and temperature and other climatic conditions in the

field (Greacen and Sands, 1980), laboratory experi-

ments can be advantageous. The effects of soil

compaction and forest litter amendment on biological

properties and processes may be best tested under

controlled and ideal temperature and soil moisture

conditions.

Our objective was to investigate the effects of soil

compaction and forest litter amendment and mixing

(referred to as forest litter amendment hereafter) on soil

microbial biomass C (MBC) and N (MBN), soluble

organic C and N, as well as C and net N mineralization

rates. We hypothesized that microbial biomass, soluble

organic C and N, and C and net N mineralization rates

would be significantly lower in soils that are compacted

and/or do not have forest litter amended than in soils

that are noncompacted or have forest litter amended.

2. Materials and methods

2.1. Soil and forest litter

Soil was an Eluviated Eutric Brunisol (Soil

Classification Working Group, 1998) that is a common

forest soil in the boreal forest region of Canada. Parent

material is lacustrine with underlying till material. Soil

(around 10 kg) was randomly collected from the

surface 10 cm of mineral soil after removing the forest

floor (on average 5 cm thick) in a lodgepole pine (Pinus

contorta Dougl. var. latifolia Engelm) stand (538170N,

1168190W) about 35 km south of Edson, in western

Alberta. The site was located in the Lower Foothills

Natural Subregion (Beckingham et al., 1996) and had a

rolling topography, with a slope of 3–5%. The

lodgepole pine stand regenerated naturally on this site

after a wildfire in 1956. The understory was dominated

by feather moss (Pleurozium schreberi) and step moss

(Hylocomium splendens) with the moss layer on

average 6.45 cm deep. The area has a mean annual

temperature of 2.0 8C and mean annual precipitation of

562 mm (Environment Canada, 2002). Forest litter was

collected from a mixedwood stand located in the same

Subregion. Trembling aspen (P. tremuloides Michx.),

white spruce (Picea glauca [Moench] Voss), and

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–86 79

balsam fir (Abies balsamea) were the dominant tree

species. The understory was dominated by bracketed

honeysuckle (Lonicera involucrata), wild red raspberry

(Rubus idaeus), and bunchberry (Cornus canadensis).

The forest floor averaged 8.3 cm thick in this stand. The

forest litter used in this experiment was primarily

composed of needle litter and deciduous foliage, along

with partially decomposed leaf litter material.

After collection, soil and forest litter samples were

placed on ice in a cooler and transported back to

the laboratory. Visible coarse fragments and roots were

removed and the soil was sieved through a 4 mm

sieve before storage in a cool room at 4 8C. The soil

had a silt loam texture, with 21% sand, 67% silt, 12%

clay, 1.40% total organic C, 0.10% total N, and a

pHH2O of 4.05. The forest litter sample had 37.7%

organic C, 1.20% total N, C:N ratio of 31, and a mean

bulk density of 0.17 Mg m�3.

2.2. Treatments and incubation procedures

Two soil compaction levels, no compaction (control,

bulk density at 1.1 Mg m�3, as observed in the field)

and severe compaction (bulk density at 1.5 Mg m�3 as

the upper limit of bulk density at which roots do not

penetrate wet soils very well, e.g., Kozlowski, 1999),

were applied to the soil with or without forest litter

amended. Thus, there were a total of four treatment

combinations: forest litter unamended and noncom-

pacted (OM0C0), forest litter unamended and com-

pacted (OM0C1), forest litter amended and

noncompacted (OM1C0), and forest litter amended

and compacted (OM1C1).

For incubation, 3 g of forest litter was mixed with the

moist soil (the weight of soil varied according to the

bulk density of the treatment, which was 55 and 75 g

(oven-dried weight) for the noncompacted and com-

pacted soil, respectively) and packed into a plastic vial,

4.4 cm in diameter and 3.3 cm deep, following the

method developed by De Neve and Hofman (2000). The

forest litter amendment rate was equivalent to 20 t ha�1

of forest litter. A 0.1 MPa pressure plate (Soil Moisture

Equipment Co., Santa Barbara, CA) was used to

determine the water content at field capacity of the soil

or the soil and forest litter mixture at �15 kPa, for both

the compacted and noncompacted treatments before the

incubation. To do this, three duplicate soil samples of

each treatment were placed into labeled rubber rings on

the plate, saturated overnight, then vacuumed to 15 kPa

for 24 h (Kalra and Maynard, 1991). Soil moisture

content was determined by oven-drying a portion of the

moist soil at 105 8C for 24 h. Using a metal cylinder

with diameter equal to the inner diameter of the vial, the

soil-forest litter mixture was uniaxially compacted to

the desired bulk density for a volume of 50 cm3,

following the method of De Neve and Hofman (2000).

For both compacted and noncompacted soils, cores with

forest litter unamended were also prepared. All the

samples were incubated at a constant room temperature

of 20 8C, and their moisture content was adjusted to

75% field capacity. Soil moisture content was main-

tained at 75% field capacity to reduce gaseous N losses

by denitrification (De Neve and Hofman, 2000). Four

replicate samples were collected from each treatment at

Days 7, 37, 68, 160, and 280.

To determine C mineralization rate, soils were

incubated in sealed 1 L Mason jars with 10 mL of 1 M

NaOH contained in a 50 mL beaker included as a CO2

trap (De Neve and Hofman, 2000). During a 9-month

incubation, NaOH traps were removed monthly and

titrated with 1 M HCl, after adding 5 mL of 1 M BaCl2,

to determine the quantity of CO2 absorbed. After the

CO2 traps were removed, glass jars were flushed with

compressed air to allow replenishment of O2 and water

was added to maintain 75% field capacity. Beakers

containing fresh NaOH were then added. The highest

rate of O2 consumption was 4 mmol 30 d�1 throughout

the incubation period, while a 1 L Mason jar holds about

9 mmol O2. An aerobic condition ought to have been

maintained for each of the monthly incubation periods.

2.3. Chemical analyses and calculations

Soil MBC and MBN were measured using the

chloroform fumigation-extraction method (Brookes

et al., 1985; Vance et al., 1987). Twenty grams of

moist soil samples from each treatment were fumigated

with alcohol-free chloroform for 24 h in an evacuated

desiccator. Fumigated and control (unfumigated)

samples were extracted with 80 mL 0.5 M K2SO4

and shaken for 1 h on a reciprocating shaker. The

extracts were filtered using Whatman No. 42 filter

papers and kept frozen at �18 8C until further analysis.

Extractable C and N were analyzed using a TOC-V

analyzer connected with a TN module (Shimadzu

Corporation, Kyoto, Japan). Soil MBC was calculated

as the difference in extractable C between the fumigated

and control samples divided by a KEC factor of 0.38

(Vance et al., 1987). Soil MBN was calculated as the

difference in extractable N between the fumigated and

control samples divided by a KEN factor of 0.45

(Jenkinson, 1988).

Extracts of the unfumigated soils were also analyzed

for NH4-N concentrations colormetrically using the

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–8680

Fig. 1. Effect of soil compaction and forest litter amendment on (a)

soil microbial biomass C and (b) microbial biomass N during a 9-

month laboratory incubation experiment. Vertical bars are S.E.

(n = 4). Treatment codes were: OM0C0, forest litter unamended

and noncompacted; OM0C1, forest litter unamended and compacted;

OM1C0, forest litter amended and noncompacted; OM1C1, forest

litter amended and compacted.

indophenol blue method (Mulvaney, 1996) and for

NO3�-N concentrations using the vanadium oxidation

method (Doane and Horwath, 2003). Soil soluble

organic C was measured as the extractable C in the

unfumigated samples, as there was no inorganic C in

the extracts based on the soil pH of 4.05 and

experimental test in the laboratory. Soil soluble organic

N was calculated by subtracting the inorganic N

(NH4+ + NO3

�) from the total extractable N in the

unfumigated samples. Net N mineralization rate was

estimated by subtracting the NH4+ and NO3

� measured

initially from those of the current month. Net

nitrification rates were calculated in the same way as

for the net N mineralization rate but based on changes in

NO3� concentrations.

2.4. Statistical analysis

The SAS software (SAS Institute Inc., 1999) was

used to perform all statistical analyses. Some

dependent variables were log (soluble organic C and

N, and C mineralization rate) or square root

transformed (net N mineralization and nitrification

rates) in order to meet the assumptions of normality

and homogeneity. The other dependent variables were

not transformed as their distribution was normal and

error variance was homogeneous. Means presented in

this paper were based on original data. Analysis of

variance was performed to test the effects of soil

compaction and forest litter amendment on soil MBC,

MBN, soluble organic C and N, C and net N

mineralization rates, and net nitrification rates. The

Proc Mixed procedure was used to analyze unbalanced

repeated measures data. Linear regression analysis was

performed to evaluate the relationships between

soluble organic C and MBC, soluble organic C and

MBN, MBN and soluble organic N by the general

linear model (GLM) procedure. The Proc Model

procedure was used to fit the measured data to zero-

and first-order kinetics for unamended and amended

soils, respectively. Differences between model para-

meters were tested using the Proc Mixed procedure

described above. In all comparisons, a = 0.05 was used

as the significance level.

3. Results

3.1. Soil MBC and MBN

In the unamended soil, soil compaction significantly

reduced MBC by 19% on Day 7 and MBN by 24% at

the end of the 9-month incubation (Fig. 1a and b;

Table 1). In the amended soil, compaction significantly

reduced MBC by an average of 26% from Days 37 to

160; MBN by 24% in the first 5-month incubation

period (Fig. 1a and b; Table 1). When there was no

compaction, forest litter amendment significantly

increased MBC and MBN during the first 160 days

of the incubation, but reduced MBC and MBN in the

last sampling date (Fig. 1a and b; Table 1). When there

was compaction, forest litter amendment increased

MBC only initially, then had no effect on either MBC

or MBN for most of the other sampling dates, but

reduced MBC and MBN on the last two sampling dates

(Fig. 1a and b; Table 1). In most cases, MBC and MBN

peaked at Day 68 and thereafter decreased with

incubation time.

3.2. Soil soluble organic C and N

Soil compaction significantly reduced soluble

organic C in the unamended soil and soluble organic

N in the amended soil in the last sampling date (Fig. 2a

and b; Table 1). Forest litter amendment significantly

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–86 81

Table 1

Analysis of variance (P values) of the effects of soil compaction and forest litter amendment on microbial biomass C (MBC), microbial biomass N

(MBN), soluble organic C and N in a 9-month laboratory incubation experiment

Source of variance Microbial biomass C Microbial biomass N Soluble organic C Soluble organic N

Compaction (C) <0.001 <0.001 0.001 0.26

Forest litter amendment (FL) 0.80 0.41 <0.001 <0.001

C � FL 0.02 0.02 0.58 1.00

Time (T) <0.001 <0.001 <0.001 <0.001

C � T <0.60 0.98 0.70 0.66

FL � T <0.001 <0.001 <0.001 <0.001

C � FL � T <0.001 0.001 0.007 0.01

increased soluble organic C and N on every sampling

date except Day 7, regardless of the level of soil

compaction (Fig. 2a and b; Table 1). Soluble organic C

decreased in the first 37 days and then increased till the

end of the incubation time, while soluble organic N

increased with time at different rates in all treatments.

Soluble organic C was negatively related to both MBC

(r2 = 0.36, P = 0.005) and MBN (r2 = 0.29, P = 0.015),

while no relationship was found between soluble

organic N and MBN (r2 = 0.13, P > 0.05).

Fig. 2. Effect of soil compaction and forest litter amendment on (a)

soluble organic C and (b) soluble organic N during a 9-month

laboratory incubation experiment. Vertical bars are S.E. (n = 4).

Treatment codes were: OM0C0, forest litter unamended and non-

compacted; OM0C1, forest litter unamended and compacted;

OM1C0, forest litter amended and noncompacted; OM1C1, forest

litter amended and compacted.

3.3. Carbon mineralization rate

Carbon mineralization in the unamended soil

followed zero-order kinetics, and the rates were

significantly lower in the compacted than in the

noncompacted soil (Fig. 3; Table 2). In the amended

soil, C mineralization followed first-order kinetics.

Soil compaction significantly reduced C mineraliza-

tion rate (P < 0.001) and the amount of readily

mineralizable C (C0) (Fig. 3; Table 2, P = 0.04).

Towards the end of the 9-month incubation period,

cumulative amounts of CO2-C evolved were signifi-

cantly greater in the noncompacted soil with forest

litter amended than in the other treatments, with 1.82,

1.57, 2.61, and 1.70 mg C g�1 soil mineralized from

OM0C0, OM0C1, OM1C0, and OM1C1, respectively

(Fig. 3).

Fig. 3. Measured (symbols, n = 4) and modeled (lines) C mineraliza-

tion rates during a 9-month laboratory incubation experiment after soil

compaction and forest litter amendment treatments were applied

(n = 4). Treatment codes were: OM0C0, forest litter unamended

and noncompacted; OM0C1, forest litter unamended and compacted;

OM1C0, forest litter amended and noncompacted; OM1C1, forest

litter amended and compacted.

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–8682

Tab

le2

Min

eral

izat

ion

rate

const

ants

asaf

fect

edb

yco

mpac

tion

and

fore

stli

tter

amen

dm

ent

So

ilco

mp

acti

on

Fo

rest

litt

eru

nam

end

edF

ore

stli

tter

amen

ded

Cm

iner

aliz

atio

nN

etN

min

eral

izat

ion

Cm

iner

aliz

atio

nN

etN

min

eral

izat

ion

k(m

gC

kg�

1d�

1)

R2

k(m

gN

kg�

1d�

1)

R2

k(d

ay�

1)

C0

(mg

Ck

g�

1)

R2

k(d

ay�

1)

N0

(mg

Nk

g�

1)

R2

No

com

pac

tio

n6

.76

0.9

90

.147

0.9

70

.005

39

20

0.9

90

.007

74

.59

0.9

7

Co

mpac

tio

n5

.90

0.9

30

.140

0.9

90

.004

21

50

0.9

90

.006

76

.51

0.9

7

Car

bon

and

net

Nm

iner

aliz

atio

nra

tes

inunam

ended

trea

tmen

tsfo

llow

edze

ro-o

rder

kin

etic

s(C

(t)

=kt

for

Cm

iner

aliz

atio

n,w

her

eC

(t)is

the

cum

ula

tive

Cm

iner

aliz

ed,an

dk

isth

era

teco

nst

ant,

t

the

tim

e;N

(t)=

ktfo

rN

min

eral

izat

ion,w

her

eN

(t)is

the

cum

ula

tive

Nm

iner

aliz

ed)

and

inam

end

edtr

eatm

ents

foll

ow

edfi

rst-

ord

erk

inet

ics

(C(t

)=

C0(1�

e�kt )

for

Cm

iner

aliz

atio

n,w

her

eC

0is

the

po

ten

tial

lym

iner

aliz

able

C;

N(t

)=

N0(1�

e�kt )

for

Nm

iner

aliz

atio

n,

wher

eN

0is

the

pote

nti

ally

min

eral

izab

leN

).R

2is

the

reg

ress

ion

coef

fici

ent.

Fig. 4. (a) Measured (symbols, n = 4) and modeled (lines) net N

mineralization rates after soil compaction and forest litter amendment

treatments were applied and (b) the effects of the soil compaction and

forest litter amendment on net nitrification rates. Vertical bars in (b)

are S.E. (n = 4). Treatment codes were: OM0C0, forest litter una-

mended and noncompacted; OM0C1, forest litter unamended and

compacted; OM1C0, forest litter amended and noncompacted;

OM1C1, forest litter amended and compacted.

3.4. Net N mineralization and nitrification rates

The general pattern of net N mineralization was

linear in the unamended soil. Soil compaction did not

affect net N mineralization rate (Fig. 4a; Table 2). In the

amended soil, net N mineralization followed first-order

kinetics. There were no differences in net N miner-

alization rate and readily mineralizable nitrogen (N0)

between compacted and noncompacted soils (Fig. 4a;

Table 2). Net N mineralization rate was generally higher

in the amended than in unamended soils throughout the

incubation (Fig. 4a). At the end of 9 months, the total

amount of N mineralized from OM0C0, OM0C1,

OM1C0, and OM1C1 was 40, 39, 61, and 59 mg kg�1

soil, respectively (Fig. 4a).

Soil compaction significantly reduced net nitrifica-

tion rates in the unamended treatment from Day 68 till

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–86 83

the end of the incubation, and reduced total NO3�-N

production by 50% at the end of the incubation time

(Fig. 4b). In the amended soil, compaction significantly

reduced net nitrification rates from Days 68 to 160 of

the incubation, but did not affect total NO3�-N

production (Fig. 4b). Forest litter amendment signifi-

cantly increased net nitrification rates on every

sampling period, regardless of the level of soil

compaction, and increased the amount of NO3�-N

produced by 177% and 429% at the end of the

incubation time in the noncompacted and compacted

treatments, respectively (Fig. 4b).

4. Discussion

4.1. Soil MBC, MBN, soluble organic C and N

Microorganisms are considered the most important

agent for litter decomposition and nutrient cycling

within the soil ecosystem (Wardle, 1992). Microbial

biomass was reported to be an early indicator of changes

caused by disturbances that result from forest manage-

ment practices (Chang et al., 1995). Under controlled

laboratory conditions in this study, we found that soil

compaction often reduced microbial biomass, particu-

larly when forest litter was amended. This was likely

due to poor aeration because compaction reduced air-

filled porosity from 36% to 13% (data not shown). Our

findings were in agreement with a study by van der

Linden et al. (1989) which was carried out in arable

soils in the Netherlands. However, Jensen et al. (1996)

did not detect any direct effect of compaction on

microbial biomass within the range of air-filled pores of

0.01–0.53 m3 m�3 that characterized soil aeration status

from anaerobic to aerobic. They indicated that soil

microbial activity was likely more sensitive to soil

compaction than microbial biomass. When forest litter

was amended, the greater negative effect of soil

compaction on microbial biomass was possibly due

to the aggravated effects of forest litter amendment on

reducing soil aeration in the compacted soil, as the

initially high C mineralization rate (Fig. 3) can quickly

deplete oxygen concentrations in compacted soil.

By adding forest litter to the soil, microbial biomass

in the noncompacted treatment markedly increased in

the early stage of the incubation as compared with the

other treatments, likely because the newly added forest

litter provided easily available C substrates that

stimulated microbial growth. After 5-month incubation,

however, microbial biomass in the forest litter amended

treatments markedly declined and microbial turnover

produced soluble organic C and N (Figs. 1 and 2).

Mineralized N was not leached but accumulated in the

forest litter amended soil. Accumulated mineral N may

have caused an inhibitory effect on microbial growth.

Application of inorganic fertilizer N has been reported

to depress microbial biomass (McAndrew and Malhi,

1992; Thirukkumaran and Parkinson, 2000).

Soluble organic compounds are extracted more

efficiently from soils by salt solutions than by water due

to the greater potential of salt solutions to replace

cations on the exchange sites (Chen et al., 2005).

Soluble organic N (e.g., amino acids) can be directly

taken up by some plants and move easily in or out of the

soil system (Jones et al., 2005; Qualls et al., 2000).

Soluble organic C and N are often regarded as a

potential source of C and N for microbial growth or as

an indicator of microbial turnover (Kalbitz et al., 2000;

Qualls, 2000). Information on the effects of soil

compaction and forest litter amendment on soluble

organic C and N dynamics in forest soils is rather

limited. In our study, soil compaction only reduced

soluble organic C and N at the last sampling date.

Comparing the temporal changes of MBC, MBN, and

soluble organic C and N, it is apparent that MBC and

MBN responded to treatments much faster than soluble

organic C and N. The C:N ratio of soluble organic

matter (between 2.4 and 7.7, data not shown) did not

differ much from the microbial C:N ratio. We also

found that soluble organic C was negatively related to

MBC, indicating that microbial biomass (through its

turnover) may be a source for soluble organic

compounds in the soil (Chen et al., 2005). Forest litter

amendment increased soluble organic C and N because

the added organic material was a direct source (Cronan

et al., 1992).

4.2. C mineralization rate

De Neve and Hofman (2000) found that mineraliza-

tion of added fresh residues was depressed under severe

compaction (bulk densities of 1.5 and 1.6 Mg m�3) in a

loamy sand soil, but was not affected when soil

compaction was less severe. The dramatic reduction in

C mineralization rate caused by soil compaction in this

study supports field measurements where total soil C

content was found to increase after soil compaction

(Powers et al., 2005; Tan et al., 2005). In contrast,

Kaiser et al. (1991) found that soil compaction

increased 14C-CO2 release from a silt loam soil with14C-labeled wheat straw added, owing to the higher

energy demand for C assimilation under anaerobic

conditions that were created by soil compaction. In a

field study, Startsev et al. (1998) found that poor

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–8684

aeration did not inhibit the decomposition of forest litter

in a boreal forest soil and speculated that some

anaerobic microbes might have proliferated under such

conditions.

In the forest litter amended soil, the decomposition

rate constant based on the first-order kinetics model was

close to the lignin decay constant of 0.006 (Paul and

Clark, 1996), and only 4 and 5% of total organic C was

mineralized during the first 10 days, in the noncom-

pacted and compacted soils, respectively. It may have

been desirable to restrict the first-order kinetics model

to the mineralization of litter C and N only (Pluth et al.,

1995). However, we felt that it would be inappropriate

to subtract the C and N mineralized in the unamended

soil from that in the amended soil to derive the C and N

mineralization rate for the forest litter, because of a

possible priming effect of forest litter addition and

variability among replications. We therefore directly

used the first-order kinetics model for C and N

mineralization in the amended soil, similar to that in

Shi et al. (2004), and model fit to data very good

(Table 2). In boreal forest ecosystems, most of the forest

litter added to the soil after timber harvesting would be

forest floor material that has a lower C availability and

rate of mineralization than fresh leaf litter (Korsaeth

et al., 2002).

4.3. Net N mineralization and nitrification rates

The lack of soil compaction effect on net N

mineralization rate in the unamended soil was

consistent with other studies (De Neve and Hofman,

2000; Li et al., 2003; Tan et al., 2005). In one case, the

absence of a significant influence of soil compaction on

net N mineralization rate was attributed to coarse soil

texture (De Neve and Hofman, 2000). In a laboratory

experiment, lower N mineralization rate with compac-

tion was attributed to N loss through denitrification due

to high water content or low air-filled porosity (Jensen

et al., 1996). At the end of our incubation period, a

significantly greater amount of NO3�-N was produced

in the noncompacted soil (Fig. 4b), indicating that

compaction limited the activities of nitrifiers due to O2

deficiency. De Neve and Hofman (2000) pointed out

that nitrification only occurs under aerobic conditions,

assuming that denitrification rates were low at 75%

water holding capacity. Adding forest litter to the soil

did not change the effect of soil compaction on net N

mineralization rate (Fig. 4a). This rejects our hypothesis

that net N mineralization rate would be lower in

compacted than in noncompacted soils with forest litter

amendment. However, a few studies have found that soil

compaction significantly reduced net N mineralization

rate in organic matter amended treatments (van der

Linden et al., 1989; Breland and Hansen, 1996; De Neve

and Hofman, 2000). Breland and Hansen (1996)

reported that soil compaction reduced mineralization

rate of 15N-labeled clover by 18%, owing to the

increased physical protection of organic material and

microbial biomass against nematode attack. Unexpect-

edly, the effect of compaction on NO3�-N production

was more pronounced in the unamended soil than in the

forest litter amended soil at the end of the 9-month

incubation. The high N concentration in the litter

amended soil might have retarded decomposition by

inhibiting microbial activities, which offset the effect of

compaction on NO3�-N production.

When forest litter is added to soil, net N mineraliza-

tion can be affected in both directions. Frey et al. (2003)

found that when forest floor material was added to the

upper 2–3 cm of mineral soil, it did not increase NH4+ or

NO3� availability; rather, it resulted in lower net N

mineralization, possibly due to increased N immobiliza-

tion. Messier et al. (1995) reported that forest litter and

mineral soil mixing did not affect decomposition and

mineralization rate in a western hemlock (Tsuga

heterophylla (Raf.) Sarg.) stand in British Columbia.

They suggested that, rather than increasing nutrient

availability, mixing may contribute to the control of

competing vegetation, and thus promote tree growth. In

general, adding organic material with a high C:N ratio to

soil will result in N immobilization (Paul and Clark,

1996). The organic material used in our study had a C:N

ratio of 31, which should have been low enough to

expect net N mineralization. Forest residues with a high

content of lignin, which is not easily incorporated into

microbial biomass, can also result in net N mineraliza-

tion at a C:N ratio as high as 50 (Paul and Clark, 1996).

Remineralization of recently immobilized N in dead

microbes is considered to contribute to net N miner-

alization in forest soils (Chang et al., 1997; Chang and

Preston, 1998). In a laboratory experiment, Pluth et al.

(1995) found that the remineralization of freshly

immobilized N from harvest litter was seven times

greater than from native forest litter. The laboratory

incubation conditions in our study were not realistic of

year-round field conditions in boreal forests, but

simulated soil temperature conditions in the mid

growing season and may provide directions for further

research. For example, a model appropriate for gauging

residual N mineralization of boreal forest soils needs to

be considered to better understand the dynamics of N in

boreal forest soils and how dynamics could be affected

by forest management practices.

X. Tan, S.X. Chang / Soil & Tillage Research 93 (2007) 77–86 85

5. Conclusions

This 9-month laboratory incubation experiment

demonstrated that soil compaction had a negative

effect on microbial biomass C and N, soluble organic C

and N, C mineralization and net nitrification rates.

Prevention and amelioration of soil compaction need to

be considered by forest managers. Forest litter

amendment increased microbial biomass initially and

soluble organic C and N throughout the incubation. A

positive effect on C and net N mineralization and

nitrification rates was found in the forest litter amended

treatment. Under field conditions, forest litter addition

and mixing through site preparation may control weed

competition by destroying the roots of the non-crop

vegetation; however, by stimulating microbial growth

and increasing nutrient availability in the early stage,

forest litter addition and mixing may increase the

competition of understory vegetation for nutrients with

crop trees. Further studies are needed to verify the

relationships found in this laboratory experiment with

applications under field conditions.

Acknowledgements

We thank the Faculty of Graduate Studies and

Research, and the Department of Renewable Resources

at the University of Alberta for financial support in the

form of a graduate scholarship, Natural Sciences and

Engineering Research Council of Canada (NSERC), the

Canadian Foundation of Innovation (CFI), the Weyer-

haeuser Company, and Weldwood of Canada Ltd. for

funding the research. We acknowledge the comments

from two anonymous reviewers and the editor that help

improve a previous version of the manuscript.

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