Patterns of Carbon, Nitrogen and Phosphorus Dynamics in Decomposing Foliar Litter in Canadian Forests

Patterns of Carbon, Nitrogen andPhosphorus Dynamics in

Decomposing Foliar Litter inCanadian Forests

T. R. Moore,1* J. A. Trofymow,2 C. E. Prescott,3 J. Fyles,4 B. D. Titus2, andCIDET Working Group

1Department of Geography and Centre for Climate and Global Change Research, McGill University, Montreal, Quebec, Canada;2Pacific Forestry Centre, Canadian Forest Service, Victoria, British Columbia, Canada; 3Faculty of Forestry,

University of British Columbia, Vancouver, British Columbia, Canada; 4Department of Natural Resource Science,

McGill University, Montreal, Quebec, Canada

ABSTRACT

We examined the patterns of nitrogen (N) and

phosphorus (P) gain, retention or loss in ten foliar

tissues in a litterbag experiment over 6 years at 18

upland forest sites in Canada, ranging from sub-

arctic to cool temperate. N was usually retained in

the decomposing litter until about 50% of the

original C remained. The peak N content in the

litter was observed at between 72 and 99% of the

original C remaining, with C:N mass quotients

between 37 and 71 (mean 55). The rate of N re-

lease from the litters was not related to the original

N concentration, which may be associated with the

generally narrow range (0.59–1.28% N) in the

litters. P was immediately lost from all litters, ex-

cept beech leaves, with critical litter C:P mass

quotients for P release being in the range 700–900.

The rate of P loss was inversely correlated with the

original litter P concentration, which ranged from

0.02 to 0.13%. The soil underlying the litterbags

influenced the pattern of N and P dynamics in the

litters; there were weak correlations between the N

and P remaining at 60% C remaining in the litters

and the C:N and C:P quotients of the surface layer

of the soil. There was a trend for higher N and P

retention in the litter at sites with lower soil C:N

and N:P quotients, respectively. Although there

was a large variation in C:N, C:P and N:P quotients

in the original litters (29–83, 369–2122 and 5–26,

respectively), and some variation in the retention

or loss of N and P in the early stages of decompo-

sition, litters converged on C:N, C:P and N:P quo-

tients of 30, 450 and 16, when the C remaining fell

below 30%. These quotients are similar to that

found in the surface organic matter of these eco-

systems.

Key words: litter; forests; decomposition; carbon;

nitrogen; phosphorus; lignin.

INTRODUCTION

The continual, long-term input of fresh litter to the

surface of the forest floor through litterfall and its

ensuing progressive decomposition as it is broken

down physically and chemically into smaller units

leads to the formation of more recalcitrant soil or-

ganic material (humus) that accumulates and

forms the different horizons in the forest floor. As

fresh litter decays along this continuum that leads

to the formation of soil organic matter (Melillo and

Received 5 March 2004; accepted 5 November 2004; published online 30

January 2006

*Corresponding author; e-mail: [email protected]

Ecosystems (2006) 9: 46–62DOI: 10.1007/s10021-004-0026-x

46

others 1989), a point is reached when there is a net

release of nutrients. This loss of nutrients from

decomposing litter is a key process governing the

availability of nutrients in ecosystems. The mass

loss of forest litters is primarily dependent on cli-

mate and initial litter quality (for example,

Meentemeyer 1978; Trofymow and others 2002).

Mass loss is approximately proportional to the loss

of carbon (C) from litter, which has indirect effects

on nutrient availability, especially of nitrogen (N)

and phosphorus (P). As the availability of N and

sometimes P generally limits tree growth in forest

ecosystems (for example, Binkley 1986), an

understanding of the factors that control release of

these two nutrients from decomposing litter is

necessary for predicting or modelling nutrient

availability and ecosystem function. Although

many studies have documented changes in C, N

and P masses and concentrations as litter decom-

poses, they have generally been based on a narrow

range of litter types incubated in the field for a

relatively short time (several years) on a small

number of sites. CE Prescott (in press) has recently

reviewed our understanding of decomposition and

mineralization of nutrients from litter and humus.

The rate of nutrient loss from litter can be de-

scribed in relation to the rate of C loss: some ele-

ments are lost faster than C is mineralized, some in

proportion with C and some more slowly than C.

After a brief leaching phase, N is usually retained

relative to C loss during the initial stages of litter

decomposition. In some ecosystems, this leads to an

increase in the total quantity of N, indicative of net

import of N into the litter (Staaf 1980a, b). The

overall result is generally (but not always; see

Tripathi and Singh 1992) a gradual increase in N

concentration and concomitant decrease in the C:N

mass ratio or quotient over time (Staaf 1980a, b;

Vesterdal 1999). Although the initial retention of N

in litter can be a function of the requirements of an

increased biomass of decomposer organisms (Berg

and Soderstrom 1979), the accumulation of N in

decomposing litter can also be a function of

chemical immobilization. For example, ammonia-N

can be chemically fixed to litter and humus, and

has a particular affinity for fresh litter and litter

with low N concentration (Axelsson and Berg

1988). Patterns of P release are more variable, with

both initial loss (for example, Monleon and

Cromack 1996; Titus and Malcolm 1999) and initial

retention (for example, Staaf and Berg 1982; Berg

and Laskowski 1997) widely reported, depending

on litter type and site (for example, Bartos and

DeByle 1981; Kelly and Beauchamp 1987). We

therefore hypothesize (H1) that N will be generally

gained during early stages of decomposition,

whereas P will be gained or be lost.

Changes in N and P and loss of C leads to a con-

vergence of the C:N and C:P quotients of litters over

time (Prescott and others 1993; Rustad 1994;

Vesterdal 1999) until a critical value is reached after

which net loss of N or P from litter begins. Until this

point is reached, the main decomposer communi-

ties in the soil are unable to obtain enough N or P

from the litter to be able to consume the associated

C they require for energy, and are hence N- or P-

limited. After this critical value, they are able to take

up C, concomitantly meet their N or P require-

ments, and in the process release these nutrients

through turnover of decomposer populations. As

they are released, these nutrients become available

for uptake by organisms in the forest floor, includ-

ing trees, through roots and mycorrhizal associa-

tions. The rate of litter decomposition until critical

values are reached is a key determinant in defining

site productivity: the faster the change in quality of

decomposing litter (that is, C:N quotient), the

greater the nutrient availability and associated site

productivity. Critical C:N quotients for forest foliar

litters can be much higher than the 25–30 generally

reported for agricultural residues (Allison 1973;

Haynes 1986), and can range between 23 and 35

(Edmonds 1979, 1980) to 80 (Hart and others

1992). Critical C:P quotients for foliar litters range

from 350 (Rustad and Cronan 1988) to 480 (Gosz

and others 1973). We thus hypothesize (H2) that

critical C:N and C:P quotients at which N and P loss

begins will vary among litter types.

As some litter contains greater fractions of re-

calcitrant material that do not readily decay,

changes in the C:N and C:P quotients of the non-

recalcitrant fraction may be sufficient for net

nutrient release to begin. However, the presence of

the recalcitrant fraction will increase the critical

value. We therefore further hypothesize (H3) that

litters with high concentrations of recalcitrant

materials will have higher critical C:N quotients.

The amount of N and P either retained or lost can

be closely related to the initial concentrations of

these nutrients in the litter. Generally, litters with

low initial concentrations of N or P retain more of

these nutrients during the early stages of decay (for

example, Edmonds 1980) before critical C:N or C:P

values are reached. Conversely, litters with high

initial concentrations of N or P have the greatest

initial loss of these nutrients (for example, Berg and

Ekbohm 1983; Stohlgren 1988), because critical

values have either been attained before litterfall, or

earlier in the decomposition process once litter

reaches the soil surface. We therefore hypothesize

C, N and P Dynamics in Decomposing Litter 47

that (H4) the initial concentration of N or P in litter

will influence patterns of nutrient loss.

The nature and amount of the organic constitu-

ents of litter can also affect nutrient cycling pat-

terns. For example, a strong positive relationship

between the initial concentration of the acid un-

hydrolyzable residue (AUR, also known as the acid-

insoluble residue or ‘‘Klason lignin’’ fraction; see

Preston and others 2000) of litter and the amount

of N retained during decomposition has been re-

ported (McClaugherty and others 1985; Upadhyay

and Singh 1985 and references therein; Berg and

Staaf 1987; White 1988). N loss is also delayed in

litter with a high lignin concentration (Berg and

McClaugherty 1989). It is thought that changes in

N in litter may therefore occur through its com-

plexing with organic molecules in the litter, and

hence (H5) we also hypothesize that the initial

AUR (that is, ‘‘Klason lignin’’) concentration of our

litter may affect N accumulation.

Finally, patterns of gain and loss of N and P can

also be influenced by exogenous availability from

the site, as suggested by the increased N content

found in decomposing litter on relatively N-rich

forest floors (Chadwick and others 1998) or on

fertilized (Berg and Tamm 1994) or clearcut (Berg

and Ekbohm 1983) sites. This suggests that these

site effects are not universal (Prescott and others

1993), and site may not influence N and P

dynamics in litter as much as the initial N and P

status of the litter (Vesterdal 1999). We therefore

hypothesize that (H6) N or P availability in the

surface soil will have smaller effects on nutrients in

litter than initial litter composition, but nutrients

will tend to be retained more in litter on nutrient-

rich sites across Canada.

We used litterbags to monitor decomposition of

ten litter types with different characteristics incu-

bated in situ for 6 years at 18 well-drained upland

sites in a range of different ecosystems across

Canada as part of the Canadian intersite decom-

position experiment (CIDET). This study was

established in 1992 to determine the controls on

long-term litter mass loss and nutrient dynamics

using 11 litter types on 21 sites across a broad range

of forested ecoclimatic regions in Canada (Trofy-

mow and others 1995; Trofymow and CIDET

Working Group 1998). CIDET therefore offers a

unique opportunity to broadly test hypotheses

regarding factors controlling nutrient dynamics in

decomposing litter. Three- and 6-year mass loss

results have been published (Moore and others

1999; Trofymow and others 2002), as has a study of

a wide variety of litter quality variables on the 3-

year mass loss (Preston and others 2000). CIDET

therefore allowed us to determine if there were

general relationships of C, N and P changes over a

relatively long time (6 years) that would increase

our understanding of the controls on nutrient

availability in forested ecosystems.

METHODS

About 11,000 litterbags containing 12 material

types (ten foliar litters and surface and buried wood

blocks) were installed at 21 locations (18 upland,

three wetland sites), representing the major eco-

climatic provinces (Ecoregions Working Group

1989) of Canada (see Trofymow and CIDET

Working Group 1998 for details). Some of the

characteristics of the ten foliar litter types and 18

upland sites used in this present study are listed in

Tables 1 and 2.

The ten foliar litter types were collected from

litter traps or senescent tissues. Litters were air-

dried, thoroughly mixed and sub-sampled to

determine air-dry to oven-dry mass conversion

factors. Sub-samples were milled to pass through a

0.2-mm mesh prior to elemental analysis (C, N and

P) and wet chemical proximate analysis of C frac-

tions (Trofymow and others 1995).

Litterbags were made of 20 cm · 20 cm poly-

propylene fabric with 0.25 mm · 0.5 mm openings

and contained 10 g of litter. Each litter type was

placed at all sites, in four replicate plots per site.

Bags were placed so that they were in contact with

the forest floor, where present, and thick standing

grass or lichen layers were moved aside. One bag

per litter type was removed from each plot once per

year, over the first 6 years, giving four replicates

per year for each litter type at each site. After col-

lection, litterbags were oven-dried at 70�C, the

remaining litter weighed and percent mass

remaining calculated. The exponential decay

parameter (k y)1, Olson 1963) was calculated from

the linear regression of the natural log of mass

remaining over time. The samples were ground and

analyzed for C, N and P concentrations and the

percent of original mass of C, N and P remaining

was calculated, on an ash-included basis. All quo-

tients (C:N, C:P and N:P) were calculated on an

elemental mass basis. The amount of N and P

remaining at the end of each of the first 6 years of

field incubation (as percent of original mass) was

plotted over the amount of C remaining (as percent

of original mass). This elucidated patterns of gain,

loss or retention of N and P relative to C, inde-

pendent of decay rate. Functional interpretations of

general stoichiometric relationships could then be

made without the confounding influence of decay

48 T. R. Moore and others

Tab

le1.

Init

ial

Com

posi

tion

of

the

Ten

Lit

ter

Types

(fro

mTro

fym

ow

an

dC

IDE

TW

ork

ing

Gro

up

1988)

an

dM

ass

Rem

ain

ing

an

dE

xpon

en

tial

Deco

mposi

tion

Para

mete

r(k

)over

6Y

ears

,A

vera

ged

acr

oss

the

18

Sit

es

Init

ial

litt

er

com

posi

tion

Deco

mp

osi

tion

Lit

ter

typ

e

Lit

ter

cod

eC

(%)

N(%

)

C:N

qu

oti

en

tP

(%)

C:P

qu

oti

en

t

N:P

qu

oti

en

t

AU

R

(%)

AU

R:N

qu

oti

en

t

AU

R:P

qu

oti

en

t

6-y

ear

mass

rem

ain

ing

(%of

ori

gin

al)

6-y

ear

deca

y

para

mete

r

(k,

y)

1)

Tre

mbli

ng

asp

en

leaves

(Pop

ulu

str

emu

loid

esM

ich

x.)

Asp

en

46.8

0.6

770

0.1

27

369

5.3

14.4

22

113

42

)0.1

40

Am

eri

can

beech

leaves

(Fa

gus

gra

nd

ifol

iaE

rh.)

Beech

47.0

0.7

166

0.0

36

1306

19.7

28.0

39

778

51

)0.1

17

Wh

ite

bir

chle

aves

(Bet

ula

pa

pyr

ifer

aM

ars

h.)

Bir

ch48.0

0.7

267

0.0

38

1263

18.9

24.0

33

632

34

)0.1

84

West

ern

redce

dar

needle

s

(Th

uja

pli

cata

Don

ex

D.D

on

)

Cedar

49.7

0.6

478

0.0

47

1057

13.6

35.6

56

757

53

)0.1

18

Bra

cken

fern

(Pte

rid

ium

aq

uil

inu

m(L

.)K

uh

n)

Fern

46.3

0.8

853

0.0

66

702

13.3

32.9

37

498

44

)0.1

44

Fesc

ue

gra

ss

(Fes

tuca

ha

lii

(Vase

y)

Pip

er)

Fesc

ue

43.8

0.7

162

0.0

58

755

12.2

11.2

16

272

32

)0.1

73

Dou

gla

s-fi

rn

eedle

s

(Pse

ud

o-ts

uga

men

zies

ii

Mir

b.

Fra

nco

)

Fir

49.6

0.7

070

0.1

05

472

6.7

30.3

43

289

46

)0.1

44

Jack

pin

en

eedle

s

(Pin

us

ba

nk

sia

na

Lam

b.)

Pin

e49.7

1.2

839

0.1

27

391

10.1

32.8

26

258

42

)0.1

48

Bla

cksp

ruce

needle

s

(Pic

eam

ari

an

aM

ill.

BSP)

Spru

ce49.5

0.7

368

0.0

79

627

9.2

28.3

39

358

36

)0.1

83

Tam

ara

ckn

eedle

s

(La

rix

lari

cin

a(D

uR

oi)

K.K

och

)

Tam

ara

ck48.8

0.5

983

0.0

23

2122

25.7

24.0

41

1770

48

)0.1

25

Qu

otie

nts

are

base

don

mass

.


Tab

le2.

Loca

tion

,E

cocl

imati

cPro

vin

cean

dSu

rface

Soil

Ch

em

istr

yof

the

18

Upla

nd

Sit

es

an

dM

ass

Rem

ain

ing

an

dE

xpon

en

tial

Deco

mposi

tion

Para

mete

r(k

)over

6Y

ears

,A

vera

ged

acr

oss

the

Ten

Lit

ters

(fro

mTro

fym

ow

an

dC

IDE

TW

ork

ing

Gro

up

1998)

Sit

elo

cati

on

an

d

acr

on

ym

Eco

clim

ati

c

pro

vin

ce

Lati

tud

e(�

N)

&lo

ngit

ud

e(�

W)

Soil

hori

zon

Soil

pH

aC

(%)

N

(%)

C:N

qu

oti

en

t

P

(%)

C:P

qu

oti

en

t

N:P

qu

oti

en

t

6-y

ear

mass

rem

ain

ing

(%of

ori

gin

al)

6-y

ear

deca

y

para

mete

r

(k,

y)

1)

Rock

yH

arb

ou

r,B

ore

al

49�3

20 ,

57�5

00

Oi–

Oe–O

a–

24.4

0.8

136

0.0

69

405

11.7

32

)0.1

76

NF

(CB

R)

E4.3

5.5

0.2

127

0.0

40

137

5.3

Ch

aple

au

,B

ore

al

47�3

80 ,

83�1

40

Oi–

Oe–O

a–

35.7

1.0

235

0.1

07

415

9.5

29

)0.2

06

ON

(CH

A)

E4.3

1.7

0.0

723

0.0

22

76

3.2

Gan

der,

Bore

al

48�5

50 ,

54�3

40

Oi–

Oe–O

a–

45.8

0.7

462

0.0

86

678

8.6

35

)0.1

76

NF

(GA

N)

E3.8

1.9

0.0

632

0.0

07

289

8.6

Gil

lam

,Su

barc

tic

56�1

90 ,

94�5

10

Oi–

Oe–O

a–

38.3

1.0

536

0.0

67

570

15.6

61

)0.0

77

MN

(GI1

)A

&B

w6.6

4.6

0.2

122

0.0

53

87

4.0

Hid

den

Lake,

Sou

thco

rdil

lera

n50�3

30 ,

118�5

00

Oi–

Oe–O

a–

38.8

1.1

235

0.0

98

397

11.4

31

)0.1

88

BC

(HID

)E

&B

s5.9

2.0

0.0

728

0.1

49

13

0.5

Inu

vik

,Su

barc

tic

68�1

90 ,

133�3

20

O–

41.7

0.9

743

0.1

51

276

6.4

77

)0.0

42

NW

T(I

NU

)A

5.2

2.7

0.1

617

0.3

26

80.5

Kan

an

ask

is,

Sou

thco

rdil

lera

n51�0

00 ,

115�0

00

Oi–

Oe–O

a38.3

1.1

633

0.1

04

370

11.2

43

)0.1

33

AB

(KA

N)

A5.0

1.3

0.0

720

0.0

23

58

3.0

Morg

an

Arb

ore

tum

,C

ool

tem

pera

te45�2

50 ,

73�5

70

Oi–

Oe–O

a–

31.6

1.1

328

0.0

80

394

14.1

23

)0.2

51

QC

(MA

R)

AE

4.0

4.4

0.1

726

0.0

20

219

8.5

Mon

tmore

ncy

,B

ore

al

47�1

90 ,

71�0

80

Oi–

Oe

–43.6

0.9

347

0.1

51

289

6.2

34

)0.1

63

QC

(MO

N)

E4.1

1.2

0.0

815

0.0

10

117

8.0

Nels

on

Hou

se,

Bore

al

55�5

50 ,

98�3

70

Oi–

Oe

–30.7

0.5

358

0.0

57

538

9.3

56

)0.0

90

MN

(NH

1)

Ej

4.8

0.6

0.0

321

0.0

09

60

3.3

Pri

nce

Alb

ert

,B

ore

al

53�1

30 ,

105�3

80

Oi–

Oe–O

a–

28.1

0.6

047

0.0

57

492

10.5

56

)0.0

95

SK

(PA

L)

AE

6.2

1.8

0.0

921

0.0

21

88

4.3

Peta

waw

a,

Cool

tem

pera

te45�5

50 ,

77�3

50

Oi–

Oe–O

a–

41.9

1.2

234

0.1

02

411

12.0

34

)0.2

24

ON

(PE

T)

E4.1

3.6

0.1

036

0.0

24

147

2.5

Port

McN

eil

,B

C(P

ML)

Paci

fic

cord

ille

ran

50�3

60 ,

127�2

00

Oi–

Oe

–47.0

1.1

242

0.0

66

717

17.0

31

)0.1

76

Oa

3.5

40.7

0.9

244

0.0

51

802

18.0

Sch

eff

erv

ille

,Q

C(S

CH

)Su

barc

tic

54�5

20 ,

66�3

90

Oi–

Oe–O

a–

36.6

0.7

648

0.0

79

465

9.6

48

)0.1

18

Bw

4.9

1.9

0.0

921

0.0

37

51

2.4

Sh

aw

nig

an

Lake,

Paci

fic

cord

ille

ran

48�3

80 ,

123�4

20

Oi–

Oe–O

a–

41.2

0.8

549

0.1

18

349

7.2

27

)0.2

11

BC

(SH

L)

E&

Bh

5.1

3.4

0.0

939

0.0

94

36

1.0

Term

un

dee,

SK

(TE

R)

Tra

nsi

tion

al

51�5

00 ,

104�5

50

Oi–

Oe–O

aA

–15.0

0.9

017

0.0

97

155

9.3

47

)0.1

27

gra

ssla

nd

A7.0

4.1

0.3

313

0.0

51

81

6.5

Tople

y,

BC

(TO

P)

Sou

thco

rdil

lera

n54�3

60 ,

126�1

80

Oi–

Oe

–39.7

1.0

538

0.1

56

254

6.7

38

)0.1

56

E&

Bw

4.8

2.7

0.1

124

0.0

69

39

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RESULTS

Litter and Site Characteristics

There was little variation in the C and N concen-

trations of the ten initial litters (Table 1), with only

pine needles substantially richer in N (1.28%) than

the other litters (0.59–0.88%). Consequently, the

range of C:N mass quotients was narrow (53–83),

except for the pine needles (39). The range of initial

P concentration was larger (0.023–0.127%), with

C:P quotients from 369 to 2,122. The initial N:P

mass quotients ranged from 5.3–25.7. There was

also a large variation in Ca concentration (0.37–

4.55%) and AUR (acid-unhydrolyzable residue, or

‘‘Klason lignin’’, 11–36%), the latter resulting in

AUR:N quotients of between 16 and 56.

The large range of vegetation and climate at the

18 sites are presented in Table 2. The surface or-

ganic horizons (usually Oi–Oe–Oa) had N concen-

trations ranging from 0.53 to 1.20%, resulting in

C:N quotients of between 28 and 62 (Table 2). Total

P concentration of the uppermost horizon was also

variable, ranging from 0.057 to 0.156%, with C:P

quotients ranging from 254 to 717. Quotients of N:P

in the upper forest floor ranged from 6.2 to 17.0. In

the underlying organic-mineral horizon (usually A

and AE), the soils are generally acid (pH in water

ranging from 3.5 to 6.0) except for the Gillam (GI1),

Prince Albert (PAL) and Termundee (TER) sites,

where pH was between 6.2 and 7.0. In these A, AE

or E horizons, C:N and C:P quotients ranged from 13

to 39 and 8 to 289, respectively.

Litter Mass Loss

The litter mass after 6 years had declined to an

average (across all sites) of 32% (fescue) to 53%

(cedar) of the original mass (Table 1). The average

exponential decay (k) rates over the 6 years ranged

from )0.117 (beech) to )0.184 (birch). Expressed

as the average value for the ten litters at each site,

the slowest rates of decomposition were at Inuvik

(INU), with 77% original mass remaining and a k

value of )0.042, and the fastest rates were at

Morgan Arboretum (MAR), with 23% mass

remaining and a k value of )0.251 (Table 2).

N and P Dynamics

We first present patterns of N and P content relative

to C content (all as percent of original content) in

the decomposing litters by combining the annual

collection data for each of the ten litters from all 18

sites (Figure 1). We examine whether critical C:N

and C:P quotients can be determined at which

there is net loss of N and P and what litter prop-

erties influence the rate of N and P loss, relative to

C loss, from the decomposing litters. The effect of

site is embedded in these patterns, and will be

treated later.

Aspen (Figure 1A) retained N relative to C,

whereas P was initially lost rapidly. The relationship

between N and C was poorly represented by a

polynomial regression (r2 = 0.20), reflecting the

variation across sites and the relatively small change

in N content (Table 3). The relationship between P

and C was well represented by the polynomial

regression (r2 = 0.85). Beech (Figure 1B) retained

P, even in litters that lost more than 50% of their C,

and there was a minor loss of N but data were highly

variable, resulting in poor polynomial regressions

(r2 = 0.10 and 0.04). Birch (Figure 1C) had similar

patterns of both N and P loss, but there was greater

variability in the P loss pattern, with r2 values of

0.72 (N) and 0.31 (P). Cedar (Figure 1D) showed

patterns of N and P similar to those for fern, but with

a great deal of variability and low r2 values (0.18

and 0.20). In fern, N and P were lost at a similar rate

with r2 values of 0.64 and 0.46, respectively (Fig-

ure 1E). Fescue (Figure 1F) lost a large proportion

(40%) of its C content in the first year at all but two

sites, and then lost both N and P at a rate similar to

the loss of C (r2 = 0.57 and 0.66 for N and P,

respectively). The patterns in fir (Figure 1G) were

similar to those of aspen, with N retention and rapid

initial loss of P (r2 = 0.67 and 0.63), which was

similar to the C loss rate over 6 years. In pine (Fig-

ure 1H), P was lost at a faster rate than N (r2 =0.72

and 0.82 for N and P, respectively). Spruce (Fig-

ure 1I) lost N more slowly than C, while P loss rate

was close to that of C and both patterns were well

represented by polynomial regressions (r2 = 0.59

and 0.78, respectively). Tamarack (Figure 1J) re-

tained N and to a lesser extent P, but there was a

great deal of variability across sites and regressions

were not strong (r2 = 0.24 and 0.11 for N and P,

respectively).

We used the polynomial regressions (Table 3) to

predict the C content remaining at the maximum

N or P content, and then established the critical

C:N or C:P quotient at that N or P value, indi-

cating the point after which there was a net loss of

N or P. Where there was a loss of N or P from the

beginning, the critical C:N or C:P value for loss

was taken to be greater than the initial litter va-

lue.


Figure 1. Continued.


Nine of the ten litters had maximum N contents

when C content remaining was between 72 and

99%, with the fern showing an overall loss of N from

the beginning (Table 3). These maximum N and

associated C values gave critical C:N quotients

ranging from 37 to 71, with an average of 55 (close to

the initial C:N quotient of the fern). Only four of the

litters (beech, birch cedar and tamarack) showed

evidence of retention of P, with critical C:P quotients

for maximum P values ranging from 650 to 904. In

the other six litters which lost P from the outset, the

initial C:P quotients ranged from 369 to 702.

We hypothesized that the loss of N and P would

be related to the composition of the initial litter,

especially the C:nutrient and AUR:nutrient quo-

tients. For instance, litters with high initial C:N

quotients would have a greater proportion of their

initial N at a given decay state than would litters

with low initial C:N quotients. To examine the rate

with which N and P was lost from the decomposing

litters relative to C content, we used the polyno-

mial regressions to predict the N and P remaining

when 25, 50 and 75% of the original C content was

remaining, across all sites (Figure 2). There was no

Figure 1. Change in C, N and P contents in each litter type over 6 years for all sites: (A) aspen leaves, (B) beech leaves,

(C) white birch leaves, (D) western redcedar needles, (E) bracken fern, (F) fescue grass, (G) Douglas-fir needles, (H) jack

pine needles, (I) black spruce needles, (J) tamarack needles. Solid circles and bold line represent N and open circles and thin

line represent P (n = 126 in each case). Dashed line represents equal losses of C and N or P. Polynomial regressions are

defined in Tables 4 and 5.


Table 3. Relationship between C Remaining (Percent of Original) and N Remaining (Percent of Original) inDecomposing Litter, by Type Across all Sites, as Described by Polynomial Regression

Litter type Regression r2 Crem (%) C (%) N (%) C:N quotient

Aspen N = )0.0081C2 + 1.27C + 51.2 0.20* 79 37 0.68 54

Beech N = )0.0098C2 + 1.55C + 40.5 0.10 97 46 0.70 66

Birch N = )0.0232C2 + 3.32C – 2.3 0.72*** 72 34 0.84 41

Cedar N = )0.0111C2 + 1.90C + 12.0 0.20* 86 42 0.60 71

Fern N = )0.0074C2 + 1.50C + 21.0 0.64*** 100 N/A N/A >53

Fescue N = )0.0128C2 + 2.03C + 24.6 0.57*** 79 35 0.74 47

Fir N = )0.0154C2 + 2.43C + 10.7 0.67*** 79 39 0.75 52

Pine N = )0.0125C2 + 2.29C – 8.4 0.72*** 92 46 1.24 37

Spruce N = )0.0100C2 + 1.80C + 15.8 0.59*** 90 45 0.71 63

Tamarack N = )0.0109C2 + 1.72C + 33.0 0.24** 79 39 0.59 65

C remaining (Crem as percent of original C) at the maximum N content and the C and N concentration and the C:N mass quotient at this value. N/A and > connotes that N lossoccurred from the start of decomposition; thus, the critical C:N quotient is assumed to have been greater than the original value, which is shown. ***, ** and * denotesignificant regressions at 0.001, 0.01 and 0.05 levels, respectively.

Figure 2. Relationship between predicted percent of original N content and percent of original P content remaining at 75,

50 and 25% of original C content remaining in litter versus C:N and C:P mass quotients (A and C) and AUR:N and AUR:P

mass quotients (B and D) of original litter, across all sites.


Figure 3. Illustrative patterns of change in N content of five litters at five sites. All represents the average pattern for the

litter type at all sites. (A) birch, (B) beech (C) fir, (D) pine and (E) tamarack. Numbers after the site code are r2 values for

the polynomial regressions depicted.


significant relationship between the rate of N loss

(relative to C loss) and either the initial C:N or

AUR:N quotient (Figure 2A and B). The range of

C:N and AUR:N quotients in the initial litters was

small (spanning 39–83 and 16–56, respectively). In

contrast, we found strong positive relationships

between the rate of P loss and both the initial litter

C:P and AUR:P quotients (Figure 2C and D). There

was a large range in initial litter C:P and AUR:P

quotients (369–2122 and 113–1770, respectively).

Thus, the rate of loss of P was related to the initial P

concentration in the litter, but the rate of N loss

was not related to the initial N concentration.

To identify the contribution of the site to the

dynamics of N and P in decomposing litter, we

plotted N (percent of original remaining) over C

(percent of original remaining) for each litter at each

site and compared this with the average for all sites.

We hypothesized that sites with low C:N quotients in

the surface soil (Oi, Oe and Oa horizons) would have

litters that retained a larger proportion of their initial

N, and those with the largest C:N quotients in the

forest floor would lose N. We chose five litters and

five sites to illustrate these relationships (Figure 3),

representative of the range of high (MAR, MON and

TER) and low (NH1 and SCH) N and P retention

patterns. To test the dependence of N retention in

litter on soil C:N quotient in the surface soil, we

calculated the N remaining at 60% C remaining,

excluding the INU and WHI sites, where the litter

decomposed slowly and had rarely lost 40% C after 6

years. The sites ranked 1–4 for greatest N retention at

60% C remaining, averaged across the ten litters,

were MAR, TER, MON and CHA and those ranked

13–16 were NH1, TOP, PMC and SCH. Of these,

MAR and TER have low C:N quotients in the Oi, Oe

and Oa horizons (28 and 17, respectively), but MON

has a high C:N quotient (47) and CHA intermediate

(36). The NH1 and SCH sites had C:N quotients of 58

and 48, respectively, in the Oi, Oe and Oa horizons

and TOP and PMC had 38 and 42 (Table 3). At the

NH1 and SCH sites, the litter was placed in contact

with lichens on the forest floor, which have very

high C:N quotients, 119 at SCH (Trofymow and CI-

DET Working Group 1998). There were only two

significant negative correlations between percent N

remaining and C:N quotient of the Oi, Oe and Oa

horizons across 16 sites (aspen and fir), but the

average percent N remaining of the ten litters was

significantly negatively correlated with the C:N

quotient of the sites (Table 4).

The analysis was repeated for P, to test the

hypothesis that P retention by tissues was related to

the C:P quotient of the organic horizons (Figure 4).

The sites ranked 1–4 for greatest P retention at 60%

C remaining, across the ten litters, were MON, TER,

HID and MAR and those ranked 13–16 were PMC,

PAL, NH1 and SCH. The C:P quotients in the Oi,

Oe, Oa horizons of the top four were 289, 155, 397

and 394, whereas those for the bottom four were

717, 492, 538 and 465 (Table 5). The lichen mat at

SCH had a C:P quotient of 1448 (Trofymow and

CIDET Working Group 1998). There were only four

significant negative correlations between percent P

remaining and the C:P quotient of the organic

horizons across 16 sites (birch, fern, fescue and

spruce), but the average of the ten litters was sig-

nificantly correlated with the C:P quotient of the

sites (Table 4). As with N, the greater the C:P

quotient in the soil surface, the less the ability of

the litter to retain P through decomposition.

Stoichiometry of C, N and P inDecomposition

Initial litter C:N quotients ranged from 39 to 83 and

showed a decrease to approximately 30, based on a

polynomial regression (Figure 5A). The average

C:N quotient in the Oi, Oe and Oa horizons of the

forest floor at the 18 sites was 39, ranging from 28

to 62. Initial litter C:P quotients ranged from 369 to

2,122 and showed a decrease to approximately 450,

based on a polynomial regression (Figure 5B). The

average C:P quotient in the Oi, Oe and Oa horizons

of the forest floor at the 18 sites was 412, ranging

from 155 to 717. This suggests that the quotient

will change little as the litter is further decomposed

into humus, because the means in the litter were

close to the means in the soil.

Table 4. Pearson Correlation Coefficient (r)between Mass of Original N and P ContentRemaining at 60% of Original C ContentRemaining in Litters versus Soil Surface LayerC:N and C:P Quotient, Respectively

Litter N P

Aspen )0.524* )0.394

Beech )0.453 )0.293

Birch )0.347 )0.599*

Cedar )0.411 )0.317

Fern )0.315 )0.547*

Fescue +0.071 )0.680**

Fir )0.513* )0.392

Pine )0.343 )0.202

Spruce +0.426 )0.409*

Tamarack )0.439 )0.410

Average )0.589* )0.607*

INU and WHI sites were excluded. ***, ** and * denote significant correlations at0.001, 0.01 and 0.05 levels, respectively.


The range of initial litter N:P quotients is large

(5–26), and there was a general convergence as C

was lost to an overall N:P quotient of about 16,

based on a polynomial regression (Figure 5C). Lit-

ters with a small initial N:P quotient (for example,

aspen, fir) tended to gain N relative to P as they

Figure 4. Illustrative patterns of change in P content of five litters at five sites. All represent the average pattern for the

litter type at all sites. (A) beech, (B) fir, (C) pine, (D) tamarack and (E) birch. Numbers after the site code are r2 values for

the polynomial regressions depicted.


Table 5. Relationship between C Remaining (Percent of Original) and P Remaining (Percent of Original) inDecomposing Litter, by Type Across all Sites, as Described by Polynomial Regression

Litter Regression r2 Crem (%) C (%) P (%) C:P quotient

Aspen P = )0.0166C2 ) 1.24C + 53.5 0.85*** 100 N/A N/A >369

Beech P = )0.0134C2 + 1.74C + 59.9 0.04 65 30 0.044 682

Birch P = )0.0166C2 + 2.40C + 23.9 0.31** 72 35 0.042 826

Cedar P = )0.0131C2 + 2.15C + 7.1 0.18* 82 41 0.045 904

Fern P = )0.0037C2 + 1.05C + 25.6 0.46*** 100 N/A N/A >702

Fescue P = )0.0045C2 + 1.25C + 19.9 0.66*** 100 N/A N/A >755

Fir P = +0.0045C2 + 0.08C + 32.1 0.63*** 100 N/A N/A >472

Pine P = +0.0031C2 + 0.45C + 17.1 0.82*** 100 N/A N/A >391

Spruce P = +0.0011C2 + 0.63C + 20.5 0.78*** 100 N/A N/A >627

Tamarack P = )0.0222C2 + 2.53C + 66.5 0.11 57 28 0.032 869

C remaining (Crem as percent of original C) at the maximum P content and the C and P concentration and the C:P mass quotient at this value. N/A and > connotes that N lossoccurred from the start of decomposition; thus, the critical C:P quotient is assumed to have been greater than the original value, which is shown.. ***, ** and * denotesignificant regressions at 0.001, 0.01 and 0.05 levels, respectively.

Figure 5. Relationship between the C:N (A), C:P (B) and N:P (C) mass quotients and percent C remaining of litters. The

solid symbols on the left represent the C:N, C:P and N:P quotients in the original litter types and the vertical lines represent

the mean ± 1 standard deviation for the data falling within the categories 80–90%, 50–60% and 20–30% of C remaining.

The curves represent the polynomial regression.


decompose, whereas litters with a high initial N:P

quotient (for example, beech and tamarack) tended

to lose N relative to P. There is also a greater

divergence of N:P ratio in the early stages of

decomposition among the beech and tamarack lit-

ters. The average N:P quotient in the Oi, Oe and Oa

horizons of the forest floor at the 18 sites was 10

and ranged from 6–17, suggesting that the quotient

will decrease as the litter is further decomposed

into soil humus.

DISCUSSION

The range of C:N mass quotients in the litters used

in our study (39–83) is slightly smaller than the

range of 31–90 in 14 tree litters employed by the

long-term intersite decomposition experiment

(LIDET, M. E. Harmon, Oregon State Univ., per-

sonal communication). Thus, our results may be

constrained by the narrow range in the initial C:N

quotient of the litters used, with only pine needles

having a low value (39). The range of C:P mass

quotients of our litters started at a lower value

(369) than the lowest LIDET ratio (664) but only

ranged up to 2,122, compared to 4,138 for LIDET

(M. E. Harmon, personal communication). The

stronger patterns established for P loss in CIDET

may reflect the influence of a lower quotient and

smaller range, as CIDET covered only about half of

the upper range of values found in LIDET.

The mesh size of the litter-bags is small and

exclusion of macroinvertebrates may affect the

rates of decomposition and N and P dynamics. At

the Shawnigan site, there was no difference in

decay rates or on soil organism communities in

litterbags with wood chips or mixed litter between

CIDET litter-bags and those with a mesh size of 5

mm (Setala and others 1996; Trofymow 1998;

Addison and others 2003a, b). Although the results

are applicable to cool temperate to subarctic forests,

they may not apply in forests where macroinver-

tebrates play an important role in the decomposi-

tion processes.

Testing our Hypotheses

The results of our study are largely consistent with

our initial hypotheses, as described in the Intro-

duction.

Hypothesis 1: N is generally gained during early stages

of decomposition, whereas P may be gained or be lost.

Nitrogen was retained in all litters, until about 40%

of the original C had been lost. Phosphorus release

was more variable, with both initial retention and

release occurring, depending on the litter type.

Hypothesis 2: Critical C:N and C:P quotients at which

N and P loss begins vary among litter type. The critical

C:N quotient ranged from 37 (pine) to 71 (cedar)

and for C:P quotient ranged from 682 (beech) to

904 (cedar). These were generally in the ranges

reported for similar litters.

Hypothesis 3: Litters with high concentrations of re-

calcitrant materials have higher critical C:N quotients.

Critical quotients increased with increasing recal-

citrance of litter, as indicated by the rate of decay

(mass loss or exponential k value) or the initial C:N

quotient. This is consistent with the suggestion that

the critical C:N quotient depends on the portion of

the litter that has decomposed (Burger and Pritchett

1984). In our recalcitrant litters, the portion that

had been decayed may have had a C:N quotient of

about 30, but the material remaining still had a

much higher C:N quotient and thus raised the

apparent critical C:N quotient.

Hypothesis 4: The initial concentration of N or P in

litter influences patterns of nutrient loss. There was a

strong relationship between the initial P concen-

tration in litter and the pattern of P release, as lit-

ters with high initial P concentrations lost P and

those with low concentrations retained P. Only

those litters with C:P quotients greater than 700

retained P. There was no such relationship for N.

This may have stemmed from the much greater

range of initial C:P than C:N quotients in our litters.

Hypothesis 5: The initial AUR or ‘‘Klason lignin’’

concentration of litter affects N accumulation. There was

little evidence to support this hypothesis based on

the weak relationship between original AUR:N

quotient and N content of litter at various stages of

decomposition (Figure 2b).

Hypothesis 6: N or P availability in the surface soil

has smaller effects on nutrients in litter than initial litter

composition, but nutrients tend to be retained more in

litter in nutrient-rich environments. There was some

evidence that the availability of N and P in the

surrounding forest floor at each site influenced N

and P release from the litter. The amount of N or P

remaining in litter was weakly but negatively cor-

related with the C:N and C:P quotients in the forest

floor. This probably reflects the degree to which

colonizing fungi can export or import nutrients in

the litter to or from the adjacent forest floor. A

positive but weak relationship was reported by

Vesterdal (1999).

Stoichiometry of C, N and P inDecomposition

Recently, there has been considerable attention

given to broad C, N and P stoichiometries, such as


in leaves related to climate (Reich and Oleksyn

2004), in plant tissues (Knecht and Goransson

2004), in food webs (Elser and others 2000) and in

C sequestration in ecosystems (Hessen and others

2004). Here, we have the opportunity to determine

if patterns are apparent across Canadian forests.

As the litters decomposed over 6 years, there was

a general decline and convergence in C:N and C:P

quotients as percent C remaining was reduced to

less than 40%, a commonly noted phenomenon in

many other decomposition studies (for example,

Melillo and others 1989; Vesterdal 1999). Our re-

sults suggest that C:N and C:P quotients move to

values of about 30 and 450, respectively, similar to

those observed in the upper organic layers of the

soil at these forested sites. In a collation of data for

terrestrial autotrophs, Elser and others (2000) noted

the wide range in C:N and C:P quotients and with

means of 31 and 375, on a mass basis. They also

noted mean C:N and C:P quotients of 6 and 45 for

terrestrial invertebrate herbivores, showing that

this decrease in quotients as litter decomposes re-

flects the nutrient demands of the decomposer

populations.

We found a convergence of N:P ratios during decay

to between 10 and 20, with a mean value of

approximately 16. This has not been reported before,

but appears to be a common occurrence, based on a

collation of N:P quotients from three sources: (1)

studies with decomposition data, (2) studies with

forest floor data, and (3) studies with both decom-

position and forest floor data. In litter decomposition

studies, the N:P quotients of beech, aspen and balsam

poplar leaves decaying over 3 to 5 years rose from 7 to

20, from 5 to 23 and from 5 to 19, respectively (Staaf

1980a, b; Louiser and Parkinson 1978). In decom-

posing red spruce needles, initial N:P ratios of 3 to 9

rose to 16 to 17 over two years (Rustad and Cronan

1988). The initial N:P quotient in a variety of litters in

three Rocky Mountain forests was 5–15 but in-

creased to between 10 and 15 after 3 to 5 years of

decomposition (Prescott and others 1993). Litter

ultimately decomposes and becomes forest floor

material, so it is not surprising that the range of N:P

quotients (10–20) that we found in our long-term

study is also consistent with several estimates of the

N:P quotientof humus found by others, ranging from

12 to 15 (Gosz and others 1976), 15 (Berg and Las-

kowski1997),17 (Goszandothers1973),18 (Lousier

and Parkinson 1978) and, from a review of the

world’s forests, 11–20 for temperate and boreal forest

floors (Vogt and others 1986). Finally, some studies

measured N and P in both decomposing litter and

forest floor. Over 1 year, Gosz and others (1973) re-

ported a convergence of N:P quotients during decay

of birch, maple and beech leaf litter from initial val-

ues of 14–29 to between 15 to 17, similar to that for

the forestfloor (17and15 in theOeandOahorizons).

The N:P quotient of litter in New Brunswick con-

verged from between 4 and 27 to between 14 and 17

after2years,whichencompasses the15–16 foundfor

the forest floor (MacLean and Wein 1978).

If we are correct in our assumption that the N:P

quotient in decomposing litter will approach 16,

regardless of initial litter values, then the pattern of

net P retention and loss during decay might be

predictable from the initial N:P quotient of the lit-

ter: loss of P from litters with initial N:P quotients

less than 16 and retention or gain of P in those

greater than 16. For example, Edmonds (1980)

measured net immobilization of P in western

hemlock leaf litter with an initial N:P quotient of 20

but observed loss of P in Pacific silver fir and

Douglas-fir with initial N:P quotients of 5. Monleon

and Cromack (1996) reviewed four studies of

Ponderosa pine needle decay and noted that there

was loss of P during the first year from all litters

with initial N:P quotients less than 11. Baker and

Attiwill (1985) reported initial retention of P in

Eucalyptus litter (initial N:P quotient 33–38) but loss

from Pinus radiata litter (initial N:P quotient 8–12)

with a convergence between 13 and 24 after 2

years of decomposition. Tripathi and Singh (1992)

found that P was retained in bamboo wood and

roots (N:P quotient 25 and 35), but not in grass

shoots and twigs (N:P quotients 14 and 15). How-

ever, they found P retention in bamboo leaves (N:P

quotient 12) but not in sheaths (N:P quotient 24),

which is not consistent with our hypothesis.

Together, these observations suggest that there is

a consistent convergence of N:P quotient to be-

tween 10 and 20 during decomposition of forest

litter in temperate and boreal forests. Although

changes in N:P quotients during litter decay have

received little attention, they are increasingly rec-

ognized as a major determinant of nutrient limita-

tion and release in ecosystems (Sterner and Elser

2002). For example, Koerselman and Meuleman

(1996) reviewed the literature on N and P limita-

tions to growth in a variety of wetland ecosystems

and concluded that a soil N:P quotient greater than

16:1 indicates P limitation while that less than 14:1

indicates N limitation for plants. Knecht and

Goransson (2004) reviewed N and P concentrations

in plant tissues, noting a range in N:P quotient from

2 to 50 and averages for coniferous, deciduous and

herbaceous tissues of 8, 11 and 10, respectively.

Elser and others (2000) compared C:N:P quotients

of autotrophs and invertebrate herbivores in ter-

restrial ecosystems and found, on a mass basis, that


autotroph C:N, C:P and N:P quotients averaged 31,

378 and 13, respectively, whereas values for

invertebrate herbivores averaged 7, 45 and 12,

respectively. Elser and Urabe (1999) suggest that

the basic predictions of nutrient release using of

stoichiometric models are generally supported by

experimental data from pelagic systems, showing

that critical N:P release quotients are primarily a

function of the N:P quotients of algae and their

grazers. Stoichiometry may apply in soils, where

food webs are becoming increasingly recognized as

important regulators of nutrient cycling. Our find-

ings suggest a stoichiometric relationship associated

with the decay process in forests, in which litters of

widely divergent initial chemistry are transformed

into humus within a smaller range of N:P quo-

tients. This may be another case of ‘‘consumer-

driven nutrient cycling’’ (Elser and Urabe 1999),

arising from differences in the N:P ratios of bacteria

and fungi and their grazers. This convergence of

N:P ratios is in keeping with the ‘‘decay filter’’

concept (Melillo and others 1989), in which

decomposition converts relatively heterogeneous

litter into homogeneous humus.

CONCLUSION

As has been found for rates of mass loss in foliar

tissues placed in diverse ecosystems (for example,

Moore and others 1999; Trofymow and others

2002), we have shown here that relationships can

be developed which are broadly applicable and can

be used to predict the fate of N and P as litter

decomposes in Canadian forests. The critical vari-

ables are primarily the composition of the original

litter, particularly the C:N and C:P quotients, and

secondarily the characteristics of the underlying

soil. Our results suggest that stoichiometric rela-

tionships between C, N and P can be developed for

the litter–humus continuum, similar to those

developed in aquatic ecosystems.

ACKNOWLEDGEMENTS

Funding from the Climate Change and Ecosystems

Processes Networks of the Canadian Forest service

supported this experiment. The CIDET Working

Group includes C. Camire, L. Duschene, L. Kozak, M.

Kranabetter, I. Morrison, M. Siltanen, S. Smith,

S. Visser R. Wein and C. Preston technical support

was provided by B. Ferris, A. Harris, L. Kutny, R.

Leach and D. White. We gratefully acknowledge

D. Anderson, C. Monreal, R. Trowbridge, M. Weber

and the late Steve Zoltai for encouragement and

assistance in establishing sites or initial litter collec-

tion. Further information on CIDET is available on

the Web at http:///www.pfc.cfs.nrcan.gc.ca/ cli-

mate/cidet.

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Patterns of Carbon, Nitrogen and Phosphorus Dynamics in Decomposing Foliar Litter in Canadian Forests