SPECIAL FEATURE

PLANT–SOIL INTERACTIONS AND THE CARBON CYCLE

Grazing triggers soil carbon loss by altering plant

roots and their control on soil microbial community

Katja Klumpp1‡, Sebastien Fontaine1*‡, Eleonore Attard2, Xavier Le Roux2†,

Gerd Gleixner3 and Jean-Francois Soussana1

1INRA, UR874 Grassland Ecosystem Research, 234, Av. du Brezet, F-63100 Clermont-Ferrand, France; 2INRA,

CNRS, Universite de Lyon 1, Microbial Ecology, UMR 5557, 43 Bd du 11 novembre 1918, 69622 Villeurbanne,

France; and 3Max Planck Institute for Biogeochemistry, Postbox 100164, D-07701 Jena, Germany

Summary

1. Depending on grazing intensity, grasslands tend towards two contrasting systems that differ in

terms of species diversity and soil carbon (C) storage. To date, effects of grazing on C cycling have

mainly been studied in grasslands subject to constant grazing regimes, whereas little is known for

grasslands experiencing a change in grazing intensity. Analysing the transition between C-storing

and C-releasing grasslands under low- and high-grazing regimes, respectively, will help to identify

key plant–soil interactions for C cycling.

2. The transition was studied in a mesocosm experiment with grassland monoliths submitted to a

change in grazing after 14 years of constant high and low grazing. Plant–soil interactions were anal-

ysed by following the dynamics of plant and microbial communities, roots and soil organic matter

fractions over 2 years. After disturbance change, mesocosms were continuously exposed to13C-labelled CO2, which allowed us to trace both the incorporation of new litter C produced by a

modified plant community in soil and the fate of old unlabelled litter C.

3. Changing disturbance intensity led to a cascade of events. After shift to high disturbance, photo-

synthesis decreased followed by a decline in root biomass and a change in plant community struc-

ture 1.5 months later. Those changes led to a decrease of soil fungi, a proliferation of Gram(+)

bacteria and accelerated decomposition of old particulate organic C (<6 months). At last, acceler-

ated decomposition released plant available nitrogen and decreased soil C storage. Our results indi-

cate that intensified grazing triggers proliferation of Gram(+) bacteria and subsequent faster

decomposition by reducing roots adapted to low disturbance.

4. Synthesis. Plant communities exert control on microbial communities and decomposition

through the activity of their living roots: slow-growing plants adapted to low disturbance reduce

Gram(+) bacteria, decomposition of low and high quality litter, nitrogen availability and, thus,

ingress of fast-growing plants. Our results indicate that grazing impacts on soil carbon storage by

altering plant roots and their control on the soil microbial community and decomposition, and that

these processes will foster decomposition and soil C loss inmore productive and disturbed grassland

systems.

Key-words: ARISA, carbon cycling, decomposition, disturbance, grassland, management,

microbial community, nitrogen cycling, particulate organic matter, PLFA

Introduction

Understanding the impact of disturbances (grazing, cultiva-

tion, climate variability) on linkages between above-ground

and below-ground biota is essential to predicting the conse-

quences of land use and global change for soil C cycling. Over

*Correspondence author. E-mail: fontaine@clermont.inra.fr

†Present address: Fondation pour la recherche sur la biodiversite, 57

rue Cuvier, CP 41, 75231 Paris cedex 05, France.

‡The first two authors have contributed equally to this work.

Journal of Ecology 2009, 97, 876–885 doi: 10.1111/j.1365-2745.2009.01549.x

� 2009 The Authors. Journal compilation � 2009 British Ecological Society

the last 20 years, theses questions have inspired various exper-

iments in forests and grasslands (Pickett & White 1986;

McNaughton, Banyikwa & McNaughton 1997; Bardgett &

McAlistair 1999; Wardle et al. 2008). Hence, we are now able

to define principles that are at least applicable to disturbances

such as grazing and cultivation (Wardle et al. 2004; Bardgett

et al. 2005). Depending on the intensity of disturbance,

ecosystems tend towards two contrasting systems that differ

in terms of functional diversity of biota, C storage and pri-

mary production. For example, intensively grazed grasslands

are dominated by fast-growing plant species producing litter

of high quality (low C ⁄N ratio and lignin content) that are

quickly decomposed by bacteria (Seastedt 1985; Berendse,

Bobbink & Rouwenhorst 1989; McNaughton, Banyikwa &

McNaughton 1997; Bardgett, Wardle & Yeates 1998). As a

result, in these productive ecosystems, C storage in the soil–

litter continuum is relatively low (C-releasing ecosystem)

(Milchunas & Lauenroth 1993). Grasslands adapted to low

grazing levels, on the other hand, are dominated by fungi and

slow-growing plants species and exhibit larger C storage

(C-storing ecosystem) and lower above-ground net primary

productivity than productive ecosystems.

However, the influence of disturbance on biotic communi-

ties and ecosystem functioning has mainly been studied in

grasslands and forests either under constant disturbance (i.e.

close to steady-state) and ⁄or along chronosequences where

many factors are likely to covary (soil mineralogy, soil

depth, climate) (Wardle et al. 2004; Bardgett et al. 2005;

Patra et al. 2006; but see Bardgett & McAlistair 1999). Con-

sequently, the mechanistic basis of the transition between

undisturbed C-storing and disturbed C-releasing ecosystems

is less known. Plant communities, litter pools, decomposer

communities and soil processes are linked in feedback loops

which make it difficult to determine the major ecological

drivers (see Bardgett et al. 2005). For example, it has been

proposed that frequent grazing favours fast-growing plant

species that allocate carbon to growth rather than to second-

ary defences (i.e lignin) and which decrease litter quality

(e.g. Wardle et al. 2004). In that case, plant species would

feedback on soil communities and processes through litter

quality (Seastedt 1985; Berendse, Bobbink & Rouwenhorst

1989; Bardgett et al. 2005; De Deyn & Van der Putten

2005). However, in some cases, grazing has been shown to

affect soil microbial communities before plant communities,

suggesting that the shift in ecosystem functioning is not nec-

essarily caused by a change in plant litter quality (Patra

et al. 2005). Finally, recent studies have shown that grasses

strongly affect litter decomposition via the presence of their

living roots which modify the environment of decomposers

(Van der Krift et al. 2001; Van der Krift, Kuikman &

Berendse 2002; Personeni & Loiseau 2004). As grazing

affects root biomass and activity (Holland & Detling 1990;

Guitan & Bardgett 2000; Hamilton & Frank 2001) changes

in soil communities and litter decomposition may be pro-

voked by a change in living roots rather than litter quality.

Given the lack of knowledge, the role of plant–grazer–

decomposer interactions on the control of ecosystem C-N

fluxes is rarely included in current geochemical models, which

partly explains discrepancies between predictions and empiri-

cal results (Fontaine & Barot 2005). A better understanding of

the nature of links between plant and soil communities and

ecosystem functioning after a change in disturbance will con-

tribute to build new models including functional diversity of

biota. Thesemodels may providemore accurate predictions on

the impact of disturbance and diversity loss on ecosystem pro-

ductivity andC storage.

A mesocosm study with semi-natural grasslands showed a

trade-off between above-ground productivity (i.e annual net

primary productivity, ANPP) and below-ground carbon stor-

age when these grasslands were submitted to a change in dis-

turbance (simulated grazing) after 14 years of constant high

and low grazing intensity. Both pre-experimental (field) and

experimental low disturbance favoured above- and below-

ground C storage (Klumpp, Soussana & Falcimagne 2007a).

Moreover, this trade-off was partly assigned to community

aggregated leaf (specific leaf area, leaf dry-matter content) and

root and rhizome traits (specific length, tissue density), which

responded significantly to (changes in) disturbance (Klumpp

& Soussana 2009).

Here, by using the same mesocosm experiment, we study

the mechanistic basis of the transition between undisturbed

C-storing and disturbed C-releasing grassland ecosystems.

Given that plant species influence litter decomposition via

the presence of living roots and that disturbance modifies

root biomass and activity (e.g. Holland & Detling 1990; Van

der Krift et al. 2001), we hypothesize that disturbance affects

soil C cycling by altering plant roots and their control on

microbial community and decomposition. To study these

mechanisms, we analysed in detail the dynamics of plant and

microbial communities, plant roots, soil organic matter frac-

tions and N leaching over 2 years. After a change in distur-

bance, mesocosms were continuously exposed to 13C-labelled

CO2, allowing to trace both the incorporation of new litter

C produced by a modified plant community and the fate of

old unlabelled litter C. Analysing the transition between

undisturbed C-storing and disturbed C-releasing ecosystems

will help identify key plant–soil interactions for C cycling

and geochemical models.

Materials and methods

EXPERIMENT WITH GRASSLAND MONOLITHS AND

MANAGEMENT

The study was carried out with grassland monoliths from a perma-

nent semi-natural mesic pasture that had been subjected to high- and

low-intensity sheep grazing without fertilization for the last 14 years

(Louault et al. 2005). High-intensity grazing, and therefore high dis-

turbance (H) prior to the experiment, meant that grassland was cut

once and sheep-grazed four times per year, whereas under low-inten-

sity grazing [pre-experimental low disturbance (L)], grassland was

grazed by sheep once a year only. Grassland under high disturbance

was dominated byHolcus lanatus, Lolium perenne, Agrostis capillaris,

Festuca arundinacea, Taraxacum officinale and Trifolium repens,

while grassland under low disturbance was dominated by Elytrigia

Effect of grazing on ecosystem carbon cycling 877

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

repens, Agrostis capillaris, Arrhenatherum elatius, Festuca rubra and

Vicia cracca (data not shown).

Procedures to select and extract grassland monoliths are described

by Klumpp, Soussana & Falcimagne (2007a). Briefly, in June 2002,

48 monoliths (L 0.5 · W 0.5 · H 0.4 m) were sampled from the two

grassland plots. Following extraction, 24 monoliths of each pre-

experimental treatment were placed in eight mesocosms closed by

transparent canopy enclosures (L 1.5 · W0.5 · H0.75 m), each con-

taining three monoliths of the same disturbance treatment. Meso-

cosms were placed in natural light and outdoor temperatures. To

reduce light transmission from the side, dark plastic edges of 10 to

20 cm height were placed around each monolith throughout the

experiment (seeKlumpp, Soussana&Falcimagne 2007a). Air humid-

ity was adjusted to outdoor conditions. Monoliths were watered one

to three times a week to target a soil water potential of c.)30 kPa.

After the transfer into mesocosms, ‘high’ grazing disturbance (H)

was simulated by cutting at 5 cm height and applying artificial urine

(5 g N m)2, see Klumpp, Soussana & Falcimagne 2007a) five times

per year. The low-disturbance (L) monoliths were neither cut nor fer-

tilized during the experiment. After 6 months of acclimatization

(from now on referred as t0), half of the monoliths of each treatment

were switched to the opposite disturbance treatment, resulting in four

replicate mesocosms each with constant low disturbance (LL), con-

stant high disturbance (HH), a shift to high disturbance (LH) and a

shift to low disturbance (HL).

All 16 mesocosms were henceforth continuously exposed to13C-labelled CO2. For d13C labelling, outdoor air was scrubbed from

H2O and CO2, which was then replaced by fossil-fuel derived CO2

depleted in 13C (d13C )34.7 ± 0.03&). Thereafter, the 13C-labelled

air was humidified, mixed with temperature-regulated cooled out-

door air (30%) and distributed to the mesocosms. During night time,

mesocosmswere providedwith unlabelled air. ThemeanCO2 concen-

tration inside the mesocosms was held close to the outdoor CO2

concentration (425 ± 39 lmol mol)1; mean difference of 13.2 ±

z9.5 mol mol)1) and had on average a d13C signature of )21.5(±0.27 &) during the two-year experiment. Air flow and CO2 con-

centration in each mesocosm was monitored continuously during the

two-year experiment (see Klumpp, Soussana & Falcimagne 2007a for

full details on themethod).

PLANT COMMUNITY STRUCTURE AND NITROGEN

NUTRIT ION INDEX

Analyses of plant community structure comprising botanical

composition and plant functional traits were carried out after mono-

lith extraction in September 2002 and then five times during the

course of the experiment (1, 6, 12, 18 and 24 months after the start of

the experiment). Measured shoot and root and rhizome traits were:

vegetative height, specific leaf area (SLA), leaf dry matter content

(LDMC) and leaf nitrogen content (LNC), specific length (SL) and

tissue density of roots and rhizomes (DENS) (for methodologies

see Klumpp, Soussana & Falcimagne 2007a; Klumpp & Soussana

2009).

The Nitrogen Nutrition Index (NNI) evaluates N availability of

the vegetation and was calculated according to Lemaire & Salette

(1983) from nitrogen content of live biomass (in N%) and green

above-ground biomass (in t DMha)1) as: (N% ⁄ (4.8DM)0.32).

SOIL ORGANIC MATTER FRACTIONS

Soil was sampled once before the start of 13C labelling (t0) and then

five times during the experiment (at 1.5, 6, 12, 18 and 24 months). At

each soil harvest, half a monolith per mesocosm was sampled. The

remaining half was sealed with a stainless steel plate and the empty

space was filled with sand. Vertical soil slices (L 0.4 · l 0.06 · H

0.3 m) of the sampled monoliths were split into three horizontal lay-

ers (0–10, 10–20 and 20–30 cm). The soil layers were air-dried and

free organic matter (OM) fractions were separated with water by

passing the soil through a series of three brass sieves of decreasing

mesh sizes (1.0, 0.2 and 0.05 mm) (wet sieving). The remaining mate-

rial in each sieve was separated into the organic and mineral fraction

by density flotation in water (Loiseau & Soussana 1999). Organic

fractions containing roots (R), rhizomes (Rh), coarse (>1 mm) and

fine (0.2 to 1 mm) particulate organic matter (POM) were oven-dried

and analysed for their C content and d13C. Hence forward, root and

rhizomewere added together (R + Rh).

GENETIC STRUCTURE OF THE EUBACTERIAL

COMMUNITY

For each soil sampling date, DNA was extracted using the Fast-

DNA� SPIN Kit for Soil (BIO 101� Systems, Qbiogene, Carlsbad,

CA, USA) and bacterial community structure was characterized by

Ribosomal Intergenic Spacer Analysis (ARISA) using a modified

method from Ranjard et al. (2000) (for details see Attard et al. 2008).

The ARISA method, which gives a molecular fingerprint of the soil

community, is a rapid method to set up and was used to follow tem-

poral changes in microbial community structure. However, this

method does not give any information on the microbial community

and its activity.

PLFA ANALYSES

To better describe the microbial community and its role on soil pro-

cesses, phospholipid fatty acids (PLFA) and their d13C signature were

determined before (t0) and 18 months after disturbance change from

fresh soil samples (depth 0–10 cm, sieved at 2 mm). PLFA and their

d13C signature were analysed according to Zelles (1999) and Kramer

& Gleixner (2006). The PLFA-methyl esters were corrected for

methyl carbon (MeOH) introduced during methylation to obtain

d13C values of PLFAs. PLFA nomenclature was used as described by

Frostegard & Baath (1996) and Zelles (1999). Saturated, branched-

chain fatty acids (14:0iso, 15:0iso, 15:0anteiso, 16:0, 16:0n, 17:0br,

17:0n) were associated with Gram-positive bacteria. Monounsatu-

rated and cyclopropyl-substituted PLFAs (16:0br, 16:1x7br, cy17:0,17:1x8br, 18:1x7 and cy19:0) were associated with Gram-negative

bacteria (Ratledge & Wilkinson 1988) and 18:1x9 and 18:2x6 with

saprophytic fungal lipids. C16:1x5, 20:3 and 20:4 indicated arbuscu-

lar mycorrhizal fungi (Jabaji-Hare 1988; Olsson 1999) and the PLFAs

10Me-C16:0 were used to indicate soil actinomycetes (Kroppenstedt,

Greinermai & Kornwendisch 1984; Brennan 1988). Individual

PLFAs and communities (fungal, gram(+), gram()), mycorrhizal

fungi and actinomycetes) are expressed in percentage peak area.

1 3C TERMINOLOGY AND ANALYSES

The fraction of ‘old’ unlabelled C (f Cold) and ‘new’ C derived from13C-labelling (f Cnew) in roots and rhizomes, coarse and fine POM,

and individual PLFAwere calculated by amass balance equation:

f Cnew ¼ ðd13Csample � d13CcontrolÞ=ðd13Cinput � d13CcontrolÞ eqn 1

f Cold ¼ 1� f Cnew eqn 2

878 K. Klumpp et al.

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

where d13Csample is the d13C of the sample, d13Ccontrol is the d13Cvalue before the start of labelling (t0) and d13Cinput is the mean

d13C value of the fully labelled green leaves during the experiment

()40.4& ± SD 2.7). d13Cinput was more depleted in winter than

in spring (P < 0.05), but did not differ between disturbance

treatments (P > 0.1) (see Klumpp, Soussana & Falcimagne

2007b for more details).

MODEL APPROACH

Rootmortality and decomposition rates of particulate organic matter

(POM) were determined by a carbon flux model. The model was con-

strained to measured quantities of old (unlabelled) and new (labelled)

carbon in root biomass and particulate organic matter in the upper 0–

10 cm soil layer during the 2-year experiment (see Model flowchart,

Fig. 1). The model comprises six compartments (expressed in mg C

g)1 soil): new C and old C in roots (Rootnew and Rootold), coarse par-

ticulate organic matter (cPOMnew and cPOMold) and fine particulate

organic matter (fPOMnew and fPOMold). The Rootnew compartment

is supplied by a fraction (s) of the daily net plant photosynthesis (Anet

in mg C g)1 soil) measured by gas exchange. Root turnover (mortal-

ity, m) releases particulate organic matter supplying carbon with a

fraction (b) to the fPOM and a fraction (1- b) to the cPOM compart-

ment. The cPOMand fPOM compartments have distinct decomposi-

tion rates, (kc) and (kf). The decomposition of coarse POM releases

CO2 with a fraction (1-a) and supplies the fPOMcompartment with a

fraction (a). Therefore, the model comprises six equations allowing to

calculate six parameter unknowns (s,m, b, a, kc and kf):

d

dtðRoldÞ ¼ �m�Rold � Tair eqn 3

d

dtðcPOMoldÞ ¼ m� ð1� bÞ �Rold � Tair

� kc � cPOMold � Tair

eqn 4

d

dtðfPOMoldÞ ¼ m� b�Rold � Tair

þ kc � a� cPOMold � Tair

� kf � fPOMold � Tair

eqn 5

d

dtðRnewÞ ¼ Anet � s�m�Rnew � Tair eqn 6

d

dtðcPOMnewÞ ¼ m� ð1� bÞ �Rnew � Tair

� kc � cPOMnew � Tair

eqn 7

d

dtðfPOMnewÞ ¼ m� b�Rnew � Tair

þ kc � a� cPOMnew � Tair

� kf � fPOMnew � Tair

eqn 8

The model was fitted to measured data of four replicate mesocosms

per treatment by using Berkeley-Madonna software. The model was

run in thermal time (Tair, in degree days) by using daily temperature

means in mesocosms. Parameters (s, m, b, a, kc and kf) are therefore

expressed in (degree days)1). Mean residence times (MRT, in years)

of carbon in roots and rhizomes, coarse POM and fine POMwas cal-

culated as 1 ⁄m, 1 ⁄ kc and 1 ⁄ kf divided by a mean (for the 2 years of

the experiment) annual temperature sum of 4500 �C.

DATA ANALYSIS

anovas were performed for particulate organic matter fractions and

PLFAs to test differences between constant low disturbance (LL),

shift to high disturbance (LH), constant high disturbance (HH)

and shift to low disturbance (HL). Differences were tested with a

Fisher-LSD post hoc test. When necessary, data was log-trans-

formed prior to analysis to satisfy Shapiro–Wilk’s test of normality.

Statistical analyses were performed with the statistica 6 package

(StatSoft Inc., Tulsa, OK, USA). With the primer software (PRI-

MER-E Ltd., Plymouth, UK) rank similarity matrices were com-

puted and used to construct ‘maps’ highlighting the similarity or

dissimilarity of bacterial and plant community structure among

samples by non-metric multidimensional scaling (MDS) (Kruskal

& Wish 1978). Similarity percentages (SIMPER) were computed to

quantify the percentage of dissimilarity between treatments,

whereas one-way analyses of similarity (ANOSIM) were performed

to test differences between treatments at each sampling by using

the primer software package (Plymouth Routines In Multivariate

Ecological Research).

TIME SCALE

Not all measures can be conducted at the same frequency because of

methodological constrains. The frequency of each analysis is reported

inAppendix S1 in Supporting Information.

Results

DESCRIPTION OF GRASSLANDS MAINTAINED UNDER

CONSTANT LOW AND HIGH DISTURBANCE

Grasslandmonoliths maintained under constant low (LL) and

high (HH) disturbance had a significantly different botanical

composition (Fig. 2a). The plant communities of the HH and

LL treatments exhibited divergent community aggregated

morphological traits (see dissimilarity Fig. 2b). The HH plant

Rootold c

Rootnew fPOMnew

fPOMold

m s 1-β

β

α

α

β

1-β

1 1-

α

kc

m Anet kf

kf

cPOMnew

cPOMold m

1-α

kc

Fig.1. Model flowchart. New and old C in roots (Rootnew andRootold), coarse particulate organic matter (cPOMnew and cPOMold) and fine par-

ticulate organic matter (fPOMnew and fPOMold). (s) is fraction of the daily net plant photosynthesis (Anet) going to Rootnew; (m) root turnover

(mortality); (kc) and (kf) decomposition rates of cPOM and fPOM compartments; (b) and (1- b) fraction of decomposed roots going to fPOM

and cPOM; (a) and (1-a) fraction of decomposed cPOMgoing to fPOMand released as CO2.

Effect of grazing on ecosystem carbon cycling 879

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

community was characterized by high specific leaf area (SLA),

high leaf nitrogen content (LNC) and low mean plant height

(mHeight) indicating a fast growth strategy (i.e. nutrient

exploitation). The LL plant community had a low SLA, low

LNC and an increased LDMC and mHeight, characterizing a

slower plant growth strategy (i.e. nutrient conservation) than

theHHplant community (data not shown).

Differences in plant community structure were mirrored by

differences in soil C dynamics and microbial community. The

amount of coarse and fine particulate organic matter (POM)

was higher at low (LL) compared to high (HH) disturbance

(Fig. 3). Moreover, during the 13C labelling experiment, the

amount of old (unlabelled) carbon in roots and rhizomes and

in coarse and fine POM declined at a slower rate in LL than

HH soils, indicating a slower microbial decomposition of

POM in the LL treatment (Fig. 3 & Table 1). The bacterial

community structure, characterized by the ARISA method,

differed significantly between soils of high and low disturbance

(Fig. 2c). This difference was also supported by PLFA data.

Low- compared to high-disturbance soil showed a higher frac-

tion of fungal (including arbuscular mycorrhizal fungi) and

individual Gram()) PLFAs, and a lower fraction of Gram(+)

and actinomycetes PLFA (Table 2).

After 18 months of 13C labelling, only a small quantity of

new C (1–3% of total) was incorporated in Gram()) and

Gram(+) bacteria PLFA irrespective of disturbance treatments

(Table 2). This result indicated that those decomposers either

fed on pre-experimental soil organic matter or most of them

were in a dormant stage. In contrast, soil fungi dependedmore

on the newly fixed plant C as the higher percentages of new C

(9–13% of total) compared to bacteria indicated. Overall, the

percentages of newC in total PLFA biomarkers were higher in

HH than in LL, demonstrating a higher microbial biomass

turnover under high disturbance.

DYNAMICS OF ECOSYSTEM FUNCTIONING AFTER A

DISTURBANCE CHANGE

Hours and days following the cut of above-ground biomass,

plant photosynthesis rate decreased in monoliths previously

adapted to low disturbance (LH). This was followed by a net

emission of CO2 during daytime for several days, indicating

that plant shoot respiration exceeded photosynthesis (for

details seeKlumpp, Soussana&Falcimagne 2007a).

1.5 month after a shift to high disturbance (LH versus LL),

plant community structure changed significantly (Fig. 2a),

Per

cent

age

of d

issi

mila

rity

Month after management change

0 2 4 6 8 10 12 14 16 18 20 22 24

0

20

40

60

Bacterial community

NSNS

NS

*** *

***

0

20

40

60

80

100 (a)

(b)

(c)

Plant community

*

NS

*

NS NSNS

* **

*

0

10

20

30

40

*

**

NS

*

***

Plant functional traits

Fig.2. Percentage of dissimilarity for plant (a) and functional traits of leaves (b) and bacterial community (c) between treatments submitted to

constant disturbance (LL versusHH,•), shift to high disturbance (LH versus LL,s) and shift to low disturbance (HL versusHH,r). Data points

are means of four replicates per treatment for percentage of dissimilarity being significant (*, P £ 0.05) or not (NS). In all three graphs, the two

constant disturbance treatments (HH versus LL,•) were significantly dissimilar (P £ 0.05) throughout the experiment. For plant species composi-

tion, percentage of dissimilarity was determined for species (n = 30) with a presence throughout the experiment (>1%). Dissimilarity of plant

functional traits was determined by using specific leaf area, leaf drymatter content and leaf nitrogen content.

880 K. Klumpp et al.

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

showing a shift fromhigh-stature plant species (i.e.Elytrigia re-

pens, Arrhenatherum elatius, Holcus lanatus, Cirsium sp., Vicia

sp.) to low-stature plant species (i.e.Agrostis capillaris, Dactylis

glomerata) supporting frequent cuts (data not shown). Species

replacement resulted in considerable changes of community

aggregated traits (Fig. 2b) that were related tomodified growth

conditions (i.e. light, nutrients, regrowth) and defoliation

intensity. For example, a shift from LL to LH increased spe-

cific leaf area (SLA) and leaf nitrogen content (LNC). How-

ever, a shift to low disturbance (HL versus HH) lead to non

significant changes in plant species community, indicating, a

shift to less frequent disturbance, modifies botanical composi-

tion only slowly (Fig. 2a).

Six months after shift to high disturbance (LH versus LL),

less old C was found in roots and rhizome due to an increased

mortality (and reserve mobilization) of root and rhizome tis-

sues (Fig. 3 and Table 1). Both, increased root mortality and

lower transfer of new C to roots and rhizomes resulted in a

reduction of total root and rhizome mass (Fig. 3). Despite the

greater input of old C into soil, caused by the increased root

and rhizome mortality, the amount of old C in cPOM and

fPOM remained unchanged (Fig. 3). Model results indicate

that microbial decomposition of old POM was accelerated

Month after start of 13C labelling

Old

R +

Rh

mg

C g

–1 s

oil

0

2

4

6

8

a

a

b b

a

b

b c

a

a

b c

a a

b c

a a b c

a

b

Old

cP

OM

m

g C

g–1

soi

l

0.0

0.5

1.0

1.5

2.0

2.5

a

a

b b

a

a a a

b

b

a

b

b a

b

a

b b b

b

a

a

Old

fPO

Mm

g C

g–1

soi

l

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

3.0

b

a

b

b

a

b

a

a

a

a a a a

a b b

a a a a

a

a

Tot

al R

+ R

hm

g C

g–1

soi

l

0

2

4

6

8

a

a

b

b

b

b

a

b c

a

a

a a

a

a

b

a

a b

c c

b*

Tot

al c

PO

Mm

g C

g–1

soi

l

0.0

0.5

1.0

1.5

2.0

2.5

b

a a a

b

a a

b b b

a

a

a ab ab b

b b b

b b b

Tot

al fP

OM

mg

C g

–1 s

oil

0 5 10 15 20 250.0

0.5

1.0

1.5

2.0

2.5

3.0

a

b a

a

b b

b

a

aaaa

a

a

a

a a

a b

b b

b

Fig.3. Total and old C in roots and rhizomes (R + Rh), coarse particulate organic matter (cPOM) and fine particulate organic matter (fPOM)

during 13C-labelling experiment in the 0–10 cm soil layer. Four treatments were studied: constant low (LL,•) and high (HH, .) experimental dis-

turbance, and shift to low (HL,r) and high (LH,s) disturbance. Data are means of four replicate mesocosms. Letters indicate significant differ-

ences (P £ 0.05) between treatments for a given date.

Table 1. Mean residence times (MRT, in years; Flowchart Fig. 1) of

carbon in roots and rhizomes, coarse (cPOM) and fine (fPOM)

particulate organic matter in pasture monoliths with constant low

disturbance (LL), constant high disturbance (HH), shift to low (HL)

and shift to high (LH) disturbance. Results are means of four

replicates per treatment. For a given compartment, letters indicate

significant differences (P £ 0.05) between treatments; n = 3 as one

replicate was randomly left out each time. When appropriate, data

was log-transformed prior to analysis to conform to the assumption

of normality

Treatment Compartment MRT (years)

LL Root & rhizomes 2.3a

LH 1.0bc

HH 0.9b

HL 1.8ac

LL cPOM 0.4a

LH 0.2b

HH 0.3b

HL 0.5a

LL fPOM 0.9ac

LH 0.7b

HH 1.0c

HL 1.3c

Effect of grazing on ecosystem carbon cycling 881

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

after disturbance was intensified (LH) (Table 1). The shift to

reduced disturbance (HL versus HH) increased root and rhi-

zome biomass through a reduction of root mortality (Fig. 3

and Table 1). Despite the reduced transfer of old C from roots

to POM, the amount of C in POM remained constant (Fig. 3),

indicating a reduced decomposition rate in this fraction

(Table 1). Changes in decomposition rates of old C in the LH

and HL treatments were parallel to a significant change in

bacterial community revealed by theARISAmethod (Fig. 2c).

Twelve months after a shift to high disturbance (LH),

markedly less old C was detected in coarse and fine POM,

confirming a substantial acceleration of the decomposition

of pre-experimental POM (Fig. 3). Modelling shows that

decomposition rates of cPOM and fPOM were accelerated by

50% and 80%, respectively, after intensified disturbances

(LH), leading to a reduction of total particulate organic C

(Fig. 3). In contrast, decomposition rates of cPOM and

fPOM were decelerated by 70% and 30%, respectively, after

reduced disturbance (HL), increasing total particulate

organic C. The acceleration of POM decomposition in the

LH treatment released large amounts of nutrients. Considering

differences in organic nitrogen of R + Rh and POM within

the 0–30 cm soil layer between the LL and LH treatment, an

amount of 14.3 g N m)2 (143 kg N ha)1) was released

through accelerated microbial decomposition (Table S2 in

Supporting Information). Most released N was retained by

grassland monoliths as loss of N through leaching was low

(0.8 g N m)2 year)1). Together with N fertilization this addi-

tional N contributed to a higher nitrogen nutrition index

(Lemaire & Salette 1983) in the LH (0.76 ± 0.05) compared

to the LL treatment (0.56 ± 0.05). In the HL treatment,

the deceleration of POM decomposition sequestrated

3.8 g N m)2 (38 kg N ha)1).

Eighteen months after management change, analyses of

PLFAs allowed a better description of changes in themicrobial

community and their consequences on decomposition pro-

cesses. Intensified disturbance (LH) decreased the fraction of

fungal PLFA by a factor of 2 and increased the fraction of

Gram(+) bacteria by a factor of 2 (Table 2). The percentage of

new labelled C in Gram(+) bacteria PLFA was less than 5%,

indicating that these microorganisms mostly decomposed old

POM deposited prior to disturbance change. Reduced

disturbance (HL) did not significantly change microbial com-

munities (Table 2). Thus, as supported by analyses of micro-

bial DNA (Fig. 1), the ‘recovery’ of a fungi-dominated soil

microbial community after reduced disturbance ismuch slower

than the proliferation of Gram(+) after intensified disturbance.

Arbuscular mycorrhizal fungi (e.g. 16:1x5, 20:3 and 20:4) and

Table 2. Mean relative abundance of individual PLFAs and percentage (%) of new C in PLFAs of fungal, Gram()) and Gram(+) bacteria,

mycorrhizal fungi and actinomycetes in pasture monoliths with constant low disturbance (LL), shift to high disturbance (LH), constant high

disturbance (HH) and shift to low disturbance (HL). For a given date, letters indicate significant difference (P-values £ 0.05, bold) between

treatments. Datawere arcsine-transformed prior to analysis to conform to the assumption of normality

Time community PLFA

t0 t18 month

LL HH LL LH HH HL

Fungal 18:1 x9 21a 19b 27a 17 b 20ab 20ab

18:2 x6 6a 5a 11a 3b 7ab 3b

Total 27a 24b 38a 20b 27b 23b

%new C 9a

13a 9a 13a

Gram()) 16:1 x7 br 11a 9a 7a 7a 11b

8a

17:1 x8 4a

3b 5ab 2a 2

a8b

18:1 x7 13a 12a 15a 11ab 12ab 8b

cy 17:0 br 2a 2a 1a 2b 2b 2b

cy 19:0 7a

13b

7a

11b 11b 12b

Total 37a

39a 35a 33a 38a 38a

%new C 1a 2a 4c 3bc

Gram(+) 16:0 n 9a

11b

6a

12b 8b 10b

17:0 br 4a 4a 1a 5b 4b 5b

n 1a 2b 0a 2b 1b 2b

a 15:0 8a

10b

6a

10bc 8ac 10bc

i 15:0 4a

6b

3a

7b 4ab 3a

Total 26a

33b

17a

34b 26c 29c

%new C 2a 3b 2a 4c

Mycorrhizal fungi 16:1 x5 7a 2b 6a 6a 5a 7a

20:3 0.4a 0 b 1a 1a 0.6a 0.4a

20:4 2a 1a 4a 5a 3ab 1b

Total 9a

3b 11a 12a 8a 9a

%new C 1a 1a 2b 2b

Actinomycetes 16:0 10Me 1a

2b 0a 0.5a 1

ab2b

%new C 0 a 0 a 0 a 0.1b

882 K. Klumpp et al.

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

actinomycetes (16:0 10Me) were not affected by disturbance

change.

Discussion

The description of grassland monoliths maintained under

constant low and high grazing disturbance for the last 14 years

was consistent with the commonly observed pattern (e.g. De

Deyn, Cornelissen & Bardgett 2008; Bardgett et al. 2005;

Wardle et al. 2004 etc). Grassland mesocosms exposed to low

disturbance were characterized by high-stature plant species

with a slow-growth strategy, fungi-dominated soil community

and slow soil C cycling (i.e. C-storing grassland) (Fig. 2,

Tables 1 and 2). In contrast, grassland mesocosms exposed to

high grazing were dominated by small-stature plant species

with a fast-growth strategy, bacteria-dominated soil commu-

nity, higher above-ground productivity and lower soil carbon

storage (i.e. C-releasing grassland) (see Klumpp, Soussana &

Falcimagne 2007a). Hence, the studiedmesocosms are a useful

proxy to investigate the mechanistic basis of the transition

between undisturbed C-storing and disturbed C-releasing

ecosystems.

HOW DOES THE TRANSIT ION OCCUR?

Changing the frequency of disturbance led to a cascade of

effects which tend to be opposed between intensified and

reduced disturbance treatments. Shift to high disturbance led,

within hours, to a sharp decrease in canopy photosynthesis

during daytime and net emission of CO2 for several days.

During this time, plant respiration exceeded plant photosyn-

thesis, and reserve remobilization and C starvation were likely

to increased root mortality (Table 1). 1.5 months after a shift

to high disturbance (LH versus LL), plant and soil community

structure changed significantly. Changes in plant community

structure were assigned to a replacement of slow-growing

high-stature plant species by fast-growing low-stature plant

species, which do support frequent defoliations.

These changes induced a decrease in root biomass and an

acceleration of old (pre-experimental) POM decomposition

within 6 months following the shift to high disturbance

(Fig. 2). The accelerated mineralization of old POM was con-

current to a proliferation of Gram(+) bacteria and a decline in

the fungal and Gram()) communities (Table 2). The fact that

only a small amount of new labelled C (<5%) was found in

Gram(+) bacteria indicated that these microorganisms mostly

decomposed old POM. Hence, the accelerated mineralization

of POMwas provoked by a proliferation of Gram(+) bacteria.

Changes in mineralization could not be ascribed to mineral

nitrogen supply as the increase in decomposition rate (50 and

80% for coarse and fine POM, respectively, Table 1) was ten

times higher than the observed effects of N addition in grass-

lands (Van der Krift, Kuikman & Berendse 2002; Manning

et al. 2008). Moreover, it is well established that N% of about

1.7 is sufficient to cover the requirements of microbes (Waks-

man & Tenney 1927; Melin 1930; Parton et al. 2007). Thus,

although decomposition of roots and coarse POMmay require

an additional source of nitrogen (N%was 1.1% and 1.4% for

roots and fine POM, respectively, see Table S1 in Supporting

Information), N addition was not likely to be the main driver

of the accelerated decomposition of fine POM (N% was 1.6).

Consequently, intensified grazing triggers proliferation of

Gram(+) bacteria and subsequent faster decomposition by

reducing plant roots adapted to low disturbance.

These results support our hypothesis that disturbance affects

soil C cycling by altering plant roots and their control on

microbial community and decomposition. At last, the acceler-

ated POM mineralization decreased soil carbon storage

(Table 1) and released plant available nitrogen, which contrib-

uted to the higher above-ground primary production of these

LH monoliths (Klumpp, Soussana & Falcimagne 2007a).

However, N losses through leaching were doubled (0.8 g and

0.4 g N m)2 years)1 in the LH compared to the LL treat-

ment), providing evidence that less frequently disturbed eco-

systems better retain nutrients.

Overall, changes in plant and soil community structure and

related processes were slower with reduced disturbance than

intensified disturbance. This difference could be explained by

contrasting growth rates of plant and microbial communities

in disturbed and undisturbed grasslands. After intensified dis-

turbance, new conditions created by defoliation (high avail-

ability of light and nutrients) allow fast-growing plant species

to dominate quickly, leading to a rapid change in soil pro-

cesses. In contrast, after reduced disturbance, an exclusion of

fast-growing plant species is slow and depends on the depletion

of key resources by slow-growing plant species. Moreover, the

slower transition may be exacerbated by the soil community

since soil fungi commonly have a slower growth rate than bac-

teria (Swift, Heal & Anderson 1979). The transition between

fast- and slow-growing communities therefore explains why,

after cessation of disturbance, the C storage capacity recovers

more slowly than this capacity is lost after intensified distur-

bance (Fig. 3).

ABOVE-GROUND AND BELOW-GROUND INTERACTIONS

CONTROLLED THROUGH LIV ING ROOT ACTIV ITY

It has been proposed that plant communities control soil

microbial communities and subsequent processes through lit-

ter quality (Seastedt 1985; Berendse, Bobbink & Rouwenhorst

1989; Merrill, Stanton & Hak 1994; Bardgett, Wardle &

Yeates 1998). Although plant species may greatly differ in their

decomposability, these differences are unlikely to affect

decomposition at ecosystem scale, as no correlation has been

found between species decomposability and fast- and slow-

cycling ecosystems (Van der Krift et al. 2001; Manning et al.

2008). Our results provide support that plant communities

exert control on soil microbial communities and related

processes through their living roots, which modify soil

resources (moisture, inorganic N, labile C) and provide rhizo-

deposits (Personeni & Loiseau 2004; De Deyn & Van der

Putten 2005). Indeed, growth of Gram(+) bacteria and decom-

position of pre-experimental POM were reduced in the pres-

ence of living roots of slow-growing plant communities.

Effect of grazing on ecosystem carbon cycling 883

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

Although the advantage of these reductions for slow-growing

plants remain unclear, maintaining a dense root system,

slow decomposition rates and, hence, low nutrient availability

may be seen as a strategy of those plants to hamper the

ingress of fast-growing plants in undisturbed ecosystems.

Moreover, mediating soil organic matter decomposition

through the activity of living roots will have a larger

impact than the production of low-quality litter. Indeed,

as shown here, the activity of living roots controlled litter

decomposition independently of their quality (Table 1 and S2).

However, further experiments, under controlled conditions,

are necessary to directly assess effects of plant species (slow-

versus fast-growing communities) on soil community and

processes.

IMPL ICATIONS FOR PRIMARY PRODUCTION, NUTRIENT

RETENTION AND CARBON STORAGE IN GRASSLANDS

Our results indicate that grazing triggers fast carbon and

nutrient cycling by altering plant roots and their control on

microbial community and decomposition. Furthermore, they

signify that slow and fast nutrient cycling have complementary

effects on biomass production. Intensified grazing stimulates

microbial decomposition, releases the previously accumulated

nutrients and increases primary production (Table 1 and S2).

However, despite returns of urine and faecal material by

herbivores, a depletion of stored nutrient in fast-cycling

(C-releasing) grasslands will occur over time, owing to lower

nutrient retention (N leaching was doubled in LH compared

to the LL treatment) and exportation by herbivores. Conse-

quently, to maintain primary production without fertili-

zation in the long-term, a return to slow-cycling (C-storing)

ecosystem is required to build new organic reserves (Table 1

and S2). Indeed, slow-cycling grasslands have a greater reten-

tion of nutrients due to higher root biomass (Fig. 3 and

Table S2).

In order to compensate climate change and food require-

ments of a growing human population, changes in land use

towards more productive (but disturbed) systems are likely to

become more frequent in the coming decades (FAO 2008).

Our results show that more productive and disturbed grass-

land systems will foster POM decomposition and soil C loss.

This concurs with a report that, for a range of European grass-

land sites, net ecosystem carbon storage declines with increas-

ing disturbance by grazing and cutting (Soussana et al. 2007).

Provided that microbial decomposition is limited to the POM

compartment, soil C loss should be low given that POM repre-

sents a small fraction of total soil organic matter. However,

studies have shown that nutrient loss of ecosystem favours the

activity of mining microbes and accelerates the decomposition

of recalcitrant soil organic matter (Carreiro et al. 2000; Fon-

taine et al. 2004; Waldrop, Zak & Sinsabaugh 2004; Fontaine

& Barot 2005). Therefore, higher N losses of ecosystems

exposed to high disturbance may lead to a long-term soil C

loss. However, further work is needed to investigate the impact

of ecosystem disturbance on the loss of recalcitrant soil organic

carbon.

Acknowledgements

We thank Patrick Pichon (UR 874), Claire Commenaux (UMR5557),

Emanuelle Personeni, and Christiane Kramer for their technical and scientific

contributions. We also thank Juliette Bloor and Nicolas Gross, Vincent Maire

and anonymous referees for helpful comments on the manuscript. This work

was supported by aMarie Curie Individual Fellowship (EVK2-CT2002-50026)

to K.K., a PhD Scholarship (INRA – Region Poitou Charentes) to E. A., the

European FP5 ‘GREENGRASS’ project (EVK2-CT2001-00105) and the

Project ANRBIOMOS.

References

Attard, E., Degrange, V., Klumpp, K., Richaume, A., Soussana, J.F. & Le

Roux, L. (2008) How do grasslandmanagement history and bacterial micro-

localisationaffect the responseofbacterial community structure to changes in

abovegroundgrazing regime?SoilBiologyandBiochemistry,40, 1244–1252.

Bardgett, R.D. & McAlistair, E. (1999) The measurement of soil fungal:bacte-

rial biomass ratios as an indicator of ecosystem self-regulation in temperate

meadow grasslands.Biology and Fertility of Soils, 29, 282–290.

Bardgett, R.D., Wardle, D.A. & Yeates, G.W. (1998) Linking above-ground

and below-ground food webs: how plant responses to foliar herbivory influ-

ence soil organisms. Soil Biology and Biochemistry, 30, 1867–1878.

Bardgett, R.D., Bowman,W.D., Kaufmann, R. & Schmidt, S.K. (2005) A tem-

poral approach to linking aboveground and belowground ecology.Trends in

Ecology and Evolution, 20, 634–641.

Berendse, F., Bobbink, R. & Rouwenhorst, G. (1989) A comparative study on

nutrient cycling in wet heathland ecosystems. II. Litter production and nutri-

ent mineralization.Oecologia, 78, 338–348.

Brennan, P.J. (1988) Mycobacterium and other actinomycetes. Microbial

Lipids (eds C. Ratledge & S.G. Wilkinson). pp. 203–298, Academic Press,

London.

Carreiro, M.M., Sinsabaugh, R.L., Repert, D.A. & Parkhurst, D.F. (2000)

Microbial enzyme shifts explain litter decay responses to simulated nitrogen

deposition.Ecology, 81, 2359–2365.

De Deyn, G.B., Cornelissen, J.H.C. & Bardgett, R.D. (2008) Plant functional

traits and soil carbon sequestration in contrasting biomes. Ecology Letters,

11, 516–531.

De Deyn, G.B. & Van der Putten, W.H. (2005) Linking aboveground and

belowground diversity.Trends in Ecology and Evolution, 20, 625–633.

FAO (2008) La Situation Mondiale de L’alimentation et de L’agriculture. FAO,

Rome.

Fontaine, S. & Barot, S. (2005) Size and functional diversity ofmicrobe popula-

tions control plant persistence and long-term soil carbon accumulation.

Ecology Letters, 8, 1075–1087.

Fontaine, S., Bardoux, G., Abbadie, L. & Mariotti, A. (2004) Carbon input to

soil may decrease soil carbon content.Ecology Letters, 7, 314–320.

Frostegard, A. & Baath, E. (1996) The use of phospholipid fatty acid analysis

to estimate bacterial and fungal biomass in soil.Biology and Fertility of Soils,

22, 59–65.

Guitan, R. & Bardgett, R.D. (2000) Plant and soil microbial responses to defo-

liation in temperate semi-natural grassland.Plant and Soil, 220, 271–277.

Hamilton, E.W. & Frank, D.A. (2001) Can plants stimulate soil microbes and

their own nutrient supply? Evidence from a grazing tolerant grass. Ecology,

82, 2397–2402.

Holland, E.A. & Detling, J.K. (1990) Plant response to herbivory and below-

ground nitrogen cycling.Ecology, 71, 1040–1049.

Jabaji-Hare, S. (1988) Lipid and fatty acid profiles of some vesicular– arbuscu-

larmycorrhizal fungi: contribution to taxonomy.Mycologia, 80, 622–629.

Klumpp, K. & Soussana, J.F. (2009) Using functional traits to predict grass-

land ecosystem change: a mathematical test of the reponse-and-effect trait

approach.Global Change Biology, DOI: 10.1111/j1365-2486.200901905.x

Klumpp, K., Soussana, J.F. & Falcimagne, R. (2007a) Effects of past and cur-

rent disturbance on carbon cycling in grassland mesocosms. Agriculture,

Ecosystems & Environment, 121, 59–73.

Klumpp, K., Soussana, J.F. & Falcimagne, R. (2007b) Long-term steady state13C labeling to investigate carbon turnover in plant soil systems. Biogeo-

sciences, 4, 385–394.

Kramer, C. & Gleixner, G. (2006) Variable use of plant- and soil-derived car-

bon by micro-organisms in agricultural soils. Soil Biology and Biochemistry,

38, 3267–3278.

Kroppenstedt, R.M., Greinermai, E. & Kornwendisch, F. (1984) Analysis

of fatty acids and other lipids of actinomycetes and coryneform bacteria.

Systematic and AppliedMicrobiology, 5, 273.

884 K. Klumpp et al.

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885

Kruskal, J.B. &Wish,M. (1978)Multidimensional Scaling. Beverley Hills, Cali-

fornia.

Lemaire, G. & Salette, J. (1983) Relation entre dynamique de croissance et

dynamique de prelevement d’azote pour un peuplement de graminees

fourrageres. I. Etude de l’effet dumilieu.Agronomie, 4, 423–430.

Loiseau, P. & Soussana, J.F. (1999) Elevated [CO2], temperature increase and

N supply effects on the turnover of below-ground carbon in a temperate

grassland ecosystem.Plant and Soil, 210, 233–247.

Louault, F., Pillar, V.D., Aufrere, J., Garnier, E. & Soussana, J.F. (2005) Plant

traits and function types in response to reduced disturbance in semi-natural

grassland. Journal of Vegetation Science, 16, 151–160.

Manning, P., Saunders, M., Bardgett, R.D., Bonkowski, M., Bradford, A.,

Ellis, R.J., Kandeler, E., Marhan, S. & Tscherko, D. (2008) Direct and indi-

rect effects of nitrogen deposition on litter decomposition. Soil Biology and

Biochemistry, 40, 688–698.

McNaughton, S.J., Banyikwa, F.F. & McNaughton, M.M. (1997) Promotion

of the cycling of diet-enhancing nutrients by African grazers. Science, 278,

1798–1800.

Melin, E. (1930) Biological decomposition of some types of litter from North

America forest.Ecology, 11, 72–101.

Merrill, E.H., Stanton, N.L. & Hak, J.C. (1994) Responses of bluebunch

wheatgrass, Idaho fescue and nematodes to ungulate grzing in Yellowstone

National Park.Oikos, 69, 231–240.

Milchunas, D.G. & Lauenroth, W.K. (1993) Quantitative effects of grazing on

vegetation and soils over a global range of environments.Ecological Monog-

raphes, 63, 327–366.

Olsson, P.A. (1999) Signature fatty acids provide tools for determination of the

distribution and interactions of mycorrhizal fungi in soil. FEMS Microbiol-

ogy Ecology, 29, 303–310.

Parton, W., Silver, W., Burke, I., Grassens, L., Harmon, M.E., Currie, W.S.,

King, J.Y., Adair, E.C., Brandt, L.A., Hart, S.C. & Fasth, B. (2007) Global-

scale similarities in nitrogen release patterns during long-term decomposi-

tion. Science, 315, 361–364.

Patra, A.K., Abbadie, L., Clays, A., Degrange, V., Grayston, S., Loiseau, P.

et al. (2005) Effect of grazing on microbial functional groups involved in soil

N dynamics.EcologicalMonographs, 75, 65–80.

Patra, A.K., Abbadie, L., Clays, A., Degrange, V., Grayston, S., Guillaumaud,

N. et al. (2006) Effects of management regime and plant species on the

enzyme activity and genetic structure of N-fixing, denitrifying and nitrifying

bacterial communities in grassland soils. Environmental Microbiology, 8,

1005–1016.

Personeni, E.&Loiseau, P. (2004)How does the nature of living and dead roots

affect the residence time of carbon in the root litter continuum? Plant and

Soil, 267, 129–141.

Pickett, S.T.A. & White, P.S. (1986) The Ecology of National Disturbance and

Patch Dynamics. Academics Press. Inc., London 472 p.

Ranjard, L., Nazaret, S., Gourbiere, F., Thioulouse, J., Linet, P. & Richaume,

A. (2000)A soil microscale study to reveal the heterogeneity ofHg(II) impact

on indigenous bacteria by quantification of adapted phenotypes and analysis

of community DNA fingerprints. FEMS Microbiology Ecology, 31, 107–

115.

Ratledge, C. &Wilkinson, S.G. (1988)Microbial Lipids. Academic Press, Lon-

don.

Seastedt, T.R. (1985) Maximisation of primary and secondary productivity by

grazers.AmericanNaturalist, 126, 559–564.

Soussana, J.F., Allard, V., Pilegaard, K., Ambus, P., Amman, C., Campbell, C.

et al. (2007) Full accounting of the greenhouse gas (CO2, N2O, CH4) budget

of nine European grassland sites. Agriculture, Ecosystems &Environment,

121, 121–135.

Swift, M.J., Heal, O.W. & Anderson, J.M. (1979) Decomposition in Terrestrial

Ecosystems. Blackwell, Oxford.

Van der Krift, T.A.J., Kuikman, P.J. & Berendse, F. (2002) The effect of living

plants on root decomposition of four grass species.Oikos, 96, 36–45.

Van der Krift, T.A.J., Gioacchini, P., Kuikman, P.J. & Berendse, F. (2001)

Effects of high and low fertility plant species on dead root decomposition

and nitrogen mineralization. Soil Biology and Biochemistry, 33, 2115–

2124.

Waksman, S.A. & Tenney, F.G. (1927) Composition of natural organic materi-

als and their decomposition in the soil: II. Influence of age of plant upon the

rapidity and nature of its decomposition-rye plants. Soil Science, 24, 317–

334.

Waldrop, M.P., Zak, D.R. & Sinsabaugh, R.L. (2004) Microbial community

response to nitrogen deposition in northern forest ecosystems. Soil Biology

and Biochemistry, 36, 1443–1451.

Wardle, D.A., Bardgett, R.D., Klironomos, J.N., Setala, H., Van der Putten,

W.H. & Wall, D.H. (2004) Ecological linkages between aboveground and

belowground biota. Science, 304, 1629–1633.

Wardle, D.A., Bardgett, R.D., Walker, L.R., Peltzer, D.A. & Lagerstrom,

A. (2008) The response of plant diversity to ecosystem retrogression:

evidence from contrasting long-term chronosequences. Oikos, 117, 93–

103.

Zelles, L. (1999) Fatty acid patterns of phospholipids and lipopolysaccharides

in the characterisation of microbial communities in soil: a review. Biology &

Fertility of Soils, 29, 111–129.

Received 10May 2009; accepted 25 June 2009

Handling Editor: Richard Bardgett

Supporting Information

Additional Supporting Information may be found in the online ver-

sion of this article:

Appendix S1.Timescale of applied methods.

Table S1. Mean N content and C ⁄N of roots and rhizomes, coarse

and fine particulate organic matter in the 0–10 cm soil layer.

Table S2. Amount of N release in the HL treatment and sequestrated

in the LH treatment from root and POM pools in the top 30 cm,

18 months after disturbance change.

Please note: Wiley-Blackwell are not responsible for the content or

functionality of any supporting materials supplied by the authors.

Any queries (other than missing material) should be directed to the

corresponding author for the article.

Effect of grazing on ecosystem carbon cycling 885

� 2009 The Authors. Journal compilation � 2009 British Ecological Society, Journal of Ecology, 97, 876–885