Small-scale variability in the contribution of invertebrates to litter decomposition in tropical rice fields

Abstract

Sustainable management of agricultural systems includes promoting nutrient cycles, which can reduce the need for application of fertilizer. As rice is one of the most important food resources in the world, sustainable management of rice paddies is increasingly in demand. However, little is known about the influence of invertebrates on decomposition processes in these ecosystems. We hypothesized that invertebrates contribute significantly to the decomposition of rice straw in paddies and that their relative contribution is affected by the distance to other landscape structures within fields. We placed rice straw in litterbags of two different mesh sizes which prevent (20 m × 20 m) or allow (5 mm × 5 mm) access of invertebrates in six irrigated rice fields for 84 days. In each field, bags were set on three transects running from the bund to the center of the field. Invertebrates significantly increased total rice straw litter mass loss by up to 45% (total decomposition: fine-meshed bags 64%; coarse-meshed bags 83%). Litter mass loss in bags accessed by invertebrates decreased with increasing distance from the bund. Such a spatial trend in litter mass loss was not observed in bags accessed only by microbes. Our results indicated that invertebrates can contribute to soil fertility in irrigated rice fields by decomposing rice straw, and that the efficiency of decomposition may be promoted by landscape structures around rice fields.

Full text

Basic
and
Applied
Ecology
16
(2015)
674–680
Small-scale
variability
in
the
contribution
of
invertebrates
to
litter
decomposition
in
tropical
rice
fields
Anja
Schmidta,∗,
Harald
Augea,b,
Roland
Brandlc,
Kong
Luen
Heongd,e,
Stefan
Hotesc,
Josef
Settelea,b,
Sylvia
Villareald,
Martin
Schädlera,b
aHelmholtz-Centre
for
Environmental
Research
–
UFZ,
Department
of
Community
Ecology,
Theodor-Lieser-Strasse
4,
06110
Halle,
Germany
biDiv,
German
Centre
for
Integrative
Biodiversity
Research,
Halle-Jena-Leipzig,
Deutscher
Platz
5e,
04103
Leipzig,
Germany
cDepartment
of
Ecology,
Faculty
of
Biology,
Philipps-University
Marburg,
Karl-von-Frisch
Strasse
8,
35032
Marburg,
Germany
dInternational
Rice
Research
Institute,
DAPO
7777,
Metro
Manila,
Philippines
eCentre
for
Agricultural
Bioscience
International
(CABI),
SE
Asia
Regional
Centre,
Serdang,
Malaysia
Received
26
February
2014;
accepted
20
January
2015
Available
online
28
January
2015
Abstract
Sustainable
management
of
agricultural
systems
includes
promoting
nutrient
cycles,
which
can
reduce
the
need
for
application
of
fertilizer.
As
rice
is
one
of
the
most
important
food
resources
in
the
world,
sustainable
management
of
rice
paddies
is
increasingly
in
demand.
However,
little
is
known
about
the
influence
of
invertebrates
on
decomposition
processes
in
these
ecosystems.
We
hypothesized
that
invertebrates
contribute
significantly
to
the
decomposition
of
rice
straw
in
paddies
and
that
their
relative
contribution
is
affected
by
the
distance
to
other
landscape
structures
within
fields.
We
placed
rice
straw
in
litterbags
of
two
different
mesh
sizes
which
prevent
(20
␮m
×
20
␮m)
or
allow
(5
mm
×
5
mm)
access
of
invertebrates
in
six
irrigated
rice
fields
for
84
days.
In
each
field,
bags
were
set
on
three
transects
running
from
the
bund
to
the
center
of
the
field.
Invertebrates
significantly
increased
total
rice
straw
litter
mass
loss
by
up
to
45%
(total
decomposition:
fine-meshed
bags
64%;
coarse-meshed
bags
83%).
Litter
mass
loss
in
bags
accessed
by
invertebrates
decreased
with
increasing
distance
from
the
bund.
Such
a
spatial
trend
in
litter
mass
loss
was
not
observed
in
bags
accessed
only
by
microbes.
Our
results
indicated
that
invertebrates
can
contribute
to
soil
fertility
in
irrigated
rice
fields
by
decomposing
rice
straw,
and
that
the
efficiency
of
decomposition
may
be
promoted
by
landscape
structures
around
rice
fields.
Zusammenfassung
Nachhaltigkeit
im
bewässerten
Tiefland-Reisanbau
ist
ein
wesentlicher
Bestandteil
zur
Sicherung
der
Nahrungsgrund-
versorgung
eines
großen
Teils
der
Weltbevölkerung.
Das
Verständnis
der
komplexen
Prozesse
im
Nährstoffkreislauf
in
Agrarökosystemen
kann
zu
einer
Erhöhung
der
Bodenfruchtbarkeit
führen
und
den
Bedarf
an
Düngemitteln
drastisch
reduzieren.
Die
Grundlage
für
eine
natürliche
Stickstoffzufuhr,
zur
Förderung
des
Pflanzenwachstums,
ist
die
Zersetzung
von
totem
organischem
Material,
was
eine
stabile
Gemeinschaft
von
Bodenorganismen
voraussetzt.
Nichtsdestotrotz
ist
das
Wissen
über
den
Einfluss
der
Makrofauna
auf
Zersetzungsprozesse
im
Boden
von
Reisökosystemen
rar.
∗Corresponding
author.
Tel.:
+49
345
5585405;
fax:
+49
345
5585329.
E-mail
address:
a.schmidt@ufz.de
(A.
Schmidt).
http://dx.doi.org/10.1016/j.baae.2015.01.006
1439-1791/©
2015
Gesellschaft
für
Ökologie.
Published
by
Elsevier
GmbH.
All
rights
reserved.

A.
Schmidt
et
al.
/
Basic
and
Applied
Ecology
16
(2015)
674–680
675
Ziel
dieser
Studie
war
es,
den
Einfluss
von
Invertebraten
auf
die
Zersetzungsrate
von
Reisstroh
zu
untersuchen
und
deren
Effektivität
in
Abhängigkeit
landschaftlicher
Strukturvielfalt
in
direkter
räumlicher
Nähe
zu
den
Untersuchungsflächen
einzuschätzen.
Um
zu
differenzieren,
wie
stark
der
jeweilige
Anteil
von
Invertebraten
und
Mikroorganismen
am
Streuab-
bau
ist,
wurden
Streubeutel
mit
zwei
verschiedenen
Maschenweiten
(20
␮m
und
5
mm)
verwendet
und
für
84
Tage
auf
die
Bodenoberfläche
bewässerter
Reisfelder
gelegt.
Der
Einfluss
der
Entfernung
vom
Reisfeldufer
auf
die
Zersetzungsrate
sollte
mit
Hilfe
von
Transekten,
die
vom
Rand
bis
zur
Mitte
von
sechs
Versuchsfeldern
gezogen
wurden,
ermittelt
werden.
Invertebraten
erhöhten
nicht
nur
die
Menge
an
insgesamt
abgebautem
Stroh
bis
zu
45%,
verglichen
mit
der
reinen
mikrobiellen
Zersetzung,
ihr
Einfluss
nahm
auch
vom
Rand
zur
Mitte
des
Feldes
hin
ab.
Die
Abbaurate
der
Mikroorganismen
blieb
innerhalb
des
Feldes
dagegen
relativ
konstant.
Unsere
Ergebnisse
zeigen
deutlich,
dass
Invertebraten
einen
großen
Einfluss
auf
die
Zersetzung
von
Reisstroh
haben
und
damit
die
Bodenfruchtbarkeit
positiv
beeinflussen
können.
Zusätzlich
konnte
eine
positive
Korrelation
zwischen
Ufernähe
und
Abbaugeschwindigkeit
von
Invertebraten
in
Reisfeldern
nachgewiesen
werden,
was
auf
eine
höhere
Nährstofffreisetzung
in
den
Randbereichen
der
Felder
hindeutet.
©
2015
Gesellschaft
für
Ökologie.
Published
by
Elsevier
GmbH.
All
rights
reserved.
Keywords:
Litter
mass
loss;
Detritivores;
Nutrient
supply;
Oryza
sativa;
Litterbags;
Philippines
Introduction
The
breakdown
of
organic
matter
is
a
crucial
mecha-
nism
for
nutrient
cycling
and
productivity
in
terrestrial
and
aquatic
ecosystems
(Cebrian
&
Lartigue,
2004).
Invertebrates
play
a
key
role
in
the
decomposition
process
in
both
terrestrial
(Swift,
Heal,
&
Anderson,
1979)
and
aquatic
systems
(Webster
&
Benfield,
1986).
Among
other
things,
invertebrates
break
down
bigger
particles
and
make
them
available
for
microorganisms
that
decompose
the
mate-
rial
further
and
are
responsible
for
nutrient
release.
These
microorganisms
are
in
turn
one
of
the
most
important
sources
of
energy
for
many
soil
(Swift
et
al.,
1979)
and
aquatic
ani-
mals
(Perry
&
Sheldon,
1986;
Hamilton,
Lewis,
&
Sippel,
1992).
Invertebrate
decomposers
are
also
known
to
act
as
scavengers
(Parmenter
&
MacMahon,
2009).
Besides
their
importance
in
the
decomposition
process,
invertebrate
decomposers
were
found
to
be
an
important
food
resource
for
predators
(Ishijima
et
al.,
2006;
Oelbermann,
Langel,
&
Scheu,
2008).
In
rice
fields,
for
example,
the
use
of
decom-
posers,
like
chironomid
larvae,
as
secondary
food
source
allows
generalist
predators,
as
e.g.
some
groups
of
aquatic
Heteroptera,
to
maintain
high
abundances
throughout
the
whole
rice
cycle
(Settle
et
al.,
1996).
Therefore,
the
role
of
invertebrate
decomposers
in
food
webs
is
crucial
for
the
main-
tenance
of
ecosystem
functions
related
to
nutrient
cycling,
habitat
structure,
and
community
dynamics.
Rice
cultivation
is
one
of
the
most
important,
stable,
and
successful
agricultural
branches
in
tropical
regions,
espe-
cially
in
Southeast
Asia
(Kurihara,
1989).
Toward
the
end
of
the
Green
Revolution,
after
the
mid-1960s,
rice
pro-
duction
was
intensified
all
over
the
world,
especially
in
Asia
(Bambaradeniya
&
Amarasinghe,
2003).
The
negative
impacts
of
these
agricultural
practices
for
invertebrate
food-
webs
in
rice
fields
have
been
shown
mainly
for
predators
and
parasitoids,
which
are
the
most
important
natural
pest
control
agents
(Schoenly
et
al.,
1996;
Ives
&
Settle,
1997;
Drechsler
&
Settele,
2001),
or
on
the
pest
species
them-
selves
(Kiritani,
1992;
Settele,
1992;
Cohen
et
al.,
1994).
In
contrast,
studies
on
the
detritivorous
invertebrate
fauna
in
rice
ecosystems
focused
solely
on
the
diversity
or
the
abun-
dance
of
invertebrate
decomposers
(Simpson
et
al.,
1993a,
1993b;
Simpson,
Roger,
Oficial,
&
Grant,
1994)
with
only
speculations
about
their
functional
role
for
decomposition
and
therefore
nutrient
dynamics.
The
lack
of
such
studies
in
rice
fields
is
surprising
since
the
soil
fauna
is
known
to
contribute
substantially
to
nutrient
dynamics
and
productiv-
ity
in
agro-ecosystems
(Benckiser,
1997).
Generally,
there
is
no
conceptual
consensus
about
the
role
of
invertebrate
decomposers
in
freshwater
ecosystems.
Moreover,
studies
in
tropical
freshwater
ecosystems
have
been
done
mainly
in
streams
(Hagen
et
al.,
2012),
and
to
our
knowledge,
virtually
no
information
on
the
contribution
of
invertebrates
to
litter
decay
in
other
tropical
freshwater
ecosystems,
such
as
rice
fields,
is
available.
In
general,
the
contribution
of
fauna
to
the
decomposition
process
in
the
tropics
is
suggested
to
be
high
both
in
terrestrial
and
aquatic
habitats
(Wall
et
al.,
2008).
However,
compared
to
terrestrial
habitats
invertebrate
activ-
ity
and
litter
characteristics
might
be
of
lower
importance
during
the
initial
phase
of
litter
decay
in
aquatic
ecosystems
as
due
to
higher
leaching
of
organic
and
mineral
compounds
mass
loss
tends
to
be
high
(Treplin
&
Zimmer,
2012).
The
decomposition
process
in
irrigated
rice
fields
may
differ
from
“real”
aquatic
systems
in
many
aspects.
For
example,
tillage
and
application
of
fertilizer
and
pesticides
can
change
soil
and
water
properties.
Various
studies
have
demonstrated
an
influence
of
nutrient
concentrations
in
water
on
microbial-driven
decomposition
dynamics,
with
prevalent
positive
effects
of
nutrient
addition
on
the
decay
rate
(Webster
&
Benfield,
1986).
Thus,
the
intensive
application
of
fer-
tilizers
may
lessen
the
relative
importance
of
invertebrates
in
the
decomposition
process.
Furthermore,
fields
are
often

676
A.
Schmidt
et
al.
/
Basic
and
Applied
Ecology
16
(2015)
674–680
irrigated
only
during
certain
periods
of
the
year
and
regularly
fall
dry.
Thus,
macro-decomposers
strictly
bound
to
a
water
habitat
may
not
establish
stable
populations
or
may
not
reach
high
abundances.
However,
most
aquatic
invertebrate
decomposers
are
not
restricted
to
aquatic
habitats
throughout
their
life
cycles.
Some
insects,
which
can
be
also
found
in
irrigated
rice
fields,
are
involved
in
the
decomposition
process
during
their
aquatic
larval
stages,
e.g.,
chironomid
larvae,
and
populate
the
surrounding
terrestrial
habitats
as
adults.
The
impact
of
surrounding
landscape
structures
on
ecosystem
func-
tions
has
repeatedly
been
shown
for
different
arable
fields
(e.g.,
Perfecto
and
Vandermeer,
2002;
Diekötter,
Wamser,
Wolters,
&
Birkhofer,
2010;
Woodcock
et
al.,
2010).
The
spatial
variability
is
often
reflected
by
a
decrease
in
diversity
and
corresponding
ecosystem
functions
(e.g.,
pollination)
in
agro-ecosystems
with
increasing
distance
from
surround-
ing
landscape
structures
(e.g.,
Klein,
Steffan-Dewenter,
&
Tscharntke,
2003;
Klein,
2009).
However,
invertebrate
decomposers
are
often
ignored
in
such
studies
despite
their
known
importance
for
ecosystem
functioning.
Here,
we
investigated
whether
invertebrate
decomposers
play
an
important
role
in
the
decomposition
process
and
if
this
function
is
mediated
by
the
distance
from
surrounding
ter-
restrial
habitats.
As
a
proxy
for
decomposition
we
measured
litter
mass
loss
of
rice
straw
in
litterbags
with
and
without
access
for
invertebrates
in
paddy
fields
surrounded
by
six
different
landscape
structures
reflecting
a
broad
spectrum
of
prevalent
structures
in
the
region
of
Laguna,
Philippines.
We
tested
the
following
hypotheses:
(1)
the
invertebrate
fauna
contributes
considerably
to
the
litter
mass
loss
of
rice
straw
in
paddy
fields,
and
(2)
the
contribution
of
invertebrates
to
the
decomposition
process
in
rice
fields
decreases
with
increasing
distance
to
the
surrounding
landscape
structures.
We
assumed
invertebrates
to
have
a
lower
influence
on
litter
mass
loss
in
the
middle
of
the
fields,
e.g.,
as
many
of
them
depend
on
surrounding
structures
in
their
adult
stage
(e.g.
chironomids).
Materials
and
methods
Study
site
The
study
was
conducted
in
the
Laguna
province
on
the
island
of
Luzon,
Philippines,
in
one
of
the
lowland,
rice-
dominated
regions
(Legato-site-label:
PH
1;
Klotzbücher
et
al.,
2015)
as
part
of
the
LEGATO
project
(Settele
et
al.,
2015).
Laguna
lies
southeast
of
the
capital
Manila
(WGS84:
14.2
N;
121.4
E)
and
is
characterized
by
a
diverse
land-
scape
structure
consisting
of
intensively
used
agricultural
areas
and
near-natural
forests,
gardens,
and
various
types
of
plantations.
Field
sites
were
located
within
an
area
of
around
100
km2.
The
soil
in
this
area
is
of
volcanic
origin,
and
a
high
proportion
consists
of
clay
and
loam.
The
aquatic
decom-
poser
mesofauna
is
dominated
by
annelids,
nematodes
and
Table
1.
Description
of
the
six
study
sites
and
their
different
sur-
rounding
structures.
Site
label
Area
(m2)
Description
of
adjacent
surroundings
“Forest”
1200
3
sides
–
forest;
1
side
–
rice
“Bushes”
2000
2
sides
–
approx.
20
m
wide
strip
of
shrub
land
(bordering
a
forest);
2
sides
–
rice
“Rice”
800
4
sides
–
rice
“Wild
meadow”
1600
1
side
–
wild
unmanaged
area;
3
sides
–
rice
“Farm”
600
1
side
–
area
with
a
house
incl.
a
small
yard
with
free-range
chickens;
3
sides
–
rice
“Vine”
500
1
side
–
vines;
3
sides
–
rice
chironomid
larvae
with
other
invertebrate
groups
in
smaller
abundances
(unpubl.).
Wet
rice
in
the
lowlands
of
Luzon
Island
(Philippines)
is
mostly
cultivated
in
two
crop
cycles
per
year,
one
in
the
dry
season
(December–May)
and
one
in
the
wet
season
(June–November).
A
crop
cycle
(without
the
fallow
period)
lasts
around
100
days.
Our
study
was
carried
out
during
the
wet
season
from
June
to
September
in
2012.
During
this
time,
the
monthly
rainfall
varied
between
75
and
465
mm,
and
the
average
temperature
varied
between
25
and
25.9 ◦C.
Weather
data
were
provided
by
the
Climate
Unit
of
the
International
Rice
Research
Institute,
Los
Ba˜
nos,
Laguna,
Philippines.
Around
20
days
after
seed
sowing,
rice
seedlings
were
transplanted
separately
in
the
field.
Paddies
were
drained
2–3
weeks
before
harvesting,
i.e.,
80–90
days
after
transplanting.
Study
design
Six
rice
fields
surrounded
by
different
landscape
elements,
representative
for
the
region,
were
chosen
(Table
1).
Field
management
was
carried
out
according
to
the
usual
man-
agement
scheme
including
fertilizers,
pesticides
etc.
It
is
important
to
note
that
this
study
was
not
designed
to
compare
different
landscape
structures,
but
to
get
an
idea
about
how
structures
around
rice
fields
in
general
can
affect
invertebrate
contribution
to
litter
mass
loss.
To
investigate
the
contribution
of
invertebrates
to
litter
mass
loss,
we
placed
10
g
of
litter
(air-dried
rice
straw;
Oryza
sativa,
variety
NSIC
Rc
222)
in
15
cm
×
20
cm
nylon
bags
of
two
mesh
sizes:
fine,
20
␮m
×
20
␮m
mesh
size,
which
gives
access
to
microbes
(bacteria,
fungi,
etc.)
only;
and
coarse,
5
mm
×
5
mm
mesh
size,
which
gives
access
also
to
invertebrates
(Tian,
Kang,
&
Brussaard,
1992).
The
filled
lit-
terbags
were
set
in
the
fields
in
June/July
2012
after
rice

A.
Schmidt
et
al.
/
Basic
and
Applied
Ecology
16
(2015)
674–680
677
seedlings
had
been
transplanted.
Pairs
of
bags,
one
fine
and
one
coarse
meshed,
were
placed
along
three
transect
lines
on
the
soil
surface
in
every
field
and
fixed
to
the
ground
by
coarse
nylon
nets
and
bamboo
sticks.
Transect
lines
in
each
field
were
4
m
apart
and
reached
from
the
bund
to
the
middle
of
the
field
(7.5
±
1.9
m,
mean
±
SD).
Due
to
the
dif-
ferent
dimensions
of
the
fields
transect
lines
varied
in
length.
Depending
on
the
size
of
the
particular
field,
5–10
pairs
of
bags
were
placed
on
each
transect
line,
with
1
m
between
each
pair;
the
first
pair
was
placed
directly
next
to
the
bund,
but
still
within
the
field.
The
two
bags
of
one
pair
were
placed
directly
next
to
each
other
with
no
space
in
between.
Litterbag
gradients
were
always
established
from
one
particular
border
of
the
field
and
if
a
site
neighbored
the
respective
structure
on
one
side
only,
this
side
was
chosen.
Litterbags
were
retrieved
84
days
after
setting
and
before
rice
harvesting
in
August/September
2012.
Soil
par-
ticles,
roots,
and
other
alien
plant
material
adhering
to
the
litter
were
removed,
and
the
remaining
straw
was
dried
at
60 ◦C
for
3
days
and
weighed
to
the
nearest
centigram.
Data
analysis
To
account
for
the
difference
in
weight
between
air-dried
and
oven-dried
straw
several
samples
of
air-dried
straw
were
weighed
before
and
after
drying
in
the
oven
for
3
days
at
60 ◦C.
The
average
weight
loss
of
all
samples
due
to
moisture
loss
was
then
subtracted
from
the
original
10
g
before
calcu-
lating
litter
mass
losses.
The
percent
loss
in
litter
mass
was
logit
transformed
prior
to
all
statistical
analyses
for
approx-
imation
of
normal
distribution
and
reduction
of
variance
heterogeneity.
Using
a
nested
general
linear
mixed
model
(GLMM)
type
III
sum
of
squares
(procedure
MIXED,
SAS
9.2.),
litter
mass
loss
was
analyzed
in
dependence
on
mesh
size,
site,
and
the
co-variable
distance
from
bund.
Transects
were
considered
random
and
nested
in
site.
The
least-square
means
were
calculated
for
the
six
levels
of
the
factor
site.
To
illustrate
the
contribution
of
invertebrate
decomposers
to
litter
mass
loss
depending
on
the
distance
to
the
bund
within
a
paddy,
the
percentage
litter
mass
loss
was
nor-
malized
to
the
specific
effects
of
the
factor
site.
This
was
done
by
subtracting
the
particular
estimated
mean
value
for
each
site
from
the
corresponding
logit-transformed
litter
mass
loss.
Results
At
all
six
sites,
the
mean
litter
mass
loss
of
the
coarse-
meshed
bags
(83
±
8%,
mean
±
SD)
was
higher
than
in
fine-meshed
bags
(64
±
7.5%,
mean
±
SD)
(Fig.
1).
Thus,
invertebrates
had
a
significant
influence
on
litter
mass
loss
(highly
significant
factor
mesh
size;
Table
2).
However,
litter
mass
loss
due
to
invertebrates
varied
across
sites
(significant
mesh
size
×
site
interaction;
Table
2).
Fig.
1.
Percent
litter
mass
loss
in
fine-meshed
and
coarse-meshed
litterbags
at
the
six
sites
with
different
surrounding
landscape
struc-
tures
(means
±
SE).
Across
all
sites,
the
overall
litter
mass
loss
in
coarse-
meshed
bags
decreased
with
increasing
distance
from
the
bund
of
the
paddy
(Fig.
2).
The
overall
litter
mass
loss
in
fine-meshed
bags,
however,
stayed
relatively
constant
at
the
different
locations
within
the
field.
These
results
indicated
that
the
invertebrate
contribution
to
litter
mass
loss
decreased
with
increasing
distance
from
the
bund.
Furthermore,
the
effect
of
distance
from
the
bund
varied
across
sites
(significant
site
×
distance
from
bund
interac-
tion;
Table
2).
At
four
sites
(landscape
structures:
vines,
rice,
bushes,
and
forest),
litter
mass
loss
decreased
with
increasing
distance
to
the
bund.
At
the
other
two
sites
(with
structures
farm
and
wild
meadow)
litter
mass
loss
increased
with
increasing
distance
to
the
bund,
but
R2values
indicated
stronger
negative
correlations
than
positive
ones
(Fig.
3).
Table
2.
Effects
of
mesh
size,
site,
and
distance
from
bund
as
well
as
their
interactions
on
mass
loss
of
rice
straw
litter
(logit-transformed)
using
a
GLMM
type
III
sum
of
squares.
Factors
Decomposition
rate
df
F
p
Mesh
size
1
180.92
<0.001
Site
5
7.83
0.002
Distance
from
bund
1
13.75
0.003
Mesh
size
×
site
5
5.13
0.009
Mesh
size
×
distance
from
bund
1
11.7
0.005
Distance
from
bund
×
site
5
4.61
0.014
Distance
from
bund
×
mesh
size
×
site
5
2.88
0.062
Factor
mesh
size
represents
the
bags
of
two
mesh
sizes
(5
mm
×
5
mm
and
20
␮m
×
20
␮m)
used
in
every
plot,
factor
site
represents
the
six
fields
with
different
surrounding
landscape
structures,
and
the
co-variable
distance
from
bund
represents
the
continuous
effect
of
the
location
of
litterbags
within
the
fields
(linearly
from
the
bund
to
the
middle
of
the
field).
The
model
also
includes
the
random
effect
transect
nested
in
site
(three
transect
lines
per
site);
the
factor
itself
and
its
interactions
are
not
shown.

678
A.
Schmidt
et
al.
/
Basic
and
Applied
Ecology
16
(2015)
674–680
Fig.
2.
Correlation
of
percent
litter
mass
loss
from
fine-
and
coarse-
meshed
bags
with
distance
from
border.
R2values
indicate
the
strength
of
the
correlations
(p
>
0.05n.s,
p
≤
0.001***).
Percentage
litter
mass
loss
was
controlled
for
site
effects.
Fig.
3.
Correlation
of
mean
percent
litter
mass
loss
at
the
six
sites
with
distance
from
border.
R2values
indicate
the
strength
of
the
correlations
(p
>
0.05n.s,
p
≤
0.05*,
p
≤
0.01**,
p
≤
0.001***).
Discussion
Although
the
potential
of
decomposition
of
straw
from
cereals
for
soil
fertility
and
site
productivity
has
been
highlighted
in
several
studies
(Mary,
Recous,
Darwis,
&
Robin,
1996;
Bhogal,
Young,
&
Sylvester-Bradley,
1997;
Yadvinder-Singh
et
al.,
2004),
the
importance
of
inverte-
brate
decomposers
in
rice
paddies
has
been
underestimated
and
rarely
studied
in
the
past
(Settle
et
al.,
1996).
The
results
reported
here
support
our
hypothesis
that
invertebrates
significantly
contribute
to
litter
mass
loss
of
rice
straw
in
trop-
ical
paddy
fields.
Hence,
invertebrates
might
be
crucial
for
the
establishment
of
sustainable
agriculture
in
rice-dominated
areas
by
increasing
the
soil
fertility
in
irrigated
rice
fields.
As
the
field
management
was
done
as
‘business
as
usual’,
pes-
ticides
might
have
had
negative
effects
on
the
invertebrate
abundance
during
our
study.
The
effect
of
the
invertebrate
fauna
on
decomposition
could
therefore
have
been
underes-
timated,
making
our
results
a
conservative
measure
of
their
contribution
and
our
conclusions
on
their
potential
role
in
ecosystem
functioning
even
more
robust.
As
our
study
was
carried
out
in
conventionally
managed
rice
fields,
our
results
reflect
the
processes
invertebrates
have
on
decomposition
in
a
much
more
realistic
way
than
a
study
on
pesticide
free
paddies
could
have
achieved.
Most
invertebrate
decomposers
in
freshwater
systems
are
not
restricted
to
aquatic
habitats
throughout
their
life
cycles.
Therefore,
the
abundance,
diversity
and
species
composi-
tion
of
invertebrates
in
rice
fields
might
be
closely
linked
to
the
surrounding
landscape.
Using
litter
mass
loss
as
proxy
we
hypothesized
that
the
contribution
of
invertebrates
to
the
decomposition
process
in
rice
fields
decreases
with
increasing
distance
to
the
bunds.
Overall,
we
found
that
invertebrate
decomposition
indeed
declined
with
increasing
bund
distance.
One
reason
for
this
could
be
the
heteroge-
neous
within-field
flooding
where
paddies
are
often
more
constantly
flooded
at
the
borders
than
in
the
middle,
which
would
negatively
affect
aquatic
invertebrates
along
with
their
decomposition
activity.
In
contrast,
litter
mass
loss
of
rice
straw
assigned
to
microbial
decay
activity
was
not
affected
by
the
distance
to
the
bunds
and
microbial
decay
processes
are
also
known
to
be
sensitive
to
changes
in
water
availability.
For
example,
studies
in
peat
soils
showed
that
microorgan-
isms
are
very
sensitive
to
water
and
oxygen
availability
(Jaatinen,
Fritze,
Laine,
&
Laiho,
2007;
Kwon,
Haraguchi,
&
Kang,
2013).
A
drop
in
water
along
with
the
consequen-
tial
increased
oxic
conditions
would
result
in
a
shift
of
the
microbial
community
structure
(Kwon
et
al.,
2013)
and
an
increase
in
microbial
biomass
(Mäkiranta
et
al.,
2009)
and
activity
(Freeman
et
al.,
1996)
and
therefore
lead
to
a
drastic
increase
of
the
microbial
decay
rate.
As
this
was
not
the
case,
the
observed
decrease
of
invertebrate
decomposition
activ-
ity
with
increasing
distance
to
the
surroundings
can
probably
be
attributed
to
the
spatial
effects
of
the
surrounding
land-
scapes
themselves
and
not
to
a
decrease
in
water
availability
in
the
middle
of
the
field.
Future
studies
should
investigate
if
different
landscape
structures
also
have
different
effects
on
decomposer
invertebrates,
which
would
account
for
the
importance
of
structural
diversity
also
at
smaller
spatial
scales
and
the
necessity
to
conserve
and
establish
a
certain
level
of
landscape
heterogeneity
for
sustainable
and
ecological
rice
agriculture.
When
we
examined
the
spatial
effects
within
the
fields
on
the
total
litter
mass
loss
assigned
to
invertebrate
and
microbial
decomposition,
differences
between
the
six
fields
became
apparent.
In
two
of
the
six
fields
the
litter
mass
loss
increased
with
increasing
distance
from
the
bund.
In
contrast,
the
other
four
fields
showed
the
reverse
pattern,
i.e.,
litter
mass
loss
decreased
with
increasing
distance
from
the
bund.
As
our
study
was
not
intended
to
describe
differences
between

A.
Schmidt
et
al.
/
Basic
and
Applied
Ecology
16
(2015)
674–680
679
specific
landscape
structures,
our
design
only
allows
us
to
interpret
the
general
patterns
a
set
of
landscape
structures
has
on
litter
mass
loss
and
how
invertebrates
are
influenced
by
them.
As
we
only
sampled
during
one
season
and
only
during
the
irrigated
phase
of
the
rice
paddy
cycle,
possi-
ble
reasons
for
the
contrasting
patterns
between
sites
could
be
different
water
depths,
which
can
range
between
5
and
30
cm,
or
varying
durations
of
flooding
(Bambaradeniya
&
Amarasinghe,
2003).
More
shallow
water
depths
result
in
higher
and
more
rapidly
changing
temperatures
and
oxygen
levels,
especially
between
day
and
night
(Bambaradeniya
&
Amarasinghe,
2003).
As
stated
above,
invertebrates
as
well
as
microorganisms
are
quite
sensitive
to
rapid
environmen-
tal
changes
which
is
why
different
patterns
of
litter
mass
losses
assigned
to
their
joint
decomposition
activities
can
arise
between
sites.
Our
study
aimed
at
demonstrating
the
general
impor-
tance
of
invertebrate
decomposers
in
rice
fields.
Future
studies
should
extend
the
period
of
investigation
up
to
at
least
two
rice
cycles
and
focus
on
a
systematic
com-
parison
of
how
differently
structured
surroundings
of
paddy
fields
influence
the
activity
of
invertebrate
decom-
posers.
Especially
the
question
whether
intensively
used
and
therefore
more
homogeneous
landscapes
support
a
lower
diversity
and
abundance
of
decomposer
invertebrates
should
receive
attention.
The
results
of
such
studies
would
help
in
establishing
and
maintaining
diverse
structures
with
sustainable
ecosystem
functions
at
local
and
regional
scales.
Conclusion
In
our
study,
we
demonstrated
that
invertebrate
decom-
posers
substantially
contribute
to
the
decomposition
process
in
irrigated
rice
fields.
Our
results
indicated
that
invertebrate
decomposers
can
be
expected
to
be
important
for
soil
fertil-
ity
and
site
productivity.
Crop
residue
management
strategies
should
consider
invertebrates
when
using
straw
to
improve
soil
conditions.
The
contribution
of
invertebrates
to
the
lit-
ter
mass
loss
of
rice
straw,
as
a
proxy
for
decomposition,
decreased
with
increasing
distance
to
the
bund
on
most
sites
tested,
which
could
indicate
that
the
surrounding
land-
scape
structure
may
influence
the
assemblages
of
invertebrate
decomposers.
Future
studies
should
evaluate
in
more
detail
how
land
management
and
landscape
structure
surround-
ing
rice
fields
contribute
to
the
maintenance
of
ecosystem
services,
such
as
nutrient
cycling
provided
by
invertebrate
decomposers.
Acknowledgements
The
study
is
part
of
the
international
project
“LEGATO”
(Land-use
intensity
and
Ecological
Engineering
–
Assess-
ment
Tools
for
risks
and
Opportunities
in
irrigated
rice
based
production
systems
–
www.legato-project.net),
funded
by
the
German
Federal
Ministry
of
Education
and
Research
(BMBF,
01LL0917A,
01LL0917L),
within
the
BMBF-Funding
Measure
“Sustainable
Land
Management”
(http://nachhaltiges-landmanagement.de/en/).
We
thank
Liberty
Vertudez
and
Rowena
Dela
Rosa
of
CESD
at
International
Rice
Research
Institute
(IRRI)
for
helping
to
prepare
the
litterbags
and
Antonio
Salamatin,
Deomedez
Izon,
and
Danilo
Vasquez
for
helping
with
setting
and
retrieving
the
bags.
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