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\begin{document}

\title{Plant roots fuel tropical soil animal communities}



\author[1]{Zheng Zhou}%
\author[1]{Jing-Zhong Lu}%
\author[1]{Jooris  Preiser}%
\author[2]{Rahayu  Widyastuti}%
\author[1]{Stefan Scheu}%
\author[3]{Anton Potapov}%
\affil[1]{University of Göttingen}%
\affil[2]{Institut Pertanian Bogor}%
\affil[3]{Georg-August-Universitat Gottingen Johann Friedrich Blumenbach Institut fur Zoologie und Anthropologie}%


\vspace{-1em}



  
  \date{May 08, 2024}


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\selectlanguage{english}
\begin{abstract}
Belowground life is traditionally considered to rely on leaf litter as
the main basal resource, whereas the importance of roots remains little
understood, especially in the tropics. Here, we analysed the response of
30 soil animal groups to root trenching and litter removal in rainforest
and plantations in Sumatra and found that roots are similarly important
to soil fauna as litter. Trenching effects were stronger in soil than in
litter with animal abundance being overall decreased by 42\% in
rainforest and by 30\% in plantations. Litter removal little affected
animals in soil, but decreased the total abundance by 60\% both in
rainforest and rubber plantations but not in oil palm plantations.
Litter and root effects were explained either by the body size or
vertical distribution of specific animal groups. Our findings highlight
the importance of root-derived resources for soil animals and quantify
principle carbon pathways in tropical soil food webs.%
\end{abstract}%



\sloppy


\textbf{Article type:} Letter

\textbf{Running title:} Roots fuel tropical soil fauna

\textbf{Article title:} Plant roots fuel tropical soil animal
communities

\textbf{Keywords:} roots; litter; land use; soil fauna; deforestation;
soil food web; carbon cycle; energy channel; basal resources; trenching

\textbf{Numbers:} 150 words in abstract; 4411 words in the main text; 81
references; 5 figures and 1 table

\textbf{Information of authors:}

Zheng Zhou: Johann Friedrich Blumenbach Institute of Zoology and
Anthropology, University of G\selectlanguage{ngerman}öttingen, Göttingen,
Germany;\emph{zzhou@gwdg.de}

Jing-Zhong Lu: Johann Friedrich Blumenbach Institute of Zoology and
Anthropology, University of Göttingen, Göttingen,
Germany;\emph{jlu@gwdg.de}

Jooris Preiser: Johann Friedrich Blumenbach Institute of Zoology and
Anthropology, University of Göttingen, Göttingen,
Germany;\emph{Jooris@gmx.de}

Rahayu Widyastuti: Department of Soil Sciences and Land Resources,
Institut Pertanian Bogor (IPB), Bogor, Indonesia;\emph{oety76@yahoo.com}

Stefan Scheu: Johann Friedrich Blumenbach Institute of Zoology and
Anthropology, University of Göttingen, Göttingen, Germany; Centre of
Biodiversity and Sustainable Land Use, Göttingen,
Germany;\emph{sscheu@gwdg.de}

Anton Potapov: Johann Friedrich Blumenbach Institute of Zoology and
Anthropology, University of Göttingen, Göttingen, Germany; German Centre
for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig,
Leipzig, Germany;\emph{potapov.msu@gmail.com}

\textbf{Correspondence:} Zheng Zhou; Telephone: + 49 (0) 162 8009139;
E-mail address: \emph{zzhou@gwdg.de}; Fax numbers: +49 (0) 551 395448

\textbf{Authorship:} AP and SS designed the research. AP, JP and RW
conducted the experiment and collected data. ZZ analysed the data with
advice from AP, JL and SS. The manuscript was written by ZZ with advice
from AP and SS. And all authors contributed substantially to revisions.

\textbf{Data accessibility statement:} Should the manuscript be
accepted, the data supporting the results will be archived in an
appropriate public repository and the data DOI will be included at the
end of the article.

Plant roots fuel tropical soil animal communities

Zheng Zhou\textsuperscript{1}, Jing-Zhong Lu\textsuperscript{1}, Jooris
Preiser\textsuperscript{1}, Rahayu Widyastuti\textsuperscript{2}, Stefan
Scheu\textsuperscript{1,3}, Anton Potapov\textsuperscript{1,4}

1. Johann Friedrich Blumenbach Institute of Zoology and Anthropology,
University of Göttingen, Göttingen, Germany

2. Department of Soil Sciences and Land Resources, Institut Pertanian
Bogor (IPB), Bogor, Indonesia

3. Centre of Biodiversity and Sustainable Land Use, Göttingen, Germany

4. German Centre for Integrative Biodiversity Research (iDiv)
Halle-Jena-Leipzig, Leipzig, Germany

Abstract

Belowground life is traditionally considered to rely on leaf litter as
the main basal resource, whereas the importance of roots remains little
understood, especially in the tropics. Here, we analysed the response of
30 soil animal groups to root trenching and litter removal in rainforest
and plantations in Sumatra and found that roots are similarly important
to soil fauna as litter. Trenching effects were stronger in soil than in
litter with animal abundance being overall decreased by 42\% in
rainforest and by 30\% in plantations. Litter removal little affected
animals in soil, but decreased the total abundance by 60\% both in
rainforest and rubber plantations but not in oil palm plantations.
Litter and root effects were explained either by the body size or
vertical distribution of specific animal groups. Our findings highlight
the importance of root-derived resources for soil animals and quantify
principle carbon pathways in tropical soil food webs.

\textbf{Keywords:} roots; litter; land use; soil fauna; deforestation;
soil food web; carbon cycle; energy channel; basal resources; trenching

INTRODUCTION

The belowground system harbours a large portion of terrestrial
biodiversity and delivers vital ecosystem services (Bardgett \& van der
Putten 2014; Guerra \emph{et al.} 2021). In terrestrial ecosystems,
80\%--90\% of the carbon fixed by plants enters the belowground system
via litter and roots, thereby fuelling the belowground food web
(Bardgett \& Wardle 2010; Gessner \emph{et al.} 2010; Schmitz \& Leroux
2020). Litter of aboveground plant compartments, in particular leaves,
is assumed to form the major source of organic matter and thereby of
crucial importance for energy flow in soil (Attiwill \& Adams 1993;
Kögel-Knabner 2002). Litter-derived carbon enters the belowground food
web through saprotrophic fungi and bacteria or via direct consumption by
litter-feeding soil fauna (Scheu \& Setälä 2002). The alternative
pathway, receiving increased attention recently, comprises
photosynthates entering the belowground system via root-derived
resources such as root exudates (Jones \emph{et al.} 2009). These
resources are taken up mainly by microorganisms including mycorrhizal
fungi and may account for up to 54\% of soil respiration in boreal
forests (Högberg \emph{et al.} 2001). Root-derived carbon has been shown
to fuel belowground food webs across trophic levels and to represent
another major source of organic matter in soil (Högberg \emph{et
al.}2001; Pollierer \emph{et al.} 2007; Bradford 2016). However, the
importance of the alternative root carbon pathway for soil animal
communities has only been investigated in temperate and boreal
ecosystems, whereas its role in tropical ecosystems with fundamentally
different rhizosphere associations and processes is unknown
(Averill\emph{et al.} 2014).

Soil fauna comprise a huge diversity of forms and functions that
supports element cycles in soil by controlling microbial communities and
soil physicochemical properties (Bardgett \& van der Putten 2014;
Briones 2014; Potapov \emph{et al.} 2022). The relative importance of
litter and root resources vary across different soil animal taxa. For
example, in temperate forests the importance of root resources for
arthropods, earthworms and fungivorous nematodes may exceed that of
litter resources (Pollierer \emph{et al.} 2007, 2012; Gilbert \emph{et
al.} 2014; Kudrin \emph{et al.} 2021), whereas in subtropical
plantations litter resources have been found to be more important than
root resources for earthworms (Chen \emph{et al.} 2020). Also, in
temperate forests oppiid mites, onychiurid springtails, proturans, and
centipedes have been found to heavily rely on root-derived resources
(Remén \emph{et al.} 2008; Endlweber \emph{et al.} 2009;
Goncharov\emph{et al.} 2016; Potapov \emph{et al.} 2016b; Bluhm \emph{et
al.}2019b). However, the relative importance of these two pathways in
fuelling soil food webs and its variation among forest types and biomes
remain unclear. For uncovering general patterns, studies across a wide
range of animal taxa and including functional traits of different soil
fauna groups are needed. For instance, animals of large body size and
high trophic position tend to feed on more diverse resources and
integrate different energy channels in soil food webs (Wolkovich 2016).
Thus, large-sized and predatory animal taxa may be less affected by
deprivation of one specific resource pathway. Indeed, it has been shown
that microarthropods are more sensitive to root trenching than
macroarthropods in temperate forests (Bluhm \emph{et al.} 2021).
Differences in the vertical distribution of soil fauna may be another
factor determining the use of litter vs. root resources (Li \emph{et
al.} 2022; Potapov 2022). In fact, root-derived resources have been
found to be more important for soil- than for litter-dwelling
springtails in coniferous forests (Potapov \emph{et al.} 2016a),
however, this pattern may not uniformly apply (Fujii \emph{et al.} 2016;
Li \emph{et al.} 2020). Overall, studies from temperate ecosystems
suggest that including functional traits of soil fauna may allow
uncovering their link to litter and root resources. Testing these
trait-based predictions in a distinct context, such as the tropics, can
provide robust evidence for their generality.

The tropics account for over half of the global annual production, and
tropical rainforests play an important role as carbon sink in the global
carbon cycle (Baccini \emph{et al.} 2017; Mitchard 2018). Agricultural
expansion is among the main threats to tropical ecosystems
(Laurance\emph{et al.} 2014; Hoang \& Kanemoto 2021) that greatly
reduces carbon storage, changes carbon cycling and redistribute the
energy flow in soil food webs (Guillaume \emph{et al.} 2018; Potapov
\emph{et al.} 2019a; Veldkamp \emph{et al.}2020). However, changes in
the importance of root-derived resources for soil animal communities
with the conversion of rainforest into agricultural land-use systems
remain unknown. Root supply varies among cropping systems (Scheunemann
\emph{et al.} 2015; Li \emph{et al.} 2020) and tree species (Zieger
\emph{et al.} 2017). Strong shifts in plant communities, changes in soil
microbial biomass and community composition, and depletion of litter
resources associated with changes in tropical land use (Krashevska
\emph{et al.} 2015; Rembold \emph{et al.} 2017) suggest that the
availability of resources for soil animal communities is also changing.
This is supported by recent studies indicating that the basis of soil
animal food webs shifts towards the living plant energy channel in
plantations (Susanti \emph{et al.} 2019; Krause \emph{et al.} 2021; Zhou
\emph{et al.} 2022), but the role of roots in this context remains
unclear.

Here, we investigate the effects of deprivation of resource input via
living roots or aboveground plant litter on soil fauna communities using
a full-factorial root-trenching and litter-removal experiment in
rainforest and plantations of rubber and oil palm in Sumatra, Indonesia.
We assessed the response of 30 high-rank animal groups in litter and
soil to evaluate the importance of living roots as an alternative major
carbon source to aboveground litter for soil animal communities in
tropical ecosystems.

Specifically we tested the following hypotheses:

\begin{enumerate}
\tightlist
\item
  Roots and aboveground litter are of similar importance for soil animal
  food webs in rainforest, whereas in plantations root resources are
  more important than litter due to depletion of litter resources in
  comparison to rainforest.
\item
  Root-trenching effects on soil animal communities are stronger in soil
  than in the litter layer.
\item
  Root trenching and litter removal restructures soil food webs through
  trait-specific effects, depending on animal body sizes, vertical
  distributions, and trophic niches.
\end{enumerate}

MATERIALS AND METHODS

Sampling sites and experimental set-up

The study was conducted in the framework of the collaborative research
project CRC990/EFForTS investigating ecological and socio\selectlanguage{english}-economic
changes associated with the transformation of lowland forest into
agricultural systems (Drescher \emph{et al.} 2016). The present study
took place in Jambi province, Sumatra, Indonesia, which is a global
hotspot of biodiversity, where over last 25--35 years rainforests have
been largely replaced by intensively managed plantations, mostly oil
palm and rubber, which lead to ecosystem degradation and biodiversity
decline (Margono \emph{et al.} 2012; Clough \emph{et al.} 2016).

The experiment was established in three land-use types, rainforest,
rubber (\emph{Hevea brasiliensis} ) and oil palm (\emph{Elaeis
guineensis} ) plantations in October 2016, and was replicated four times
in each land-use type, resulting in a total of 12 independent sites
spread across an area of ca. 35 km diameter with adjacent sites being
spaced by 0.5--5 km. Four experimental treatments (plots) were
established at each experimental site: control, root trenching, litter
removal, and both root trenching and litter removal (Fig. 1); each
experimental plot measured 75 x 75 cm. Root trenching was performed by
digging a trench around the treatment area and establishing a 0.6 mm
think plastic screen to a depth of 60--70 cm around the plot. All weeds
were removed regularly (every two weeks) from the trenched plots
throughout the experiment. Litter removal was performed by removing the
litter layer and installing a roof (metal mesh of 5 mm and plastic mesh
of 2 mm; Fig 1) above the experimental plots to avoid fresh litter to
enter the plots. In the litter removal plots the litter was replaced by
plastic bamboo leaves to fully cover the soil surface and minimize
confounding effects due to erosion and soil drying, and to focus on
effects of litter as food resource. Experimental plots were checked
every two weeks and litter occasionally fallen into the plots from the
side was removed.

Sampling, extraction and classification of soil fauna

Soil fauna was sampled after one year in September--October 2017. In
each plot one 16 x 16 cm sample was taken with a spade and divided into
three layers: (1) litter (plastic leaves were not sampled), (2) 0--5 and
(3) 5--10 cm of soil, resulting in total of 112 samples. The samples
were transported to the laboratory and animals were extracted under a
temperature gradient between 45degC above and 15degC\textsuperscript{}below
the substrate (Kempson \emph{et al.} 1963) until the substrate was
completely dry (6--8 days). Animals were collected in a glycerol: water
(1 : 1) solution and subsequently stored in 80\% ethanol. Animals were
classified into 30 high-rank taxonomic groups (Oribatida, Collembola,
Protura, Mesostigmata, Pauropoda, Hymenoptera, Hemiptera, Diptera,
Thysanoptera, Prostigmata, Psocoptera, Auchenorrhyncha, Symphyla,
Pseudoscorpiones, Lepidoptera, Diplopoda, Campodeidae, Isopoda,
Formicidae, Opiliones, Schizomida, Araneae, Japygidae, Heteroptera,
Coleoptera -- predators, Coleoptera -- Staphylinidae, Coleoptera --
herbivores, Isoptera, Orthoptera, Dermaptera, Chilopoda, Blattodea,
Lumbricina), roughly representing trophic/functional groups (Potapov
\emph{et al.} 2022).

Statistical analysis

All analyses were done in R 4.0.3 (R Core Team 2020). To assess the
effects of litter removal and root trenching on total abundance and on
each animal group, we fitted linear mixed-effects models (LMMs) using
log-transformed abundance values or generalized linear mixed-effects
models with Poisson distribution (GLMMs) and then applied contrasts
between resource exclusion (either no litter or no root) and resource
inclusion (either litter or root) to estimate effect sizes. We focussed
on the effects of the litter removal and root trenching instead of the
full-factorial design as explorative models did not show significant
interactions between the two treatments. The litter removal effect
represents the difference between `control' + `trenching' versus `litter
removal' + `litter removal and root trenching'. The root trenching
effect represents the difference between `control' + `litter removal'
versus `trenching' + `litter removal and root trenching'. We checked
model assumptions of the most parsimonious models by fitting model
residuals versus the results of fitted models, and simplified models
based on the Akaike information criterion (AIC).

Firstly, we estimated the effect sizes of litter removal and root
trenching on the total abundance and richness in different layers using
separate models as litter was absent in the litter removal treatment.
The GLMMs (with Poisson distribution) included litter (removal/no
removal), root (trenching/no trenching), land-use type (rainforest,
rubber and oil palm plantations) and layer (litter layer, 0-5 and 5-10
cm soil depth for the root trenching model, and 0-5 and 5-10 cm soil
depth for the litter removal model) as fixed effects, with treatment
nested within plot as random effect to account for interdependence of
layers from the same soil core and treatments from the same plot. In
addition, we estimated the effect sizes of litter removal and root
trenching on the total abundance and richness (pooled across layers).
The GLMMs included treatment and land-use type as fixed effects, with
plot as random factor to account for interdependence of treatments from
the same plot.

Next, we estimated the effect size of litter removal and root trenching
on the abundance of each taxonomic group (lumped data across layers) by
LMMs. The model included litter (removal/no removal), root (trenching/no
trenching), land-use type, taxonomic group as fixed effects, and plot as
random effect. In this model we excluded groups which occurred less than
5 times among all 120 samples, namely Auchenorrhyncha, Lepidoptera and
Dermaptera. Besides, data that were 0 in the control and the treatment
were excluded from the model since effect size for double zero data
cannot be reliably estimated.

To relate effects of the litter/root exclusion to traits of soil animal
groups, we used the effect sizes of litter removal and root trenching
for each taxonomic group from the above described models. We used the
following traits/characteristics assigned at animal group level: average
body mass, abundance in the control treatment, vertical distribution in
the control treatment, and trophic niche as indicated by stable isotope
values (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N). The vertical
distribution was represented by scaled values between 0 (the group only
in litter) and 1 (the group only at 5-10 cm depth), accounting for the
abundance in each layer. For the average body mass of taxonomic groups
and trophic niche (\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C and \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N
values), we used our previous data from the same study sites
(Potapov\emph{et al.} 2019c; Zhou \emph{et al.} 2022). Stable isotope
values of animals were calibrated to the respective values of litter in
the respective plot. \selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{15}N values reflect the trophic
position (Post 2002; Potapov \emph{et al.} 2019b) and
\selectlanguage{greek}δ\selectlanguage{english}\textsuperscript{13}C values indicated the use of different basal
carbon resources, e.g. fresh and old organic matter (Pollierer \emph{et
al.} 2009; Potapov \emph{et al.} 2019b).

Principal component analysis (PCA) and PERMANOVA were used to analyse
the influence of land use and treatments on soil animal community
composition (pooled data across layers). We used Hellinger and
Bray--Curtis distance transformation prior to PCA and PERMANOVA,
respectively, to test the effects of land use and treatment on animal
community composition.

\emph{Vegan} package was used for PCA and PERMANOVA analysis
(Oksanen\emph{et al.} 2020); \emph{nlme} and \emph{lme4} package were
used to fit LMMs and GLMMs, respectively (Bates \emph{et al.} 2015;
Pinheiro\emph{et al.} 2022), and \emph{emmeans} package was used to
conduct planned contrasts (Lenth 2021). All mixed models were visually
checked to meet the assumption of residual homogeneity of variance.
Results were visualized using the \emph{ggplot2} package (Wickham 2016).

RESULTS

1. Effects of litter removal and root trenching on faunal abundance and
richness

Root trenching reduced the total abundance of soil fauna by 42.6 \selectlanguage{ngerman}±
12.7\%, 28.5 ± 15.7\% and 29.9 ± 15.6\% (estimated means ± standard
errors) in rainforest, rubber and oil palm plantations, respectively
(Fig. 2; Table S1). Litter removal reduced the total abundance of soil
fauna in rainforest and rubber plantations by 63.6 ± 8.7\% and 60.0±
9.3\%, respectively, but the effect was not significant in oil palm
plantations. Generally, effects of root trenching on soil fauna
abundance were stronger in soil than in litter. Soil fauna abundance was
reduced by 64.2 ± 8.8\% and 50.4 ± 12.5\% in 0--5 and 5--10 cm soil
depth in rainforest, and by 36.0 ± 15.3\% and 36.1 ± 15.5\% in 0--5 cm
depth in rubber and oil palm plantations, respectively, while the
abundance in the litter layer did not change significantly. Litter
removal generally did not change soil fauna abundance in soil. Total
richness of soil fauna (30 taxonomic groups) declined in rainforest by
both root trenching (23.1 ± 10.9\%) and litter removal (27.6 ± 10.2\%).
In rubber plantations root trenching reduced the richness of soil fauna
by 43.1 ± 14.9\% in the litter layer, whereas in oil palm plantations it
was reduced by 33.7 ± 12.6\% in the 0--5 cm soil. Litter removal did not
change the richness of soil fauna in soil across land-use types.

2. Soil animal community structure

Soil animal community composition varied significantly among the three
land-use types (PERMANOVA; F = 5.54, \emph{p} \textless{} 0.001) as well
as experimental treatments (PERMANOVA; F = 3.50, \emph{p} \textless{}
0.001), with the difference among treatments being most pronounced in
rainforest and least in rubber plantations (PERMANOVA; Land use ×
Treatment interaction, F = 2.74, \emph{p} \textless{} 0.001; Fig. 3).
The effects of root trenching on soil animal abundances were generally
independent of litter removal (Table 1). The root-trenching effects on
the abundance of each group were universal across different land-use
types (Table 1), but the average effects across soil animal groups were
stronger in rainforest (-29.6 ± 9.8\%) than in rubber (-17.4 ± 8.1\%)
and oil palm plantations (-18.4 ± 8.9\%) (Fig. 4a). Litter-removal
effects on the abundance of each group varied across land-use types
(Table 1). In contrast to effects of root trenching, the average effects
of litter removal across groups were negative in rainforest and rubber
plantations (-25.0 ± 11.5\% and -27 ± 6.5\%, respectively), but positive
in oil palm plantation (57 ± 38.4\%) (Fig. 4b).

The response of animal groups to root trenching and litter removal
varied with land-use type (significant three-factor interaction for
both; Table 1). Root trenching significantly decreased the abundance of
Protura, Mesostigmata, Prostigmata, Symphyla, Schizomida and Japygidae
in rainforest, that of Prostigmata, Psocoptera, Formicidae and Araneae
in rubber, and that of Protura, Pauropoda, Hemiptera and Formicidae in
oil palm plantations (Fig. 4c, Table S2). Litter removal significantly
decreased the abundance of nine animal groups in rainforest including
Oribatida, Mesostigmata, Diptera, Thysanoptera, Psocoptera,
Pseudoscorpiones, Diplopoda, Campodeidae and herbivorous Coleoptera, and
decreased six groups in rubber including Oribatida, Collembola,
Mesostigmata, Thysanoptera, Psocoptera and Formicidae. Conversely,
litter removal increased the abundance of Prostigmata in oil palm
plantations, but it decreased the abundance of Hemiptera (Fig. 4c, Table
S3).

3. Linking effects of litter removal and root trenching to animal traits

In root trenching treatments the decline in animal density (measured as
effect size) was more pronounced in small- than large-sized taxa;
although this was also the case in litter removal treatments in
rainforest and rubber, overall this was not significant (Fig 5a, b;
Table S4). Accordingly, abundant taxonomic groups (usually represented
by groups with smaller body size) declined more with both litter removal
and root trenching than rare ones (Fig 5c, d; Table S4). Litter-removal
effects were much stronger in taxa inhabiting predominantly the litter
layer, whereas root-trenching effects were not related to the vertical
distributions of animal taxa (Fig 5e, f; Table S4). Both litter and root
effects were not universally related to the trophic position
(\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values) or use of basal resources
(\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values) of animal taxa; however, in litter
removal treatments the response was more pronounced in animal taxa with
low \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N values but only in rainforest and rubber, and
in trend this also applied to animals with low \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C
values but only in rainforest and oil palm (Fig. 5g-j; Table S4).

DISCUSSION

We analysed the effects of litter removal and root trenching on soil
animals in rainforest and plantations, for the first time testing the
importance of living root carbon supply for tropical soil fauna
communities. We found that root-trenching effects are of similar
magnitude and more uniform across land-use types than litter-removal
effects. Root-trenching effects were more pronounced in soil than in the
litter layer, while litter removal little affected animal abundance in
soil. Litter removal decreased animal abundance in rainforest and rubber
plantations but not in oil palm plantations. Root and litter exclusion
shaped soil food webs through different mechanisms. Root trenching
affected stronger small-sized and abundant animal groups, with the
effect being independent of the initial vertical distribution. By
contrast, litter removal affected more abundant groups that inhabited
litter in control plots.

1. Litter and root resources across land-use types and soil layers

Compared to litter removal, root-trenching effects on soil animals were
more universal across land-use types and in oil palm plantations they
even exceeded effects of litter removal. This suggests that living roots
are of similar importance than leaf litter in fuelling soil animal food
webs in tropical ecosystems, and are even more important than litter in
oil palm plantations, which supports our first hypothesis. Root-derived
carbon predominantly comprises easily available carbon compounds
entering the belowground food web via mycorrhizae and root exudates,
which are rapidly consumed by microorganisms and thereby propagated to
higher trophic levels in the rhizosphere (Bradford 2016; Zieger \emph{et
al.} 2017). By contrast, leaf litter comprises a variety of complex
compounds including lignin and waxes but also secondary compounds
deterring incorporation of its compounds into animal consumers (Vitousek
1984; Pollierer \emph{et al.} 2007), and this may in particular apply to
leaf litter material in the tropics (H\selectlanguage{ngerman}ättenschwiler \& Jørgensen 2010;
Butenschoen \emph{et al.} 2014; Marian \emph{et al.} 2018).
Litter-removal effects on soil animals varied with land use, changes of
both total abundance and mean effect size across animal groups indicated
that litter resources were more important in rainforest and rubber
plantations but less in oil palm plantations with poorly developed
litter layer. In rubber plantations the trees were tapped for collecting
latex, which likely reduces the input of assimilates to roots, and
therefore soil organisms may rely more on litter resources than in
rainforest, although the litter layer in rubber plantations also is
reduced compared to rainforest. However, as opposed to rainforest and
rubber plantations, the mean effect of litter removal across groups in
oil palm plantations was positive (increase in effect sizes by 57\%) and
the total abundance was not significantly affected. Litter not only
serves as food resource for detritivore soil animals, but also comprises
the habitat of litter living species (Sayer \emph{et al.} 2006;
Fujii\emph{et al.} 2020). To consider the role of litter as habitat we
not just removed the litter layer but replaced it by plastic leaves.
Covering the nearly bare soil in oil palm plantations by these leaves
may have reduced desiccation and thereby beneficially affected soil
animals. The results suggest that in oil palm plantations litter-derived
food resources are of minor importance and that the physical absence of
the litter layer expose soil animals to detrimental environmental
conditions. This points to the great restoration potential of e.g.
mulching for soil animal communities in oil palm plantations which will
improve the buffering ability, habitat structure, and provide food
resources for soil fauna (Tao \emph{et al.} 2018; Potapov \emph{et
al.}2020).

Root-trenching effects were stronger in soil than in the litter layer,
which supports our second hypothesis. Soil biota are known to
essentially rely on root-derived resources and this applies to both soil
microorganisms as well as soil animals (Pollierer \emph{et al.} 2007;
Bluhm \emph{et al.} 2019a, 2021). Trenching cuts off the input of `green
energy' into the soil from above the ground thereby eliminating
mycorrhizal fungi but also reducing saprotrophic rhizosphere
microorganisms (Díaz-Pinés \emph{et al.} 2010; Bluhm \emph{et
al.}2019a). However, it also leaves the cut roots inside of the trenched
plots, which may increase the supply of resources to the decomposer
system. Nevertheless, soil animal abundance was strongly reduced by root
trenching reflecting the overwhelming importance of resources derived
from living roots. Meanwhile, trenching little affected animals in the
litter layer although decomposers in litter layer may also benefit from
root-derived resources transferred via fungal hyphae from the soil into
the litter (Frey \emph{et al.} 2003; Wallander \emph{et al.} 2006).
Litter removal little affected animal abundance in soil suggesting that
similar to root resources in litter, litter resources are of very
limited importance for the nutrition of soil living animal taxa.
Overall, this suggests that both root-derived and litter resources are
consumed mainly in close vicinity where they are located with very
limited translocation to other layers. This points on potential spatial
compartmentalisation of soil animal food webs, i.e. litter and soil
animal communities are partly independent and fuelled by food resources
channelled to the belowground system via different pathways.

2. Trait-specific responses of soil animals

Partially confirming our third hypothesis, the exclusion of basal
resources restructured the belowground food web in respect to body size,
vertical distribution and abundance as well as trophic niches of
taxonomic groups. Deprivation of root-derived resources detrimentally
affected in particular small-sized groups. Resources from living roots,
particularly labile compounds such as root exudates, are readily used by
rhizosphere microorganisms and thereby transferred to higher trophic
levels such as mesofauna depending heavily on microorganisms as food
(Albers \emph{et al.} 2006; Sokol \emph{et al.} 2019; Li \emph{et
al.}2021). In fact, the root-derived energy channel is viewed as fast
energy channel in the belowground system (Pollierer \emph{et al.} 2012;
de Vries \& Caruso 2016). Small-sized soil animals typically are
characterized by faster energy turnover than large-sized species
(Brown\emph{et al.} 2004; Potapov \emph{et al.} 2021b) underlining that
in soil the fast energy channel based on root-derived resources is
particularly important for small-sized animals. Further, the porous
structure of soil may restrict the access of root-derived resources by
large-sized animals (Erktan \emph{et al.} 2020), and large-sized animals
are likely to more intensively integrate different energy channels by
foraging at larger spatial scales (Wolkovich 2016) allowing them to more
flexibly respond to the exclusion of root resources. The effect of
root-trenching was more pronounced in abundant animal groups as
abundance typically scales negatively with body size (Brown \emph{et
al.} 2004; White \emph{et al.} 2007).

Exclusion of litter little affected animal groups inhabiting the mineral
soil, whereas the effects of root trenching were independent of the
vertical distribution of soil animal groups. Stable isotope labelling
studies reported that root-derived resources also propagate into animal
species typically inhabiting the litter layer (Pollierer \emph{et al.}
2007), suggesting that soil animals benefit from root-derived resources
independent of their vertical habitat preferences. Notably, as discussed
above, root trenching in particular reduced total abundance in soil and
not in the litter layer, which means the faunal decline in soil layer
were not because the groups with deeper vertical distribution were
affected more by trenching, but because the animals initially inhabiting
soil layers migrated up to the litter layer after trenching lowered the
food-resources availability in the soil.

Root-trenching effects were independent of the trophic niche of soil
animals as indicated by \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N and
\selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C values suggesting that root-derived resources are
of similar importance for taxa across food chains (Glavatska \emph{et
al.} 2017; Zieger \emph{et al.} 2017). The litter-removal effects,
however, were less pronounced in low trophic level taxa in rainforest
and oil palm plantations, and also in taxa using `older' microbially
processed carbon in rainforest and rubber plantations. This suggests
that predators are in general less affected by deprivation of litter
resources than primary decomposers, at least in certain ecosystem types.
However, our stable isotope analysis represented only two dimensions of
the trophic niche of species (Potapov \emph{et al.} 2021a) and studies
on other trophic niche-related dimensions, e.g. animal stoichiometry,
need to be included in future.

\section*{CONCLUSIONS}

{\label{conclusions}}

For the first time we evaluated the importance of litter and
root-derived resources for the soil animal food web in tropical
ecosystems including rainforest and plantations. The response of a wide
range of soil animal taxa indicates that both litter and root-derived
resources shape belowground food webs in tropical ecosystems. Our
results document the importance of living root supply as an alternative
to leaf litter resource pathway in soil animal food webs of tropical
ecosystems, which is even more important than litter-based resources in
oil palm plantations. Beneficial effects of the addition of artificial
leaves in oil palm plantations point to the potential of improving
habitat structure, e.g. via mulching, to promote soil food webs and the
services they provide. Root-derived resources altered the body size
structure of soil animal communities by favouring in particular small
and abundant taxa, reflecting that living roots essentially structure
soil food webs and their functioning. Our study sheds light on the
principle carbon pathways in tropical soil animal food webs and how they
change with anthropogenic land use. This knowledge provides the basis
for animal-cantered carbon modelling, ecosystem-friendly agricultural
management, and conservation of soil animal biodiversity in the tropics.

\section*{ACKNOWLEDGEMENTS}

{\label{acknowledgements}}

This study was funded by the Deutsche Forschungsgemeinschaft (DFG),
project number 192626868--SFB 990 in the framework of the collaborative
German-Indonesian research project CRC990. Z.Z is supported by China
Scholarship Council (CSC) (202004910314).

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DF, numerator degrees of freedom; den DF, denominator degrees of
freedom.\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
& num DF & den DF & F-value & p-value\tabularnewline
\midrule
\endhead
Root & 1 & 556.34 & 50.62 & \textless{} 0.001 ***\tabularnewline
Litter & 1 & 453.54 & 18.90 & \textless{} 0.001 ***\tabularnewline
Land Use (LU) & 2 & 7.43 & 2.78 & 0.125\tabularnewline
Group & 23 & 561.00 & 92.98 & \textless{} 0.001 ***\tabularnewline
Root ? Litter & 1 & 556.37 & 0.03 & 0.855\tabularnewline
Root ? LU & 2 & 556.36 & 2.47 & 0.086\tabularnewline
Litter ? LU & 2 & 464.59 & 12.75 & \textless{} 0.001 ***\tabularnewline
Root ? Group & 23 & 556.93 & 1.51 & 0.060\tabularnewline
Litter ? Group & 23 & 557.62 & 2.95 & \textless{} 0.001
***\tabularnewline
LU ? Group & 46 & 560.44 & 4.37 & \textless{} 0.001 ***\tabularnewline
Root ? Litter ? LU & 2 & 556.36 & 2.97 & 0.052\tabularnewline
Root ? Litter ? Group & 23 & 556.25 & 1.14 & 0.296\tabularnewline
Root ? LU ? Group & 46 & 556.85 & 1.69 & 0.004 **\tabularnewline
Litter ? LU ? Group & 46 & 557.42 & 1.42 & 0.040 *\tabularnewline
\bottomrule
\end{longtable}

Asterisks denote significance levels: * p \textless{} 0.05; ** p
\textless{} 0.01; *** p \textless{} 0.001.

\textbf{Figure 1.} Experimental design. Control (C), litter removal (L),
root trenching (R), and combined treatment (RL) were established at 12
sites, four of each rainforest, rubber and oil palm plantations in Jambi
Province, Sumatra.

\textbf{Figure 2.} Effect sizes of litter removal and root trenching on
abundance and richness of soil fauna in each layer and in total. Effect
sizes are given as log-response ratios of litter or root exclusion
compared to litter or root inclusion, respectively {[}ln (value in
litter or root exclusion /values in litter or root inclusion){]}.
Asterisks indicate significant effects, with (*) \emph{p} \textless{}
0.1, * \emph{p} \textless{} 0.05, ** \emph{p} \textless{} 0.01 and
***\emph{p} \textless{} 0.001. Bars represent 95\% confidence intervals
(n = 8).

\textbf{Figure 3.} Principal component analysis of soil animal community
composition varying with land-use types across experimental treatments
(a), and among the different experimental treatments in rainforest (b),
rubber (c) and oil palm plantations (d). Experimental treatments are
coded by different line types: Control (solid line), litter removal
(long dash), root trenching (dot dash) and litter removal + root
trenching (dotted).

\textbf{Figure 4.} Effects of litter removal and root trenching on
different taxonomic groups. Relative changes in abundance due to (a)
root trenching and (b) litter removal, values represent mean effects
across groups. (c) Effects of litter removal and root trenching on
abundance of individual taxonomic groups. Effect sizes are given as
log-response ratios of litter or root exclusion compared to litter or
root inclusion {[}ln (value in litter or root exclusion /values in
litter or root inclusion){]}. Soil fauna abundance were summed across
three depths (litter, 0--5 and 5--10 cm soil depth). Asterisks indicate
significant effects, (*) \emph{p} \textless{} 0.1, * \emph{p}
\textless{} 0.05, ** \emph{p} \textless{} 0.01, *** \emph{p} \textless{}
0.001. Bars represent 95\% confidence intervals (n = 8). Taxonomic
groups are ordered by increasing mean body mass.

\textbf{Figure 5} Relationship between the responses of animal groups to
litter removal or root trenching and animal traits or community
parameters: body mass (a, b), abundance (c, d), vertical distribution
(e, f), \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{13}C (g, h) and \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{15}N (i, j).
Black dashed lines denote overall model fits and coloured lines indicate
different land-use types. The model outputs are reported in Table S4.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig/Fig}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-2-Total-abundance-and-richness/Fig-2-Total-abundance-and-richness}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-3-PCA/Fig-3-PCA}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-4-Each-group/Fig-4-Each-group}

\end{center}
\end{figure}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Fig-5-Traits/Fig-5-Traits}

\end{center}
\end{figure}

\selectlanguage{english}
\FloatBarrier
\end{document}

