INTRODUCTION
Plant litter is important in the
formation of soil organic matter in terrestrial ecosystems (Pan et
al. 2011). Decomposition of plant
litter is fundamental in maintaining soil health, carbon sequestration
and plant productivity (Hättenschwiler et al. 2005). Litter
decomposition is primarily caried out by microbes (fungi and bacteria),
microfauna (e.g. nematodes), mesofauna (e.g. collembola and mites) and
macrofauna (e.g. termites, earthworms, isopods and millipedes) (Lehmann
& Kleber 2015). These organisms break down plant materials to small
particles, mineral salts, carbon dioxide and water which are then used
by plants and soil organisms for
nutrients and energy (Swift et
al. 1979; Griffiths et al.2021a).
Generally in ecology microbes are considered the major decomposers since
they can produce cellulase and lignases that degrade structural
polysaccharides in plant litter (i.e. cellulose and lignin)
(Papanikolaou et al. 2010; Pausas & Bond 2020). Another important
component of the soil biome is the invertebrates whose effects on litter
decomposition are often underestimated (Filser et al. 2016;
Pausas & Bond 2020). However, micro-organisms are the only soil biota
currently included in global biogeochemistry models (Cotrufo et
al. 2010; Wieder et al. 2015).
This omission limits our ability to
model global nutrient and carbon budgets, which are crucial for
understanding the links between climate change and ecosystem services
(Grandy et al. 2016; Bishop et al. 2021).
Recently there has been an increase in understanding of invertebrate
decomposer assemblages and how they affect carbon cycling and nutrient
availability (Chang & Lai 2018; Shelomi et al. 2019; Jolyet al. 2020). Studies have shown decomposer invertebrates can
increase litter mass loss and nutrient release either in partnership
with symbiotic microbes or using endogenous cellulases (Eggleton &
Tayasu 2001; Ashton et al. 2019; Griffiths et al. 2019).
Earthworms, beetles, isopods and millipedes can produce extracellular
cellulases in partnership with gut symbionts (Pauchet et al.2010; Ni’matuzahroh et al. 2022). In addition and apart from gut
symbionts, some of these groups are also capable of producing endogenous
cellulases to digest dead organic matter through invertebrate-enzymatic
decomposition without the support of microbes (Pauchet et al.2010; Griffiths et al. 2021a).
In addition to affecting litter decomposition directly through the
breakdown of organic material, soil invertebrates are important
bioturbators that enhance microbial decomposition by reshaping soil
physical and chemical properties. Termites and earthworms, for example,
build huge networks of soil galleries that promote water infiltration
and transportation (Jouquet et al. 2011; Hoeffner et al.2019). These water-filled pores and
water films are ideal habitats for aquatic microbiota, such as protists,
nematodes, bacteria and fungi (Hoeffner et al. 2019). Foraging
and nesting behaviors of invertebrates improve soil nutrient
heterogeneity, creating hotspots of essential plant-nutrients such as
nitrogen and phosphorus (Griffiths et al. 2018; Ashton et
al. 2019; Phillips et al. 2021). Invertebrate faeces influence
soil nutrients and ambient microbial communities (David 2014). The
wide-ranging ways in which invertebrates influence soil properties are
complex, with ecosystem-wide effects mediating microbial decomposition
and plant growth (Swift et al. 1979; Adejuyigbe et al.2006). Although a growing body of work highlights the importance of soil
invertebrates in decomposition, there remains large gaps in our
understanding of the biogeography of soil invertebrate contributions to
carbon and nutrient cycling.
The ecological functions carried out by soil invertebrates vary across
regions, such as the tropics and non-tropics (Brussaard et al.2012; Phillips et al. 2021; Kass et al. 2022).
Evolutionary history and habitat
have led to regional differences in soil faunal taxa (van den Hoogenet al. 2020). In tropical forests, macrofauna (e.g. termites,
earthworms, isopods and millepedes) dominate the decomposer invertebrate
assemblages (Swift et al. 1979; Lavelle et al. 2022). The
termites are especially important for decomposition yet are a group that
is largely absent from temperate regions. Termites remove large
quantities of dead organic material from the forest floor (Bignell &
Eggleton 2000). In forests outside the tropics (e.g. temperate,
subtropical, and boreal) where microbes are accepted as the dominant
decomposers, the mesofauna (e.g. mites and collembolas) and macrofauna
(e.g. earthworms, isopods and millepedes) are also important
contributors to litter decomposition (Anderson 1978; Korboulewskyet al. 2016; Heděnec et al. 2022). The quality and
palatability of plant-derived resources affect soil invertebrate
diversity (Cebrian 1999; Kurokawa et al. 2010; Bastida et
al. 2020). Plant materials with a high proportion of nitrogen relative
to structural polysaccharides (e.g. low C:N or lignin:N ratios) usually
shows high palatability and decomposability (Swift et al. 1979;
Kurokawa & Nakashizuka 2008). In addition, most invertebrates are
sensitive to climate. They are usually more active in warm or humid
biomes such as tropical forests, limited by cold and dry conditions
(García-Palacios et al. 2013). As most published observations are
from non-tropical regions (Xu et al. 2020; McCary & Schmitz
2021), current models and their conceptual framework may not assess
invertebrate effects on decomposition accurately. Accordingly, an
understanding of the regional differences in invertebrate-mediated
decomposition and how specific decomposer invertebrates, climate, and
soil mediate regional differences are important for us to estimate
invertebrate effects on biogeochemistry.
We have performed a meta-analysis to compare the effects of soil
invertebrate on leaf litter decomposition in forests contrasting the
tropics and the non-tropics. We then analyzed whether and how
invertebrate diversity, litter traits, climate, and soil pH influence
the global patterns of soil invertebrate-mediated decomposition. Using
this approach, we address whether soil invertebrate contributions to
forest leaf litter decomposition vary across regions. Based on previous
work on soil invertebrate-mediated decomposition, we hypothesize that
(1) soil invertebrate effects on forest litter decomposition will be
significantly higher in tropical forests than non-tropical forests; (2)
high termite diversity, warm and humid environments will be the major
drivers of high tropical invertebrate contributions to forest litter
decomposition.