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.