Spatial distribution of soil invertebrate effects on forest leaf
litter decomposition
Forest litter decomposition by
invertebrates was the highest in the tropics and decreased with
increasing absolute latitude (Fig. 2a). Although overall microorganism
effect (69% on average) on forest litter decomposition twice that of
invertebrate effect (31% on average), the contribution of invertebrates
to litter decomposition in the tropics approaches that of
microorganisms. In boreal forests, the invertebrate contribution is low,
with microorganisms acting as the major decomposers (Fig. 2a). Overall,
invertebrates had significantly positive effects on forest litter
decomposition, leading to a 31% increase in decomposition where there
was invertebrate access
(LRR++ = 0.368, Qt =
37687.144, df = 475, P < 0.001) (Fig. 2b, Table S1). Soil
invertebrates increased litter decomposition significantly by an average
of 40% in tropical forests (LRR++ = 0.513, df = 161, P
< 0.001) compared with 26% in non-tropical forests
(LRR++ = 0.303, df = 311, P < 0.001),
respectively (Fig. 2b, Tables S1, S2). Tropical invertebrates
contributed 1.5 times more to forest litter decomposition in situthan did non-tropical invertebrates (Fig. 2b, Table S2).
Across different forest biomes, soil invertebrates contributed
significantly to litter decomposition in all forest biomes, increasing
decomposition by 43% in tropical wet forests (LRR++ =
0.554, P < 0.001), 28% in evergreen broadleaf forests
(LRR++ = 0.323, P = 0.001), 26% in
deciduous forests
(LRR++= 0.303, P < 0.001), 25% in coniferous forests
(LRR++ = 0.293, P < 0.001), and 24% in cold
or dry woodlands (LRR++ = 0.274, P < 0.001)
(Fig. 2b). Compared with non-tropical forests, tropical wet forests held
the highest invertebrate-mediated litter decomposition (Table S2). Among
forests in different zoogeographic realms measured, we found the higher
soil invertebrate effects on decomposition in Neotropical forests (48%,
LRR++ = 0.656, P < 0.001) compared with those
in the Nearctic (29%, LRR++ = 0.338, P = 0.001), the
Palearctic (25%, LRR++ = 0.292, P < 0.001),
and the Sino-Japanese forests (22%, LRR++ = 0.247, P
< 0.001), respectively (Fig. 3, Table S2). There were no
significant differences in invertebrate effects in the Neotropical
forests compared with forests in the Oriental (29%,
LRR++ = 0.344, P < 0.001), Panamanian (46%,
LRR++ = 0.625, P < 0.001), and Australian
(35%, LRR++ = 0.426, P = 0.004) data, respectively
(Table S2).
Drivers of soil invertebrate
effects on forest leaf litter decomposition
Mixed-effect meta regression showed a significantly positive
relationship between termite diversity and invertebrate effect sizes (Qm
= 17.842, P < 0.001, df = 446, Fig. 4a). For litter traits,
our meta regressions showed significantly negative relationships between
invertebrate effect sizes and C: N ratios in litter (Qm = 5.475, P =
0.019, df = 266, Fig. 4b) and
lignin: N ratios (Qm = 5.634, P = 0.018, df = 153, Fig. 4c), whereas no
significant relationships were observed between effect sizes and litter
C, N, P contents (as percentage of dry litter weight) (Table S1). In
terms of climatic variables, there were significant positive
relationships between invertebrate effect sizes and mean annual
temperature (Qm = 8.133, P = 0.004, df = 474, Fig. 4d) and mean annual
precipitation (Qm = 25.672, P < 0.001, df = 475, Fig. 4e). We
also found that soil pH negatively correlated with invertebrate effects
on forest litter decomposition (Qm = 11.226, P = 0.001, df = 450, Fig.
4f).
In our dataset, the decomposition durations of 73% of observations are
≤ 1 year, and the remaining 23% are from 1 to 2 years. Mixed-effect
meta regression showed that soil invertebrate effects weakened with the
increasing of decomposition duration
(Qm = 5.942, P = 0.015, Table S1), indicating soil invertebrates
contribute differentially to the early stages of leaf litter
decomposition (Fig. S1a). We did not find a significant impact of faunal
exclusion protocol (chemical vs. physical) on invertebrate effects
(Table S1). The reliability of our results was supported by Egger’s test
for funnel plot asymmetry (z = 0.8708, P = 0.3838), showing that
invertebrate effect sizes were not affected by publication bias (Fig.
S2).