2.5 Statistical analyses
All analyses were performed using R version 4.0.5
(https://www.r-project.org/). We analyzed litter mass loss using linear
mixed effects models (LMMs). Data of the deciduous and coniferous
forests were analyzed separately. In each LMM, type of deposited
compounds (control, N, Na, PAHs), earthworms (with and without), mesh
size (fine and coarse) and time (five sampling dates) were treated as
fixed factors.
Mesh
size was nested in mesocosms and included as random factor to account
for non-independence of litterbags within mesocosms and
repeated
sampling. To evaluate the effects of type of deposited compounds,
earthworms and mesh size, we used planned contrasts between the control
and the respective treatment. Data were log(x + 1) transformed to meet
normality if needed. Due to the response variables being log
transformed, contrasts are analogous to log response ratios
(Piovia-Scott et al., 2019). We used ‘nlme’ to fit mixed-effect models
and ‘emmeans’ for planned contrast. The changes in litter mass loss, C
and N loss were shown as percentages and were calculated as\(\left|m_{t}-m_{c}\right|\times 100\%\), with \(m_{t}\) and\(m_{c}\) are the mass loss percentages of the treatment and control,
respectively. In another word, the percentages shown in the results are
true mass percentages to total litter mass (not the ratio of treatment
to control, or the ratio of the difference in treatment and control to
control). To evaluate effects of the addition of E. fetida , we
modelled abundance and biomass of earthworms at the end of the
experiment using generalized linear models with quasi-Poisson
distribution to account for model over- or under-dispersion.
Structural equation models (SEMs) were used to inspect pathways linking
earthworms and deposited compounds to litter mass loss (see Fig. 4 and
Fig. S4; Tian et al., 2018, Yin et al., 2022). To compare the effects of
different types of deposited compounds and earthworms on litter mass
loss, we merged six models for each forest as one including the three
types of deposited compounds and two mesh sizes. In each model,
deposited compounds and earthworms were included as categorical
variables (with and without) and the other variables as numeric;
values
of all variables were scaled to 0 - 1 before modeling. Direct effects of
Na on litter mass loss were built-in models of the coniferous forest
according to the modification indices in R; direct effects of earthworms
on SIR were removed to improve fitting (Shipley, 2009; Yin et al.,
2022).
3 Results
3.1 Changes in the
abundance of earthworms
The addition of E. fetida resulted in significantly increased
abundance and biomass of total earthworms including original existingE. fetida in both deciduous and coniferous forests (P <
0.001, Table S1). Specifically, the addition of E. fetidaincreased the abundance of total earthworms
by
factors of 4.66 and 2.93 in mesocosms of deciduous and coniferous
forests, respectively (Table S2).