3.2 Changes in litter mass loss, C and N loss
Deposited compounds uniformly
reduced litter mass loss in both deciduous and coniferous forests, with
the effects being independent of mesh size (Table 2, Figs 1,
2). The effects were strongest
for Na, which decreased mass loss by 9.13% and 5.60% in deciduous and
coniferous forests, respectively. Similar to mass loss, deposited
compounds also reduced litter C and N loss (Table S3). Again, the
effects were strongest for Na, which reduced litter C loss by 7.15% and
litter N loss by 7.63% in the deciduous and coniferous forests,
respectively (Figs 3). Addition of N and PAHs reduced litter mass, C and
N loss but less than Na, e.g., N and PAHs reduced litter mass loss by
4.82%, 7.68% in deciduous forest and 3.42%, 1.46% in coniferous
forest, respectively (Figs 1, 2, 3).
Earthworms generally accelerated
litter mass loss in both deciduous and coniferous forests, with the
effect being independent of deposited compounds (Table 2, Figs 1, 4).
The positive effect of earthworms on the loss of litter mass and litter
C was similar but varied between forests. In the deciduous forest,
earthworms significantly accelerated litter mass loss and C loss in
coarse (6.66% and 6.20%) but not in fine bags
(Figs 1, 4, 5, S2). In the
coniferous forest, earthworms significantly accelerated the litter mass
loss and C loss in both coarse and fine bags, but the effect was less
strong than in the deciduous forest (2.02% and 2.80%; Table S3, Figs
1, 4, 5, S2).
3.3 Pathways affecting
litter mass loss
The pathways of positive earthworm effects on fauna- and microbe-driven
mass loss varied with the types of deposited compounds (N, Na, PAHs) and
forests (deciduous, coniferous; Figs 6, S4, Table S4). Earthworms
directly increased fauna-driven mass loss in the deciduous forest, but
increased microbe-driven mass loss in the coniferous forest. Further, in
the deciduous forest,
earthworms
increased litter mass loss driven by fauna and microbes’ decomposition
by promoting soil pH in the Na addition treatment and by promoting soil
microbial biomass in the PAH treatment.
The
negative
effects of deposited compounds on litter mass loss were mainly due to
reduced soil pH in both the deciduous and coniferous forests (Fig. 6).
Further, the addition of PAHs reduced fauna-driven litter mass loss via
reducing soil microbial biomass in both the deciduous and coniferous
forests. By contrast, in the
coniferous forest Na addition directly decreased both litter mass loss
driven by fauna and microbes without changing soil pH and microbial
biomass.
4 Discussion
Human activities increase the
input of atmospheric deposited compounds into natural ecosystems, the
deposited compounds including inorganic and nonmetal ions, metal ions
and organic contaminants (Li et al., 2016). These compounds may have
different influence on nutrient cycling of natural ecosystems based on
the difference of their chemical properties. Soil engineers, earthworms
play a key role in nutrient cycling of terrestrial ecosystems (Blouin et
al., 2013). However, the lacking of studies about the interactive effect
of deposited compounds and earthworms on nutrient cycling limits our
understanding of the mechanism of deposited compounds affecting
terrestrial C and N cycling, and also limits the bioremediation such as
apply earthworms to forests contaminated by atmospheric deposited
compounds. This study investigated how the effects of earthworms are
affected by different types of deposited compounds during major nutrient
cycling in forests such as litter decomposition. Unexpectedly,
earthworms uniformly increased litter mass loss irrespective of
different deposited compounds. Notably, the pathways earthworms affect
litter mass loss varied among different types of deposited compounds due
to different pathways of these compounds affecting litter mass loss.