4.1 Effects of earthworms on litter mass loss irrespective of
deposited compounds
As hypothesized, N, Na and PAHs
all decreased litter mass loss. Effects of deposited compounds on litter
mass loss have been shown to vary with their concentrations, with
detrimental effects typically increasing at higher concentrations (Knorr
et al., 2005; Ji et al., 2020). Our N and PAHs treatments doubled the
deposited amount of N and PAHs at our study sites and the negative
effects were in line with previous studies, but were relatively small.
Fast cycling and losses of N via leaching (Kreutzer et al., 2009; Wang
et al., 2021) might have been responsible for the weak N effect.
Although increased by a factor of two, the concentration of PAHs in this
study (1.813 μg g-1 dry soil) may have little toxic
effects on soil decomposers according to previous studies
(Rodriguez-Campos et al., 2014; Zhang et al., 2016). Further,
earthworms and lignin-degrading
fungi, known to stimulate the degradation of PAHs (Haritash & Kaushik,
2009; Rodriguez-Campos et al., 2014),
may have contributed to the weak
effects of PAHs on litter mass loss. Compared to N and PAHs, Na addition
more strongly reduced litter mass loss,
potentially due to the rather
high concentrations of Na added (0.5% NaCl solution, 3.28 g
Na+ m-2 month-1)
(Kaspari et al., 2009, 2014; Jia et al., 2015).
In the region of Nanjing, the
input of Na of predominantly natural (marine) origin into terrestrial
ecosystems is only 40.79 μg m-2month-1 (Li et al., 2016). The results support earlier
findings that in contrast to small Na input, high amounts of Na inhibit
both faunal and microbial activity, and thereby decreases litter mass
loss (Jia et al., 2015). Overall,
the results suggest that the effect of deposited compounds on litter
mass loss depends on the type of compounds with the effects of low
concentrations of deposited compounds affecting litter mass loss in
subtropical forest ecosystems only moderately.
Notably, the positive effect of earthworms on litter mass loss was not
significantly modified by deposited compounds, this is opposed to our
second hypothesis. The species characteristics such as epigeic group and
highly contamination tolerance may contribute to the stable role ofE. fetida on litter mass loss under different deposited
compounds. Effects of earthworms on litter mass loss have been shown to
vary among ecological groups of earthworms, with the effects of epigeic
species may be bigger than others (Suarez et al., 2006). Correspondingly
the epigeic species E. fetida increased litter mass loss, which
is in line with previous studies (Heungens, 1969; Rajapaksha et al.,
2013). Previous studies found E. fetida to be able to live in
highly contaminated soil (Geissen et al., 2008) suggesting that E.
fetida is rather insensitive to soil contaminations and may even
contribute to the decontamination of soils (Rodriguez-Campos et al.,
2014). Thus E. fetida increased litter mass loss regardless of
deposited compounds although the compounds such as PAHs are toxic and
high concentrations of Na inhibit animal activity (Peng et al., 2008;
Jia et al., 2015).
In deciduous and coniferous forests, earthworms’ positive effects on
litter mass loss were both not affected by different types of deposited
compounds. Considering the pathways earthworms stimulated litter mass
loss varied between the deciduous and coniferous forest (also the
different soil conditions), suggesting earthworms get rid of negative
effects of deposited compounds on litter mass loss via multiple ways.
However, we lacked replication of forest types and only studied a single
deciduous (Q. variabilis ) and a single coniferous (P.
massoniana ) forest, and the effects of earthworms cannot be compared in
a straightforward way as we added a larger number of earthworms to the
deciduous than the coniferous forest and it has been shown that the
effect of earthworms on litter mass loss increases with earthworm
abundance in forests (Gonzalez et al., 2003; Szlavecz et al., 2011;
Huang et al., 2020).