4.2 Pathways linking the effect of deposited compounds and
earthworms to litter mass loss
Partly supported the third hypothesis, our SEM indicated that N, Na and
PAH all indirectly inhibited litter mass loss in the deciduous forest by
acidifying the soil, but they
little affected soil microbial biomass in particular in fine litterbags
suggesting that deposited
compounds changed the activity rather than biomass of microorganisms. N
and PAHs also indirectly inhibited fauna-driven litter mass loss by
acidifying the soil. Zhang et al.
(2016) reported that N addition does not affect fauna-driven mass loss,
supporting our results that
acidification due to N addition (Lin et al., 2017) was responsible for
the detrimental effects of N (and PAHs) on fauna-driven litter mass
loss, potentially due to deteriorating habitat properties of
detritivores. Notably, PAHs also indirectly inhibited fauna-driven
litter mass loss by reducing microbial biomass in both forests. Since
PAHs may accumulate in soil organisms (Jonker & van der Heijden, 2007;
Muijs & Jonker, 2009), the negative effects of PAHs aggravate with time
indicating that in the long-term their detrimental effects may exceed
those of N and Na depositions.
The lack of effects of Na on
fauna-driven litter mass loss may have been due to detrimental effects
of high concentrations of Na being cancelled out by Na functioning as
essential element for soil animals (Kaspari et al., 2014).
Results of our SEM further
indicated that in the coniferous forest Na directly inhibited litter
mass loss suggesting that it also affects animal and microbial activity.
Overall, our SEM suggest deposited compounds acidified the soil and
thereby inhibited the activity of soil detritivores and microorganisms,
but the pathways of the detrimental effects of these compounds on litter
mass loss varied among the types of deposited compounds studied. In the
long-term detrimental effects of PAHs are likely to exceed the negative
effects of N and Na due to their accumulation in litter and decomposer
organisms.
Earthworms increased litter mass loss by directly affecting fauna- and
microbe-driven litter mass loss, and indirectly affecting soil pH and
soil microbial biomass, supporting the third hypothesis. The pathways
earthworms promoted litter mass loss differed between forests, and
corresponded to the different responses of earthworms to deposited
compounds. Earthworms directly promoted fauna-driven litter mass loss in
the deciduous forest, but promoted microbe-driven litter mass loss in
the coniferous forest. Since earthworms prefer to feed on high quality
litter (Rajapaksha et al., 2013)
and earthworm abundance and
litter mass loss correlate positively (Huang et al., 2020),
earthworm abundance and litter
quality may have contributed to the different effects of earthworms on
litter mass loss in the deciduous and coniferous forest. Further,
the mitigation of the negative
effect of Na on litter mass loss in the deciduous forest by earthworms
likely was due to earthworms reducing the negative effect of Na on soil
pH. Earlier studies also found earthworms to alter soil pH (Sackett et
al., 2013), but our results indicate that this may depend on the type of
deposited compounds. By contrast,
the mitigation of the negative effect of the addition of N on litter
mass loss (and soil pH) in the coniferous forest by earthworms according
to our SEM was due to earthworms directly increasing litter mass loss.
Further, the negative effect of PAHs on fauna-driven litter mass loss
was mitigated by earthworms via beneficially affecting pH and microbial
biomass, but also directly by increasing litter mass loss.
5 Conclusion
This study provided novel and
detailed insight into how atmospheric deposited compounds and earthworms
affect the nutrient cycling in interactive ways. Importantly, using SEM
we identified mechanisms and pathways responsible for these
interactions. Our results suggest that the positive effects of
earthworms on litter mass loss are not modified by different types of
deposited compounds, and the mechanisms vary between the types of
deposited compounds. Although not replicated, the results further
suggest that the mechanisms responsible for earthworms maintain uniform
and positive effects on litter mass loss under different types of
deposited compounds vary with forest type. The results highlight the
importance of studying the effects of deposited compounds and earthworms
on terrestrial nutrient cycling in concert as earthworms may persist in
their beneficial roles and mitigate the influence of detrimental
deposited compounds.
Acknowledgements
We thank H Jiang for field support, ZJ Xie and HF Yin for helpful
comments on previous versions of the manuscript. This work was supported
by the National Natural Science Foundation of China (31870598, 32001300,
32160356); Strategic Priority Research Program of the Chinese Academy of
Sciences (A) (XDA19050400); Jiangsu Forestry Science and technology
innovation and promotion project (LYKJ[2021]16); the Key Program of
Scientific Research projects of Hunan Provincial Education Department
(21A0334); key specialized research and development breakthrough program
in Henan province (222102320289), Scholarship of China Scholarship
Council (202006190207). The publication of this study was supported by
the Open Access Funds of the Göttingen University.
Conflict of Interest
The authors declare that they have no conflict of interest.
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