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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