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).