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.