**4. Discussion**

Following our first hypothesis, we verified that the process of humidification promoted by soil fauna differed on species. Except for willow, we found soil fauna significantly promoted the accumulation of humic substances in litters of birch, fir, and cypress (Figure 4), indicating soil fauna could promote the process of humidification depending on species [37,41]. Additionally, we found soil fauna individual density significantly related to humification degree besides cypress (Figure 8), possibly by the presence of high concentration of volatile organic compounds in cypress, which could inhibit the activity of soil decomposer [42]. Overall, our result clearly demonstrated that soil fauna promotes the process of litter humification but varied by species and environmental factor.

Species, sampling, stage and their interactions had significant effects on the humic substances accumulations as litter humification proceeding in the alpine forests (all *p* < 0.001, Table 1). The positive effect of soil fauna on the accumulation of humic substances is high in the stages of GS and LF, which is in accordance with the densities of individual and group of soil fauna, indicating that soil fauna is favorable to the accumulations of humic substance of birch, fir, and cypress. Our result showed that soil fauna plays vital roles during the period of accumulation of humic substances, agreemen<sup>t</sup> with previous studies [19,43]. However, soil fauna (*Prostigmata* and *Oribatida*, etc.) also feed on humic substances and reduce the content of humic acid through the intestines, thereby converting organic matter [16], but these activities are varied among seasons and resulting in the capability of producing humic substances are difference. Interestingly, soil fauna had a positive effect on the accumulation of humic substance of willow litter in each stage aside from the stage of OF, while inhabited other foliar litters (Figure 4). The presence of scavenging soil fauna was found in birch, fir, and cypress litter, but not in willow litter (Supplementary Materials Table S1), so the humic substances might be highly consumed and decomposed by both soil microbes and scavenging fauna [8,10]. Moreover, we found the overall contribution rate of soil fauna to humic substance in coniferous litters (fir and cypress) is higher than that in broadleaf litters (birch and willow), which is related to litter quality. As higher

contents of lignin in coniferous litters are more conducive to the formation of humus than broadleaf litters [43,44].

The contribution rate of litter humic substance response to soil fauna differs among sampling time, which is affected by litter qualities and environmental factors (Table 2). For fir litter, soil fauna had positive effects on both humic acid and fulvic acid, indicating a net accumulation of humic substances on the whole year, which was influenced by litter substrate quality [43]. Moreover, temperature and humidity can affect the biodiversity of soil fauna [44], and the growing season increases with soil fauna density and individual density, resulting in the contribution rate of soil fauna on fulvic acid being much higher than humic acid, as the contribution of soil fauna to humic acid was negative in the stage of GS (Figure 5). More importantly, after one-year incubation, the contribution rate of litter humic acid to soil fauna seems related to elevation, as positive contribution of birch and fir litters appeared in the plantation and negative values of willow and cypress litter in the primary forest. Differently, the contribution rate of litter fulvic acid to soil fauna was related to species, which showed negative contributions in coniferous litters and positive values in broadleaf litters. The divergent responses of humic acid and fulvic acid to soil fauna in different litter species highlighted that environmental factors and litter quality jointly controlled the content of humic substances [4,20,44].

Generally, environment exerts significant effects in the early decomposition stages due to physical effects [39]. When litter components that are susceptible to physical effects are consumed, the influence of the environment on litter decay is weakened. Otherwise, environmental factor directly affected the individual density and group density of soil fauna in each stage (Table 2), thus affecting the process of litter humification. We found the contribution rates of humic acid and fulvic acid to soil fauna reached maximum values in litters of birch, fir, and cypress at the stages of GS and LF. The individual density and group density of soil fauna both reached their peak values under the influence of temperature in these two stages, so their contribution rate reached the maximum. Moreover, Larionova et al. (2017) found there was an optimal temperature (12 ◦C) for the formation of humic substances [24]. In our site, the mean air temperature is 11 ◦C in the growing season, which provides good conditions to promote the humic substances of the fallen leaves accumulate rapidly. The annual contribution rate of soil fauna to the accumulation of humic acid in the plantation forest is higher than that of primary forest, and the contribution rate of the fulvic acid accumulation in coniferous litter is higher than that in broad-leaved litter, which indicates that the formation of humic acid by soil fauna is mainly affected by environmental factors, while the formation of fulvic acid is mainly affected by the quality of the matrix.

Over one-year field incubation, only willow litter showed a positive effect on the humification degree by soil fauna, and other litters were negative values (Figure 6). The different responses of humification degree to soil fauna in litters appeared significantly in the stages of OF and DF (Figure 7). The positive effects in willow litter at the stages of OF (16%) and DF (22%) indicate that soil fauna also play an important function, improving humification degrees even in cold winter. Previous studies have suggested a nonnegligible process of litter humification in winter, but their humification degree depends on litter quality [4]. Otherwise, the humification degree is positively correlated with the individual and group density of soil fauna (Figure 8), but this relationship is also controlled by litter quality and type [8,20]. We found the individual density of soil fauna in the litter of birch and cypress significantly correlated with humification degree, and the group density for willow and cypress litters was significantly correlated with the humification degree. In contrast, the fir litter showed no significant effect to litter humification mediated by soil fauna.

For litters of birch, fir and cypress, the annual contributions of soil fauna to the humification degree are negative, ranging from −48% to −8% after a year field incubation (Figure 7), indicating soil fauna may not be a key factor controlling the humification degree of these litters; other factors, such as initial litter quality and environmental factory, are also important [8,11,45]. The contribution rate of soil fauna to humification degrees reached the maximum values in the stage of LF (Figure 7), which are in accordance with the patterns of individual density and group density (Figure 3 and Supplementary Materials Table S1). Other studies also showed that the population of soil fauna has a positive correlation with the humification degree of litter [4,45].
