Research Highlights: Natural regeneration is important in pine–oak mixed forests (Pinus armandii Franch., Pinus tabuliformis Carr., and Quercus aliena Bl. var. acuteserrata Maxim.ex Wenz.), but allelopathy as a limiting factor has not been studied. Our research provides insights into allelopathy in pine–oak mixed forest litter. Background and Objectives: Allelopathy among tree species occupying the same ecological niche in mixed forests may adversely affect regeneration. We studied allelopathy in pine and oak forest litter to determine the effect on regeneration, whether it is offset by adding activated carbon or plant ash, and what allelopathic substances are present. Materials and Methods: We used laboratory seed culture and field seeding to determine pine and oak litter regeneration effects on P. tabuliformis and P. armandii in the Qinling Mountains, China. In the laboratory, we irrigated seeds with three different litter concentrations. A fourth treatment incorporated activated carbon. The field study established small quadrats in mixed forest to study how removing or retaining litter and spreading plant ash affected sown seeds. High performance liquid chromatography–mass spectrometry was used to compare differences in chemical substances in extracts with and without activated carbon. Results: Litter extracts significantly affected germination rates in both species. Seedling morphological and physiological indexes showed that litter extracts negatively affected growth in both species, but activated carbon alleviated this inhibitory effect on P. armandii. Forest stand and litter did not affect P. armandii seed germination. Pinus tabuliformis germination rates were significantly higher in plots with removed litter than when litter was retained or plant ash spread, and lower in oak than pine forest. Allelopathic substances detected in pine forest were trioctyl trimellitate, amyloid β-Peptide 10–20, and triisobutyl phosphate, potentially affecting P. armandii seed germination and growth. Conclusions: Appropriate removal of litter in mixed forests can improve the natural regeneration ability of P. tabuliformis.

Belowground multitrophic communities and their potential associations are important biological foundations for the maintenance and recovery of soil ecosystem functions. However, it remains unclear how soil micro-food web composition and network complexity are linked to soil ecosystem multifunctionality across different post-cropland restoration states. In this study, we used a space-for-time substitution approach and selected cropland and three post-cropland restoration states in the Taihang Mountains, including shrub–grassland, Populus spp. plantation, and Robinia pseudoacacia plantation, each with five plots, to examine changes in bacterial, fungal, and nematode communities, soil micro-food web network complexity, and soil ecosystem multifunctionality in topsoil and subsoil. Compared with cropland, bacterial diversity was higher in the shrub–grassland, Populus spp. and R. pseudoacacia plantations, whereas fungal diversity showed a marked decline in the Populus spp. plantation. Furthermore, the topsoil of the R. pseudoacacia plantation exhibited much higher micro-food web network complexity and soil multifunctionality than cropland, whereas differences among land-use types were relatively small in the subsoil. Structural equation modeling indicated that vegetation type, soil depth, soil moisture content, microbial diversity, nematode diversity, and soil micro-food web network complexity jointly explained 94% of the variation in soil ecosystem multifunctionality (R² = 0.94). Further random forest analysis identified soil depth as the most important predictor of soil ecosystem multifunctionality, followed by soil micro-food web network complexity; among biological variables, fungal diversity, bacterial diversity, omnivorous-predatory nematode diversity, and herbivorous nematode diversity also showed relatively high importance. Overall, these results suggest that changes in soil ecosystem multifunctionality across post-cropland restoration states were strongly soil-depth-dependent, and that soil micro-food web diversity and network complexity can serve as important biological indicators for assessing restoration outcomes.