Abstract:

Objective: This study investigated the driving mechanisms of tree species diversity and environmental factors on soil multifunctionality in the mid-temperate forests of northeast China, and further elucidated the variations in the relationship between tree species diversity and soil multifunctionality along environmental gradients, aiming to provide a scientific basis for forest conservation and management in this region. Method: In this study, based on data from 279 sample plots in mid-temperate forests of northeast China, five soil function indicators, including total organic carbon content, total nitrogen content, total phosphorus content, available nitrogen content, and available phosphorus content, were selected. The averaging method was used to calculate the soil multifunctionality index. Multiple linear regression and structural equation modeling were employed to investigate the effects of atmospheric nitrogen deposition flux, climate, and topographic factors on the relationship between tree species diversity and soil multifunctionality, as well as the driving mechanisms of soil multifunctionality. Additionally, the interactive effects of tree species diversity and the aforementioned environmental factors were analyzed. Result: 1) Tree species diversity had a significant positive effect on soil total organic carbon content, total phosphorus content, total nitrogen content, available nitrogen content, and soil multifunctionality index (β=0.31、0.27、0.34、0.34、0.22, P<0.05), but a significant negative effect on soil available phosphorus content (β=?0.16, P<0.05). Nitrogen deposition flux showed a significant negative correlation with soil total organic carbon content, total phosphorus content, total nitrogen content, available nitrogen content, and soil multifunctionality index (β=?0.14、?0.35、?0.17、?0.22、?0.18, P<0.05). Soil pH was significantly positively correlated with total phosphorus content, available phosphorus content, and soil multifunctionality index (β=0.17、0.44、0.15, P<0.05). Soil carbon-to-nitrogen ratio exhibited a significant positive correlation with total organic carbon content, total nitrogen content, and soil multifunctionality index (β=0.38、0.16、0.13, P<0.05). 2) The relationship between tree species diversity and certain soil single-functionality indicators and soil multifunctionality index was regulated by the interaction between nitrogen deposition flux and soil carbon-to-nitrogen ratio. It was manifested that the positive effects of tree species diversity on soil total organic carbon content, total phosphorus content, total nitrogen content, and soil multifunctionality index weakened with increasing soil carbon-to-nitrogen ratio under medium-to-high nitrogen deposition environments, but strengthened under low nitrogen deposition environments. 3) The structural equation model revealed that nitrogen deposition flux was able to directly negatively affect soil multifunctionality index (β=?0.19, P<0.01), and soil pH and carbon-to-nitrogen ratio also exerted indirect negative effects on the soil multifunctionality index. Climate factors primarily indirectly influenced soil multifunctionality index via tree species diversity and soil pH, while topographic factors indirectly influenced soil multifunctionality index through tree species diversity, soil pH and carbon-to-nitrogen ratio, and soil pH indirectly regulated soil multifunctionality index by affecting tree species diversity. Conclusion: This study reveals that tree species diversity, nitrogen deposition flux, soil pH, and carbon-to-nitrogen ratio are key direct drivers of soil multifunctionality. Climate and topographic factors exert indirect influences by regulating both biotic and abiotic components. Notably, the positive effect of tree species diversity on soil multifunctionality is regulated by nitrogen deposition flux and soil carbon-to-nitrogen ratio. The findings of this study highlight the importance of regulating soil pH and soil carbon-to-nitrogen ratio under the context of atmospheric nitrogen deposition, which contributes to biodiversity conservation and the maintenance of forest ecosystem functions.

Key words: soil multifunctionality, tree species diversity, environmental factors, soil carbon-to-nitrogen ratio, soil pH