1. Introduction
The dramatic increase in atmospheric emissions of reactive nitrogen (N) from human activities (industrial and agricultural activities and fossil fuel combustion) since the Industrial Revolution has also led to the rapid increase in N deposition to terrestrial and aquatic ecosystems ((Ackerman, Millet , & Chen, 2019). N deposition provides a new source of fertilizer for plants, but excessive N input can also affect biogeochemical cycles and alter ecosystem structure and function (Yu et al., 2019). Forest ecosystem species diversity is an important indicator reflecting the relationship between plants and the environment, determining the structure and ecological functions of forest communities, which are mutually constrained and synergistic with environmental factors. Moderate N input will alleviate forest N limitation to a certain extent and promote vegetation growth. On the other hand, excessive N deposition induces ecological problems such as soil acidification, reduction of biodiversity, and degradation of forest functions(Gao et al., 2019; Hong, Gan , & Chen, 2019; Vuorenmaa et al., 2018). The results of a worldwide network of long-term monitoring of N deposition since the twentieth century show that China faces more severe N deposition than the United States, European countries, and other countries in East Asia(Zhang et al., 2021). Nitrogen deposition remains a threat to the biodiversity and stability of forest ecosystems (Weldon, Merder, Ferretti , & Grandin, 2022).
Herbaceous plants have the highest species diversity in forest ecosystems and contribute significantly to forest ecosystem structure and function, but are more sensitive to atmospheric changes and N deposition(GILLIAM, 2007; McDonnell, Clark, Reinds, Sullivan , & Knees, 2022; Thrippleton, Bugmann, Kramer-Priewasser , & Snell, 2016). It has been demonstrated that excessive N input can have complex effects on herbaceous community structure and plant biological characteristics, such as reducing herbaceous diversity and altering herbaceous root chemistry and biomass(Li et al., 2015). However, most studies have focused on tropical forests and temperate grasslands, and not enough studies have been conducted on herbaceous plants in temperate forests.
The functions of soil microorganisms in forest ecosystems should not be underestimated, among which AM fungi can establish symbiotic relationships with most herbaceous plants and play an important role in the establishment and maintenance of plant communities((Mariotte et al., 2013; Smith & Read, 2008; Wang et al., 2019). Arbuscular mycorrhizal (AM) fungi can promote plant growth((Hoeksema et al., 2010; Xie et al., 2022)and improve plant resilience(Chen, Arato, Borghi, Nouri , & Reinhardt, 2018; Ruiz-Lozano et al., 2016), and mycelium secretion-GRSP can also promote soil aggregation(Gao, Wang , & Wu, 2019), affecting host plants in a range of direct and indirect ways. Most mycorrhizal fungi depend on the host plant for existence and reproduction and AM fungi are thought to be susceptible to environmental conditions, such as climate((Compant, van der Heijden , & Sessitsch, 2010), plant species((Kivlin, Hawkes , & Treseder, 2011), and soil properties, etc. Excessive N input indirectly affects the structure and diversity of AM fungi through changes by soil factors (Boeraeve et al., 2022; Ma et al., 2021), and thus affects the ecological function of AM fungi. The results of the studies are not the same around the world, which is due to the influence of the experimental conditions. A global meta-analysis showed that the negative effect of N addition on AM fungi was mainly a reduction in total AM fungal abundance and did not significantly affect the diversity and structure of AM fungi(Han, Feng, Han , & Zhu, 2020). However, in a tropical simulated N deposition experiment, it was found that N addition mainly reduced the diversity and abundance of AM fungi(Camenzind et al., 2014). AM fungi have been shown to contribute to the maintenance of forest herbaceous diversity under N application conditions(Smith & Stephan, 2021), But studies in northern forest lands are scarce.
In this experiment, AM fungi, herbaceous plants, and soil properties were investigated in a northeastern temperate Korean pine plantation ecosystem that was subjected to a 7-year N addition experiment. We hypothesized that (1) Long-term nitrogen fertilization will change the soil pH value, increase soil GRSP content and thus improve the stability of soil aggregates. (2) Long-term N addition will change the structure and diversity of AM fungal communities, thus affecting the ecological functions of AM fungi. (3) Long-term N addition would directly change AMF diversity by reduce pH, and indirectly by change herbaceous plant composition.