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.