Summary
- Research
has indicated that increases in nitrogen (N) deposition can greatly
affect ecosystem processes and functions. There is limited information
about the effects of long-term N addition on soil nematodes and their
functional composition, although nematodes are the most abundant
multicellular animals on Earth.
- We conducted a field experiment in 2004 with four levels of N addition
(0, 60, 120, and 240 kg N ha-1yr-1) in a subtropical Cunninghamia
lanceolata forest. Soil samples
with three depths (0-20, 20-40 and 40-60 cm) were collected and the
community structure, diversity and trophic groups of soil nematodes
were determined in 2014.
- N addition significantly increased the abundance of bacterial- and
fungal-feeding nematodes, but decreased the abundance of plant-feeding
nematodes at the 0-20 cm soil layer. Accordingly, the plant parasite
index and enrichment index decreased but the basal index and channel
index increased, which weaken the importance of the plant-based energy
channel, but enhance the importance of the fungal-based energy
channel. N addition had no effects on the diversity of soil nematodes
in three soil depths. Structural equation modeling analysis indicated
that N loading directly changed plant-feeding (totalr2 =0.42) nematodes, or indirectly affected
bacterial- (r2 =0.43), fungal-
(r2 =0.31) and plant-feeding nematodes via
change soil nutrients, soil water content and pH.
- These findings suggest that N addition can change the community
structure and energy channels soil nematodes, which would affect soil
processes and food web functions in forest soils under future
environmental change scenarios.
Key-words:nitrogen
deposition, soil biology, soil nematodes, forest ecosystem, global
change
Introduction
The deposition of reactive nitrogen (N) from the atmosphere to the
Earth’s surface has been increasing globally (Vitousek et al.1997; Li et al. 2020; Xie et al. 2020) and is predicted to
increase to as much as 200 Tg N yr−1 by 2050 (Vitouseket al. 1997; Galloway et al. 2008). In China, the
increases in N deposition caused by rapid agricultural and industrial
development and its pollution on ecosystems has been of wide concern
over the past decade (Cui et al. 2013; Liu et al. 2013; Yuet al. 2019). The effects of N deposition on terrestrial
ecosystems include changes in carbon (C) and N cycling (Isbell et
al. 2013; Zhang et al. 2020), plant species richness (Luet al. 2010; Wu et al. 2013), soil community structure (Liet al. 2019; Yang et al. 2020) and human health (Chenet al. 2019; De Marco et al. 2019). The effects of N
deposition on soil nematode communities, however, are incompletely
understood.
Nematodes are the most abundant multicellular animals on Earth (van den
Hoogen et al. 2019). They are important components of soil food
webs (De Long 2017). Abundance and diversity of nematodes are correlated
with many geochemical processes such as ecosystem succession, litter
decomposition and N cycling (Fu et al. 2000; Neher et al.2012; Zhao et al. 2012).
Several related studies have shown that the development and
characteristics of nematode communities were greatly affected by plant
community characteristics (Li et al. 2007; de la Peña et
al. 2016; Shao et al. 2016; Zhao et al. 2019). Because
nematodes are sensitive to external disturbances (e.g., land-use change
and environmental pollution) and nematode communities are generally
regarded as effective indicators of the health and stability of soil
food webs (Bongers 1990; Powell 2007; Zhao et al. 2013).
Researchers have therefore concluded that the assessment of an
ecosystem’s renewal efficiency and adaptability after changes in site
conditions should include belowground components including soil
nematodes (Todd et al. 2006). In recent years, studies of
nematode communities have focused on their responses to changes in
agricultural management
and
land-use. For example, cropping systems have obvious effects on soil
nematode communities (Treonis et al. 2018). In addition,
fertilization can alter nematode communities by affecting the soil
microbial communities and the soil abiotic properties (Liu et al.2016; Li et al. 2018a; Zhao et al. 2018).
The effects of excess N deposition on soil nematodes have not been well
understood. Previous reports indicated that N deposition increased the
abundance of a particular taxon or of several trophic groups (Lianget al. 2009; Shaw et al. 2019). In another report, N
deposition generally decreased the abundance of nematodes in N-rich but
phosphorus-poor soils (Zhao et al. 2014). Other researchers found
that long-term N enrichment reduced the abundance of bacterial-feeding,
fungal-feeding, plant-feeding and omnivorous-predatory nematodes because
of soil acidification (Chen et al. 2015) or because of biotic
interactions (Shao et al. 2017; Shaw et al. 2019).
Generally, changes in the community compositions of soil nematodes may
alter energy fluxes in soil food webs (Shao et al. 2019), where
the energy flow and stability of food webs can be closely linked (Rooney
and McCann, 2012; Schwarz et al. 2017).
In the current study, we focus on the effects of N addition on soil
nematodes in subtropical forest. Tropical and subtropical forests
contribute one-third to terrestrial primary production and have high
species richness, which function as important C sinks (Brookshireet al. 2012; Ferreira et al. 2018). However, global change
phenomenon such as N deposition could nevertheless have substantial
effects on tropical and subtropical forest ecosystems (Hedin et
al. 2005; Lewis et al. 2009; Gerber et al. 2010).
Therefore, exploring the responses of soil nematode community and the
changes in the energy channel in soil food webs to a long-term N
addition in tropical and subtropical forests is of great importance to
ecosystem stability. In the present study, we conducted a long-term
N-loading experiment to determine how N addition affects the community
structure of soil nematodes in a subtropical forest. We hypothesized
that 1) N addition will alter the energy channel that indicated by the
structure of nematode community; and 2) different trophic groups of
nematodes would have different responses to N addition with the direct
or indirect pathways. These hypotheses were suggested by previous
findings, which reported that N deposition and other environmental
factors can affect soil nematode communities and energy channels (Zhaoet al. 2014; Shao et al. 2017; Shao et al. 2019).