Discussion
In the present study, long-term N loading affected the abundance of
nematode trophic groups in shallow soil of a subtropical Chinese fir
forest. The N3 treatment, for example, significantly increased total,
bacteria- and fungal-feeding nematode abundances but significantly
decreased the abundance of plant-feeding group (Fig. 1), which is
consistent with the first hypothesis. Because the population dynamics of
nematode trophic groups are closely related with their functional roles
(Mueller et al. 2016; Shaw et al. 2019), N addition would
also affect nematode ecological indices, i.e., MI25, EI, SI, BI, and CI
(Fig. 3). For example, N loading significantly decreased the MI25 value
at 0-20 cm soil depth but not at deeper soil layer, which supported that
N loading only affected bacterial-feeding nematodes in the shallow soil
layer. The values of MI25 are determined by c-p2 to c-p4 values (Zhao et
al. 2014), however, the values of c-p3 and c-p4 guilds did not respond
to N loading (Fig. S1 and S2), so
the increase of bacterial- and fungal-feeding nematode abundances to N
loading was caused by the increase in the abundance of
c-p2
values in our study. Therefore, we considered that the long-lived
functional guilds with higher c-p values, e.g., the OP group, are more
vulnerable to environmental changes and require longer time to recover
from disturbances than fungal- and bacterial-feeding nematodes (Ferriset al. 2001; Liang et al. 2009). Consistent with that
view, our results showed that N addition tended to reduce the abundance
of OP group. Furthermore, the SI value, which is mainly determined by
the abundance of omnivorous and predatory nematodes (Ferris et
al. 2001), also declined at 0-20 cm soil depth in response to N loading
(Fig. 2d). The SI value is generally found to be high in undisturbed
natural systems (Ferris et al. 2001; Zhao et al. 2013),
which suggests that N addition may bring some negative effects on soil
ecosystem of the subtropical forests.
Another interesting finding is that N loading decreased the EI value,
but increased the CI value (Fig. 3). EI reflects resource availability
and decomposing capacity of primary decomposers of soil food web, while
CI is an indication of fungal-feeding nematodes as the dominant energy
channel in soil food web (Ferris et al. 2001; Shaw et al.2019). Therefore, the consideration of both indices provides useful
information on soil nutrient status. A previous study suggested that a
higher EI value and a lower CI value indicated a resource-rich soil
environment dominated by bacterial-feeding nematodes (Liang et
al. 2009). Our finding that N loading decreased the EI value and
increased the CI value indicates that N loading diminished the relative
importance of a bacterial-dominated energy channel. Moreover, the, A
significant increase in the BI value indicated a positive effect of N
loading on both Fu2 and Ba2 guilds
(Fig.
S2), since BI value is mainly determined by Fu2 and Ba2 guilds (Zhaoet al. 2014). N loading also increased the abundance of
fungal-feeding nematodes, suggesting
that the relative importance of fungal energy channel increased with N
loading. However, N loading significantly reduced the abundance of
plant-feeding nematodes (Fig. 1). The results differed from those of
Shao et al. (2017), who found that N inputs did not significantly
affect the abundance of plant-feeding nematodes. The reason would be
that we used a gradient of N-loading rates, which probably provides more
information than a single rate in that Shao et al. (2017). Because the
quantity of nitrogen applied regulated the responses of soil nematode
communities (Lokupitiya et al. 2000; Wei et al. 2012; Liet al. 2013).
It is worth noting that N addition had no effects on the
diversity
of soil nematodes (Fig. 2). Both the Shannon-Wiener diversity and
Margalef richness were similar with or without N loading and
significantly decreased with soil depth. Our results are similar with
previous results from a secondary tropical forest (Zhao et al.2014) but not with results from a temperate forest (Sun et al.2013) or a grassland (Wei et al. 2012), where N addition reduced
soil nematode diversity. The Simpson dominance index in N2 treatment was
significantly higher at 40-60 cm soil depth than at the other two soil
depths, indicating that a certain concentration of N addition might
increase the dominant position of some nematode species in deep soil.
Our results indicated that nematode diversity only showed some dynamics
along different soil depth but not N treatments. The explanation would
be that the changes of soil nematode community structure did not always
reflect the diversity, since the complexity of soil micro-food web in
natural ecosystems (Rooney and McCann, 2012; Li et al. 2013). Our
recent report also showed that changes in soil microbial community
composition would mainly contribute to the litter decomposition process
compared with soil microbial diversity (Wu et al. 2019), which
supports the phenomenon of soil nematode in this study.
Structural equation modeling was performed to test the second hypothesis
how N addition affects the trophic groups of soil nematodes with direct
or indirect pathways. We found that N loading reduced the abundance of
plant-feeding nematodes either directly or indirectly via an alteration
in soil pH. The negative effect of N loading on plant-feeding nematodes
may result from the negative effects on plants, for that N addition may
reduce plant diversity, aboveground biomass, and fine root biomass of
understory vegetation (Lu et al. 2010; Lu et al. 2011; Wuet al. 2013). A decrease in the aboveground biomass and fine root
biomass of understory vegetation may lead to a reduction in food
resources for plant-feeding nematodes (Wu et al. 2013; Fanet al. 2014). Our previous reports also indicated that N loading
decreased soil pH in the studied region (Wu et al. 2013; Shenet al. 2019) and other report also proved it in Northeast China
(Liang et al. 2009). Soil acidification may indirectly affect
plant-feeding nematodes by decreasing soil base cations and altering the
abiotic environment (Chen et al. 2015).
N loading increased the abundance of fungal- and bacterial-feeding
nematodes apparently through an increase in the concentration of soil
available nutrients, which presumably supported increases in food
resources for them. On one hand, the decrease of soil pH in our study
did not exert a negative impact on the fungal- and bacterial-feeding
nematodes, which was supported by the SEM analysis.
Indeed,
an increase in the individuals of bacterial-feeding nematodes was
measured. Commonly, soil acidification induced by N addition can inhibit
soil microbial growth and bacterial biomass accumulation (Rousk et
al. 2010; Chen et al. 2015; Li et al. 2018b), which is
supposed to lead a reduction of food resources for soil fungal- and
bacterial-feeding nematodes, and eventually result in a decline in their
abundance. On the other hand, N loading may change the community
structure of bacterial-feeding nematodes by changing their environmental
adaptations. Generally, soil condition with plentiful food resources are
beneficial to those nematodes with short generation cycles (Bongers,
1990; Bongers & Ferris, 1999; Liang et al. 2009), i.e., Ba1 and
Ba2 guilds. The elevated N stress made bacterial-feeding nematodes
change their survival strategy and transform community structure to
abundant r -strategy colonizer (Ba1 and Ba2 guilds) under N
addition (Zhao et al. 2015a). The co-occurrence network of
nematodes taxa analysis showed that nodes and edges varied in different
N treatments (Fig. 4), which supported our inference
that
N addition can change the community structure and stability of soil
nematode communities (Zhao et al. 2015b). All these results
indicate
that
nematodes changed by N addition, whether they can finally reach a new
equilibrium in response to gradually elevated N loading requires more
studies.