Summary
  1. 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.
  2. 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.
  3. 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.
  4. 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).