Ecologists are increasingly recognizing that above- and belowground communities are tightly linked and that both communities greatly affect ecosystem processes and properties (Wardle et al. 2004; Bardgett et al. 2005). For example, plants influence soil biota by providing substrates in the form of litter and root exudates (Zak et al. 2003; Wardle et al. 2006), while decomposers regulate plant growth and community composition by mineralizing nutrients (Van Der Heijden et al. 2008). The feedbacks between above- and belowground communities are different among ecosystems and are affected by environmental factors (Bever 2003; Bardgett and Wardle 2010). Little is known, however, about the simultaneous responses of above- and belowground communities to changes in the global environment including changes in nitrogen (N) deposition (Suding et al. 2008; Bardgett and Wardle 2010).
Anthropogenic activities, such as the burning of fossil fuels, the application of artificial fertilizers, and the cultivation of N-fixing legumes, have altered the global N cycle (Vitousek et al. 1997; Galloway et al. 2004, 2008). Over the past century, atmospheric deposition of reactive N (mainly, nitrogen oxide and ammonia) has increased three- to fivefold (IPCC 2007), and N deposition is expected to further increase in many areas of the world (Galloway et al. 2004). N deposition could influence above- and belowground communities by altering soil chemical properties, plant community composition, and litter inputs (Bååth and Anderson 2003; Allison et al. 2007; Manning et al. 2008). Because N loading increases the quantity and quality of litter input and thus promotes decomposer abundance and activity (Manning et al. 2008; Cusack et al. 2011), N deposition can directly affect plant growth and soil communities (Manning et al. 2006). N deposition can also affect plant community composition by enhancing competitive release of N-limited species (Gilliam 2006; Bobbink et al. 2010). However, more research is needed on the responses of plants versus microorganisms to changing environments (Suding et al. 2008) because the mechanisms underlying the responses and interactions remain unknown. This is especially true for tropical and subtropical forest ecosystems, which not only harbor great biodiversity (Bobbink et al. 2010; Janssens et al. 2010) but which also have been confronted with increasing N deposition (Lamb et al. 2005; Galloway et al. 2008).
Substantial research has documented that increases in atmospheric N deposition could strongly affect plant community composition (Clark and Tilman 2008; Duprè et al. 2010; Maskell et al. 2010). Although N enrichment could increase plant growth by eliminating N deficiency (Vitousek and Howarth 1991; Lebauer and Treseder 2008), plant growth might not always increase in response to N addition because excess N may not be invested in the carboxylation step of photosynthesis (Bauer et al. 2004). In addition, plant responses to N supply depend on plant functional type; when the N supply is low, for example, increases in biomass are greater for herbaceous than for woody species and for trees than for shrubs (Xia and Wan 2008).
N deposition could also affect microorganisms in different ways in different ecosystems (Allison et al. 2007; Keeler et al. 2009). Long-term experiments in temperate forests often showed that soil microbial biomass was negatively correlated with N input (Wallenstein et al. 2006; Treseder 2008). In a mature tropical forest, Mo et al. (2008b) also reported that high nitrogen deposition significantly reduced soil microbial biomass after a 3-year treatment. However, results are yet to be consistent – there was one study found that large amounts of N fertilizer had no effect on soil microbial characteristics (Keeler et al. 2009) – which indicates that more detailed studies should be conducted on this topic.
Recent research in tropical ecosystems showed that N deposition affected the soil microbial community (Cusack et al. 2011), understory plant diversity (Lu et al. 2010), N cycling (Fang et al. 2011), and ecosystem carbon dynamics (Mo et al. 2008b). To our knowledge, however, no study has simultaneously measured the responses of plant and soil microbial communities to N deposition in subtropical regions.
This study concerns the effect of N on plant and soil microbial communities in Chinese fir plantations. Chinese fir (Cunninghamia lanceolata) is one of the most important reforestation species in subtropical China (Sheng et al. 2010). By 2009, Chinese fir plantations occupied approximately 1.12 × 107 ha and represented the third largest reforested-plantation type in China (China Forestry Administration 2010). Although a recent report indicated that Chinese fir is sensitive to N deposition (Wei et al. 2012), it remains unclear how plant and soil microbial communities in Chinese fir plantations would respond to N deposition. The current report describes a long-term field experiment that included 8 years of continuous N application in a 20-year-old Chinese fir plantation. The objective was to test the effects of N deposition on both plant and soil microbial communities in the plantation.