Soil fertility is the inherent capacity of a soil to provide essential nutrients for plant growth in adequate amounts and suitable proportions. As “built-in” soil regulators and catalysts that contribute to recycling nutrients into available inorganic forms, soil microorganisms mediate soil fertility and nutrient uptake processes. Recently, increasing attention has been focused on soil fungi, which link soil processes with nutrient uptake by plants (Tedersoo et al. 2015). However, fungal activity is affected not only by substrate availability but also by soil environmental conditions (e.g., texture, pH, moisture) (Blagodatskaya and Kuzyakov 2008). Previous studies have suggested that fertilization reduces microbial diversity in the topsoil; however, few studies have considered the vertical spatial distribution of microbes in the soil. Different environmental conditions between the topsoil and subsoil result in distinct microbial community structures (Baath and Anderson 2003; Glassman et al. 2017), particularly in the fungal community. However, the effects of the interaction between soil fertility and depth on both the abundance and function of soil microbial communities, and plant–microbe interaction dynamics, throughout the soil profile remain unclear.
Soil fungi, including saprotrophic and symbiotic fungi (mycorrhizal, pathogenic, and endotrophic), exhibit significant vertical patterning, and are affected by substrate resource availability (McGuire et al. 2013). Saprotrophic fungi are primarily carbon (C) decomposers, and thus occur in the topsoil, where organic matter from plant litter accumulates (Rousk et al. 2009). By contrast, symbiotic fungi occur in subsoils, obtaining C entirely from plant roots, and are directly involved in plant nutrient uptake (Bonfante 2018). The interactions between saprotrophic and symbiotic fungi influence overall fungal community function and plant-soil ecological processes (Baar et al. 1999). Based on physiological differences, fungal species have different requirements with respect to the quality and quantity of C substrates (e.g., labile and recalcitrant C content, carbon-nitrogen (C-N) ratio), resulting in differential growth responses among taxa to C substrates of varying chemical recalcitrance (Goldfarb et al. 2011). Due to substrate preferences, studies also suggest that soil fertility affects microbial interactions by increasing the dominance of certain groups and reducing network connections (Yadav et al. 2015).
Soil fungal community assemblages are shaped by specific environmental factors, particularly soil water content, C-N ratios, and pH, which also affect soil fertility and vary among soil layers (Peigné et al. 2018). Soil water content affects the transportation of fertilizer inputs from surface soils to subsoils as well as the availability of nutrients to plants and soil fungi (Griffin 1963). Carbon enrichment in surface soils increases rates of N cycling compared to subsoils. Clay content affects fungal growth and enzyme activity (Tedersoo et al. 2014). Soil pH is decreased by chemical fertilizer inputs, and is higher in subsoils than in surface soils (Yan et al. 2018), benefiting fungi that decompose soil organic matter (Tanjang et al. 2009). As with substrate utilization, soil fungal species have distinct ecological niches, and are consequently affected by the interactions between fertility and soil environmental factors.
Most attempts to identify the processes that structure natural communities have focused on conspicuous macroorganisms, whereas those which structure microbial communities remain relatively unknown. Two main theories have emerged to explain these processes: niche theory, which highlights the importance of deterministic processes, and neutral theory, which focuses on stochastic processes (Cira et al. 2018). Environmental stress selects microbes by substrate availability, and species persist according to the availability of their ecological niche (O’Malley 2008). Microbial interactions in turn affect populations via predation, competition, or facilitation (Dumbrell et al. 2010). The balance between selective forces, both extrinsic and intrinsic, shape community composition and function. Soil nutrient availability is considered a key environmental driver and has been shown to shape microbial community assembly and behavior. With increased soil organic inputs, community composition becomes increasingly complex as a result of nutrient limitations (Garcia-Orenes et al. 2016). However, most of these studies focus on surface soils, which are considered hotspots of microbial activity (Lazcano et al. 2013). Little attention has been given to deeper soils, and it is unclear whether fungal communities in deeper soils are shaped by similar assembly processes.
We investigated the community composition of soil fungi along a vertical profile (i.e., soil depths of 0–60 cm) in three tea plantations with different fertilizer input levels, as well as in a nearby forest, and explored changes in the fungal community along the vertical gradient and under different input treatments. We hypothesize that soil fungal diversity is positively correlated with fertilizer inputs, with a more pronounced increase in surface soils compared to the subsoils as fertilizer inputs may promote microbial groups that are currently nutrient-limited. In addition, we expect microbial composition to change, with saprotrophic microbes increasing as fertilizer inputs select for certain fungal groups, particularly saprotrophic fungi, that can use labile C and nutrients. Finally, we hypothesize that fertilizer inputs affect fungal communities directly through changes in soil chemical properties. Fungal diversity and composition may be shaped directly by environmental factors or indirectly through interspecific interactions. These processes were variously affected by fertilizer inputs.