Black locust (Robinia pseudoacacia) is a leguminous tree which can rapidly fix nitrogen (N) from the atmosphere via Rhizobium (Zhang et al. 2019) and further alter soil properties by increasing mineral N (Medina-Villar et al. 2016). R. pseudoacacia is able to disperse quickly and colonize a broad range of xeric habitats, including steep rocks or toxic man-made substrata (Cierjacks et al. 2013), and has been extensively naturalized in the temperate regions of North America, Europe, and Asia (Sabo, 2000; Lee et al. 2004; Vítková et al. 2017; Yang et al. 2019). Natural reproduction of R. pseudoacacia plantations is primarily vegetative through root suckering and stump sprouting, allowing vigorous regeneration after coppicing and disturbance (Peng et al. 2003). However, after two or three rotations, the productivity of R. pseudoacacia coppice plantations tends to decline (Cierjacks et al. 2013), which may further jeopardize its ecological role in soil and forest restoration.
Plant community structure and productivity in natural environments depends, among other factors, on soil nutrient availabilityand soil microbial communities (Reynolds, H. L., & Haubensak, K. A. 2009; Vitkova et al. 2015; Liu et al. 2018; Chen et al. 2020). Soil nutrient availability can alter soil processes catalyzed by soil microbial communities (Yang et al. 2016). Therefore, changes in soil microbial community composition can affect the plant community (Balota et al. 2013; Ma et al. 2018) and nutrient absorption by plants (Weidner et al. 2015; Zhang et al. 2018b). In turn, plants can directly and indirectly influence microbial communities environment by effect of root exudation (Sasse et al. 2018). So soil ecological transformation may provide a simple means of identifying stable state within the ecosystem (Macdonald et al. 2019).
Rhizosphere is a critical interface supporting the exchange of resources between plants and the surrounding soil environment, which provides microhabitats and niches for diverse microorganisms and microbial species (Philippot et al. 2013; Mendes et al. 2013). Rhizosphere microorganisms play a key role in plant growth and soil properties, especially in the rhizosphere niche (Philippot et al. 2013; Zhang et al. 2018a), which influences several plant physiological processes such as growth and energy metabolism affecting overall plant health (Fonseca et al. 2018). Generally, there are significant differences between rhizosphere and bulk soil microenvironments, the most obvious of which is that the higher nutrient content and root exudates in the rhizosphere contribute to improving soil carbon and nitrogen concentrations (Yin et al. 2018). Such differences may affect the composition of the rhizosphere microbial community (Neumann et al. 2014). Soil properties and their ecological processes provide a scientific basis for understanding the interaction between root physiological activity and soil physical and biological environments. At the same time, rhizosphere micro-ecology may be a key driver for predicting tree growth mechanisms.
Previous research has reported the high capacity of R. pseudoacacia for nitrogen fixation (Buzhdygan et al. 2016), and higher N mineralization and nitrification rates in black locust plantations compared to surrounding soils (Williard et al. 2005). Moreover, the excess of N can accumulate in the soil (Berthold et al. 2009) by means of root exudates, contributing to increasing soil fertility (Joëlle et al. 2010). The main nitrogen form uptaken by plants is inorganic nitrogen including nitrate and ammonium. R. pseudoacacia benefits from nitrogen fixation associated with symbiotic rhizobia in root nodules (Cierjacks et al. 2013). The reduction of soil N availability induces nodulation and biological nitrogen fixing of R. pseudoacacia in order to sustain the required nitrogen amounts for plant growth (Mantovani et al. 2015). Therefore, both bacteria and N play an important role in the growth and development of R. pseudoacacia plantations.
With the development of R. pseudoacacia coppice plantations, unexpected problems have arisen in Mount Tai (China) forest ecosystems, including the decline of landscape quality, soil erosion and plant dwarfing, in line with previous research suggesting tree growth decline and trunk shape worsening (Geng et al. 2013). However, to date, most studies have attempted to investigate the effects of conversion from natural forests to plantations on soil properties, soil microbes and their community structure (Zhang et al. 2017; Yang et al. 2018). But there is a gap in knowledge concerning the effects of the transtion from seedling plantations to coppice stands. Radtke et al. (2013) showed that repeated clear cuttings every 20–30 years favored the spread of R. pseudoacacia. Yet, the effects of shift from seedling to coppice plantations on soil properties and soil microbes are not yet well understood, and information is scarce. The aim of this study was to (1) shed light on the effects of shifting from seedling to coppice stands in black locust plantations on soil properties and soil bacterial community composition, especially Rhizobium, and (2) investigate the relationships beween soil properties and bacterial community composition in seedling and coppice plantations, respectively. The study was performed in first generation seedling plantation stands (F), first generation coppice plantations (S) and second generation coppice plantations (T) in Mount Tai, China. We hypothesized that (1) the changes caused by the conversion of seedling to coppice stands lead to decline of soil quality, and to alterations in soil bacterial community composition, (2) nutrient availability plays an important role in shaping the bacterial community, and (3) the relative abundance of Rhizobium decreases in coppice plantations.