1 INTRODUCTION
China‘s livestock industry has undergone a vast transition during the last four decades, providing the majority of meat and other dairy products supporting the rapidly growing human population. However, increasing demand for livestock generated large amount of bio-waste. It has been documented that over 3.8 billion tons of livestock waste was produced per year in China and the nitrogen (N) in the waste accounted for 38% of the agricultural N released into the environment (data from China Rural statistical Yearbook, 2014). Additionally, the increased availability of synthetic N (mostly chemical fertilizers) contributed to the abandonment of livestock waste instead of re-using in agricultural system (Bai et al., 2018). As a consequence, large areas of lands are necessary for those redundant livestock manure disposal (Jia et al., 2018), with severe environmental issues for terrestrial and aquatic ecosystems.
Livestock waste confers benefits to soils if managed and used properly as it contains abundant carbon (C), N, phosphorus (P) and other trace elements, improving soil fertility and quality. However, livestock production, consuming 1/3 of global agricultural land and 32% of world’s freshwater, is a primary source of greenhouse gases such as nitrous oxide (N2O) and methane (CH4), and of N and P in water leading to eutrophication (Yu et al., 2019). A large amount of N in livestock manure is finally converted into nitrate (NO3-) by microorganisms and accumulates in the soil profile (Ju et al., 2007; Yang et al., 2006; Zhou et al., 2016). Concerning the severe impact of nitrate on aquatic and terrestrial ecosystems, it is of particular importance to distinguish various nitrate sources, e.g. from sewage or manure. Dual isotope approach using nitrogen and oxygen isotopic composition has been successfully applied in differentiating the numerous nitrate sources and identifying the N role in various processes (Krapac et al., 2002; Liu et al., 2013; Wang et al., 2017). However, knowledge about the time-dependent changes of manure-derived 15N and 18O in the soil profile is limited.
Soil harbor enormously diverse microorganisms driving a wide range of ecosystem processes including soil nutrients cycling, organic matter decomposition and detoxification of contaminants (Bardgett & van der Putten, 2014). Hence insights into the role of microbial community can also explain how livestock manure influences ecosystem processes and function, as well as provide a more effective strategy to recycle and utilize the nutrients (Feng et al., 2015). Numerous studies have reported that organic fertilization like animal manure can improve soil fertility, impact microbial diversity and alter community structure and composition compared with chemical fertilization (Ashworth et al., 2017; Hartmann et al., 2015; Lin et al., 2019; Sun et al., 2015; Tian et al., 2015; Yang et al., 2017). However, few studies have explicitly addressed the succession feature of bacterial community along the soil profile seasonally, especially from the start of manure application. Previous research suggested that environmental filtering is more important than stochastic process in agricultural system due to the manure disturbance (Yu et al., 2019), and vice versa for a less disturbed environment, like forest (Mayfield & Levine, 2010). It has been widely accepted that microbial community assembly is driven by deterministic or stochastic processes (Jiao et al., 2018; Stegen et al., 2012), while more and more studies suggested that the assembly process generally performed together but not independently (Powell et al., 2015; Zheng et al., 2013). Therefore, understanding the factors influencing the microbial community assembly process along the soil profile with the addition of manure is critical to assessing the environmental consequences of livestock waste disposal. Livestock manure is a considerably labile and C-rich source of resource and energy, which would likely activate r-strategy microbial taxa (Hartmann et al., 2015; Ho et al., 2017). In our previous study, we found shifts of soil bacterial community structure in organic manure treated soil using finger-printing methods (Shen et al., 2010). Feng et al (2015) further suggested that species Bacillus asahii responded most distinctly to manure addition, which become dominant in organic manure fertilized soils. However, which species would disappear along the application of manure is still unknown. Here we investigated the spatio-temporal variation of bacterial community in a long-term cattle manure loaded site near a dairy farm. We also collected the adjacent forest soil for comparison. We hypothesized that along the soil profile, nutrient availability might drive variation of bacterial community composition with r-strategy taxa responding most strongly with time. The aim for this study was to investigate (1) nitrate accumulation and leaching potential along the soil profile; (2) the most distinctly bacterial group in response to long-term manure loading; (3) the seasonal and vertical assembly of soil bacterial community and their factors in a sandy loam soil. Our study provides important information for understanding how the complex of soil bacterial community responds to anthropogenic disturbance, especially the abandonment of livestock waste and land use transformation.