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.