Introduction
To
meet increasing population demands, agricultural and forest ecosystems
have been more intensively managed during the last five decades
(Sengupta et al., 2015). However, the problem of soil degradation caused
by long-term intensive management has become increasingly prominent
(Guo, et al., 2010). Fertilization is the main practice employed in
intensive management. For decades, many studies have reported that the
increased
application
of single mineral fertilizers improved soil fertility and crop yields
(Zeng et al., 2016; Wang et al.,2017b). However, this practice also
accompanied various negative effects such as soil acidification,
greenhouse gas emissions, nutrient losses, and deterioration of soil
structure (Dalal, Wang, Robertson, & Parton. 2003; Zeng et al.,2016).
Soil microbes responsible for most of the soil biochemical processes are
also affected by changes in nutrient availability, pH and organic matter
content resulting from heavy fertilization (Wang et al.,2017). It was
reported that the input of mineral fertilizer reduced bacterial richness
and disturbed soil microflora communities (Sun, Zhang, Guo, Wang, &
Chu,2015).
Reductions
in microbial biomass was closely related to the duration and amount of N
input in both field and lab-based studies (Treseder, 2008). Thus,
long-term and large-scale application of mineral fertilizers is
considered to be a key reason for reduced microbial biodiversity
associated with intensive management (Wang et al.,2017). Therefore,
land conservation and soil fertility recovery are very important to
intensive agricultural system.
Bacteria-mediated fixation of C and N play an important role in
sustaining soil fertility in agricultural ecosystems
(Fan,2019;
Kekulandara, Sirisena, Bandaranayake, Samarasinghe, & Suriyagoda,
2019). Autotrophic microorganisms along with algae contributed about
40% and between 4-10% of CO2 fixation in oceans and
wetlands ecosystem, respectively (Cannon et al., 2001; Stanley, Johnson,
& Ward, 2003). Many studies have focused on the assessing the
importance of autotrophic bacteria in fixing atmospheric
CO2 into soil OC(Ge et al., 2012;Wu et al., 2014;Yuan
et al., 2015). Autotrophic bacteria in soil annually capture about
0.6–4.9 Gt C, which represents 0.5–4.1 % of total terrestrial C
fixation (Falkowski et al., 2000). The C fixed is first imported into
unstable OC pools as microbial biomass carbon (MBC) and
dissolved
organic carbon (DOC) (Ge
et
al., 2012).
Biological
N
fixation
(BNF) process has been considered, both economically and
environmentally, as a source of N for plant growth. They play an
important role on N supply for most ecosystems, especially in
low-fertility soils (Norman & Friesen,2017).
Globally,
current estimates suggest that N fixed by BNF (~300Tg
nitrogen yr-1) is much higher than that produced
industrially (~125 TG nitrogen yr-1)
(Kuypers , Marchant, & Kartal, 2018). It was estimated that soil N
input via BNF accounted for 16% of global N2 input
annually (Ollivier et al., 2011). Soil OC and total nitrogen (TN)
contents were shown to increase significantly following inoculation with
diazotrophic Azotobacter and Bacillus (Kheirfam, Sadeghi,
Homaee, & Zarei, 2017). Introduction of one Azotobacter sp.
strain, a free-living bacterium with excellent ability of N fixing, was
estimated to save 50-75% of the mineral N and P
fertilizer
in 2 year field experiment (Dadrasan, Chaichi, Pourbabaee, Yazdani, &
Keshavarz-Afshar, 2015). Besides enhancing N, C and P levels in
agricultural
systems,
N-fixation bacteria could indirectly improve soil physical properties
(Zhao, Qin, Weber, & Xu,2014), including declined soil density and
increased water holding capacity, hydraulic conductivity and mean weight
diameter(MWD)(Nisha, Kaushik, & Kaushik, 2007).
Bamboo is an important ecological, industrial and cultural resource. The
total output value of the national bamboo industry reached 117.3 billion
RMB in 2010 (National Bamboo Industry Development Plan 2011-2020).Moso bamboo (Phyllostachys pubescens ) covered 4.6778
million ha accounting for 73% of the total bamboo area in China by 2018
(the 9th national forest investigation) is a
significant component of forest ecosystems (State Forestry
Administration of the P.R. China, 2018). Due to its high economic
return, Moso bamboo has received intensive management to enhance
its productivity in the past few decades
(Liu
et al., 2011; Li et al., 2013). The intensive management (IM) practices
employed with Mosobamboo
forests are primarily annual fertilizer application and removal of
understory herbs and shrubs. Farmers usually prefer to use mineral
fertilizers which are more efficient and convenient than organic
fertilizers, especially in mountain and hill land where farmer are
difficult to practice. However, as with agricultural systems, long-time
application of mineral fertilizers has resulted in the decline of soil
fertility in bamboo plantation (Qin et al., 2017). As a result of
observed ecological problems due to the sole use of inorganic
fertilizers, combined applications of mineral fertilizers and manure
(MCM) have been introduced into Moso bamboo management to prevent
land from negative effects of mineral fertilizer.
The application of MCM has been proved to be a potentially superior land
management practice than the application of mineral or
organic
fertilizers alone. Some field experiments confirmed that MCM played an
important role in maintaining soil health, improving soil fertility, and
promoting the restoration of biotic and abiotic soil properties (Wang,
Lai, Wang, Pan, & Zeng, 2015). Meta-analysis and modelling data from
upland soils and paddy-upland rotation soils across the major
agricultural zones in China revealed that the MCM increased the SOC
content and crop yields substantially (Jiang et al., 2018). The
long-term fertilization experiment showed that application of MCM
improved SOC significantly
(Liang, Yang, He,
& Zhou, 2011). It
was demonstrated that SOC was positively correlated with crop yields
following more than twenty years of continuous winter wheat–summer corn
rotation cultivation (Yang,
Zhao,
Huang, &
Lv, 2015). There are
several direct or indirect factors associated with MCM that contribute
to improved crop yields. For example, the application of MCM has
directly increased SOC and improved mineral N utilization efficiency by
accelerating microbial SON mineralization activity
(Pan
et al., 2009). The higher SOC contents resulting from MCM treatment led
to a greater
cation
exchange capacity (CEC) when compared with soils receiving with no or
only inorganic fertilizers in a low-productivity paddy field (Mi et al.,
2018).
The MCM could potentially increase and modify microbial biomass, enzyme
activities, or community composition by providing an OC energy source
and nutrients in organic form (Zhao et al., 2016). MCM has generally had
positive effects on bacterial CO2 and N2fixation. Fertilization increased cbbL abundance, with the
highest cbbL copy number and RubisCO enzyme activity in NPK plus
rice straw soil (Yuan et al., 2012). Long-term mineral NPK fertilization
decreased the diversity of diazotrophic community, whereas NPK plus rice
straw and NPK plus chicken manure treatments maintained the diversity of
diazotrophic community (Liao, Li, &
Yao,
2017). However, positive results have not always be observed. For
example, Lin et al. (2018) reported
that
long-term application of
inorganic
fertilizer plus organic material (pig manure) suppressed the abundance
and diversity diazotrophs and altered community structure, while
inorganic fertilizer combined with plant residue (rice straw or radish)
had no effect on the community structure of diazotrophs. The
inconsistent results may be due to environmental heterogeneity and the
type organic materials applied.
In the past few decades, long-term intensive management of Mosobamboo has been reported to cause soil deterioration, including soil
erosion and nutrient leaching (Shinohara & Otsuki, 2015), soil
acidification (Qin et al., 2017), and reduction of soil C and N storage
(Li et al., 2013). It also caused a general decrease in microbial
diversity and shifts of microbial community structure (Xu, Jiang, & Xu,
2008) and more specifically reduced the abundance and altered the
community structure of arbuscular mycorrhizal fungi (AMF) (Qin et al.,
2017). Soil CO2 and N2 fixation bacteria
are considered sensitive to changes in soil nutrients, pH and organic
matter content caused by heavy fertilization (Wu et al., 2014; Tang et
al., 2017). It was observed that the abundance of
CO2fixation bacteria in topsoil increased at the first 10 years of
application of mineral fertilizer
inMoso bamboo planation, and then decreased (Liu et al., 2018).
However, the abundance and diversity of
diazotrophic
bacteria decreased at first and then increased (He et al.,2015). Thus,
it is necessary to better understand how these bacterial groups respond
to applications of MCM. We hypothesized that MCM could lead to a
positive effect on CO2 and N2 fixation
by microbial communities in Moso bamboo
planation.
The method of space-for-time substitution was used to establish a
chronosequence of Moso bamboo stands with different durations of
MCM management. We tracked changes in the genes cbbL andnifH , which respectively encode a component of
ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) and a
nitrogenase reductase subunit, as these have been use previously to
investigate the abundance and composition of CO2fixation (Videmšek et al., 2009; Yuan et al.,2015) and
N2 fixation bacteria (Mmm, Marchant, & Kartal, 2018).