4 DISCUSSION
Intensified livestock production system contributes significantly to
environmental impacts including water, air and soil quality by altering
the biogeochemical cycling of carbon, nitrogen and phosphorus
(Bai et al., 2018;
Leip et al., 2015;
Pelletier & Tyedmers, 2010). In this
study, higher amount of nitrate was detected along the soil profile
applied with cattle manure compared with the adjacent non-manured
woodland, indicating higher risks of nitrate pollution even down to the
subsurface soils with continued manure application. This result was in
good agreement with a meta-analysis based on over 7000 cropland samples,
which showed large accumulations of nitrate in the soil deep down to 4
meters (Zhou et al., 2016). Soil
δ15N-NO3- value
further corroborated the high proportion of manure-derived nitrate in
the maize field, which is consistent with previous research working on
nitrate contamination sources in various environments
(Fenech et al., 2012). While some
researchers mentioned that the effect of manure addition on nitrate
leaching varied with soil texture, climatic condition as well as the
applied amounts (Gai et al., 2019;
Maeda et al., 2003). Over-use of manure
results in nitrate redundancy that gives rise to environmental concerns
(Herrero & Thornton, 2013), e.g.,
groundwater contamination, soil nitrogen losses. Similar result was also
obtained from a long-term field study carried out in Nebraska in USA
applied with different rates of manure application
(Nguyen et al., 2013). However, in the
above mentioned study, they suggested that manure if applied below or at
the rates equal to crop N requirements has many benefits for food
production and environmental protection. Recently, the possibility of
substituting manure for commercial fertilizer has been addressed based
on 141 published studies and concluded that recycling of livestock
manure in agroecosystems would reduce nitrogen losses and increase food
production especially in upland soils (Xia
et al., 2017). Critical land management in crop production is therefore
necessary in keeping sustainability of food production and agricultural
systems.
Soil microbial diversity exhibited distinct response to the environment
disturbance (Falkowski et al., 2008). In
this study, manure addition had no significant impact on bacterial OTU
richness and Shannon diversity in the maize field compared to woodland
for both seasons, indicating that the change of land use in this site
had no significant impact on bacterial α diversity. Generally,
agricultural soil microbial diversity tend to be spatially homogeneous
when compared to natural habitats because of the interference from human
activities (Kennedy & Smith, 1995). Soil
bacterial diversity showed a decreasing trend from woodland to cropland
(Zheng et al., 2017), which is contrast
to our observation. Previous research also observed that soil bacterial
diversity responds strongly to the land use change of Amazon rainforest
and resulted in biotic homogenization in agricultural soils
(Rodrigues et al., 2013). This
discrepancy regarding the little difference of bacterial diversity
between these two land uses was probably attributed to the addition of
manure in the maize field. Additionally, not all the change of land uses
would alter bacterial α diversity (Jesus
et al., 2009). For example, soil bacterial diversity kept constant
following the conversion of cropland to orchard in a degraded karst
system (Liao et al., 2018). Our results
suggested that the conversion of woodland to cropland did not
significantly change bacterial α diversity.
Contrasting to the result of bacterial OTU richness between these two
land uses, clear separation of bacterial structure between the maize
field and woodland was detected even to the deep layers, which was
further confirmed by PerMANOVA analysis. This difference may be
explained by the following reasons. Firstly, manure contains numerous
microbial resources including liable C and N, providing soil nutrients
for microbial growth, especially for the bacteria
(Enwall et al., 2007;
Sun et al., 2004). Secondly, manure
application will change soil physical and chemical properties by
altering soil aggregate stability, water and gas exchange, which in turn
improving microbial living conditions (Guo
et al., 2019). Additionally, manure harbors a large amount of
microorganisms (Meng et al., 2019), and
some of which will be introduced and persisted in soils from several
days to several months depending on soil types and manure sources
(Udikovic-Kolic et al., 2014). All these
indicated that manure application favor microbial growth, enabling a
shift in soil bacterial community composition
(Peacock et al., 2001;
Shen et al., 2010). The differences
between these two land uses were more apparent in the network analysis
as maize field networks harbor more modular than woodland. The
co-occurrence network in woodland harbored a denser and highly connected
bacterial community compared with that in the maize field, nevertheless,
the bacterial community in maize field is more specialized with
different ecosystem function.
Most interestingly, the shift of bacterial composition kept consistent
along the soil profile, as site-specific pattern of bacterial community
structure was detected in this study. Differences in spatial
distribution of soil bacterial community was largely affected by the
vertical gradients of soil properties (Jiao et al., 2018). The increase
of ecological heterogeneity with increasing depth in our studied site
may be attributable to the significant response of bacterial β diversity
along the soil profile. Similar to our study, researchers in four Alaska
soil cores also indicated that microbial assembly process was mainly
determined by environmental factors rather than depth
(Tripathi et al., 2018). In both land
uses, the ses.MNTD mean values deviated significantly from zero,
indicating that bacterial assemblages had higher phylogenetic clustering
than expected by chance. This finding is consistent with other previous
studies (Horner-Devine & Bohannan, 2006;
Stegen et al., 2012), which indicated
that soil bacterial community were more structured by environmental
filtering in a wide range of environments. Significant correlation
between ses.MNTD and depth further suggested that bacterial community
are shifted from more phylogenetically clustered assembly to less along
soil depth. Similar results have been found in various habitats, like
Alaskan soil cores (Tripathi et al., 2018), Tibetan Plateau
(Chu et al., 2016), which showed that
environmental filtering are more important in the assembly of bacterial
community in surface soils than in deeper soils.
In this study, we found higher concentrations of nitrate in the maize
field with long-term applications of livestock waste. This raises the
question about which bacterial species responded mostly promptly or
persisted longer following manure addition in the context of high
nitrate scenario. Pearson correlation analysis between the dominant
bacterial groups and nitrate content suggested that nitrate was
significantly correlated with phyla of Bacteroidetes (P< 0.001), Nitrospirae (P < 0.01),Firmicutes (P < 0.001), andGemmatimonadetes (P < 0.001) in the maize field
(Table S3), but not correlated with special groups in the woodland.
Furthermore, many bacterial groups at phylum or order levels showed
significant differences in the two land uses in the spring than in the
autumn, especially Bacteroidetes . Accumulating evidence showed
that Bacteroidetes is widely present in a range of habitats
including plant (Thomas et al., 2011),
soil (Lauber et al., 2009) and animal gut
(Tajima et al., 1999). Their mammal
origin might explain the higher abundance and sensitive response in the
maize field especially in spring. Our result is consistent with a
previous study by Wolinska and his colleagues
(Wolinska et al., 2017), who suggested
that Bacteroidetes could be recommended as a sensitive indicator
for agricultural soil type. Additionally, Bacteroidetes is
specialized in decomposing high molecular weight compounds such as
polycyclic
aromatic hydrocarbons and xenobiotics
(Fernandez-Gomez et al., 2013), which
could be explained by its copiotrophy life strategy exhibiting more
responsive to rich available nutrients (Ho
et al., 2017; Pepe-Ranney et al., 2016).
The enrichment of this microbial group in response to the addition of
exogenous manure, suggests their ability in processing complex organic
matter in soils and therefore improving soil fertility.
CONCLUSIONS
Significant changes in soil nutrient availability with the addition of
manure contributed to the separation of soil bacterial community
structure between different land uses in our study. More sensitive
bacterial groups were detected in response to the manure addition
especially in the beginning. Bacteroidetes phylum was one of the
most responsive groups to the addition of manure addition, which could
be a potential bio-indicator for agricultural usage. These results
provide solid and necessary information for a better understanding of
the succession characteristics of bacterial diversity as well as the
driving factors in the maize field receiving cattle manure over a long
term. Bacterial diversity is beneficial to terrestrial ecosystem in
increasing disturbance tolerance and maintaining ecosystem services.
Network analysis further highlights strong differences in network
structure between the maize field and woodland that is more related to
ecosystem function. We propose that future research should focus on the
microbial diversity and function in assessing environmental risks of
manure disposal as well as strategies to reduce nitrate leaching.