Denitrification-related microbial functional groups in river
water are shaped by anthropogenic land uses
As
decomposers, microbiota play vital roles in
N and
P cycles in ecosystems (Dijkstra et
al., 2012). Soil erosion can increase N and P in water (Ma et al.,
2016). When
N
concentrations increase in the water environment,
nitrifying
and denitrifying bacteria could colonize in large numbers (Geets et al.,
2007). Here, we observed that the N concentrations in river water
increased with increases in proportions of agricultural and urbanized
land area (Fig. 4). We speculate that anthropogenic land uses may
stimulate nitrification and
denitrification-related microbial
functional groups. The results of
functional gene prediction showed that the microbial functional groups
were associated with denitrification, N cycle, and Fe and Mn respiration
in the river areas under mainly agricultural and urbanized land uses,
with their relative abundances positively correlated with TN,
NH4+-N,
NO3--N, and
NO2--N concentrations in river water
(Fig. 5).
Through metagenomic analysis, Wang et al. (2022) found that denitrifying
bacterial community abundance was markedly improved near a sewage plant.
In the present study, the impacts of anthropogenic disturbance on
denitrifying bacteria in the freshwater river ecosystems were variable.
The processes of N respiration,
nitrate respiration, and nitrite respiration constitute denitrification,
which is accompanied by the production of gaseous N such as nitric oxide
(NO), N2O, and N2 (Torralbo et al.,
2017). Potential strategies of reducing the emission of
N2O, a GHG, have
attracted considerable attention (López et al., 2013, Rafique et al.,
2014). Considering N pollution and GHG emissions, it is essential to
reduce TN,
NH4+-N,
NO3--N, and
NO2--N concentrations in river areas
disturbed by agricultural and urbanized land use activities.
Microbial responses to environmental change are complex (Scheuerl et
al., 2020). Recently, Xun et al. (2021) found that bacterial communities
with higher phylogenetic diversity tend to be more stable, implying that
microbiota with higher biodiversity are more resistant to disturbance.
In the present study, we predicted the microbial functions involved in
the synthesis of major and trace elements required for bacterial growth.
In the river area mainly under forest land, the major functional groups
were related to
methylotrophy
(Tami et al., 2015), methanol
oxidation (Pastawan et al., 2020),
anoxygenic
photoautotrophy S oxidation, anoxygenic photoautotrophy
H2 oxidation, aerobic anoxygenic phototrophy (Sasikala
et al., 1995), oxidation, nitrite
ammonification
(Howie-Esquivel et al., 2010), nitrate ammonification, nitrification,
aerobic nitrite oxidation,
anoxygenic photoautotrophy, aerobic NH3 oxidation, and
microelement (Mn and Fe) oxidation. Most of the functions are involved
in the synthesis of organic compounds with complex structures to support
bacterial growth. The relative abundances of
carbon metabolism-related functions
were also higher in the river area under mainly forest land use,
compared with that under mainly agriculture and urbanization (Fig. 5b).
Therefore, microbial community structures under agricultural sites were
simpler than those under forest sites in the river ecosystems.
In the present study, microbial community composition and functional
groups were influenced not only by land-use types but also by elevation
in the river areas of the Qinling tributaries. The alpha diversity of
microbiota in river water was the lowest and the microbial functional
groups were the fewest in the Qinling tributaries. Due to the associated
low temperature and nutrient concentrations (Fig. S3), high elevations
are not conducive for the normal growth of microbiota (Zhang et al.,
2013). Consequently, severe natural disturbance can also reduce
microbial stability in freshwater river waters.
Land-use
disturbance by agriculture and urbanization has long-term impacts on
microbial functional groups in river sediment
It is generally considered that microbial community functions in
sediment are more diverse than those in the water column. Sediment
microbiota mainly participate in
organic decomposition, N and P cycling, and water pollution remediation
in rivers (Kallmeyer et al., 2012, Meng et al., 2019 and Wu et al.,
2021). Long-term natural evolution and human activities shape microbial
community composition in sediment (Huang et al., 2021, Hung et al., 2021
and Marshall et al., 2019). In the present study, we observed that the
functional composition of sediment microbiota was altered by different
land-use types in the freshwater rivers. There were numerous functional
groups associated with denitrification and human (e.g., multiple
denitrifications, human pathogens, human gut metabolism, mammal gut
metabolism, and organic synthesis) in the river ecosystems, mainly under
agricultural and urbanized land use (Fig. S6). However, only
organic
synthesis and nitrification-related functional groups were observed in
the river ecosystems under mainly forest land use, with few functional
groups associated with human and denitrification.
Considering the environmental factors and microbial community
composition, land-use type shapes microbial functional groups in rivers.
Furthermore, the human-related microbial functional groups detected in
the sediment provide strong evidence of the impact of anthropogenic land
use on microbial functional groups in river sediment (Fig. 6c-d).
Similarly, microbial functional groups have been applied in the
measurement of microbial water quality responses to land-use type using
fecal indicator bacteria and molecular source tracking in rivers and
near-shore surface waters
(Verhougstraete et al., 2012).
In the present study, the major functional groups of sediment microbiota
were similar to those predicted in river water; the relative abundances
of functional groups associated with C metabolism and metal metabolism
were lower in sediment in river ecosystems under mainly agricultural and
urbanized land uses than under forest land use, with opposite trends
observed for functional groups associated with N metabolism and human or
mammal gut metabolism. Through
LEfSe
analysis, we identified the microbial functional groups that were
characteristic of river sediment (Fig.
6a-b).
Denitrification and human-related
microbial functional groups were relatively abundant in the sediment of
rivers mainly under agricultural and urbanized land uses, whereas few
human-related microbial functional groups were observed in the sediment
of rivers mainly under forest land use. Overall, the results indicate
that the microbial functional groups in freshwater river sediment were
influenced by land-use type.