Soil pH and nitrate content contribute to observed soil bacterial
community in our study. Soil pH determining bacterial community was
widely reported and the effects of soil pH as a primary determinant of
microbial community composition and diversity have been widely
documented in previous studies (Lauber et al. 2009, Rousk et al. 2010).
The diversity and richness of soil bacterial communities were proved to
be differed by ecosystem type, and these differences could largely be
explained by soil pH based on a study using 98 soil samples from across
North and South America (Fierer and Jackson 2006). Bacterial structure
was also reported mainly affected by nitrate nitrogen (Li et al. 2021).
In our study, agricultural practices significantly increased soil pH and
nitrate content in the transition from forest to crop soils (Table 1),
which further proved that agricultural practices altered soil abiotic
traits, thus affected soil microbiomes and their potential effects on
ecosystem functions.
Soil organic matter % and soil pH together contributed highest to the
observed different nir K functional gene community in our study
(Correlation = 0.443). Limited studies have examined how environmental
factors affect the denitrification functional genes. In general, thenir K dinitrifers are heterotrophic, therefore organic matter may
contribute to their metabolic pathway, which may explain a high
contribution of soil organic matter % to observed difference ofnir K community under various vegetation types. The response of
dinitrifiers to soil pH may be similar to other soil heterotrophs, which
usually function best near neutrality(Cavigelli and Robertson 2001).
Soil pH has been reported affecting the nitrification rate (Kyveryga et
al. 2004). Forest soil with lower soil pH also performed lower
nitrification rate (Nugroho et al. 2007). Furthermore, denitrifier
diversity was closely related to the rates of nitrous oxide consumption
in a terrestrial ecosystem, and denitrifier community composition alone
can potentially influence in situ N2O production
(Cavigelli and Robertson 2000, 2001). Land-use change due to
agricultural intensification is one of the most significant
anthropogenic activities that greatly affect soil microbial communities
by altering edaphic variables (Geisen et al. 2019). A more stable soil
chemical and biological composition was observed in soils subjected to
low human inputs than in those with high human input, which is likely to
be one of main drivers of biodiversity changes (Bevivino et al. 2014).
We focused on how agricultural practices alter the microbial diversity,
and the shifting in soil microbiomes are critical for their protentional
roles in regulating N cycling and their effects including
microbial-mediated nitrous oxide (N2O) emission for
global warming, nitrate (NO3-)
leaching for groundwater pollution, as well as microbial community in
the mitigation of soilborne diseases for soil health.
Soil nitrogen and nitrogen forms have been intensively studied in
relation to host nutrition and disease severity (Huber and Watson 1974).
However, nitrogen forms affected soilborne diseases differently.
NO3- stabilizes rhizosphere fungal
community in suppress Fusarium wilt disease (Gu et al. 2020). In
contrast, using ammonium instead of nitrate has been reported in reduced
incidence of the Southern blight disease (Jenkins and Averre 1986). We
observed significant high NO3-contents in crop than in forest soils (Table 1), which may affect the
development of soilborne disease, such as Southern blight
(Sclerotium rolfsii ) disease, and soil health for agricultural
production (Jenkins and Averre 1986, Milner et al. 2019).
We found significant higher AOB amo A gene relative abundance
(Table 4) and lower nir K richness and Shannon diversity (Table 5)
in crop than in forest soils, suggesting agricultural practices
potentially increase nitrification as well as negatively affect
denitrification communities and processes. Higher nitrification rate has
been reported relating to higher N2O to atmosphere and
NO3- leaching to ground water
(Butterbach-Bahl et al. 2013, Barta et al. 2017). Agricultural
activities such as use of N fertilizers and animal manure are the main
anthropogenic sources responsible for the atmospheric
N2O increase (Bouwman et al. 2002, Davidson 2009).
Greenhouse effect of N2O has an almost 300-fold greater
potential than CO2 (Thomson et al. 2012). We are still
far from fully understanding microbial regulation of the processes for
nitrogen losses in agricultural production. It is now well recognized
that microbial activities in soils are a major contributor to
atmospheric loading of N2O (Snyder et al. 2009, Thomson
et al. 2012). Similarly, NO3-, as the
most mobile form of N in soil, leaches easily from the soil ecosystems
and, therefore, has emerged as one of the most alarming and widespread
contaminants of groundwater and surface water resources. The
NO3- leaching usually originates from
diffuse sources, such as intensive agriculture and unsewered sanitation
in densely populated regions or point sources such as irrigation of land
by sewage effluent. Fertilizer application and subsequent leaching from
cropland is reported to have the highest contribution (60%) toward
NO3- leaching into the groundwater.
Groundwater contamination by NO3- is a
globally growing problem and more than 80 pounds of nitrogen per acre
per year has been estimated leaching into groundwater underneath
irrigated lands, usually as NO3-(Subbarao et al. 2006, Margalef-Marti et al. 2021).
5. Conclusions
Our study discovered significant effects of agricultural practices on
soil microbial and N functional diversity compared with adjacent forest
soils. Agricultural practices performed a negative effect on soil
bacterial diversity and the diversity of denitrification nir K
functional gene, whereas the relative abundance of a nitrification OTU
was significantly increased by agricultural practice, suggesting the
higher potential of N2O emission for greenhouse gas
effects as well as the higher NO3-leaching rate for ground water contamination in crop than in forest
soils. Soils are the foundation of a healthy ecosystem. Soil microbiomes
play critical roles in regulating soil nutrient especially N cycling
processes and contribute to climate change such as microbial-mediated
N2O emission for global warming,
NO3-leaching for underground water contamination, and mitigation of
soilborne diseases for soil health. Clearly understanding the microbial
diversity changes from forest transition to crop soils will offer great
potential to gauge the health of soil ecosystems subjected to
N2O emission for global warming and
NO3- leaching for ground water
contamination, as well as soil microbiomes and N forms to suppress
soilborne diseases.