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.