Land-use type influence environmental characteristics of freshwater rivers
Previous studies exploring the impacts of land-use type on N and P dynamics have focused on soil environments, with a few studies conducted in freshwater river ecosystems (Enanga et al., 2011 and Leblanc, 2008). In the present study, environmental data were obtained from freshwater rivers under disturbance by different land-use types. We observed that land-use type could induce shifts in the ecological environments of rivers. For example, Cond, TN, NH4+-N, and NO3--N in water samples increased with an increase in the proportion of agricultural land (Fig. 4). The sensitivity of the environmental factors to land-use type may indicate increases in soil erosion and agricultural or municipal sewage discharge. As a tributary of the Yellow River and Loess Plateau, Weihe River is associated with severe soil erosion (Wei et al., 2010).
Agricultural land use increases water demand, whereas rapid urbanization and population growth exacerbate water pollution (Song et al., 2015). Through Pearson’s correlation analysis, we observed that Turb, Cond, TN, TP, NH4+-N, and NO3--N in water samples all significantly increased with an increase in disturbance by agricultural and urbanized land use (Fig. 4). Urban and agricultural land border river basins and runoff from such land areas loads nutrients into the adjacent canals, and ultimately flow into Weihe River.
Anthropogenic land uses reduce microbial diversity and alter microbiotacomposition in rivers
Based on 16S rRNA gene sequencing, we observed that agricultural land use could induce microbial diversity loss and community compositional changes in river water and sediment. The alpha diversity of microbiota in water samples was lower in the river area mainly under agricultural land use. We speculated that due to soil erosion in agricultural land, abundant N was released into water (Li et al., 2017), negatively affecting microbial diversity, which is consistent with the finding of a former study that reported changes in microbial diversity with an increase in N concentration in a freshwater river (Tai et al., 2013). Microbial community composition in sediment samples showed trends similar to those observed in water samples, which could be attributed to the long-term fusion of microbiota between river water and sediment (Kristensen, 1984).
We also explored the impacts of land-use type on microbial community composition in the river ecosystems. The NMDS and UPGMA clustering results showed that microbiota in water samples were congregated in the river area dominated by agricultural land. Notably, high levels ofAcinetobacter spp. occurred at a few agricultural and urbanized land sites in spring.Acinetobacter spp. reportedly utilize a wide range of organic compounds as sole sources of carbon and energy (Hommel et al., 2014), so that they can degrade a variety of organic pollutants (Zhang et al., 2021; Hommel et al., 2014), which is consistent with the environmental characteristics of our study area; in spring, TN and NO3--N concentrations in water samples were as high as 6.94±0.97 (mg/L) and 5.22±1.32 (mg/L), respectively, which were 14.3% and 40.3% higher than those in autumn. The increased N nutrients could stimulateAcinetobacter spp. proliferation. In many previous studies, N has been considered to have a prominent impact on microbial community composition in freshwater environments (Xing et al., 2020 and Maritz et al., 2019).
We subsequently studied the relationship between environmental factors and microbial community composition using the Mantel’s test. According to the results, both TN and NO3--N concentrations in water samples increased with increases in proportions of agricultural and urbanized land areas; furthermore, microbial community composition was shaped by specific environmental factors. In contrast, the proportions of forest and shrub land areas were negatively correlated with N concentrations, and shaped microbial community composition. He et al. (2016) also observed that land-use type considerably influenced microbial community composition in a karst underground river.