Abstract
Microbiota play essential roles in nitrogen (N) cycling in freshwater river ecosystems. However, microbial functional groups associated with N cycling (especially denitrification) in freshwater rivers under anthropogenic disturbance are still poorly understood. Here, we studied the impacts of different land-use types on denitrification-related microbial communities in Weihe River, Hanjiang River, and their tributaries, in the Qinling Mountains, China. The major land-use types in the three river areas were divided into natural (forest, shrub, grassland, and open water) and anthropogenic (agricultural and urbanized land) types. A landscape survey of microbiota in the river water and sediment was carried out with extensive sample sources based on deep 16S rRNA gene sequencing, which yielded operational taxonomic units for predicting functional groups. With increases in proportions of agricultural and urbanized land areas, electrical conductivity, total N, ammonium-N, and nitrate-N all increased in water samples. Conversely, microbial diversity exhibited a decreasing trend in water samples, whereas the relative abundance of denitrification-related functional groups increased, with increases in the proportions of agricultural and urbanized land areas. The relative abundances of denitrification- and human-related microbial functional groups in sediment samples were distinctively higher in Weihe River (mainly under agriculture and urbanization), when compared with those of Hanjiang River and Qinling tributaries (dominated by forests). The results indicate that anthropogenic land-use types, such as agricultural and urbanized land, result in simple microbial community structure and stimulate microbe-mediated denitrification in freshwater rivers.
Keywords: Land-use type, Denitrification, Microbiota, Freshwater ecosystems, Agricultural land, Urbanized land
Background
Freshwater is a fundamental component of aquatic ecosystems and is essential for coupling of biogeochemical cycles between continents and oceans (Aufdenkampe et al., 2011). The emission of greenhouse gases (GHG) from freshwater ecosystems is source of concern globally (Searchinger et al., 2008, Robertson et al., 2000 and Liu et al., 2020). Freshwater systems emit 0.3 Tg N yr−1 to the atmosphere in the form of nitrous oxide (N2O) under natural conditions (Tian et al., 2020), with implications for global warming. N2O is one of the most long-lived GHGs (Prinn et al., 2018). The atmospheric N2O burden increased from 1,462 Tg N in the 1980s to 1,555 Tg N in 2007–2016, with anthropogenic N2O emissions at 7.3 (4.2–11.4) Tg N yr−1 and natural N2O emissions at 9.7 (8.0–12.0) Tg N yr−1, respectively (Tian et al., 2020). Consequently, it is essential to explore the mechanisms of nitrogen (N) cycling in freshwater systems, which could facilitate the minimization of the impacts of N2O emissions on global warming.
Microbiota play active roles in the conversion of N in freshwater ecosystems (Mosier et al., 1998). Generally, organic N is transformed into molecular N via two pathways. First, ammonium-N (NH4+-N) is oxidized into nitrate-N (NO3--N) or nitrite-N (NO2--N) by nitrifying bacteria; second, NO3--N or NO2--N is reduced to gaseous N by denitrifying bacteria (Shen et al., 2021). In addition, N fixation refers to the reduction of N2 molecules to NH3 or organic N by N-fixing bacteria (Ryu et al., 2020). In freshwater environments, nitrification, denitrification, and N fixation processes mediated by distinct microbiota establish the N cycle alternately or simultaneously. The denitrification process produces N2O as an intermediate. N2O emissions are higher in areas with intense anthropogenic disturbance than in areas with minor disturbance (Zhao et al., 2021).
Microbiota as the foremost decomposers in nature drive the decomposition of biological remains and maintain biogeochemical cycling (Zaan et al., 2010 and Xu et al., 2014). Microbiota colonize suitable environments (Harrison et al., 2014) and respond sensitively to environmental stimuli, such as toxic substances, sewage treatment, and natural self-purification (Pei et al., 2018 and Wang et al., 2021). In addition, aquatic bacterial diversity is influenced by evapotranspiration, elevation, and temperature (Zhang et al., 2021). A large number of denitrifying bacteria exist in high-N wastewater (Pai et al., 1999; Liu et al., 2020). In the past, denitrification was mainly considered to occur in anaerobic or hypoxic conditions; however, recent studies have reported that some bacteria can also perform denitrification under aerobic conditions (Wan et al., 2009). According to Alessandra (2017), N2O emissions are released from the hyporheic–benthic zone and benthic–water column zone in the Kalamazoo River, Michigan, USA.
Anthropogenic disturbance disrupts the intensity, frequency, and timing of natural disturbance regimes that maintain the ecological integrity of river ecosystems (Cabezas et al., 2009). River ecosystem characteristics are often governed by interactions between hydrological connectivity and local environmental conditions. Such interactions, coupled with intensive agricultural and urbanized land uses, severely alter river hydrodynamic and biogeochemical gradients (Valipour et al., 2012 and Campo et al., 2014). Despite the impacts of anthropogenic disturbance on microbiota have been explored in marine and terrestrial ecosystems (Archer et al., 2020), it remains unclear how different land-use types influence microbial community structure in freshwater ecosystems.
Numerous factors influence microbe-mediated N cycling and denitrification in aquatic ecosystems. For example, saltwater intrusion can alter the interactions between biotic and abiotic components in tidal wetland ecosystems and therefore influence denitrification rates (Neubauer et al., 2019). Long-term sulfide inputs in freshwater lake sediments enhance chemoautotrophic denitrification, rather than dissimilatory NO3--N reduction into NH4+-N (Ypa et al., 2021). However, there is still a dearth of studies assessing the impacts of different land-use types on denitrification efficiency and associated microbial communities in freshwater river ecosystems.
The present study evaluated the impacts of different land-use types on the characteristics of river environment, microbial community structure, and denitrification-related functions in a freshwater ecosystem. The results of the present study could offer novel insights into microbe-mediated denitrification in freshwater rivers under anthropogenic disturbance.