Abstract
Riverbank soil ecosystems are important zones in terms of transforming inorganic nitrogen (N), particularly nitrate (NO3−-N), in soils to nitrous oxide (N2O) gases. Thus, the gasification of N in the riverbank soil ecosystems may produce a greenhouse gas, N2O, when the condition is favourable for N2O-producing microbes. One of the major N2O-producing pathways is denitrification. Thus, we investigated the denitrification potentials along Shibetsu River, Hokkaido, Japan. We sampled riverbank soils from eight sites along the Shibetsu River. Their denitrification potentials with added glucose-carbon (C) and NO3−-N varied from 4.73 to 181 μg·N·kg−1·h−1. The increase of the denitrification after the addition of C and N was negatively controlled by soil pH and positively controlled by soil NH4+-N levels. Then, we investigated the changes in 16S rRNA bacterial community structures before and after an anaerobic incubation with added C and N. We investigated the changes in bacterial community structures, aiming to identify specific microbial species related to high denitrification potentials. The genus Gammaproteobacteria AeromonadaceaeTolumonaswas markedly increased, from 0.0 ± 0.0% to 16 ± 17%, before and after the anaerobic incubation with the excess substrates, when averaged across all the sites. Although we could not find a significant interaction between the denitrification potential and the increase rate of G. AeromonadaceaeTolumonas, our study suggested that along the Shibetsu River, bacterial response to added excess substrates was similar at the genus level. Further studies are needed to investigate whether this is a universal phenomenon even in other rivers.
Highlights
Riverbank ecosystems are very important ecological zones in relation to nitrogen (N) cycle
For the topsoil samples (0–15 cm, eight samples in total), we investigated the changes in 16S rRNA bacterial community structures before and after the anaerobic incubation with NO3−N and glucose-C, with two replications. e incubation procedure and substrate concentrations were the same as the method used for the measurement of denitrification potential except that the incubation length was 48 h, and the number of replication was two for this 16S rRNA experiment
Post hoc Tukey’s test was performed when there was a significant interaction between the sampling sites and the treatments (p < 0.05) to evaluate the effect of treatments for each sampling site. en, the relationship among the denitrification potentials and other soil characteristics was investigated using linear regressions
Summary
Riverbank ecosystems are very important ecological zones in relation to nitrogen (N) cycle. Is process reduces NO3−-N to gases such as nitrous oxide (N2O) and di-nitrogen (N2). Us, it is important to evaluate the factors controlling denitrification potential in riverbanks along river ecosystems. While denitrification can potentially reduce NO3−-N from the riverbank ecosystems, it can negatively impact the environment because N2O is a greenhouse gas. Nitrification process (the oxidation of ammonia to NO3−-N) is an important N2O-producing process when soils are well aerated [5], but denitrification is more important when soil is wetter or submerged with water [6]. Carbon and N contents in soils and the amount of readily available inorganic-N positively control the denitrification potentials because these are substrates for the activity of denitrifying microbial communities [7, 8]. Soil pH controls denitrification potentials the relationship between pH and denitrification is markedly influenced by other factors
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