Salinity alters N2O production pathway in lake sediments on the Qinghai–Tibet Plateau

Abstract

Qinghai–Tibet Plateau (QTP) with abundant lakes is considered as an important area for nitrous oxide (N₂O) emissions. Although lakes on the plateau occur on a gradient of salinity, how variable salinity affects the N₂O production pathways in natural lakes remains poorly understood, possibly leading to over- or underestimates when predicting N₂O budgets. In this study, we investigated 19 QTP lakes on a salinity gradient (freshwater, saline, hypersaline) to determine the effects of salinity on relative abundances of microbes involved in nitrogen (N) transformation processes, using 16S rRNA gene high-throughput sequencing of lake sediments combined with absolute abundance of functional genes in N₂O metabolism determined by quantitative real-time polymerase chain reaction and N species analyses. Our results revealed nitrogen species and contents varied with differences in salinity. At relatively high salinity, nitrification and assimilatory nitrate reduction were inhibited, whereas dissimilatory nitrate reduction to ammonium and N fixation were stimulated. Abundances of nirK and nirS genes in denitrification indicated nitrifier denitrification encoded by nirK gene was dominant in freshwater lakes, whereas incomplete denitrification encoded by nirS gene was prevalent in hypersaline lakes. The ratio of (nirK+nirS)/nosZ gene abundances implied the N₂O emission potential in hypersaline lakes was greater than that in freshwater lakes. Overall, the results suggest that salinity can alter the N₂O production pathway and affect N₂O emission budgets in lacustrine ecosystems.