Soil nitrogen biogeochemistry and hydrological characteristics shape the nitrate levels in a river.

The high levels of nitrate (NO) in the surface water have contributed to eutrophication and other eco-environmental damages worldwide. Although the excessive NO concentrations in rivers were often attributed to anthropogenic activities, some undisturbed or slightly disturbed rivers also had high NO levels. This study utilized multi-pronged approaches (i.e., river natural abundance isotopes, N-labeling techniques, and qPCR) to provide a comprehensive explanation of the reason for the high NO levels in a river draining forest-dominated terrene. The river natural abundance isotopes (δN/δO-NO) indicated that the soil source (i.e., soil organic nitrogen-SON and chemical fertilizer-CF) were the primary contributors to the NO, and the NO removal was probably prevalent in the basin scale. The N-labeling techniques quantitatively showed that denitrification and anammox were stronger than nitrification in the soils and sediments. Structural equation models suggested that nitrification in the soils was regulated by NH-N contents, which, in turn, were closely related to fertilization in spring. Denitrification and anammox were largely controlled by elevation and functional gene abundances (i.e., nirK and hzsB, respectively). The hydrological isotopes (i.e., δD/δO-HO) indicated that the transport of NO from soil to the river was related to the intensity of runoff leaching to the soil, In contrast, the riverine NH was largely from point sources; thus, increasing runoff led to a dilution effect. This study clearly showed that soil biogeochemistry and hydrological condition of a river basin jointly shaped the high NO levels in the almost undisturbed river.