River bed carbon and nitrogen cycling: State of play and some new directions

The significance of freshwaters as key players in the global budget of both carbon dioxide and methane has recently been highlighted. In particular, rivers clearly do not act simply as inert conduits merely piping carbon from catchment to coast, but, on the whole, their metabolic activity transforms a considerable fraction of the carbon that they convey. In addition, nitrogen is cycled, sometimes in tight unison with carbon, with appreciable amounts being ‘denitrified’ between catchment and coast. However, shortfalls in our knowledge about the significance of exchange and interaction between rivers and their catchments, particularly the significance of interactions mediated through hyporheic sediments, are still apparent. From humble beginnings of quantifying the consumption of oxygen by small samples of gravel, to an integrated measurement of reach scale transformations of carbon and nitrogen, our understanding of the cycling of these two macro elements in rivers has improved markedly in the past few decades. However, recent discoveries of novel metabolic pathways in both the nitrogen and carbon cycle across a spectrum of aquatic ecosystems, highlights the need for new directions and a truly multidisciplinary approach to quantifying the flux of carbon and nitrogen through rivers. ► We consider rivers in a landscape setting and how, through exchange, the cycling of carbon and nitrogen are influenced. ► Respiration in rivers follows temperature, and, with warming, could generate even more CO2 and CH4 per unit carbon carried. ► A new synthesis predicts the global amount of denitrification in rivers as a fraction of the carbon respired (33 Tg N y–1). ► Novel metabolic pathways for the production of N2 gas beyond denitrification are considered. ► Chemosynthetic production by methane oxidation could represent a paradigm shift in how we view production in rivers.