Abstract
The coupling between nitrification and N2 gas production to recycle ammonia back to the atmosphere is a key step in the nitrogen cycle that has been researched widely. An assumption for such research is that the products of nitrification (nitrite or nitrate) mix freely in the environment before reduction to N2 gas. Here we show, in oxic riverbeds, that the pattern of N2 gas production from ammonia deviates by ~3- to 16-fold from that predicted for denitrification or anammox involving nitrite or nitrate as free porewater intermediates. Rather, the patterns match that for a coupling through a cryptic pool, isolated from the porewater. A cryptic pool challenges our understanding of a key step in the nitrogen cycle and masks our ability to distinguish between sources of N2 gas that 20 years’ research has sought to identify. Our reasoning suggests a new pathway or a new type of coupling between known pathways in the nitrogen cycle.
Highlights
The coupling between nitrification and N2 gas production to recycle ammonia back to the atmosphere is a key step in the nitrogen cycle that has been researched widely
Combinations of Eqs. (1) to (4) recycle ammonia back into atmospheric N2 gas and this coupling between aerobic nitrification and anaerobic N2 gas production is a key concept in the nitrogen cycle, controlling ecosystem production and the abundance of life on Earth[6,7]
We show that the pattern of N2 gas production from ammonia in these oxic riverbeds violates the prevailing concept that coupled nitrification and N2 gas production is a two-step process with free nitrite or nitrate as intermediates
Summary
The coupling between nitrification and N2 gas production to recycle ammonia back to the atmosphere is a key step in the nitrogen cycle that has been researched widely. We added 15N-ammonia to oxic sediment microcosms (see Methods) to trace the coupling between nitrification and N2 gas production both with and without the inhibitor of aerobic nitrification, allylthiourea[19] (~80 μM ATU in the porewater, Treatments 1 & 2, Table 1 and Methods) that does not inhibit denitrification or anammox[2,20].
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