High spatial resolution of distribution and interconnections between F e‐ and N ‐redox processes in profundal lake sediments

Abstract

Summary The F e and N biogeochemical cycles play key roles in freshwater environments. We aimed to determine the spatial positioning and interconnections of the N and F e cycles in profundal lake sediments. The gradients of O 2 , NO 3 − , NH 4 + , pH , E h , F e( II ) and F e( III ) were determined and the distribution of microorganisms was assessed by most probable numbers and quantitative polymerase chain reaction. The redox zones could be divided into an oxic zone (0–8 mm), where microaerophiles ( G allionellaceae ) were most abundant at a depth of 7 mm. This was followed by a denitrification zone (6–12 mm), where NO 3 − ‐dependent F e( II ) oxidizers and organoheterotrophic denitrifiers both reduce nitrate. Lastly, an iron redox transition zone was identified at 12.5–22.5 mm. F e( III ) was most abundant above this zone while F e( II ) was most abundant beneath. The high abundance of poorly crystalline iron suggested iron cycling. The F e and N cycles are biologically connected through nitrate‐reducing F e( II ) oxidizers and chemically by NO x − species formed during denitrification, which can chemically oxidize F e( II ). This study combines high resolution chemical, molecular and microbiological data to pinpoint sedimentary redox zones in which F e is cycled between F e( II ) and F e( III ) and where F e and N ‐redox processes interact.

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