Humans have more than doubled the natural cycle of nitrogen (N) by N2 fixation causing serious threats to the atmosphere, soils, surface and groundwaters. Specially affected are coastal oceans as their productivity is often nitrogen limited, which is not the case in lakes. On the way to the oceans, soils and lakes are the great denitrifiers that relieve the rivers from a part of their N burden, transforming it into N2, and thus exercise an important control over coastal eutrophication. Denitrification and burial of N in the sediments are essential ‘ecosystem services’ of lakes to downstream rivers and coastal oceans. Even though the net effects of nitrogen turnover can be estimated quite accurately, little is known about specific processes and microbial actors controlling it.
While empiric evidence shows that N removal in lakes is proportional to load and water residence time, there is no clear view of the processes regulating denitrification rates or the degradation, burial and reservation of N-containing organic compounds in settling particles in individual lakes. Combining geochemical, microbiological and modelling approaches we will provide answers to these questions, culminating in the implementation of a generalizable mechanistic model for N transformation, and specifically N removal, in lakes.