Figure 4. Cycle of biologically driven N transformations that occur in natural and human-influenced terrestrial and marine environments. Nitrogen (N2) fixation (step 1) and N assimilation (from ammonium, nitrate, or organic N, step 2) are anabolic processes, whereas mineralization (step 3), nitrification (steps 4−6), DNRA (steps 7, 15), denitrification (steps 7−11), and anammox (anaerobic ammonium oxidation, steps 12−14) are (or are a consequence of) catabolic processes. Abiotic sources of fixed N enter the biological N cycle as ammonium and nitrate. Note, the typo “nitrate oxidation” above arrow 7 in the original published version of Figure 4 has been corrected to display “nitrate reduction” in the revised figure shown here.