A, COD and BOD all represent the oxygen consumption for decomposing organic matter. The higher the oxygen consumption, the more the organic matter content, so A is correct.

B, plants can absorb inorganic salts such as N, P and K from water, thus reducing inorganic nutrition in water, so B is correct;

C, there are emergent plants, floating plants and submerged plants in this wetland, which reflects the vertical structure of the community, and the horizontal structure of the community can also be caused by the differences of nutrients in water and light intensity, so c is correct;

D. The position of sewage inlet and outlet of the constructed wetland leads to a large amount of oxygen in the water, so the decomposition of substances mainly depends on aerobic microorganisms, so D is wrong.

D is chosen.