Azolla is used as a biofertiliser because it

Assertion Nitrogen-fixing bacteria of legume root nodules survive in oxygen depleted cells.

Reason Leghaemoglobin completely removes oxygen from nodule cells.

Assertion Magnesium is important in photosynthesis and carbohydrate metabolism.

Reason $$\mathrm{Mg}^{2+}$$ is involved in the synthesis of nucleic acids.