Soil Biology and Microorganisms Questions and Answers

Answer: D

The C:N ratio of humus is about 10:1.

As organic matter decomposes, micro-organisms release carbon as carbon dioxide while holding on to nitrogen, so the C:N ratio narrows. Fresh crop residues may have ratios of 40:1 to 80:1 or more, but stable humus settles at about 10:1 (roughly 10 to 12:1), close to that of the microbes themselves.

20:1 is roughly the threshold below which residues release nitrogen as they decompose, not the ratio of humus. 20:3 and 10:2 are not standard figures.

Answer: D

The enzyme is Nitrogenase.

Nitrogenase, made by the Rhizobium bacteroids inside the root nodules, reduces atmospheric nitrogen gas to ammonia, using a large amount of ATP. The enzyme contains iron and molybdenum and is destroyed by oxygen, which is why nodules contain leghaemoglobin, the pink pigment that binds oxygen and keeps its level low around the enzyme.

Rubisco fixes carbon dioxide in photosynthesis. Nitrate reductase reduces nitrate taken up from the soil to nitrite, a step in nitrate assimilation, not fixation. Hydrogenase recycles the hydrogen given off by nitrogenase and only improves the efficiency of fixation.