The diagram shows a simplified nitrogen cycle. Which row shows the correct labels for P, Q, R, and S?

Explanation

The conversion of nitrates in the soil to nitrogen is called the denitrifying(P) process, which is carried out by denitrifying bacteria. Then this nitrogen is fixed(Q) into the legume plants, which can form ammonia either by ammonification by saprotrophic fungi(S) or decay of leaf tissue(R). Hence the answer is A. ( p ) Denitrification. Denitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere. Dinitrogen gas (N2) is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist. ( q ) Legumes can form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is to form nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant. ( r ) The process of ammonification is carried out by certain bacteria just as Bacillus ramosus, B. vulgaris, Clostridium, Actinomyces, etc. In this process, these organisms release a specific amount of nitrogen compounds like ammonia. However, saprophytic bacteria will not be involved in this process. ( s ) Plant and animal wastes decompose, adding nitrogen to the soil. Bacteria in the soil convert those forms of nitrogen into forms plants can use. Plants use the nitrogen in the soil to grow. People and animals eat the plants; then animal and plant residues return nitrogen to the soil again, completing the cycle.