The diagram shows a simplified version of the nitrogen cycle in an agricultural ecosystem.

Name the processes WWW, YYY and ZZZ shown on the diagram.
Process ZZZ occurs most rapidly in waterlogged, compacted soils (anaerobic conditions). What is meant by anaerobic conditions?
Name the group of organisms that carry out process ZZZ.
Farmers can use a crop rotation system that includes growing legumes (such as alfalfa or clover). Explain how the presence of legumes undergoing process XXX increases the nitrate concentration of the soil for the next crop in the rotation.
Give one ecological problem caused by the leaching of nitrate-containing fertilisers from agricultural land into nearby ponds or rivers.