Notes

Nitrogen in air. Lightning and nitrogen-fixing bacteria turn nitrogen into nitrates, which are absorbed by plants and use them for growth. The plants are eaten by animals, which will defecate and die. They are then digested by decomposers, who will convert the compounds into compounds that can be digested by plants, like urea to ammonia. Denitrifying bacteria turn nitrates into nitrogen gas. Nitrogen-fixing root nodule bacteria is found in legume plants: these two organisms have a mutualistic relationship as plants protect bacteria while the bacteria provide them with nitrates. Farmers can use this to their benefit in crop rotation: by planting legumes then ploughing more nitrates will be available in the next rotation.

Nitrogen in air. Lightning and nitrogen-fixing bacteria turn this into nitrates, which are absorbed by plants and used for growth. These are then absorbed by animals during feeding. The animals will excrete and die: decomposers will decay them and convert compounds into ammonia, which can be absorbed by plants. Denitrifying bacteria turns nitrates back into nitrogen. Nitrogen-fixing root nodule bacteria is found in legumes. These have a mutualistic relationship with the bacteria: the plants protect the bacteria while receiving nitrates directly from it. The nitrogen cycle is added to by humans: farmers spread manure over their fields to act as a fertiliser as it will contain urea that can be converted into ammonia. They also use crop rotation: by planting legumes then ploughing there will be more nitrates available in the next rotation.