(a)(i) Ecosystem: -

This is a self-supporting unit/ system.
it consists of all living organisms and the non-living factors, that interact with each other/are interdependent.
Each component influences the properties of the others.
The living organisms make up the biotic components, e.g. plants and animals
The non-living factors make up the abiotic components; e.g. soil/edaphic factors/climatic factors.

(ii) Population: This refers to the total number of organisms/individuals of the same kinds/ species living together within a habitat/area at a particular time. The size of the population varies due to different factors; e.g. natality/mortality/immigration/emigration.
(iii) Ecological succession in plants: - This is the orderly series of changes occurring in plant communities over a given period of time, until a stable/climax community is established. It usually commences in a newly formed open/barren habitat/abandoned farmland, which is usually as a result of human activities/natural disasters/ catastrophes. The pioneer/primary colonizers have short life cycles/span, which get replaced by other plants which thus derive nutrients from their remains. These latter secondary colonizers have longer life cycles/span. - The initial pace of a succession is rapid but it gradually slows down until a final/stable/climax community is reached
(b) Ways by which Nitrogen is made available to soil include:
(1) Through nitrogen fixation
(2) Through thunder storms and lightening
(3) By putrefaction/decay of organic matter
(4) By nitrification
(5) By application of artificial fertilizers
(6) By application of farmyard manure/organic fertilizers/poultry, droppings/animal dungs/green manure.