a. Primary productivity : amount of biomass/organic matter produced per unit area over a time period by the plant during photosynthesis.
Factors : availability of nutrients / quality of light available / availability of water / temperature of the given place / type of plant species of the area/photosynthetic capacity of the plants.
b. Oxygen increases rate of decomposition.
Decomposition is an oxygen consuming process thus soluble substances such as sugars.
Chemical : decomposition is slow when chitin and lignin are present.