The oxygen level in the garden is higher because the plants are constantly releasing oxygen through photosynthesis.
Photosynthesis is the process by which the plants make their food with the help of sunlight. This food helps the plant to live and grow. The by-products created by photosynthesis are glucose and oxygen. The glucose is used by the plants for growth and oxygen is released in the air.
The one component in photosynthesis that can't be recycled but must be constantly available is energy.
Light energy, mostly from sunlight, is the main requirement for photosynthesis to occur.
Photosynthesis is a process wherein light energy is converted into chemical energy.