It is possible that there are side effects to a garden. One possible side effect is the excessive amount of moisture which could loosen up the soil. Also a bunch of bugs will begin to appear that could just eventually kill the garden in the long run if not treated properly
Invasive species is the easiest way to answer this, with invasive species the diversity will likely decrease
Photosynthesis gives out oxygen and takes in carbon dioxide
Aerobic respiration uses the oxygen and gives out carbon dioxide
-Making a cycle
(Without the respiration the cell would die which would stop photosynthesis)
Hope this helps!!