Year after year, Xavier raises a vegetable garden in the same plot of soil. He observes that the plants grow less abundantly from one year to the next, and they produce fewer vegetables. Which of these processes could help explain Xavier's observations?

A. the rate at which soil bacteria perform photosynthesis
B. the rate at which soil bacteria perform denitrification
C. the rate at which soil bacteria perform nitrogen fixation
D. the rate at which soil bacteria perform chemosythesis