The Nitrogen Cycle Practice Test

A wetland becomes waterlogged after heavy rains, creating low-oxygen conditions in the soil. Over time, nitrate levels in the wetland soil drop and nitrogen gas ($\text{N}_2$) release to the atmosphere increases. Which nitrogen-cycle process best explains these observations?

A farmer plants soybeans (a legume) in a field that has low available nitrogen. After the growing season, soil tests show increased ammonium ($\text{NH}_4^+$) even though no fertilizer was applied. Which nitrogen-cycle process most directly explains the increase in soil ammonium, and what is the key biological agent involved?