In natural systems, nitrogen is lost by denitrification. predict which of the following processes will be most responsible for replacing lost nitrogen in an ecosystem.

a) erosion

b) fixation

c) combustion

d) weathering

Which of the following best describes the purpose of the parathyroid hormone? a. it decreases the blood calcium level and decreases blood phosphorus level. b. it decreases the blood calcium level and increases the blood phosphorus level. c. it increases the blood calcium level and increases blood phosphorus level. d. it increases the blood calcium level and decreases blood phosphorus level.