Nature's Cycle

Whenever one or more +1/+1 counters are placed on a creature, add {g} to your mana pool at the beginning of your next precombat main
phase.
Whenever a creature dies, add {b} to your mana pool at the beginning of your next precombat main phase.
{1}{b}{g}, Sacrifice a creature: Put two +1/+1 counters on target creature.