Carbon cycle is returned to the atmosphere primarily by the respiration of consumer.

Carbon is produced by producers, consumed by consumers, released by consumers, and returned to the air and soil by decomposers.

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.

aquifers are bounded above and below by impermeable rock. the driving force of groundwater movement in confined aquifers is pressure, rather than gravity.