Heterotrophs evolved to utilize O 2 as an electron acceptor. The atmosphere for the past 0.5 billion years or so has been balanced with respect to O 2 . 4. Humans are reversing the past 2. 5 billion years of history by accelerating respiration. In all of these compounds, C provides the major skeleton. All other essential elements depend upon C for their assimilation, movement through ecosystems, and losses from ecosystems.
Carbon drags other elements around most of the cycle.
C inputs are limited by resource availability. C has important gaseous phase, inputs and outputs are high from ecosystems. N content controls N fixation through a competitive advantage for N-fixers, as well as a chemical inhibition of N fixation by N availability. N has several important inorganic phases that can be leached or volatilized. P has no important gaseous phase, - and has high affinity for other soil minerals 5.