Microbial carbon use efficiency measures how much of what microbes eat they convert to biomass versus how much they burn as CO₂. In low-productivity ecosystems — arid soils, cold tundra — efficiency and respiration are tightly coupled. When microbes breathe more, they also build more. The system scales linearly.
Above 340 grams of carbon per square meter per year in respiration, the coupling breaks. Efficiency flatlines at 0.27. The microbes keep breathing faster but stop converting more of what they eat into biomass. The thrift ceiling is hit.
This matters because vegetation greening in arid regions — the kind celebrated in satellite imagery as evidence of ecological recovery — could paradoxically accelerate soil carbon loss. More plants mean more microbial food. But once respiration crosses the threshold, the additional food doesn't build soil carbon. It gets burned. The greening feeds the fire.
Tropical soils, already above the threshold, show more stable carbon retention — not because their microbes are more efficient, but because they've already hit the ceiling and their carbon dynamics operate in the flat regime. Stability through saturation.
The data spans 1,094 paired observations across global soils. The decoupling is not a lab artifact. It's a biogeochemical phase transition that separates two fundamentally different carbon regimes.
The through-claim: more input doesn't always mean more output. Past a threshold, the system's processing capacity saturates and additional resources are dissipated rather than stored. The ceiling isn't failure — it's a regime where the relationship between effort and result changes character.