'One physical system': Tansley's ecosystem as Earth's critical zone.

Area of Science:

• Earth Science
• Ecology
• Biogeochemistry

Background:

• Integrative concepts like biosphere, ecosystem, and Earth's critical zone link matter, energy, and organisms for a systems-level planetary understanding.
• Tansley's (1935) ecosystem concept of 'one physical system' is congruent with Earth science's critical zone across diverse spatial-temporal scales.

Observation:

• Congruence between ecosystems and critical zones extends vertically, encompassing the full depth of weathering profiles.
• Ecosystem metabolism, including respiratory CO₂, propagates to the critical zone's base, influencing soil formation and ecosystem evolution.

Findings:

• Full accounting of photosynthetically fixed carbon must include respiratory CO₂ and carbonic acid reaching the critical zone base.
• Downward CO₂ diffusion, though a small fraction of respiration, significantly impacts soil formation rates, ecosystem evolution, and resilience.

Implications:

• Current ecosystem boundaries are often too shallow, hindering a complete understanding of structure and function.
• Further research should explore connections between upper and lower critical zone ecosystems, watersheds, and deep microbial communities influencing mineral weathering.