Cycles, balances and disturbances

The cycles of matter

Energy crosses the ecosystem and dissipates. Matter is not lost: it cycles indefinitely.

The fundamental difference

   ENERGY                           MATTER
   enters: sunlight                 comes from nowhere
   leaves: heat                     goes nowhere
   one-way FLOW                     closed CYCLES
   must be constantly renewed       recycles indefinitely

An ecosystem can recycle its atoms for millions of years; it cannot run for a day without an energy input. This is why Earth is a system closed to matter and open to energy.

The carbon cycle

                    ATMOSPHERE (CO₂)
                    ^      |      ^
        respiration |      | photosynthesis
        decomposition|     v      | combustion
                    |  LIVING THINGS
                    |      |
                    +------+
                           |  burial (millions of years)
                           v
                    FOSSIL FUELS

The natural cycle is balanced: photosynthesis removes about as much carbon as respiration and decomposition return.

Burning fossil fuels breaks that balance. It re-injects within a century carbon stored over hundreds of millions of years — a release about a million times faster than the storage that produced it. This mismatch of speed, not of nature, is the most direct physical description of climate disruption.

The nitrogen cycle

Nitrogen illustrates a point often misunderstood: a resource can be abundant and inaccessible.

   atmospheric N₂: 78 % of the air     yet UNUSABLE as such
        |
        |  FIXATION by bacteria (including those in legume roots)
        v
   ammonium, then NITRATES  ->  taken up by plants
        |
        |  consumption, then decomposition
        v
   return to the soil
        |
        |  DENITRIFICATION by other bacteria
        v
   atmospheric N₂

Plants are bathed in nitrogen without being able to use it: the triple bond of N₂ is too stable. Only certain bacteria can break it. This is what makes legumes — clover, alfalfa, peas — so valuable in agriculture: their root nodules host these bacteria and enrich the soil.

The Haber-Bosch process, which has performed this fixation industrially since 1913, has fed a world population multiplied by four. It now consumes about 1 to 2 % of world energy, and the resulting nitrate excess causes eutrophication of rivers and coastal zones.

The water cycle

   oceans, soils  --evaporation-->  atmosphere
                                        |
                                   condensation
                                        v
                                  precipitation
                                        |
                          run-off, infiltration
                                        v
                                  back to the oceans

Living things take an active part: an Amazonian forest recycles through evapotranspiration a major share of the rain it receives. Deforestation therefore does not merely remove trees: it can dry out an entire region, since the forest partly makes its own rain.

Summary