HOW SOC, PHOTOSYNTHESIS, WATER VAPOUR AND ATMOSHERIC OH SUPPORT SUSTAINABLE PASTURE-BASED DAIRY FARMING.

The author recently posed the question to Google, what is the relationship between soil carbon (SOC), photosynthesis, water vapour and the atmospheric hydroxyl (OH) radical. Quoting a large number of scientific papers, Google summarized the relationship as follows: ‘’Soil carbon, photosynthesis, water vapour and atmospheric OH form a highly interconnected feedback loop that bridges the terrestrial biosphere, the hydrological cycle, and atmospheric chemistry. Plants drive this entire system by exchanging gases with the atmosphere, which simultaneously regulates soil biology, moisture levels, and the cleansing capacity of the air’’. The interaction between these four components can schematically be depicted as an interconnected feedback loop:

[Photosynthesis] ― (Transpiration) → [Water Vapour]

             │                                                                  │

 (Carbon input)                                          (UV photolysis)

            ↓                                                                ↓

  [Soil Carbon] ← (Microbial Activity) ― [OH Radicals]

Photosynthesis is a key driver, with the vegetation absorbing carbon dioxide (CO₂) from the atmosphere, thereby creating biomass by converting the CO₂ into sugars and other carbon substances which build plant tissue and roots. These also feed soil microbes, the net effect being stable soil organic matter (SOM), primarily humus. In the higher layers of the soil, the carbon acts as a sponge, which dramatically increases the soil’s ability to hold water. If well-hydrated, the carbon-rich soil prevents plant water stress, thereby enabling sustained photosynthesis and consistent transpiration.

The transpiration supplies a massive portion of the global atmospheric water vapour. The water vapour together with solar ultraviolet (UV) light is central in the formation of hydroxyl radicals (OH). In the lower atmosphere, called the troposphere, UV light breaks down ozone (O₃) into ‘’excited’’ oxygen atoms (referred to as O(¹ D)). The latter reacts directly with water vapour (H₂O) to create hydroxyl radicals (OH). The hydroxyl radicals are the primary chemical oxidant (atmospheric ‘’cleanser’’) that destroys greenhouse gases such as methane (CH₄) and chlorofluoro-hydrocarbons, in the case of CH_4, it is broken down to CO₂ and water vapour over an approximate 9-year lifespan. The CO₂ is then through the biogenic cycle, again available to photosynthesis. This procedure is optimal when sufficient water vapour is present and OH is not required to react with other pollutants and substances in cleansing. If OH levels drop, CH₄ will accumulate, which may lead to warming the earth, causing droughts, stalling photosynthesis, and accelerating soil carbon loss through wildfire or microbial decay. Bare soils, such as with degraded rangeland, will have a similar effect.

To elaborate: All pathways of OH formation are directly and indirectly tied to moisture and biogenic activity. In ecosystems rich in vegetation, organic volatiles, soil microbes, and photochemical flux — essentially “alive air” — OH production can be orders of magnitude higher than in dry or desert-like air. More humid ecosystems produce a lot more hydroxyl radicals during both day and night. So, when the hydroxyl radical concentrations are increased, the half-life and life span of methane and other hydrocarbons can be reduced, with less implication to global warming.

To assure maximum benefits and protection against vegetation and soil deterioration, dairy pasture and other rangeland systems should maximise active biomass production and SOM, as this will provide optimum water vapour for OH formation, transpiration and photosynthetic products. Photosynthesis in the grazing context is maximized by rotational grazing through active regrowth and SOM by mix pasture species which optimize below ground microbial activity – these principles imply regenerative agricultural practices.