☔️ Water series: Interactions of flora with water and climate

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A simplified diagram of the water cycle or “hydrologic cycle” (Wikimedia).

The source of all our food which is also involved in most of the cooking techniques we use every day is, in fact, this basic molecule essential to life: water (H2O). It is also one of the major environmental concerns, when disputed as a ressource for humans or as a milieu for wild species. How water is the honorable “background” of every form of life on Earth, and how our relationship with it might change while facing global warming, are some of the questions that led us to this “☔️ Water series”, divided in 4 chapters:

  1. ☔️ Water series: The primordial soup of life: oceans water
  2. ☔️ Water series: Interactions of flora with water and climate
  3. ☔️ Water series: How mankind uses water? From food to energy
  4. ☔️ Water series: Future extreme dynamics of water use

Water and ecosystems

Thanks to plants transpiration, the pressure from roots to leaves allow water to be used as a conveyor for nutrients: no heart needed here (unknown source).
Common fertilisers are usually a mix of Nitrogen, Phosphates and Potassium.

Salts and soil

As much as we vertebrates need to regulate our bodies salinity by peeing, “transpiration” help plants evacuate the excess of soil water they pump from the ground to convey nutrients through their roots.[1] Those nutrients are known as the NPK[2] trinity, and are present in the form of dissolved salts and/or fertilisers extracted from the ground: Nitrogen[3] (N) in the form of nitrates, Phosphorus (P) in the form os phosphates, and Potassium (K) in the form of cream of tartar (potassium bitartrate).[4] What we call “evapotranspiration” describes the combined evaporation of the rain that just fell on vegetation and plant’s transpiration. Evapotranspiration, caused by the sun energy, concerns a mass of water that is called “green water”, while the fresh water that ends up stored in the ground or streaming on the surface to form lakes and rivers is named “blue water”.[2] Seemingly, the evaporation of seawater condensates when it reaches a high altitude and precipitates in the form of rain when clouds droplets reach a critical mass.[5]

Erosion and salts

Depending on the geography of the land where it falls down, this evaporated water would have different effects on the land and ecosystem. Afterward, water will pursue the “hydrologic cycle”[5], either by running back to oceans, evaporating, or being fixed for a time[6] as groundwater or ice. However, much more water is stored than actually moving through this cycle. Among the effects of rain on land, erosion is the most double-edged one. Erosion happens when water falls on a surface that is either already saturated with water, or simply waterproof (artificial covered soils or agricultural compact soil); making urbanisation and agriculture foster floods and erosion of the soil, which could destroy vegetation and the humus soil layer, thus entire ecosystems. Erosion is however the process that let seawater and fresh water to grab nutrients (salts) from the rocks it runs on, allowing the life soup to stay rich and salty.

Water saturation in ecosystems

Along with the climate and geography of the land, the way water is distributed affects greatly what type of ecosystems would exist: if water is scarce and hidden in the ground (deserts), saturating the air and the soil (equatorial and continental forests), flooding vegetation permanently (wetlands) or frozen with the soil as permafrost (toundra and taïga); water “decides” who will thrive and where. In most of those ecosystems, the plants growing thanks to water will have an important retro-effect on water distribution: when rain falls on vegetation, a share of it will never reach the soil and be captured by leaves, while the excess of water that the soil can’t absorb wont run away, because tree trunks and plants will slow it down. Combined with plants inner water, we can state that vegetation is like a big sponge slowing the water from joining streams and rivers locally, thus regulating floods and erosion.[5]

Water and climate

Different water distribution will allow different ecosystems to exist (The Soft Protest Digest).
Peat soils are mostly situated in the Northern part of the world (Parish 2018).
Diagram illustrating the way carbon is sequestrated in wetlands (The Soft Protest Digest).
The way carbon is absorbed and distributed in oceans is a complex matter that is not fully understood, and involves chemical dynamics like the “Solubility pump” (Wikimedia).

Wetlands

Wetlands are almost constantly saturated water, so this water stays available for nearby valleys and prairies by leaking through groundwater or slowly streaming in rivers. Hence, this water benefits as much wetlands ecosystems as fields of local farmers[7]. Considered useless and dangerous in the ancient times[8], wetlands used to be massively drained to obtain more agricultural lands; but wetlands are today recognised as the most important type of ecosystems to preserve, protect and restore. Why is it so?[9]

  • The first reason lies in plain sight: they are considered the most biologically diverse of all ecosystems.[10]
  • The second reason is the ability of some wetlands to filter water by retaining nutrients and trap heavy metals that can be processed by some plants, thus limiting the pollution of rivers and groundwater.
  • The third and last reason is the ability of wetland to store atmospheric CO2 in the form of vegetation and soil.

Among all types of soils that can be found in wetlands, peat soils[11] are the most effective at storing the excess of atmospheric carbon (for as long as they don’t dry or burn). When plants that extracted atmospheric carbon from the air die, their organic matter composed of carbon is trapped in the water under anaerobic conditions that allows the annual rate of biomass production (plant growth) to be greater than the rate of decomposition.[5] This means that the accumulation of dead plants in the soil is so fast that it grows faster than plants and soil animals can process it. Thus, a humus depth of a few meter can form, when most soils only accumulate a few decimetres of humus.[12] Water keeps this “unused” carbonic organic matter in a state of stability that makes it such an efficient carbon sink.[5]

Oceans

Water has an even bigger role to play in atmospheric carbon sequestration, with oceans being the largest carbon sink, even though their annual sequestration is lower than the one of vegetation and soil combined. Here, no need for organic matter, as CO2 simply dissolves in sea water before to be transformed in diverse elements known as dissolved inorganic carbon (the “solubility pump”)[2]. While it seems to be a good news for anthropogenic climate change, one of those inorganic carbon elements unfortunately is carbonic acid. It causes ocean acidification, which might have disastrous consequences on marine life by weakening shells of molluscs. Those shells are made of limestone, and the acidity of water dissolves it. If shell molluscs were to go extinct, this potential biomass collapse would result in a huge amount of carbon released. Even thought the scale of this acidification stays uncertain, and mitigation methods are experimented, studies suggested that the global warming of oceans might limit their sequestration capacity until they get saturated with carbonic acid.[5]

Notes

  1. Plants roots actually absorbes water thanks to osmotic laws, by the use of metabolic energy in cells to increase the salts’ concentration inside the roots, so that the soil water is less concentrated and gets in. However, this process called “active” is less important that of the “passive” absorption of water caused by the leaves’ transpiration that creates a suction from bottom to top, throughout the plants tissues. source: https://en.wikipedia.org/wiki/Absorption_of_water
  2. 2.0 2.1 2.2 Ghislain de Marsily, «Les chemins de l’eau dans le sol», Le sol, une merveille sous nos pieds, Belin, 2016.
  3. Some plants also extract Nitrogen from the air by collaborating with bacterias in their roots.
  4. Salt is a chemical compound made of the assembly of cations and anions; which makes it an electrically neutral compound. Sea water contains a majority of Sodium (Na) and Magnesium (Mg) ions, but also Potassium (K) that is a primal fertilizer. Magnesium (Mg) is also used in agriculture in the form of Magnesium sulfate poured in the soil. source: Wikipedia page “Salts” https://en.wikipedia.org/wiki/Salt_(chemistry)
  5. 5.0 5.1 5.2 5.3 5.4 5.5 source: Wikipedia pages:
  6. The residence time of water depends on where it is stored: see table (approximations). source: https://en.wikipedia.org/wiki/Water_cycle#Residence_times
  7. Still, the water saturation often drives farmers to drain their soil to grow crops.
  8. Marshes and swamps were considered as disease reserves, and unsuitable for most agricultural system, because unworkable.
  9. In any given ecosystem, appart from tropical rainforest, carbon stocks are significantly higher in the soil than in plants and animals. source: https://fr.wikipedia.org/wiki/Fichier:IPCCStockCarboneSolsV%C3%A9g%C3%A9tationFr.jpg
  10. From marine and coastal wetlands like salt marshes, sea cliffs, mangroves and lagons; to inland wetlands like rivers, lakes, seasonally flooded prairies, ponds and meadows.
  11. Peat can be used as a fuel, and is similar to coal an oil, as it is old undecayed organic matter, but younger thus unfossilised.
  12. Humus is the top layer of the soil, which is made of decaying organic matter consumed by decomposers. It is vital for plants to grow, but also subject to erosion.