(Image above courtesy of Understanding Ag)
One of the bigger problems to overcome today is reductive thinking. This problem is especially bad in a world where everyone wishes to reduce complex concepts to memes. So to get people to system think, let alone think in terms of multiple interconnected cyclical systems, is nearly impossible to do. Most people in modern Western societies don’t think this way including and especially many scientists.
With multiple system thinking, not only do you have to understand various systems, you also have to understand how these multiple systems are interconnected and interdependent. Thus, in the “green zone” as noted in my prior blog, for example, you need to understand how the carbon, small water, and nitrogen cycles work independently and interdependently. So needless to say, there are a lot of moving parts and shifting variables in such multi-variable equations and models.
Such explorations also require a multi-disciplinary understanding of how botany, soil science and atmospheric chemistry are also interconnected and interdependent like for example how soil carbon sequestration through root exuded carbon metabolites increase water retention and thus plant growth leading to more water vapor and biological volatile organic [BVOC] emissions from plants resulting in more hydroxyl radical [OH] formation and the consolidation of water vapor into rain clouds as further described below. Yes, everything is interconnected….and not just a long run on sentence.
BVOC’s monoterpenes are emitted by plants out of their stomata. Different monoterpenes are emitted by different plants (e.g. pinene by grasses, and .isoprene by trees). As I noted in my prior blog, citing Monk, P.S. et al. 2015, emitted BVOC’s from the stomata of plants also interact with naturally occurring nitrogen oxides (volatized nitrogen from the soil via the nitrogen cycle) to form the tropospheric surface level ozone [O3] needed for the excited oxygen [O+] to interact with the water vapor [H2O] needed for hydroxyl radical [OH] formation. O3 zapped by the sun becomes O+ and O2, the O+ interacts with H2O to become 2 OH molecules.
Hydroxyl radicals collide with all kinds of trace gases in the troposphere including biogenic forms of methane CH4. When this happens CH4 is then broken eventually back to CO2 and water vapor H2O. This CO2 is then fixed again via photosynthesis including the Calvin Cycle into glucose and then a myriad of carbon metabolites. These sources of carbon cycle through oxidation or methanogenesis to again produce CO2 or biogenic forms of CH4 which are cycled over and over again. On the other hand, thermogenic forms of CO2 and CH4 (fossil fuels) trapped in the ground, that haven’t been in the atmosphere for millions and hundreds of millions of years, exceed this biogenic carbon cycle (i.e. more carbon emitted than can be fixed via photosynthesis). Therefore such thermogenic emissions last longer in the atmosphere, reduce hydroxyl radical availability and or end up absorbed by the oceans (leading to ocean acidification). I’ll discuss methane persistence due to reduced hydroxyl availability in my next blog post.
Getting back to the second pathway for BVOC’s, when emitted, they also collide with previously formed hydroxyl radicals in the troposphere. These collisions result in the formation of secondary organic aerosols or SOA’s. These SOA’s function as another source of ice nuclei to seed rain clouds as noted in this paper: Wolf et al, 2020, A biogenic secondary organic aerosol source of cirrus ice nucleating particles. Water vapor and clouds need nuclei to consolidate and form rain clouds.
There’s water vapor from hydroxyl oxidation of CH4 in the troposphere. There’s also the water water transpired from the stomata of plants. So the SOA’s seed the consolidated water vapor (clouds) from both plant transpiration and hydroxyl oxidation of biogenic forms of methane [CH4 + OH –> CO2 + H2O] including enteric methane. And thus these SOA seeded clouds also produce rain.
To see a more detailed explanation of tropospheric hydroxyl oxidation, please read my old blog: WTF happens to all that methane?
Without healthy soils to allow rain fall to permeate the soil and be retained, there’s less plant growth. Thus there’s less transpired water and BVOC’s emitted from plants ….which, in turn, means there’s also less OH and SOA formation and thus less water vapor consolidation and less rain fall. Conversely, with increased carbon metabolite root exudation, there’s better soil structure, more water infiltration and retainage, more plant growth, more BVOC and water vapor emissions, more hydroxyl radical and SOA formation…and thus more rainfall as well as more atmospheric CO2 being fixed into plants and pumped into the soil to feed fungi and microbes. The necromass from the fungi and microbes is associated with minerals and particles and thus becomes both mineral and particle associated organic matter. This reduces the atmospheric load of CO2 by fixing that CO2 into biomass and soils.
More plant cover and more rainfall means more local cooling…one micro-climate at a time. So soil carbon sequestration is about a lot more than carbon credits and carbon sequestration.