So here I put together what appears as a rather strange idea to write about — what is common between us and soils, specifically soil carbon accrual and… weight gain? And what can we learn about mathematical modeling from there? I am confident this will be extremely insightful to anyone interested in our relationship with soils, and to better understand what’s all the fuss and buzz about “soil carbon sequestration” aka. capturing atmospheric C and locking it in the soil — something you have probably heard a lot on climate change-related news.
First and foremost, it is helpful to know that the common thing between the two is — soil C accrual means the soil system is gaining (organic) carbon, whereas in the context of weight gain, your body system is also gaining organic carbon. In other words, changes in soil C or body mass are both governed by the input and output relationship of organic matter conceptually (i.e., the simplest model abstraction).
Basically, we are all “leaky vessels” that ingest food (i.e., high-energy organic C compounds), food here is an “input” in modeling terminology¹. If we eat too much, our body weight increases, via temporarily having more materials in our gut or on a longer term, the storage of more fat, muscles or bone collagen, etc. after the food is processed. Conversely, if we lose organic matter (via defecation, respiratory CO₂ loss, tissue turnover i.e., dead cells) more than we eat, we lose weight.
The same is happening in the “soil vessel”, it takes in dead organic matter from plants and animals (commonly called “litter” or “detritus”), which are ultimately originated from atmospheric CO₂. Some of this organic matter is lost via respiration, some are leached out down at the bottom of the soil vessel (much like defecation), some are eroded away (like dead skin cells sloughing off) and if the total loss is less than the gain over a period of time, soil C accumulates. If a piece of land is deforested i.e., there is little food for the soil to eat, it will always suffer a loss of C, same when we died and therefore stop eating, our body would decay away gradually. To sum up, that is to say — a good model should capture the correct magnitude of these gain and loss pathways.
Here comes the specifics governing the magnitude of these gains and losses. Firstly, not all food and litter are the same, i.e., different inputs “behave” differently in the vessel. Some food makes you gain more fat/glycogen (high sugar food) whereas others help you gain more muscle mass (high-protein food/balanced diet) or other tissues. Moreover, to maintain body functions, breaking down fats is a preferred source of energy than muscle fibers, we say in modeling terminology — fat is more “labile” (i.e., have a higher turnover/decay rate) than muscles. You can probably imagine blood glucose and glycogen are the most “labile” and therefore stay only for a short time in our body.
Similarly, organic C with different qualities or “palatability” in the soil are digested by soil microbes and animals for energy and are transformed into different products (similar to fats, peptides, sugars). Some organic C is soluble, highly labile molecules that get broken down or lost quickly like glucose (i.e., high turnover rate), some are chunks of bulky partially-decomposed plant matter much like our chunks of body fat (commonly called “particulate organic matter”), whereas some become “sticky” dead microbial cells and biofilm that can get attached tightly to soil mineral surfaces (like skeletal muscles attaching to bones) which then become quite resistant to break down.
(Question for you: Do you think (eating) more is better? Given the climate emergency, should we dump tons of organic waste to every small piece of land to help sequester carbon?)
Secondly, not all humans and soils are the same to begin with (i.e., me and you have different properties, which means having different “vessel-specific” model parameters that affect various organic matter transformation processes). Some of us are endowed with a large skeletal frame (larger vessel size) which helps build muscle mass more easily. Some of us (folks in the tropics) are less likely to build up fats in part attributable to genetic or physiological differences, since storing fats is evolutionarily more advantageous under cold climates.
Same for soils, different soil minerals around the globe originate from different parent rock types and geochemistry and thus have different surface areas and binding strength towards different organic molecules (you can think of soil minerals as a scaffold/matrix that organic matter attaches to, like our skeleton), or different fertility and acidity levels which affect the functioning of microbes (≈ digestive cells), sometimes making them unable or excel at breaking down certain organic matter. In general, a “medium to fine texture” (loamy) soil that is rich in pedogenic metallic oxides is good at holding carbon.
Thirdly, our propensity to gain weight changes over time i.e., we gain weight more easily from toddler till midlife and lose muscle mass towards old age. Not surprisingly, soils also age, a freshly formed or exposed rock with minimal fine soil particles cannot hold much organic C. As weathering (≈ aging) proceeds, the rocks are turned into finer and more reactive particles that comprise more pore space and reactive surface to increase the capacity to stabilize carbon. When the soil gets too old (e.g., highly-weathered soils are common in the tropics), however, many of the reactive minerals are gone and the soil loses quite a bit of its capacity to stabilize carbon, much like old people losing their ability to accrue muscles.
Finally, even for the same human, when we move from one place to another e.g., with different climate, body metabolism changes (how it changes depends on stress factors and homeostasis i.e., we tend to increase metabolism when we face stress in order to regain the internal “optimal” condition). Similar for soils, under a different temperature and precipitation (hence soil moisture content), the microbial decomposition rate of different fractions of organic C changes, and models need to capture this by relating the rate of these processes with temperature and moisture (environmental factors like climate are called “boundary conditions” in modeling terminology).
(Question for you: how would soil carbon change in a hotter climate?)
In summary, we have pretty much covered all the essential factors that affect soil C accrual: 1) the inputs — various dead organic matter from plants and animals; 2) the soil-specific parameters — inherent soil properties; 3) the model process parameters — turnover rate and partitioning parameters that control the fate of different types of organic matter; 4) the external boundary condition — e.g., climate. Hurray! Now you know exactly how to model soil carbon, as well as body weight.
At this point, I wonder, does it surprise you that we are in many ways quite similar to the mere dirt beneath our feet? Of course, there are some differences between our bodies and soils. For example, a human body is quite functionally-specialized and good at homeostasis, enabling our digestive system to have a rather stable activity rate, whereas a soil profile is quite functionally-diverse and plastic² due to their rich diversity of microbes, as well as being “stratified” in a sense (facing fairly different environmental conditions at different depths). Nonetheless, I hope this article gave you some food for thought about our similarity with soils. I also hope you now have a better idea of how soil scientists think and build models to predict how much C will be lost/gained from the soil, say in the next 50 years under climate change. You can probably imagine this is not easy. Depending on the model scope, it is like to predict the precise trajectory of body weight over time, sometimes of all 8 billion people.
Last but not least, you should realize that this type of model can extend to an unimaginably large number of “budget” systems, not just our body mass and soil carbon mass (Can you think of some?). So I am certainly looking forward to all of you being inspired to become the next modeler (not fashion model…), chat me up in the comment section if you have any questions!
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1. The values/quantities of these inputs can be prescribed by the model user or it can be “prognostic”. For example, litter is an “input” to the soil model, but the “output” of a plant model (if there is one available), the soil and plant models can be coupled and litter quantity (amount of food) is thus endogenously determined.
2. “Plasticity” in biology means the ability to adjust physiological processes to changing environment.
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