avatarMarmotian

Summary

The article compares the process of soil carbon accrual to human weight gain, highlighting the similarities in input and output relationships and the importance of mathematical modeling in understanding these processes.

Abstract

The article discusses the conceptual similarities between soil carbon accrual and human weight gain, both of which are governed by input and output relationships of organic matter. The author explains that humans and soils are both "leaky vessels" that ingest food and litter, respectively, and that changes in soil carbon or body mass are determined by the input and output relationship of organic matter. The article also discusses the specifics governing the magnitude of gains and losses in both systems, including the quality of inputs, the characteristics of the vessels, and the propensity to gain weight or carbon over time. The author concludes by summarizing the essential factors that affect soil carbon accrual and highlighting the importance of mathematical modeling in predicting soil carbon loss or gain.

Opinions

  • The author believes that there is a vacant niche in effective science communication and aims to fill this gap by providing a concise and deep explanation of the relationship between soil carbon accrual and weight gain.
  • The author suggests that understanding the relationship between soil carbon accrual and weight gain can help better understand the fuss and buzz about "soil carbon sequestration" in climate change-related news.
  • The author emphasizes the importance of capturing the correct magnitude of gain and loss pathways in mathematical models to accurately predict soil carbon loss or gain.
  • The author highlights the importance of considering the quality of inputs, the characteristics of the vessels, and the propensity to gain weight or carbon over time in mathematical models.
  • The author suggests that mathematical models can extend to an unimaginably large number of "budget" systems, not just soil carbon mass and body weight.

Are We and Soil The Same? — A Modeling Perspective of Soil Carbon Accrual and Weight Gain

After all, we and Mother Nature come from the same life force.

Building soil carbon in a healthy way is similar to building muscles for a healthy body. (Image on the left came from Piqsels public domain, on the right came from Pxhere public domain)

Early on in my research endeavor, I really hate the fact that there are very few well-written “starter pack” that is concise and at the same time deep enough about a science subject. Scientific articles do contain good information but are very scattered i.e., you may find small pieces of information that are useful to you here and there. Long review papers are too cumbersome and frankly, too boring for a starter. So I often feel there is a vacant niche left hanging to be filled, to fill the gap of effective science communication.

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).

Essentially, humans are “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 in the 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 time, soil C accumulates. One can imagine if a piece of land is deforested i.e., depriving the soil of food to eat, it will always suffer a loss of C. Same when we die 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.

Us eating broccoli is like soil eating plant debris. The soil could theoretically eat the whole tree, but like us, it does not over-eat under normality. The organic material eaten is not stored permanently, some are inevitably lost from our metabolic activities. (Artwork by the author Marmotian)

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 is digested by soil microbes and animals for energy and transformed into different products (similarly 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: Given the climate emergency, should we dump tons of organic matter on 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 characteristics, 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 an evolutionary adaptation 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 soil microbes (≈ digestive cells and gut microbiota), sometimes making them unable or excel at breaking down certain types of organic matter. (FYI, 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 rock 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 climates, body metabolism changes. Similar for soils, under a different temperature and precipitation (hence soil moisture content), the 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 — 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 its 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 similarities 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 predicting 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 (but not fashion model…), chat me up in the comment section if you have any questions!

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Footnotes:

  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.

Check out how soils will “gladly eat” us too if we don’t behave:

Climate Change
Science
Sustainability
Education
Weight Loss
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