Carbon Cycle for Dummies
April 20, 2012 9:16 PM   Subscribe

Calling all soil scientists: Could someone explain the carbon cycle diagram on this page to me like I'm a three year old?

I'm looking for simple steps - eg. Then worms and bugs and bacteria poop out all the decomposed leaves they ate and that makes...proteins and polysaccharides. Would you believe I'm actually trying to write a song about this? And grade 11 chem/bio was so long ago...

Also: I presume this is the cycle as it occurs in a healthy crop - in basic terms what would get thrown off balance if farmers continue to plant the same crop over and over?
posted by stray to Science & Nature (9 answers total)
 
> in basic terms what would get thrown off balance if farmers continue to plant the same crop over and over?

It would deplete the soil, and each crop would be less successful until failure. This is why slash and burn subsistence agriculture is based on moving to a new plot every few years and letting the soil recover over many more years as unharvested vegetation and other organisms build it back up.

Of course if you are adding to the soil and fertilizing each year, you could get away with it a lot longer.

Note: Not A Soil Scientist, just a student.
posted by Listener at 9:43 PM on April 20, 2012


IANASoilScientist, but maybe starting with a simpler but bigger picture would help? Here's a diagram of the global carbon cycle, not just soil, which shows how carbon moves from the atmosphere into the ocean and land, then back again. Maybe it will make a little more sense when you see where your diagram fits into the bigger picture.

Without knowing what kind of song you're trying to write it's hard to pitch the information to the appropriate level, but here's my take on the diagram.

1. Plants grow and absorb CO2 and water.
2. Plants die and get eaten by insects, worms, bacteria, and fungi.
3. The solid waste products of these critters is called humus, which sounds better than bug poop.
4. Immobilization: humus has a nice spongy, absorbent texture that helps absorb rainwater and keeps soil from eroding or blowing away.
5. Humus is broken down further by micro-organisms, which can't use everything they find. The stuff they can't use (excess nitrogen, phosphorus and sulfur - N, P and S), they dump back into the soil. Voila - mineralization.
6. But wait, the micro-organisms are a heap of biomass themselves, and when they die their proteins and polysaccharides return to the soil. You've heard of protein, but you also know polysaccharides as "carbs" ... sounds a lot like food, huh?
7. Now you have nice nutritious topsoil and you can plant the next crop.

Please post your song when you're finished! In return, I'll leave you with The Biochemist's Songbook by Dr. Harold Baum of the University of London. You can listen to mp3 files here, or get the book and sing 'em yourself. The Krebs Cycle was never so much fun!
posted by Quietgal at 10:17 PM on April 20, 2012 [1 favorite]


Well, I'm sorry to say that's kind of a crap diagram... it's an oversimplification and not really what anyone calls a "Carbon Cycle" in a soil science class.

The vast majority of carbon broken down by decomposers is released back to the atmosphere, where the plants take it in via stomata in the leaf. That is the sort of interior cycle in the diagram. The rest is more complex, and isn't necessary cyclical: Plant roots put carbon into the soil as they shed plant cells, and the roots of plants and hyphae of fungi also produce the proteins and sugars. Plants also shed carbon by dropping leaves on the surface of the soil. That debris is broken down by decomposers, its final stable form being humus, basically a complex, colloidal mix of mostly insoluble humic acids. Humus can be stable for hundreds, if not thousands of years, and it is a major component of soil structure. It's not entirely certain how plants interact with humic acids, exactly, they may take them up, they may have a hormonal effect, or some other crazy chemistry. At any rate, there should probably be an arrow from humus to crop. Proteins and polysaccharides are also created during humification, so... yeah, I don't like this diagram. Immobilization is when carbon (or other nutrients) is converted into biomass, as in plants, or microorganisms in the soil. In any case, it is not available for plants to take in, hence "immobile". When the biomass is decomposed, those nutrients are released and become mobile- this is also referred to as mineralization. These inorganic nutrients are in a form that plants can take up, hence "mobile".
posted by oneirodynia at 10:37 PM on April 20, 2012


That is not a great diagram. I'm reasonably familiar with nutrient cycling (though IANASS) and I find it totally impenetrable. Quietgal's diagram is much better!

Here is the most basic version of the carbon cycle:
  1. photosynthetic organisms (e.g. plants) take carbon from the air (as CO2 and use it to build themselves.
  2. other organisms (e.g. herbivores) eat the plants, taking their carbon and using it as fuel.
  3. when organisms use that carbon for fuel, they breathe out the exhaust as CO2, releasing it back into the air.
  4. go to step 1.
There are lots of other important pathways which are shown in Quietgal's graphic, but that's sort of the classic process, the one taught in high school biology. There are other cycles for other essential biological elements such as oxygen, nitrogen, and phosphorous. They are intimately connected to but somewhat different from the carbon cycle. It's possible that you're being a bit thrown off by that, since it seems like you're looking at just the carbon aspect of things.
posted by Scientist at 10:41 PM on April 20, 2012


Response by poster: Ooo, thanks for the great clarifications all! In case it influences other responses, I'm particularly interested in decomposition as it affects the creation of topsoil, more so than the carbon cycle in general.

Thanks - you've all helped already!
posted by stray at 10:48 PM on April 20, 2012


Well I don't have a comprehensive understanding of that but ne interesting thing (to me) about topsoil formation and soil cycling is that a lot of the early research was done by none other than Charles Darwin!

At one point he got really interested in how soil formed and he had the idea that it might be from worms. So he engaged in a series of experiments with worms and long-term observations of the fields around his home and figured out that the soil was being built up from worm castings. Worms make their way through the soil underground, ingesting it as they go. At night, they surface and deposit the remains on the surface as castings (i.e. worm poop). These castings are super nutrient rich and eventually break down and become an important part of the topsoil. This movement of material from underground up to the surface by worms is also what causes things like boulders to gradually sink into the fields around them.

Now I'm not claiming that this is a totally comprehensive explanation of how topsoil is formed, but it's pretty neat and it comes to us courtesy of probably the greatest biological mind in history so I think it's super interesting.
posted by Scientist at 10:59 PM on April 20, 2012


in basic terms what would get thrown off balance if farmers continue to plant the same crop over and over?

Traditionally, as I understand it, the big problem was that the soil would be depleted in nitrogen compounds; not all plants can [or rather, have symbiotic root bacteria which can] fix atmospheric nitrogen into a biologically-useful form, and if your crop plant is one of the many that don't, the soil will run out of nitrates. Crop rotation has other benefits as well but if you're trying to recover a dim 11th-grade memory about crop rotation it's probably about nitrogen.

(In industrial agriculture the nitrogen comes from the Haber process or the like, instead of being fixed biologically. This is one of the ways modern agriculture is indirectly dependent on fossil fuels.)
posted by hattifattener at 12:43 AM on April 21, 2012


Others are mentioning nitrogen, which is generally the first limiting macronutrient and the primary reason we add manure. Look up the nitrogen cycle, eg in Wikipedia, and you will be closer to understanding what's happening with plant life. And check out rhizobium, a great word to sing and a common nitrogen-fixing organism - takes nitrogen out of the air and puts it in the soil. There are also micronutrients, so you can google both of those terms macro and micro.
posted by Listener at 7:13 AM on April 21, 2012


Best answer: That is not a great diagram. I'm reasonably familiar with nutrient cycling (though IANASS) and I find it totally impenetrable. Quietgal's diagram is much better!

Two different processes at two entirely different scales, so you can't really compare them.

I'm particularly interested in decomposition as it affects the creation of topsoil, more so than the carbon cycle in general.

What we call topsoil, in soil science would be the intersection of the O and A horizons (some people will say it is the A horizon only. The boundaries are not perfectly discrete, so I include the intersection with the O). O stands for the organic material on the surface, including the more recent, less decomposed and more identifiable organic material, and the more decomposed. Where it meets the A horizon is where organic material is incorporated into soil. The A horizon is the area where humus meets the weathered parent material of the rock underlying the horizons, or the deposited (weathered) material from somewhere else (an alluvial plain is formed when soil is washed down from other places, for example. These soils are highly fertile). The most biological activity occurs in the A horizon, so this soil is composed of minerals, organic material, and microorganisms. Keep in mind that the A horizon is highly variable, sometimes very shallow, and sometimes extremely deep. The processes that create this layer are weathering of parent rock, deposition, mineralization and immobilization of nutrients (through decomposition, that's what your original diagram was sort of showing), and plant processes (chemical and physical actions as roots actively take in nutrients and slough off cells, or die outright). Chemical processes in the soil along with precipitation, move clay particles and/or chelate and transport iron into lower soil horizons.

The decomposers in the soil consist of insects, bugs, works, grubs, fungi, bacteria, and other microorganisms. (There are soils where part of the formation and structure of the soil is highly modified by animals like prairie dogs;where they are disappearing the soil is changing, holding less water and becoming less fertile, and modifying the local ecology as well.) The larger organisms break down the initial organic material into smaller pieces that are more easiely attacked by other processes, and their excrement is also broken down. Fungi use enzymes to break down really difficult things like cellulose and liginin, and the end result is finally humus, with the other nutrients becoming mineralized (changed to inorganic forms, making them available for plant processes), taken up by plants (immobilized), returned through senescence of organic material to the soil surface or sloughed by roots, then consumed by decomposers (immobilized when the nutrients become part of their bodies, and mineralized when they are excreted, generally.) Carbon is excreted into the atmosphere by organisms that use oxygen (like us, and bugs and aerobic bacteria) and taken in by things that photosynthesize, like plants.

Humus is important because it helps soil particles aggregate (clump together), allowing for gaseous exchange with the atmosphere and water infiltration. When organic material is taken out of the cycle, such as in cropping systems in which organic material is removed, soil formation is changed. No more humus is formed, and if you combine that with mechanical destruction of soil structure you end up with compacted soils, and the cycle of immobilization/mineralization nearly ceases completely. Soil organisms go away, nutrients must be applied topically, and you set yourself up for dustbowl conditions as there's nothing to hold the soil together. At that point you no longer have an A horizon, you no longer have topsoil.

Anyway, I don't know if that's any more illuminating, because it's complicated. Hopefully there were some helpful bits!
posted by oneirodynia at 9:27 AM on April 21, 2012 [1 favorite]


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