What happens to the carbon in rotting grasses?

  • #1
Ivan Seeking
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The subject of cow flatulence and methane causes me to wonder what happens to the carbon in the grass that grows and dies naturally. When grass rots, how much of the carbon is converted to soil thus trapping the carbon, and how much carbon is released as CO2 or methane? I assume that this depends on whether the rotting process involves aerobic or anaerobic bacteria, but is there any way to guess at typical numbers?

I realize that deforestation goes hand in hand with the issue of cattle in many areas, but it seems to me that cows are otherwise getting a bad rap. Since grass grows to replace the grass that has been eaten, thus absorbing as a minimum the equivalent in carbon release by the cow, isn't a free-range cow carbon neutral at worst?

Edit: I should say cow burps - eructation. It seems that the flatulence bit is also a little unfair to our friends of the bovine persuasion. :biggrin:
 
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  • #2
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I'm thinking the best place to start looking is in statistics on methane digesters.

Dry grass doesn't rot, and I'm wondering if all rotting of grass is essentially anaerobic. It may have to be saturated in moisture, which would exclude air, to rot.

I once experimented with raising worms to see how easy or difficult it was. I collected them in the back yard at night during the rainy season and they always seemed to have their heads poked into the blackened, rotting grass that was plastered to the ground beneath the live standing grass. I assume they were eating it, and I suspect, therefore, alot of dead grass is incorporated into the soil as worm droppings.
 
  • #3
Ouabache
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zoobyshoe said:
Dry grass doesn't rot, and I'm wondering if all rotting of grass is essentially anaerobic. It may have to be saturated in moisture, which would exclude air, to rot.

Think about composting which is an aerobic process. Typically you might add any non-woody vegetation to a compost pile. (from the kitchen; vegetable and fruit scraps, even coffee grinds and egg shells, but no meat or dairy). Grass qualifies as compostable vegetation.

The reference I gave, describe the dominant organisms that decompose vegetation, delineated by temperature range (in degr F)..

Aerobic Decomposition
Below 42° F is biological zero -- little to no active bacterial processing takes place
From 42° to 67°, you get psychrophilic (moldering) processing actinomycetes and (other) fungi).
From 68° to 112°, mesophilic bacteria are dominant.
From 113° to 160° thermophilic bacteria take over.

http://www.michigan.gov/mda/0,1607,7-125-1566_1733_22582_22592-68858--,00.html (including nightcrawlers) do play an roll in aerobic decompostion
 
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  • #4
Ouabache
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Moisture definately plays a role in whether decomposition is aerobic or anaerobic. If too wet, it can be adjusted by adding dry leaves. Not only are the leaves dryer, it also has higher C:N (carbon to nitrogen ratio) and helps promote aerobic decomposition.

If rotting grass is not kept at the proper moisture level (such as too wet) it will smell like ammonia. With its high nitrogen content, part of it is converted to NH3.
 
  • #5
jim mcnamara
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If grass did not decompose, the world would be thousands of feet deep in grass "carcases" -

If you look at soils, you'll find the top layer (especially in temperate climes) has a leaf litter layer, then under that what is called duff. The duff layer, tiny particles of leaves, is chock full of small invertebrate organisms, plus fungi, bacteria and friends, that chop up the organic matter into smaller and smaller bits. A lot of the fixed carbon is lost to respiration during this process.

So, the penetration of fixed carbon from the duff layer into soils is small because there isn't much leftover, and it doesn't have anything other than percolation or surficial insect activity to embed the remaining carbon down in the soil.

In bog conditions, arctic conditions and wherever else those little guys don't live there is no chewing action. Resident fixed carbon levels in these soils are high, eg., permafrost can be 10% carbon by dry weight.

And.

The reason the chernozem soils (dark prairie soils) are dark is because a large percent of grasses biomass is underground in rhizomes - underground stems. When the plant dies, the carbon slowly is broken down, in the absence of the the duff-layer chewers, into carbon-rich compounds.
 

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