“The first incorrect assumption is that carbon dioxide (CO2) breathed out by livestock adds to the flow of carbon dioxide in to the atmosphere. This is wrong because if livestock didn’t eat the carbon from the plants and breathe it out other organisms including insects, earthworms and soil bacteria would do so anyway. About 85% of carbon absorbed by plants returns naturally to the atmosphere due to respiration by other organisms and more is lost due to fires and other disturbances to the land.” (David Steele raises similar objections.)
The statement about earthworms and bacteria is correct, but isn’t the devastating refutation that it initially appears. We need to look at the total oxidation effect of livestock. Suppose you have a pasture with no cows on it. The plants die and the soil bacteria send 85% of their carbon that the cows would have eaten in the form of grass back into the atmosphere. Then, you add cows that eat the grass and breathe. Have you increased the total oxidation, or does it remain the same?
Walsh’s objection implies that if the cows eat the grass, the soil bacteria, bereft of organic carbon from dying grass, will just stop oxidizing (or do it proportionately less). But there is no shortage of organic soil carbon — in fact, it’s several times the amount of carbon in the atmosphere. I suspect the worms and bacteria would just find something else to munch on, so the cows would add to the net oxidation process.
The real question is, does livestock respiration displace something else that was already doing the oxidation, or are they an additional source of net oxidation? (This objection is analogous to a similar one, that livestock displace wildlife.)
I didn’t do an extensive analysis, but I did take a look at Natural Grasslands (edited by Robert Coupland, Elsevier, 1992), which contains a few references to this question. In some grasslands that were studied, grazing did not cause significant changes in mycofloral populations (Blanka Úlehová, p. 114), or actually resulted in an increase in the number of abundant species (W. K. Lauenroth and D. G. Milchunas, p. 212). There is apparently no effect, or possibly a positive effect, of livestock on the biomass of soil organisms in the soil of these natural grasslands.
To refute LCC you’d need to show that the livestock displace other oxidizers in the soil, offsetting some or all of the effects of their respiration, or perhaps that they stimulate plant growth which increases carbon sequestration. Actually, very light grazing might have some effects in this direction, but the overall deleterious effects of grazing on vegetation is well documented. And at the other end, turning the land into a desert might really destroy soil bacteria and worms (although it would also destroy all the plants as well).
We can argue, I suppose, about where the burden of proof lies. But prima facie it would seem that cattle overall do not have any obvious effect in either depressing the soil oxidation or increasing carbon sequestration that would be there in the absence of cattle. So I think that the estimate of cattle respiration is the relatively best estimate of what this net carbon oxidation is. This doesn’t mean that LCC is necessarily right, but that this is where science is right now.
Eventually, if livestock agriculture gets totally out of hand, the soil will be depleted of carbon and the plants, livestock and humans will all die; but because there’s so much carbon in the soil, we’ll have runaway global warming long before that happens. Thus, livestock are critical to the climate change issue.