Is pastoral agriculture carbon neutral?

You might think that because all the carbon in cattle farming comes from the grass that the cattle is fed on, then the beef produced should be carbon neutral. Although the carbon is recycled, some of it is recycled as methane (CH4) which is a more powerful greenhouse than carbon dioxide (CO2). Not all carbon compounds have the same effect on the atmosphere.

Photosynthesis is the process plants use to take sunlight, water and carbon dioxide, and convert it into plant matter.

In the photosynthesis that occurs in the growth of pastures used for grazing livestock, about 1% of the converted CO2 is belched back into the atmosphere as CH4 by cattle during digestion. As a greenhouse gas, CH4 is 25 times more potent than CO2 (see Global Warming Potential). Pastoral farming is not carbon neutral due to the amount of CH4 cattle emit into the atmosphere. Pastoral farming's carbon footprint is increased further by the presence of nitrous oxide (N2O), which is produced in soil from the nitrogen deposited as animal excrement and fertiliser.

The following diagram shows that the CO2 flows from the atmosphere to the soil and is respired back again in its original form. A small amount of CO2 is converted as CH4 and belched out. 

Diagram showing the movement of carbon in pastoral agriculture. Carbon flows reflect a near-steady state (units:t(C)/ha/yr). Flows to or from the atmosphere are CO2 except where indicated. Source: Tony Parsons, AgResearch, based on Parsons & Chapman (2000).