Dairy NZ

The short answer to the above question is NO and I will take you through the reasons why that is the case.

The story so far

We have all heard the argument that methane becomes carbon dioxide which will become grass which the cow eats and that carbon goes round and round with no net increase in the atmosphere. It is further argued that the methane a cow emits does not add to the methane in the atmosphere, it simply replaces it. Some also argue that carbon stored in pasture, products and soil offset or balance methane emissions from grazing animals. Thus there is a strong belief in some circles that farms are at the least carbon neutral. In discussing these issues it is important that we understand what happens to the carbon from the time it is fixed into grass by photosynthesis to the time that it is released back into the atmosphere through decomposition and both plant and animal respiration.

What about all the carbon fixed by pasture

Plants absorb carbon dioxide from the atmosphere during photosynthesis, but approximately three quarters of this is released back into the atmosphere through plant respiration and decomposition of plant and root material.  The remaining carbon in plant material is consumed by the animal and provides the energy needed for its maintenance and milk/meat production with a small amount being converted to methane. The carbon in faeces and urine is also eventually released back to the atmosphere as carbon dioxide. So rather than being stored in the pasture, carbon is cycled around from the atmosphere, into the plant, and back to the atmosphere in a continuous process. The residence time of this carbon in the soil is not long enough for it to be considered a permanent carbon sink under current rules.

As an example, current research suggests that a typical New Zealand grazed pasture fixes approximately 20 tonnes of carbon* per hectare (20 t C/ha/y) from the atmosphere of which 10 t C/ha/y is returned by respiration leaving a net balance of 10 t C/ha/y (Figure 1). Of the net 10 tonnes of carbon removed from the atmosphere (i.e. photosynthesis minus respiration) about 5 tonnes is retained in plant shoots that are consumed by grazing animals and the other half is retained in plant roots and in plant litter deposited on the soil surface. Of the 5 tonnes eaten by the animal, 2.7 t C/ha/y is returned back to the atmosphere as CO2 through animal respiration while a small amount is incorporated into animal products such as milk and meat. About 1.6 t C/ha/y is excreted onto soils in the form of dung and urine. Another 0.2 t of the carbon consumed by the animal is converted to and released to the atmosphere as methane, a gas with a global warming potential of more than twenty times that of carbon dioxide over a 100 year timeframe.

What about all the carbon fixed by pasture and stored in the soil?

Soils contain more carbon than vegetation and the atmosphere combined. It is estimated that in a hectare of grazed dairy pasture, soil typically contains 80-100 t/carbon in the upper 30 cm. However, soils tend to remain in a steady equilibrium unless significant changes such as deforestation or cultivation take place. This means that, on average, under current management practices the quantity of carbon stored in New Zealand pastoral soils is constant with no net carbon gain or loss over time. Since there is no net increase in carbon storage in soils, it is unlikely that soils can actively sequester enough carbon to offset methane emissions from grazing animals.

So what can we conclude from all this?

As can be seen from the above discussion, carbon goes through a series of cycles through pastoral systems, and that ruminant animals do not add any new carbon to the atmosphere. But that is really unimportant because ruminants convert carbon dioxide to methane, the greenhouse gas that is more than twenty times as powerful as carbon dioxide. Thus farming ruminants tend to increase the proportion of carbon in the methane form rather than in the carbon dioxide form.

Having considered the pastoral system as a whole, we can conclude that the carbon fixed by pasture is not enough to compensate for the losses through plant respiration, decomposition of biomass and the conversion of carbon dioxide into methane. 

Footnote: *the carbon is fixed as carbon dioxide but for simplicity this is referred to as carbon.

For more information please email: climatechange@dairynz.co.nz