Tuesday, November 08, 2011

9t/ha/yr? True or False?

You know who your friends are when someone tells you what everyone else has been thinking. A friend who happens to be a highly regarded soil scientist recently raised an issue about an assertion in one of our press releases, ie. Col Seis's rate of increase of soil carbon at 9t/ha/yr, as follows:

"Using photosynthesis alone and growing plants alone this would be very, very difficult. Just to grow 9t/ha/yr of above ground biomass would be a good effort in many areas, let alone convert that to soil carbon. The only possibility is to bring in a carbon source from outside as a mulch or compost. Then it would probably require large additions of mulch or compost to get increases of this amount."

Our response:

Re 9t/ha/yr, you're right - incorporation of litter is not enough to explain this rate of soil carbon sequestration. As I said in the press release, the nature of carbon farming is such that multiple techniques of soil management are applied at the same time. In the case of Col Seis's well-studied soils, he has used/is using several techniques such as grazing management, pasture cropping, and compost teas. He has been managing his soils this way for 10 years. He doubled his soil carbon in 8 years then doubled it again in 2 years. We have other data from experienced carbon farmers - none as dramatic as 9t/ha/yr, but much of it significantly different to that measured by scientists. There are several possible explanations for this:

1. Science has yet to study the impact of multiple soil management practices.

2. Science has yet to study these combined techniques over the time period required for maximum response.

3. There are phototrophic and autotrophic bacteria that do not need organic matter to create energy, capable of photosynthesis.

4. Soil microbial activity is stimulated by practices that encourage root zone action, including exudates and nutrient fixing.

5. The combined effect of the techniques triggers a compounding or multiplier effect in the soil.

6. Soil carbon could have emergent properties which impact on sequestration rates. (Ie. properties that emerge as soil carbon levels increase – such as increased biodiversity in soil microbial communities – that can drive these increases faster and wider).

I suspect many non-farmers would favour one or other of the following:

7. Carbon farmers routinely misreport their soil carbon results.

8. Carbon farmers are not competent to take soil samples.

There is so much we don't know. We do know that the science is not in on the use of combinations of sequestration practices and won't be for some time. Meanwhile we must do what we can with what we have.




PS. We would not expect that rate of increase to continue. Some farmers have noticed that there would appear to be a tipping point at around 7 years when changes in soil management really kick in, at least with grazing management. Soil carbon levels appear to bounce up and down in an upward direction - like a basketball bouncing upstairs. Which is why a system of averaging over 5 years is a sound approach.

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