Wednesday, 22 April 2009

Replanting the Rainforests: Carbon Sequestration in Soils

I've already talked about Biochar at length. It seems that there are more ways apart from Biochar to sequester carbon in the soils.

Here's simple set of data enough to make things clear:

• One hectare = 10,000 sq. metres
• Soil 33.5 cm deep (1 foot approx)
• Bulk density = 1.4 tonnes per cubic metre
• Soil mass per hectare = about 4,700 tonnes
• 1% change in soil organic matter = 47 tonnes
• Which gives about 27 tonnes Soil Carbon
• This captured 100 tonnes of atmospheric CO2

This is from a report by Soil Carbon. Have a look at it, it's pretty good. There's a plenty of information on this subject. But the first article argues that: 'As reasonable as a simple “1 % increase” may sound, it appears not to be scientifically valid.'

The FAO (Food and Agricultural Organization of the UN) estimates that the soils can contain twice as much carbon as the atmosphere. (The extensive report by the FAO can be found here.) It says (page 19):

Soils are the largest carbon reservoir of the terrestrial carbon cycle. The quantity of C stored in soils is highly significant; soils contain about three times more C than vegetation and twice as much as that which is present in the atmosphere(Batjes and Sombroek, 1997). Soils contain much more C (1 500 Pg of C to 1 m depth and 2 500 Pg of C to 2 m; 1 Pg = 1 gigatonne) than is contained in vegetation (650 Pg of C)and twice as much C as the atmosphere (750 Pg of C). Carbon storage in soils is the balance between the input of dead plant material (leaf and root litter) and losses from decomposition and mineralization processes (heterotrophic respiration). Under aerobic conditions, most of the C entering the soil is labile, and therefore respired back to the atmosphere through the process known as soil respiration or soil CO2 efflux (the result of root respiration – autotrophic respiration – and decomposition of organic matter – heterotrophic respiration). Generally, only 1percent of that entering the soil (55 Pg/year) accumulates in more stable fractions (0.4 Pg/year) with long mean residence times.

The report is 129 pages long. It's useless posting excerpts from every chapter. You had better read it. I will highlight some of the key points here.

As I was saying earlier, the FAO report doesn't exactly validate the statistics provided by Soil Carbon. The carbon contents of the soil vary with the condition of the soil, i.e. dry or wet. It says that carbon content ranges between 7 tons and 24 tons in normal (non-depleted) soils, depending on the climate zone and vegetation. If we make a few calculations we'll find that an increased carbon sequestration of 14 tons/hectare is the best possible value – much less than the value SC provides.

Soil Carbon reckons that ruminant grazing plays an essential part in restoring the soils, sequestering carbon and, of course, keeping an ecosystem healthy. But the FAO doesn't agree wholeheartedly with this either. It argues that while their role is important, ruminant creatures don't increase the net amount of carbon in the soils; they merely shift carbon from one piece of land to another. The FAO also highlights the fact that cattle release methane (a much more dangerous GHG than carbon dioxide) into the atmosphere.

But such disagreements between the FAO and SC are beside the point here. And that too, is just a difference of numbers. I think both the FAO and SC agree on one common environment-friendly point. We can (and we need to) sequester more carbon in soils. That can act as a weapon against GHGs and Global Warming. The farmlands will also be improved. To cut a long story short, it'll benefit us almost as much as Biochar.

That's it!

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