For climate change mitigation, there must be an additional transfer of carbon from the atmosphere to the soil, not just redistribution within the soil or landscape. In other words, there needs to be an increase in the absolute quantity of carbon.
Spreading organic manure, for example, while beneficial for the field it is spread upon, does not introduce new carbon. It merely redistributes carbon already sequestered from the atmosphere by the plants the animals have consumed.
Minimum tillage might increase carbon stocks in the top few inches of soil, but this may be at the expense of less carbon deeper down. This is an example of depth redistribution. We must also remember that the level of soil carbon does not increase indefinitely. At some point, it will reach equilibrium.
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Doing the right things
For example, farmers who have done all the right things for many years, such as introducing livestock and grassland to an arable farm, may have reached equilibrium.
No matter what such farmers do, they cannot increase soil carbon stocks. It seems unfair that they no longer qualify for any rewards unless they plough all the fields and start again.
Where you farm also affects how long it might take to build up soil carbon stocks. Many of our best soils are sandy loams. Sandy soils do not hold on to organic matter as clay soils do. When it comes to measuring carbon stocks, it is essential to measure not only the concentration of soil carbon but also soil bulk density. Measuring soil carbon stocks is important, especially with the development of monetary incentives.
With this in mind, let us consider the facts as presented by David Powlson. The main hurdle to overcome is how to measure any significant changes in soil carbon levels over time. Rothamsted has been measuring changes to soil organic matter since 1843.
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Two decade study
However, a more recent experiment spanned 22 years. Straw was either removed or returned on sites at Rothamsted and Woburn, representing clay and sandy soils respectively. On the clay soil, soil organic carbon (SOC) increased from 1.84% to 1.94%, and on the sandy soil, from 1.00% to 1.10% – an increase of 0.1% over 22 years. To put this into context, the margin of error when testing soils is 0.1%.
David suggested that trying to measure small increases in carbon was like using a battleship to measure the weight of the captain. One could, in theory, weigh the battleship with and without the captain on board, but in David’s words, ‘that would be silly’.
It takes a long time to be able to measure any significant increase in soil carbon stocks. Government-funded soil testing schemes, both here in Scotland and in England, do not measure carbon stocks. One suspects this may be because measuring soil bulk density is considered too hard.
But should we be using carbon stocks anyway? Powlson tends to agree with William H Schlesinger when he states that ‘Carbon sequestration in agricultural soils is not likely to offer any net storage of carbon that can be marketed as a credit to emissions from other sectors of the economy’.
Powlson proposed a different way to recognise all the good things farmers are doing. A better system would be to reward farmers who adopt practices that will help improve or retain organic matter and reduce GHG emissions.
Chopping straw
When Rothamsted looked at soil microbiological activity and soil structure after incorporating straw over an extended period, both were improved.
Microbes live on soil organic matter and release nitrogen, phosphorus and sulfur into the soil, as well as gluey substances that help bind soil particles together.
The net result is better water infiltration, less need for fertilisers, less leaching of nutrients, better soil structure, and more stable yields in drought or wet seasons. So, while the increases in soil organic matter may be very small, there are huge benefits to soil health.
Governments and carbon credit companies could recognise and reward actions farmers take to increase soil organic matter rather than trying to measure small increases in soil carbon or organic matter (carbon makes up about 50% of organic matter). Planting trees on grassland would result in no additional transfer of carbon from the atmosphere, but it would reduce food production. Policymakers should take the urgent need for food security into account. We need to ensure every hectare of agricultural land is devoted to sustainable food production.
End to set-a-side
Powlson told us the days of setting land aside from growing crops should be well and truly over. He wants his grandchildren, and grandchildren all over the world, to have enough food to eat.
There is one mitigation measure farmers should be adopting right now, and it’s a very simple message 'use less nitrogen'.
David would rather see a nitrogen policy than a carbon policy.
One tonne of nitrous oxide (N2O) is equivalent to almost 300 tonnes of CO2.
Soils can provide 50% of crop needs, and farmers can increase the amount of nitrogen soils provide by adopting land management practices to increase soil organic matter.
I would urge readers to join SSCR, as the £15 fee will not break the bank, and you can watch a recording of David Powlson on the SSCR website. David gave us all plenty of food for thought.
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