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Will soil organic matter save the planet?

Limitations of Soil Carbon Credits

I have previously commented on the limited scope for addressing climate change through soil carbon sequestration, and carbon credits/payments in particular. This Statement from WWF supported by several environmental and research organisations sets out the reasons why Carbon Dioxide emissions certificates and payments are not appropriate as a means of offsetting emissions.

The fact is that carbon is highly mobile in the soil, with a high rate of turn over, dependant partly on management and is a natural part of the nutrient cycle involved in growing crops. Mineralisation of organic matter and release of carbon dioxide and nitrogen is partly, but not solely a consequence of cultivations. The situation is further complicated by the lack of effective means of monitoring soil carbon change over time. The widespread claims by some farmers of doubling soil organic matter levels after a couple of years of cover crops or some other management change are clearly nonsense and point to inaccurate sampling and monitoring procedures. Or wishful thinking. Managing, maintaining and in some circumstances increasing soil organic matter is of course an entirely good thing for all sorts of reasons but the current unholy rush to pay farmers for carbon sequestration to address climate change is misplaced and should be challenged. It will lead to some very red faces and funders will question where their money has gone, particularly in those inevitable cases where SOM has declined. Carbon sequestration and soil cultivation I have also commented on the use of the plough versus non inversion tillage, and I have remained a supporter of the shallow plough, despite some optimistic claims made for higher carbon sequestration from Zero or Non Inversion Tillage. In these cases the soil sampling methods have often proved to be poor and in particular sampling to inadequate depth. I have visited a number of trials where the claimed advantages of Min or Zero tillage have not been substantiated, but one should never rely in the results of a single trial. Recently another trial conducted on several sites has also shown limited carbon sequestration advantages from shallow and or zero tillage. In this case measured to a more reliable 60 cm. “ Variable impacts of reduced and zero tillage on soil carbon storage across 4–10 years of UK field experiments Overall there is limited benefit in using shallow minimum tillage and zero tillage practices in the UK to increase soil carbon storage when a soil profile of 60 cm is considered but other benefits associated to these systems, such as speed of working and timeliness of operations, should be considered.” Another report recent report comes up with similar conclusions: “ Modest capacity of no-till farming to offset emissions over 21st century Environmental Research Letters (2021): These results indicate that the global potential for SOC sequestration from NT adoption may be more limited than reported in some studies and promoted by policymakers “ The potential for increasing soil carbon is highly dependent on soil type and is probably more influenced by additions of soil carbon in the form of green manures, leys, manures and compost than by the cultivation method.

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