The quantity of carbon (C) held in organic matter in the world’s soils to a depth of 1m is estimated at about 1,500 Gt, about twice that currently in atmospheric CO2, and thus significant in the global C cycle and in influencing climate change. The ‘4 per mille’ initiative, launched at the Paris Climate Conference in 2015, proposed that current annual CO2 emissions from fossil fuel burning could be completely offset if the global stock of soil organic carbon (SOC) was increased annually at the rate of 4 parts per 1000 (4‰ or 0.4%) of the current value.
At Rothamsted Research we used SOC data from our long-term experiments, providing 114 treatment comparisons on three soil types over 7 to 157 years, to assess the rate at which SOC could be increased. Treatments included organic additions (manure, straw, cover crops),
nitrogen (N) fertilisers, introducing pasture leys into continuous arable systems, and converting arable land to woodland. With drastic changes to soil management or land use, the annual 4‰ rate of SOC increase was met or exceeded, in some cases over many years. But within management practices more easily adopted in practice it is very difficult to reach this rate of increase. Practices that were very effective at sequestering C included (a) removing land from arable cropping and converting to woodland and (b) changing from continuous arable cropping to a ley-arable rotation comprising eight years pasture followed by two years of arable crops. However, both changes would have major impacts on food security and farmers’ incomes. Manure applications caused large SOC increases but there are major practical and environmental constraints to applying the large rates (35 t fresh material ha-1 annually) used in the longest running experiments. It is also questionable whether manure applications represent genuine climate mitigation as opposed to relocation of organic C; though manure delivers numerous soil quality benefits. Changing to zero tillage and the wider use of cover crops are practical measures that offer potential to increase SOC. However, we consider that the 4‰ annual rate of SOC increase is unachievable in most agricultural situations and cannot be regarded as a major contributor to climate change mitigation. It is more realistic to promote practices for increasing SOC because they improve soil quality and functioning, especially as small increases can have disproportionately large beneficial impacts, though not necessarily translating into increased crop yield. Improved management of nitrogen in agriculture, decreasing direct and indirect nitrous oxide emissions, is probably a more effective means of mitigating climate change in agriculture.