Increasing the organic matter content of soil benefits both crop growth and climate change. About 50% of this so-called soil organic matter is made of carbon. Hence, the entire global soil organic matter stock contains about twice the amount of carbon as atmospheric carbon dioxide.
Unfortunately, clearing natural vegetation to establish agriculture has led to losses of soil organic matter with much carbon emitted to the atmosphere as carbon dioxide. This adds to the impact of climate change caused by fossil fuel burning.
In the US Midwest, it is estimated that 40-60% of initial soil organic carbon has been lost. In principle, climate change could be slowed if some of the carbon lost from soils was restored through changes in land management practices.
How much additional organic carbon can be sequestered in agricultural soils?
Global estimates of the quantity of carbon that could be sequestered through changed cropping practices vary from 1% to 35% of current fossil fuel emissions The new paper by Córdova et al. sets out to measure soil carbon sequestration actually achieved from management changes during a 25-year experiment in the US Midwest on a loamy soil with high content of sand and silt.
The study provides realistic values for soil carbon sequestration, at least in one environment. Based mainly on a well-designed replicated experiment, the authors measure soil bulk density in the treatments; this is essential if total carbon stocks are to be calculated but is often not done.
The study followed “conventional” management using chisel ploughing and locally recommended applications of fertilizers and crop protection chemicals in a rotation of maize-soybean-winter wheat. Throughout 25 years, under “conventional” management, soil organic carbon remained unchanged at around 40 Mg carbon ha-1 to a depth of 100 cm.
Presumably the soil organic carbon decline since the start of cropping in the mid-1800s had come to an end as the soil reached a new equilibrium value: this trend is seen in other long-term studies such as the 182-year-old Broadbalk Experiment at Rothamsted, UK. Compared to the “conventional” treatment under no-till soil, organic carbon had increased slightly at an average rate of 0.2 Mg carbon ha-1 yr-1 in the 0-100 cm depth, mainly in the 0-10 cm layer. In contrast to other studies, soil organic carbon did not change in the soil just below the former plough layer under no-tillage.
In systems which included two cover crops in the 3-year rotation prior to spring sowing of maize or soybean, soil organic carbon increased at 0.4-0.5 Mg carbon ha-1 yr-1. This was despite reduced fertilizer inputs in these treatments which presumably led to lower yields compared to “conventional” and no-tillage and, thus, decreased organic inputs from roots and crop residues.
Soil organic carbon sequestration rate slows over time
In systems in which soil organic carbon was increasing due to management change compared to “conventional”, the annual rate of increase declined over time. For example, under no-tillage the annual rate of carbon accretion was 1.3 Mg carbon ha-1 yr-1 during the first 12 years. Yet, when averaged over the whole 25 years, it was only 0.2 Mg carbon ha-1 yr-1.
This “levelling off” of soil organic carbon accrual is nothing new and needs to be remembered. Some companies (and even over-enthusiastic scientists!) promoting payments for soil carbon sequestration only quote the rapid increases during the first few years while implying that these rates will continue indefinitely.
Soil organic carbon changes in perennial crops
The authors also measured soil organic carbon changes under perennial plants including alfalfa, poplar grown for bioenergy or conversion of cropland to trees. Soil organic carbon increases under these were mostly greater than any treatment changes to row crops. From a soil carbon viewpoint, including perennials in crop rotations would be beneficial.
But, again, the rate of sequestration declined over time, as seen in other studies. For example, under poplars with an 8-10-year harvest cycle, annual carbon sequestration rate was 1.7 Mg carbon ha-1 yr-1 during the first 12 years but only 0.4 Mg carbon ha-1 yr-1 averaged over 25 years.
How much can increasing soil organic carbon offset climate change?
The increases in soil organic carbon measured in these treatments, compared to “conventional”, will almost certainly be beneficial for soil health by providing improved conditions for crop roots to grow – though this was not included in the study. Yet, the measured rates of soil organic carbon increase in predominantly arable systems indicate that the contribution to slowing climate change through offsetting carbon emissions from fossil fuels is small.
