Changes in soil organic matter over 70 years in continuous arable and ley–arable rotations on a sandy loam soil in <scp>E</scp> ngland

The sequestration in soil of organic carbon ( SOC ) derived from atmospheric carbon dioxide ( CO 2 ) by replacing arable crops with leys, has been measured over 70 years on a sandy loam soil. The experiment was designed initially to test the effect of leys on the yields of arable crops. A 3‐year grazed grass with clover (grass + clover) ley in a 5‐year rotation with arable crops increased percentage organic carbon (% OC ) in the top 25 cm of the soil from 0.98 to 1.23 in 28 years, but with little further increase during the next 40 years with all‐grass leys given fertilizer nitrogen ( N ). In this second period, OC inputs were balanced by losses, suggesting that about 1.3% OC might be near the equilibrium content for this rotation. Including 3‐year lucerne ( M edicago sativa ) leys had little effect on % OC over 28 years, but after changing to grass + clover leys, % OC increased to 1.24 during the next 40 years. Eight‐year leys (all grass with N or grass + clover) in 10‐year rotations with arable crops were started in the 1970s, and after three rotations % OC had increased to ca. 1.40 in 2000–2009. Over 70 years, % OC declined from 0.98 to 0.94 in an all‐arable rotation with mainly cereals and to 0.82 with more root crops. Applications of 38 t ha −1 farmyard manure ( FYM ) every fifth year increased % OC by 0.13% by the mid‐1960s when applications ceased. Soil treated with FYM still contained 0.10% more OC in 2000–2009. Changes in the amount of OC have been modelled with RothC ‐26.3 and estimated inputs of C for selected rotations. Little of the OC input during the 70 years has been retained; most was retained in the grazed ley rotation, but 9 t ha −1 only of a total input of 189 t ha −1 . In other rotations more than 98% of the total OC input was lost. Despite large losses of C , annual increases in OC of 4‰ are possible on this soil type with the inclusion of grass or grass + clover leys or the application of FYM , but only for a limited period. Such increases in SOC might help to limit increases in atmospheric CO 2 . Highlights Can leys sequester significant amounts of atmospheric CO 2 in SOM and contribute to the 4‰ initiative? Changes in the percentage and amount of OC were measured and modelled over 70 years and OC losses estimated. Three‐year grass or grass + clover leys increased % OC , but only to an equilibrium level that was then maintained. Despite large losses, sequestering CO 2 ‐ C at 4‰ year −1 by growing grass or grass + clover leys is possible.