Does organic farming accumulate carbon in deeper soil profiles in the long term?

Organic farming systems provide the opportunity to deliver more soil ecosystem services than conventional practices. One such service could be soil organic C (SOC) accumulation, but recent debates suggest that this is unclear. Furthermore, organic farming potential to accumulate SOC for the entire soil profile is not well known. We quantified the cumulative SOC stocks, aggregate-associated SOC, and particulate organic matter (POM) concentrations for the 0-100 cm depth of the soil profile in a comparative crop rotation experiment in eastern Nebraska after > 20 yr of management. We studied: 1) conventional farming, 2) conventional farming with diversified rotation, 3) organic rotation with alfalfa (Medicago sativa L.) as green manure, and 4) organic rotation receiving 37 Mg ha− 1 yr− 1 cattle manure. All the treatments had been managed for 40 yr except the organic rotation with green manure, which had been in place for 20 yr. Organic farming increased SOC stock, aggregate-associated SOC, and POM concentrations but only in the 0-15 cm soil depth. The SOC stock under organic cattle manure system was 19% greater than under conventional farming (33.1 Mg ha− 1) and 13% greater than under conventional farming with diversified rotation (34.8 Mg ha− 1). The SOC stock under organic rotation with green manure (36.6 Mg ha− 1) was 10% greater than in conventional farming. Results suggest that organic cropping systems accumulated SOC at 0.16 Mg C ha− 1 yr− 1 with cattle manure and 0.18 Mg C ha− 1 yr− 1 with green manure. Growing alfalfa for only two years in the 4-yr organic rotation probably limited its potential to increase SOC for the whole soil profile. Overall, in the long term, organic farming increases SOC stock but only in the upper 15 cm of the soil profile.