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

• Published in: Geoderma 2017-02, Vol.288, p.213-221
• Main Authors: Blanco-Canqui, Humberto, Francis, Charles A., Galusha, Tomie D.
• Format: Article
• Language: English
• Subjects: Cattle manure; Conventional farming; Diversified rotation; Green manure; Organic farming; Particulate organic matter; Soil organic C
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2016.10.031

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 0–100cm depth of the soil profile in a comparative crop rotation experiment in eastern Nebraska after >20yr 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 37Mgha−1yr−1 cattle manure. All the treatments had been managed for 40yr except the organic rotation with green manure, which had been in place for 20yr. Organic farming increased SOC stock, aggregate-associated SOC, and POM concentrations but only in the 0–15cm soil depth. The SOC stock under organic cattle manure system was 19% greater than under conventional farming (33.1Mgha−1) and 13% greater than under conventional farming with diversified rotation (34.8Mgha−1). The SOC stock under organic rotation with green manure (36.6Mgha−1) was 10% greater than in conventional farming. Results suggest that organic cropping systems accumulated SOC at 0.16MgCha−1yr−1 with cattle manure and 0.18MgCha−1yr−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 15cm of the soil profile. •Organic farming increased soil C pool only in the upper 15cm depth after >20yr.•Organic rotations with cattle and green manure increased soil C pool by 10% to 15%.•Organic farming increases near-surface particulate organic matter concentration.