Organic carbon and stable 13C isotope in conservation agriculture and conventional systems

Conservation agriculture might have the potential to increase soil organic C content compared to conventional tillage based systems. The present study quantified soil organic carbon (SOC) and soil C derived from C-3 (wheat) and C-4 (maize) plant species using delta C-13 stable isotope. Soil with 16 y of continuous application of zero tillage (ZT) or conventional tillage (CT), monoculture (M) or rotation (R) of wheat and maize, and with (+r) and without retention (-r) in the field of crop residues were studied in the central highlands of Mexico. The highest SOC content was found in the 0-5 cm layer under ZTM and ZTR with residues retention. The soil cultivated with maize showed a higher SOC content in the 0-10 cm layer with residue retention than without residue. In the 10-20 cm layer, the highest SOC content was found in the CT treatment with residue retention. The SOC stock expressed as equivalent soil mass was greatest in the 0-20 cm layer of the ZTM (wheat and maize) and ZTR cultivated treatments with residue retention. After 16 y, the highest content of soil delta C-13 Was found in ZTM + r and CTM + r treated soil cultivated with maize; -16.56 parts per thousand and -18.08 parts per thousand in the 0-5 cm layer, -18.41 parts per thousand and -18.02 parts per thousand in the 5-10 cm layer and -18.59 parts per thousand and -18.72 parts per thousand in the 10-20 cm layer respectively. All treatments had a higher percentages of C-C-3 (derived from wheat residues or the earlier forest) than C-C-4 (derived from maize residues). The highest percentages of C-C-4, was found in ZTM + r and CTM + r treated soil cultivated with maize, i.e. 33.0% and 13.0% in 0-5 cm layer, 9.1% and 14.3% in the 5-10 cm layer and 5.0% and 6.8% in 10-20 cm layer, respectively. The gross SOC turnover was lower in soil with residue retention than without residues. It was found that the ZT system with residue retention and rotation with wheat is a practice with a potential to retain organic carbon in soil. (c) 2009 Elsevier Ltd. All rights reserved.