Storage and forms of organic carbon in a no-tillage under cover crops system on clayey Oxisol in dryland rice production (Cerrados, Brazil) Aure ´lie Metay a,c, * , Jose ´ Aloisio Alves Moreira b , Martial Bernoux c , Thomas Boyer c , Jean-Marie Douzet d , Brigitte Feigl e , Christian Feller c , Florent Maraux f , Robert Oliver d , Eric Scopel g a ISTOM (College of Advanced studies in International Agro-Development), 32, boulevard du Port, 95094 Cergy-Pontoise, France b EMBRAPA-CNPAF, Rodovia Goia ˆnia a Nova Veneza, km 12, Fazenda Capivara, C.P. 179, 75375-000 Santo Anto ˆnio de Goia ´s, GO, Brazil c IRD, UR041 Seq-C, Laboratoire MOST, BP 64501, 34394 Montpellier Cedex 5, France d CIRAD TA 40/01, Avenue Agropolis, 34398 Montpellier Cedex 5, France e CENA/USP, C.P. 96, 13400-970 Piracicaba, SP, Brazil f CIRADVisiting Scientific in FAO, B719-AGLW Section, Via delle terme di Caracalla, 00100 Roma, Italy g EMBRAPA, Cerrados Rodovia Brası ´lia Fortaleza, BR 020, Km 18, Planaltina, DF, CEP 73301-970, Brazil Received 31 August 2005; received in revised form 21 June 2006; accepted 16 July 2006 Abstract The management and enhancement of soil organic carbon (SOC) is very important for agriculture (fertility) as well as for the environment (carbon (C) sequestration). Consequently, changes in soil management may alter SOC content. No-tillage (NT) practices are potential ways to increase SOC. We studied the SOC from agricultural soils in the Cerrados in Central Brazil. We compared two different tillage systems: conservation agriculture with no-tillage under cover crops (NT) and disc tillage (DT) for 5 years in a context of rainfed rice production. The soil is a dark red Oxisol with high clay content (about 40%). The objectives of the study were: (i) to evaluate the short-term (5 years) impact of tillage systems on SOC stocks in an Oxisol and (ii) to better understand the dynamics of SOC in different fractions of this soil. We first studied the initial situation in 1998, and compared it to the 2003 situation. NT with cover crop (Crotalaria) was found to increase the storage of C in the topsoil layer (0–10 cm) compared to DT. The difference observed for the 0–10 cm layer under NT in comparison with DT represented C enrichment under no-tillage amounting to 0.35 Mg C ha À1 year À1 and corresponding to less than 10% of cover crops residues returned to the soil. A particle-size fractionation of soil organic matter (SOM) showed that differences in total SOC between NT and DT mainly affected the 0–2 mm fraction and, to a smaller extent the 2–20 mm fraction. This specific enrichment of SOC in the silt and clay fraction was attributed to (i) the storage of a water soluble C in the field and (ii) the effect of soil biota and especially fauna activity. The mean residence time of carbon associated with the fine fractions being rather long, it might be assumed that the preferential storage in fine fractions resulted in a long-term carbon storage. This study suggests a positive short-term effect of a no-tillage system on C sequestration in an Oxisol. # 2006 Elsevier B.V. All rights reserved. Keywords: Oxisol; No-tillage; Carbon sequestration; Particle-size fractionation of SOM; Cerrados; Brazil www.elsevier.com/locate/still Soil & Tillage Research 94 (2007) 122–132 * Correspondence at: ISTOM, 32, boulevard du Port, 95094 Cergy-Pontoise, France. Fax: +33 1 30756261. E-mail address: a.metay@istom.net (A. Metay). 0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2006.07.009