Soil Biology & Biochemistry 38 (2006) 2753–2761 Soil organic matter dynamics along a rice chronosequence in north-eastern Argentina: Evidence from natural 13 C abundance and particle size fractionation Thierry Desjardins a,Ã , Patricia J. Folgarait b , Anne Pando-Bahuon c , Cyril Girardin d , Patrick Lavelle c a Institut de Recherche pour le De´veloppement (IRD), UMR137/Universidade Federal Rural da Amazoˆnia (UFRA), CP 917, 66077-530 Bele´m (PA), Brazil b Unidad de Interacciones Biolo´gicas, Centro de Estudios e Investigaciones, Universidad Nacional de Quilmes, Roque Saenz Pen˜a 180, B1876BXD, Bernal, Buenos Aires, Argentina c Institut de Recherche pour le De´veloppement (IRD), UMR 137, 32 rue Henri Varagnat, 93143, Bondy, France d Laboratoire de Bioge´ochimie et Ecologie des Milieux Continentaux, Centre INRA, INA-PG, BP1, 78850 Thiverval-Grignon, France Received 15 June 2005; received in revised form 7 April 2006; accepted 11 April 2006 Available online 15 May 2006 Abstract We studied the consequences of rice cultivation and its subsequent abandonment for soil organic matter (SOM) dynamics in north- eastern Argentina. Two chronosequences, which included a pristine grassland with C4 vegetation as a control, and several stages of rice (C3) fields abandoned for 1, 2, 4, 6 and 15 years were selected, and soil samples from the first 10 cm were gathered from each plot. Natural 13 C abundance coupled with particle-size fractionation were employed to characterize SOM changes through time discriminated by SOM origin. Soil samples up to 50 cm were also collected throughout one chronosequence. Most changes in SOM occurred on the first 20 cm layer and, bulk density, carbon and nitrogen content, as well as d 13 C remained similar at greater depths. After the rice cropping, the bulk density was slightly greater than in the natural grassland, and remained stable after the abandonment. Carbon and nitrogen contents remained almost stable in the surface layer during the cultivation. d 13 C varied accordingly with the changes in vegetation cover with a C4 signature in the natural grassland and mainly a C3 signature in the rice fields. The abandonment of the rice cropping induced a decrease of the soil organic matter content, mainly of natural grassland origin, during the first 4 years. When the abandonment extended, the SOM content (from C4 origin) increased slowly and after 15 years, was almost the same as that of the natural grassland. The carbon turnover was greater in the coarser fractions than in the finer ones, confirming that soil organic carbon in the sand fraction was relatively labile. However, all the fractions were affected by inputs and outputs of C derived from rice and natural grassland. This fact could indicate that the former protected carbon could become less stable due to cultivation. r 2006 Elsevier Ltd. All rights reserved. Keywords: Argentina; Natural grassland; Rice fields; Soil organic matter; Stable carbon isotope; Turnover 1. Introduction Soil organic matter (SOM) plays an important role for soil fertility because it influences many soil properties which are vital for soil functioning (Feller and Beare, 1997). It is a source of energy for the soil biota and simultaneously a source and a sink for nutrient elements; it has charge properties which make it a site of ion exchanges, and its chemical and physical properties promote aggrega- tion with mineral particles, governs the structure stability and influence soil water regimes (Swift and Woomer, 1993). The level of organic matter in soil is determined by the equilibrium between the factors that determine its forma- tion and those which promote its breakdown (Greenland and Nye, 1959). Among those factors, the influence of the abiotic conditions of the soil as well as the presence of key ARTICLE IN PRESS www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2006.04.029 Ã Corresponding author. Tel.: +55 91 32 74 19 42; fax: +55 91 32 72 19 42. E-mail address: thierry.desjardins@ufra.edu.br (T. Desjardins).