Agronomie 19 (1999) 349-364
DOI: 10.1051/agro:19990503

Evolution of soil carbon with various cropping sequences on the rolling pampas. Determination of carbon origin using variations in natural ¹³C abundance

Adrian Andriulo^a, Jérôme Guérif^b and Bruno Mary<sup>b

a</sup>  INTA, Estación Experimental de Pergamino, CC 31(2700), Pergamino, Argentina
^b  Inra, unité d'agronomie de Laon-Péronne, rue Fernand Christ, 02007 Laon cedex, France

Abstract - The evolution of organic matter is studied in the soils of the humid central Argentinean pampas, which are extensively cultivated, without the use of fertilisers. The organic carbon contents of the soils were recorded on two sites for 13 years. At Oliveros, five rotations were compared after 60 years of cropping: wheat monoculture (Triticum aestivum L.), maize monoculture (Zea mays L.), wheat/soybean (Glycine max L.), wheat/soybean-maize and wheat/soybean-sunflower (Helianthus annuus L.). At Pergamino, a soybean monoculture followed directly after ploughing in the original pasture. Measurements of the variation in the ¹³C isotopic composition were made to determine the proportions of 'young' C (derived from crops) and 'old' C (present at the start of the experiments) in the total reserve of organic C. At Oliveros, the soil C contents fell markedly during the first 5 years of the experiment (a loss of about 9 %) and then seemed to have stabilised for all the rotations. The amount of young C formed by the end of 13 years varied from 3.9 to 9.9 Mg ha^-1, i.e. 10-17 % of the returned carbon. At Pergamino, ploughing the pasture caused a large fall in the organic carbon (28 %) during the 13 years of soybean monoculture. The amount of old C fell rapidly, its renewal time being 24 years. (© Inra/Elsevier, Paris.)

Résumé - Évolution des stocks de carbone des sols de la pampa argentine. Détermination de l'origine du carbone en utilisant les variations d'abondance naturelle ¹³C. L'évolution des matières organiques est étudiée dans les sols de la Pampa centrale humide argentine, cultivés de façon extensive, sans apport d'engrais. Les stocks de carbone organique des sols ont été suivis sur deux sites pendant 13 années. A Oliveros, cinq rotations sont comparées après 60 ans de mise en culture : monoculture de blé (Triticum aestivum L.), monoculture de maïs (Zea mais L.), rotation blé/soja (Glycine

Key words: soil carbon / ¹³C abundance / rolling pampas / cropping / rotations

Mots clés : carbone du sol / abondance de ¹³C / pampa ondulée / mise en culture / rotations.