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

The evolution of organic matter is studied in the soils of the humid centra l 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 year s of cropping: wheat monoculture (Triticum aestivum L.), maize monoculture (Zea mays L.), wheat/soybean (Glycine max L.), wheat/soybean-maize and whea t/soybean -sunflower (Helianthus annuus L.). At Pergamino, a soybean monocu lture followed directly after ploughing in the original pasture. Measuremen ts of the variation in the C-13 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 Olivero s, the soil C contents fell markedly during the first 5 years of the experi ment (a loss of about 9 %) and then seemed to have stabilised for all the r otations. 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, plo ughing 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. ((C) Inra/Elsevier, Paris.)