Organic matter turnover in a calcareous clay soil from Syria under a two-course cereal rotation

Total organic C and microbial biomass C were measured in soils from a two-course rotation experiment at the International Center for Agricultural Research in Dry Areas (ICARDA), Syria. Six rotations were sampled: wheat-vetch, wheat-lentil, wheat-wheat, wheat-chickpea, wheat-fallow and wheat-medic. Ten years after the experiment started in 1983-1984, soil samples (0-20 cm) from the wheat-vetch, wheat-lentil, wheat-wheat and wheat-chickpea rotations all contained similar quantities of organic C and microbial biomass C. The soil under the wheat-medic rotation had gained organic C and biomass C, compared with the wheat-wheat 'rotation', whereas both organic C and biomass C had fallen in the wheat-fallow 'rotation'? although of all these differences, only the wheat-medic gain was significant (p = 0.05). With one exception. there were no measurable differences in either soil organic C or biomass C between treatments in which crop residues were grazed in situ or ungrazed. Radiocarbon was measured in two soil samples from the wheat-wheat 'rotation': the mean radiocarbon age was 550 years. An updated version (ROTHC-26.5) of the Rothamsted model for the turnover of organic matter in soil was used to simulate these measurements. Using ROTHC-26.5, the calculated annual inputs of plant C (in roots, stubble, root exudates, etc.) to the soil, averaged over the 2 years of each rotation, were: wheat-vetch, 0.87, wheat-lentil, 0.71, wheat-wheat, 0.71, wheat-chickpea, 0.75, wheat-fallow, 0.45, wheat-medic, 1.20 t C ha(-1) year(-1). The modelled turnover time of soil organic C (excluding inert organic C) in the wheat-wheat rotation was 19.2 years. The response to a change in management is slow in these Syrian soils: ROTHC-26.5 predicts that it will take 67 years for the soil under the wheat-medic rotation to move half-way to its final equilibrium value. Microbial biomass measurements were simulated to within experimental error, (C) 1999 Elsevier Science Ltd. All rights reserved.