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–fentil, wheat–heat 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 `rotationʹ, whereas both organic C and biomass C had fallen in the wheat–fallow `rotationʹ, 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 `rotationʹ: 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.