The aim of this study was to understand the effects of lime and gypsum on nitrogen and carbon turnover of the soil. A pot experiment was conducted in parallel with a field experiment which was set up in 1989 in a declining forest of the French Ardennes. A dystric cambisol, associated with a moder and mull humus separately, was used to study changes in the soil chemistry as a result of added lime and gypsum top-dressing. The lime was applied to the surface of an acid mull humus of an oak (Quercus petraea) stand and of a moder humus of a spruce (Picea abies) stand. A quantity of 2.8 t ha(-1) equivalent CaO was supplied as CaCO3, CaCO3+MgO and CaSO4.2H(2)O. The experiment was installed in an open-air nursery for 20 months, during which the organic carbon and nitrogen in the solution were analysed monthly. They were analysed in the solid phase after 20 months. At the end of this period the changes in the soil and leachate depended mainly on the type of the material added. The leachate was enriched with nitrogen from the third month of the experiment under lime treatments and in the control. The same pattern was found under the two humus types but the magnitude was higher in soil with a mull humus. The nitrogen was mostly leached as NO3--N in the carbonate treatments and in the control, whereas it was predominantly NH4+-N under gypsum. The NO3-N was 50% higher than NH4+-N in the control and CaCO3, CaCO3+MgO treatments. In the CaSO4 treatment this phenomenon was reversed. The leaching of organic carbon was greater under gypsum than under the other treatments whatever the humus. In the solid phase of the soil (organic layers) the organic carbon and nitrogen concentration decreased significantly after liming, especially in the mull humus. Consequently it induced a decrease in C:N ratio of about 18% with respect to the control.
