Résumé
Several long-term studies on tropical soils have shown that legume residue incorporation increases soil nitrogen (N) sequestration more than that of carbon (C), resulting in a fall in the C/N ratio. This study was designed to assess the short-term effect of legume litter addition on N supply and the long-term effect on soil organic matter (SOM) formation and soil C/N decrease. The long-term effect was evaluated in a 2-year mesocosm experiment with high and frequent organic inputs from two types of legume litter with different C/N ratios, using stable isotope techniques. The short-term effect of litter was analysed using four different litters in 3-month laboratory incubations. A model of litter decomposition was used to describe C and N kinetics in the laboratory experiment and to verify whether the long-term effect of litter may be predicted from short-term incubations. The results of the mesocosm experiment confirmed that legume inputs increased soil organic N (mean +21%) more than organic C (mean +15%) (P = 0.05). Although no differences between litters were observed for C dynamics, N sequestration (14% and 28%) and the final soil C/N (12.0 and 10.8) varied with litter C/N (34.4 and 16.1, respectively). The laboratory experiment and model outputs confirmed these findings and indicated that the higher N sequestration was due to a change in the parameters describing humification of C and N coming from litter. This change depended on litter quality and was greater for litters with low C/N-C humification 0.66 g C g(-1) C and N humification 0.76 g N g(-1) N for litter C/N 16.1. Carbon and N sequestration were greater in the laboratory experiment, due to a higher mineralisation of the new SOM derived from litter in the mesocosm experiment-32% and 15% for the mesocosm and the laboratory experiments, respectively. Our results indicated that the decrease in soil C/N and the rapid mineralisation of new SOM should be considered in models of litter decomposition to respond correctly to the long- and the short-term effects of legume litter inputs in tropical soils.