Decomposition of Medicago sativa debris incubated at different depths under Mediterranean climate

Reports on the dynamics of carbon and nitrogen at different depths in soil profiles usually describe a higher mineralization of both at upper horizons . Nevertheless, under Mediterranean climate, upper soil horizons are strong ly affected by drought, especially in summer, and may offer pedoclimatic co nditions less favorable to microbial activity than deep soil layers. Theref ore, decomposition could be slower in the upper soil layers. To test this h ypothesis, mixtures of Medicago sativa ground plants and soil material were incubated in the field, at 5, 20, and 40 cm depth, in nylon mesh bags. Min eralization of carbon and nitrogen was studied for two years. At 5 cm, mine ralization of both elements was lower, and no differences were found betwee n 20 and 40 cm. After two years of field incubation, the remaining carbon ( as a percentage of the initial content) was 27.95% +/- 0.88 at 5 cm depth, 19.87% +/- 0.77 at 20 cm, and 18.78% +/- 1.19 at 40 cm. Mineralization of n itrogen exceeded that of carbon. After two years of field incubation, the r emaining nitrogen (as a percentage of the initial content) was 17.62% +/- 3 .06 at 5 cm depth, 12.17% +/- 0.94 at 20 cm, and 11.26% +/- 0.99 at 40 cm. The biodegradation rate in upper layers was lower for all biochemical fract ions (water-soluble compounds, lipids, polysaccharides, lignin). This contr asts with the usual findings on this topic, but is consistent with our prev ious results with forest litter samples. Mineralization clearly followed do uble-exponential kinetics, with a labile and a recalcitrant pool of both ca rbon and nitrogen. The labile pool of carbon accounted for about 50% of the total initial carbon, whereas that of nitrogen accounted for about 60%. No clear effect of depth on the proportion between the labile and the recalci trant pool was observed, neither for carbon nor for nitrogen. It was not po ssible to identify both pools with the biochemical fractions, suggesting th at these pools should be interpreted in physical terms (unprotected vs. pro tected). No direct effect of depth on the global retention of N was detecte d.