| Abstract: | 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 strongly affected by drought, especially in summer, and may offer pedoclimatic conditions less favorable to microbial activity than deep soil layers. Therefore, decomposition could be slower in the upper soil layers. To test this hypothesis, 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. Mineralization of carbon and nitrogen was studied for two years. At 5 cm, mineralization of both elements was lower, and no differences were found between 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 nitrogen exceeded that of carbon. After two years of field incubation, the remaining 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 fractions (water-soluble compounds, lipids, polysaccharides, lignin). This contrasts with the usual findings on this topic, but is consistent with our previous results with forest litter samples. Mineralization clearly followed double-exponential kinetics, with a labile and a recalcitrant pool of both carbon 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 recalcitrant pool was observed, neither for carbon nor for nitrogen. It was not possible to identify both pools with the biochemical fractions, suggesting that these pools should be interpreted in physical terms (unprotected vs. protected ). No direct effect of depth on the global retention of N was detected. |
