Fate of microbial residues during litter decomposition as affected by minerals

Minerals may protect organic matter against microbial decay, either by direct chemical and physical interactions or by inhibitory effects on the soil microbial community. To clarify the effects of minerals on organic matter cycling by microorganisms, we used amino sugars as tracers for C and N in dead microbial cells after 0, 15, 29, 90, 239, and 498 days of incubation of beech leaf litter mixed with quartz sand, Fe oxide, Al hydroxide, or Mn oxide. Beech leaf litter without addition of any mineral phases was used as the control. The results show that amino sugar concentrations increased as litter decomposition proceeded. Decreasing ratios of glucosamine to muramic, acid and of glucosamine to galactosamine indicated that bacterial products accumulated increasingly relative to fungal cells with increasing incubation time. As the presence of Mn oxide promoted losses of plant-derived C, there was a more pronounced selective accumulation of the microbial-derived amino sugar C than in the other treatments. Aluminium hydroxide and Fe oxide inhibited synthesis of bacterial amino sugars by a factor of two. This resulted in lower amino sugar C proportions. compared with the other treatments. Consequently, the amino sugar C proportions were sensitive to both increasing amino sugar synthesis and C mineralization rates. In contrast, the amino sugar N proportions were not affected by any mineral additions. Thus, the mere presence of minerals did not affect the cycling of N through the, amino sugar pool, but minerals. altered the relative proportions of N sequestered within residues of bacteria and fungi.