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Mineralisation processes and Nitrogen Retention

Because heterotrophic bacteria have much lower C/N ratios than detritic plant material, its mineralisation may create a bacterial demand for mineral N. This possibility was studied in microcosm experiments; two different scenarios of primary producer mineralisation were studied: i.e., Zostera marina roots as representative of rooted phanerogam communities, and Ulva lactuca thalli as typical for eutrophied coastal lagoons. The enrichment of the sediment with root material from Zostera marina as well as with Ulva lactuca thalli stimulated the oxidation of organic carbon and bacterial growth as compared to the unamended sediment (Lomstein et al. a,b, Extended Report, Vol. II, p. 371, p. 379). In contrast to the surface addition of Ulva lactuca, which stimulated the fluxes of phosphorus and ammonia from the sediment surface to the water column (Lomstein et al.a, Extended Report, Vol. II, p. 379), Zostera marina root addition into the sediment increased the retention of ammonium compared to carbon. Hence, it was confirmed that additions of Zostera marina roots to the sediment resulted in increased bacterial ammonium assimilation rates thus making inorganic nitrogen less available (at least temporarily) for plant growth. Hence, the effect is a reduction of the effect of external N-loading (Lomstein et al.b, Extended Report, Vol. II, p. 379).

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Université Bordeaux 1
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