Possible positive-feedback mechanisms: plants change abiotic soil parameters in wet calcareous dune slacks

In this paper the results are presented from a mesocosm study of the effects of typical dune slack plants on the soil solution nutrient contents. In dune slack succession, early successional species often show radial oxygen loss (ROL) whereas their successor species do not show ROL. ROL has impact on abiotic soil parameters and therefore, affect the competitiveness of both species. Mesocosms with Littorella uniflora and Carex nigra, used as respectively a ROL and a non-ROL species, showed remarkable differences in soil solution parameters. Special attention was given to nitrogen, as it is the limiting resource in dune slack succession. Mesocosms with L. uniflora showed a higher nitrate content in the soil than mesocosms with C. nigra and the control. Moreover, estimating the nitrogen balance, a significantly higher fraction of nitrogen was missing in L. uniflora (57%) than in C. nigra (5%). The enhanced nitrogen loss in mesocosms with L. uniflora could act as a positive-feedback mechanism for early successional stages that slows down the vegetation development in early stages of dune slack succession towards the more-productive later stages. The mechanism could even lead to alternative stable states in dune slack succession.