Kan in een kalkrijke natte duinvallei de vegetatiesuccessie worden geremd door een microbiële mat?

In wet dune slacks a pioneer situation can exist for a long time. One of the causes for this slow-down of the succession could be the existence of a microbial mat. The upper layer of the mat, consisting of algae and cyanobacteria, develops a tough layer due to excretion of extracellular polymeric substances. Moreover the CO2 fixation and sulfate reduction cause the pH to raise and therefore calcium carbonate is precipitating which strenghtens the top layer still more. Due to respiration of heterotrophic bacteria the oxygen concentration in the soil is diminishing. The redox potential will decrease and sulfate will be used as an electronacceptor. It is then reduced to toxic sulfide. It is assumed that pioneer species can cope better with these circumstances than later species appearing in the vegetation when the microbial mat is absent. This investigation consists of two experiments. The first one examines if growth of Carex nigra, a later species, and Littorella unWora, a pioneer species is affected by a microbial mat. For this purpose plants of these species were grown in the glasshouse in mesocosms filled with sand with a microbial mat and without one on top. The soil was kept moist with artificial ground water. It appears as expected from literature that Littorella uniflora is leaking oxygen by the roots in the soil, a process called Radial Oxygen Loss. Although this has a positive effect on soil oxygen concentration and redox potential there is no evidence found that Littorella could grow better in microbial mats than Carex nigra. Probably this is due to the experimental design which was not reducing enough. The mineral composition of the water supply to the mesocosms changed considerably due to the influence of a pump. Oxygen caused a nitrifying process in which the ammonia in the water changed to nitrate. Instead of sulfate reducing probably nitrate was reduced in the mat which caused the redox potential to stay relatively high.The second experiment investigated the dependence of survival of seedlings on the presence of a microbial mat. Seedling of pioneer species and later successional species were planted with there roots in tiny holes in the microbial mat while others were just put on the surface. During 15 weeks the survival of these 8 species was counted and compared to the survival in pots with only sand. Parnassia palustris appeared to be affected by the microbial mats in both treatments. Samolus valerandi was affected stronger if the seedling were put just on the surface as Centaurium littorale, Juncus alpino-articulatus and Pedicularis palustris which were only affected in this situation. Seedlings of Agrostis stolonifera, Carex oederi and Calamagrostis epigejos could establish themselves well in all situations. S. valerandi possibly caused toxic sulfide to disappear in the microbial mat by Radial Oxygen Loss but generally in the pots with microbial mats sulfide was present. In this experiment there is no clear distinction between later and earlier successional species but more experiments are needed to clarify the role of the life history of the plants.