Changing soils : belowground food webs during secondary succession

Abstract

Reclaiming arable land is a method to increase and connect nature areas in the Netherlands. A frequent applied method is land abandonment of sandy soils to promote secondary succession towards a heathland. In practice this development takes place very slowly. The rate of transition from an agricultural to a natural vegetation depends on above-and belowground processes and the interactions between the above- and belowground compartments. Changes in vegetation composition will influence the quantity and quality of roots and soil organic matter (SOM) that form the basic resources of the soil food web. In turn, the soil food web may affect the development of the vegetation directly through root herbivores and indirectly by mineralisation, thereby providing nutrients for plant uptake. In this thesis changes in the soil food web during secondary succession after land abandonment were analysed. Biomasses of soil organisms and C and N mineralisation rates were measured in three ex-arable fields, taken out of production 2, 9 and 22 years prior to measurements, and in a reference heathland that has not been in production. To analyse changes in soil food web structure, I defined trophic levels, i.e. groups of organisms that gain their energy from the same adjacent trophic level, and I defined energy channels, i.e. groups of organisms gaining food originating from the same primary energy source. Changes in soil food web structure were analysed using the measurements. The role of the soil food web in mineralising inorganic nutrients such as carbon and nitrogen and the stability of the web was analysed using both measurements and a mathematical model. The biomass of the bacterial and fungal energy channel both increased. Also the biomass at the two lowest trophic levels increased, while the biomass at the two highest trophic levels did not increase. Both bacteria and fungi contributed equally to carbon mineralisation, while fungi contributed most to nitrogen mineralisation. Soil fauna at higher trophic levels had minor contributions to carbon mineralisation, while they had significant contributions to nitrogen mineralisation. The stability of the soil food web was very high in all fields. It was found that changes in soil food web structure and functioning were very slow and seem to follow the changes in vegetation. I did not find strong evidence of an indirect facilitating role by the soil food web supplying the plants with nutrients, as the mineralisation rates increased and the vegetation changed towards a vegetation with plant species that preferably grow in nutrient poor conditions. It was concluded that although the chosen methodology is open for improvement, the study provided a reasonable picture of changes in soil food web structure and functioning