The soil life cycle : food webs and ecosystem services during soil transformations

Soil is one of the most important natural resource for life on Earth and provides important ecosystem services, such as food production, carbon sequestration, water regulation and contaminant attenuation. Soil quality, defined as the soil’s ability to provide these services, is drastically reduced in many locations and regions worldwide due to human activities. This loss in soil quality ultimately leads to soil degradation, erosion and desertification, imposing a severe and increasing risk for the growing human population. It is therefore essential that we are aware of the importance of protecting soil, and at the same time understand processes that build up and regenerate soil. The key objective of the present study was to obtain a better understanding of soil food web structure and functioning, and how these develop in stages along the soil life cycle. Using field surveys, I investigated the soil food web structure and functioning in different sites along the soil life cycle, including soils developing in glacial chronosequences, productive soils under different land use and management, and soils under risk of degradation. The soil food web was expected to build up in biomass and structure, be highest in the intermediate soils, and decrease in soils at or nearby degradation. This was indeed the case when comparing developing soils in the chronosequences, and comparing productive soils with degrading soils. But also land use type turned out to be very important for the structure of the soil food web. Biological measures such as biomass, activity and diversity of soil organisms, especially that of soil microarthropods, were found to be indicative for soil quality in all sites. I also investigated the possible role of soil organisms in the soil ecosystem functioning, in terms of soil structure formation and C and N mineralisation. Although soil organisms are known to have an important role on soil structure formation, no clear indications of such a role were found for that function in the studied sites. However, soil microbial biomass and activity, and the biomass of other trophic groups, did play a crucial role in soil ecosystem process rates, especially the C and N mineralisation rates. In conclusion, I have found that soil food webs assemble in a directive manner: organism biomass and activity increase with soil productivity. In productive soils, land use type and land management are the main drivers affecting soil food web structure and functioning, although this effect is limited to the topsoil. Under harsh conditions, soil organisms reach a relatively low biomass and are sensitive to aspects of intensive agricultural land use.