Treatment effects on community structure of the soil fauna – Work package 3.2

The community structure of the soil fauna will be analyzed in order to detect seasonal and treatment-induced shifts in diversity, relative abundance of species and functional groups. Some faunal groups (primarily protozoans and nematodes) are feeding on roots, root exudates and bacteria and thus closely associated with a rapid turnover of organic C in the rhizosphere. Other functional groups such as collembola and mites are feeding on fungal hyphae and decaying plant material and part of a slow turnover of organic C. It is interesting to see if shifts in the dominance between these functional groups will occur under changing environmental conditions, or whether the soil fauna community is relatively resilient to such changes. These studies will include continuous measurements of various parameters suitable to investigate the effects of extreme events on community structures as well as effects of naturally occurring extreme climate events e.g. cold and heat stress, which may interact with the experimental stresses. For instance, warming may increase the vulnerability of species to frost damage and drought may increase the sensitivity to heat stress. We expect to find a high and different responsiveness to climatic changes among organisms and species and therefore we expect to discern community changes related to the climate manipulations already after three treatment years.

The community structure of the soil fauna will be analyzed through extraction of collembola, mites, enchytraeids, nematodes and protozoa from soil cores collected at each plot. The depth of soil cores will be determined on basis of the depth distribution of soil fauna measured in the pre-treatment profiling described above. Collembola and mites will be extracted by a modified MacFadyen dry funnel gradient extraction, enchytraeids by Bermann wet funnel technique and nematodes with a micro-version of the wet funnel technique followed by enumeration in a Doncaster counting dish. Collembola and enchytraeids will be determined to species whereas mites will be determined to higher taxonomic levels. Nematodes will successively be characterized to a sufficient taxonomic level to establish feeding groups (feeding on bacteria, fungi, omnivorous, carnivorous, plant roots). Protozoa will be cultivated in microtiter plates and enumerated with the Most Probable Number technique. This technique also allows us to estimate the dominating forms among the protozoa (flagellates or naked amoebae). Sampling campaigns in the first (pre-treatment) and second year will cover the main seasons spring, summer, autumn and winter, while a more frequent sampling throughout the year in the fourth year will be conducted to describe phenology more comprehensively.

The flow of N and C through the plant–microbe–fauna food web will be investigated at specific campaigns by determination of stable isotope profiles (¹⁵N and ¹³C) of selected species. This activity is coordinated with WP4.