Organic matter cycling in grassland soils of the Swiss Jura mountains: biodiversity and strategies of the living communities

Several characteristics of the soils of three nutrient-poor grasslands, pertaining to their biological activity, have been measured three times during the vegetation cycle of 1994 in the Swiss Jura. Properties measured were: abundances of the faunal groups Collembola (Insecta), Oribatida, Gamasida and Actinedida (Acarina); PLFA richness, abundance and diversity (indicating microbial diversity); ATP content (an index of soil biomass), soil respiration (CO2 production); and alkaline phosphatase, urease, chitinase, xylanase and laminarinase activities, as biochemical tracers of biotic activity. Most of these variables showed significant differences from site to site. In particular, the biochemical descriptors ATP content and phosphatase and urease activities varied in accordance with the amount of the clay-humic complex. The soils showed two strategies of litter degradation: (a) an enzymatic strategy, prevailing at the end of winter, when the edaphic climate is unfavourable; (b) a biotic strategy, based upon the work of the whole living community (fauna and microflora), at its maximum at the end of summer. Diversity of the microbial communities (as estimated by the PLFA richness) was highest in spring, when the carbon sources are complex (young litter), and subsequently decreased in inverse relationship with the specific activity of the biomass (viz. CO2 production-to-ATP ratio). This behaviour is attributed to an increase of the amount of simple organic compounds resulting from enzymatic activity and also to the activity of microphytophagous microarthropods.