Field mesocosms for assessing biotic processes in soils: How to avoid sideeffects

Field mesocosms can overcome the simplicity and deficiencies of laboratory based experimental designs. This study deals with a number of possible side effects of a mesocosm technique that involves deep-freezing of soil monoli ths to eliminate soil Fauna, wrapping in nets of various mesh-size to contr ol faunal immigration and replanting in the field. We used Berlese-Tullgren sets in the field to directly inoculate mesocosms with microarthropods. Af ter 6 months of exposure, the number of collembolans equalled control level whereas immigration and inoculation of oribatids accounted for only 30 % o f the control. The number of ciliates, their distribution into heeding grou ps, and the numbers of nematodes, tardigrades and rotifers were not signifi cantly affected by the elimination of mesofauna. We also did not detect sig nificant treatment specific effects on microclimatic conditions within the litter layer of the mesocosms. Furthermore, we compared the monolith approa ch with a technique using sieved soil as a time-saving alternative. Water c apacity and infiltration rate of mesocosms made of sieved soil did not diff er from mesocosms made of monoliths, hut NH4+ losses were significantly hig her in sieved soil when defaunated by deep-freezing. We conclude that the i nvestigated mesocosm technique has little side effects and recommend the us e of monoliths in mesocosm studies. (C) 2000 Editions scientifiques et medi cales Elsevier SAS.