HATCHING OF ENCHYTRAEIDAE (OLIGOCHAETA) FROM EGG COCOONS IN AGRICULTURAL SOIL EXPOSED TO DIFFERENT DROUGHT REGIMES - A LABORATORY STUDY

The hypotheses tested in this investigation were: 1.) Enchytraeidae eg g cocoons are present in agricultural soils in high numbers. 2.) Egg c ocoons survive dry periods to a certain extent. 3.) Drought susceptibi lity differs between species or genera. Soil samples were taken in win ter from a verge along an agricultural field near Uppsala in Sweden. T he enchytraeids were extracted from the soil with wet funnels, whereaf ter the soil samples were incubated in the laboratory at 20 degrees C for 10 weeks under different moisture regimes. It was assumed that all individuals present at the end or the experiment had been recruited f rom ''the egg-cocoon bank''. The seven treatments were: 10 weeks with 60 % moisture (field capacity) and 1, 2, or 9 weeks at 20 % or 10 % mo isture, respectively (simulating different levels of summer drought), followed by 60 % moisture for up to totally 10 weeks. The abundance of enchytraeids in the field (0-5 cm depth) was 6.41 ind. per sample (2 300 ind, m(-2)), After incubation there was an average of 90 ind. per sample in the wettest treatment, which corresponds to 32 000 eggs m(-2 ) in the sampled soil, whereas only 9 ind. per sample were found in th e driest one. Intermediate numbers were present in the intermediate tr eatments. Ten genera of Enchytraidae were found: Enchytraeus, Frideric ia and Henlea were present in all treatments; Buchholzia and Mesenchyt raeus were lacking in the driest; Enchytronia and Hemifridericia were found only in the wettest. The variation between individual samples wa s great. We conclude that the soil contained a substantially high dens ity of enchytraeid eggs and that the eggs are negatively affected by d rought. There are indications that the survival of egg cocoons differe d between, genera. Compared with other investigations the extraction e fficiency was reasonably good; thus most of the individuals found shou ld have originated from egg cocoons that hatched during incubation. Fu rther investigations of egg production, survival and longevity are nec essary to better understand the population dynamics of enchytraeids an d other soil animals.