EFFECTS ON PLANT-PRODUCTION AFTER ADDITION OF LABILE CARBON TO ARCTICALPINE SOILS/

Biomass production was analysed in Festuca vivipara, grown for 3 month s in pots with non-sterilized or sterilized soil after factorial addit ion of three levels of labile carbon combined with high and low levels of N and P. The soil was a nutrient-poor subarctic heath soil. In the non-sterilized soil plant biomass production increased strongly only in the treatment with high levels of both N and P, which suggests that both nutrients limited plant growth. In the sterilized soil addition of a high level of N without P addition gave almost the same growth re sponse as in the combined NP treatment. This was because of a more tha n 30-fold increase of inorganic phosphorus in the soil as P was releas ed from the killed microbial biomass after sterilization. Sugar additi on reduced plant growth in all treatments. The reduction in plant grow th was dose dependent within the range of 0-450 mu g C g(-1) soil adde d to the non-sterilized soil, but the response levelled off at 233 mu g C g(-1) soil in the soil that had been sterilized at the start of th e experiment. The plant response, together with observed depletion of soil inorganic N and P, indicated that the microbial biomass immobiliz ed nutrients efficiently and reduced plant growth when extra labile ca rbon was added. The inhibition of growth was lower, however, in the so il which had been sterilized, probably because of a slow recovery of t he microbial populations in it. Two of the nutrient-carbon solutions c losely matched the N, P and C concentrations in a solution containing leaf extracts of Cassiope tetragona and Betula tortuosa that had been used previously to test for possible allelopathic effects of compounds in the leaf extracts. These extracts also reduced plant growth. The g rowth reduction was equally large or larger after nutrient-sugar addit ion than after addition of leaf extracts in three out of the four poss ible combinations of species and sterilized or non-sterilized soil. In the fourth case (Betula extract added to sterilized soil), the effect was larger when leaf extract was added than after addition of the nut rient-carbon solution. This could be due to a low rate of microbial de gradation of phytotoxic substances in this soil because of a slow reco very of the microbial populations after sterilization. The generally s tronger or equal effect of the nutrient-sugar addition compared to the leaf extract addition leads to the conclusion that microbial nutrient immobilization and microbial competition for nutrients increased as a function of labile carbon addition with the extract. Hence, it appear s that enhanced microbial activity and microbial nutrient immobilizati on rather than phytotoxic effects was the primary reasons for the redu ced biomass production in F. vivipara even after addition of the leaf extracts.