INHIBITION OF GROWTH, AND EFFECTS ON NUTRIENT-UPTAKE OF ARCTIC GRAMINOIDS BY LEAF EXTRACTS - ALLELOPATHY OR RESOURCE COMPETITION BETWEEN PLANTS AND MICROBES

Previous research has shown that plant extracts, e.g. from boreal dwar f shrubs and trees, can cause reduced growth of neighbouring plants: a n effect known as allelopathy. To examine whether arctic and subarctic plants could also be affected by leaching of phytochemicals, we added extracts from the commonly occurring arctic dwarf shrubs Cassiope tet ragona and Empetrum hermaphroditum, and from mountain birch, Betula pu bescens ssp. tortuosa to three graminoid species, Carex bigelowii, Fes tuca vivipara and Luzula arcuata, grown in previously sterilized or no n-sterilized arctic soils. The graminoids in non-sterilized soil grew more slowly than those in sterilized soil. Excised roots of the plants in non-sterilized soil had higher uptake rate of labelled P than thos e in sterilized soil, demonstrating larger nutrient deficiency. The di fference in growth rate was probably caused by higher nutrient availab ility for plants in soils in which the microbial biomass was killed af ter soil sterilization. The dwarf shrub extracts contained low amounts of inorganic N and P and medium high amounts of carbohydrates. Betula extracts contained somewhat higher levels of N and much higher levels of P and carbohydrates. Addition of leaf extracts to the strongly nut rient limited graminoids in non-sterilized soil tended to reduce growt h, whereas in the less nutrient Limited sterilized soil it caused stro ng growth decline. Furthermore, the N and P uptake by excised roots of plants grown in both types of soil was high if extracts from the dwar f shrubs (with low P and N concentrations) had been added, whereas the P uptake declined but the N uptake increased after addition of the P- rich Betula extract. In contrast to the adverse extract effects on pla nts, soil microbial respiration and soil fungal biomass (ergosterol) w as generally stimulated, most strongly after addition of the Betula ex tract. Although we cannot exclude the possibility that the reduced pla nt growth and the concomitant stimulation of microbial activity were c aused by phytochemicals, we believe that this was more likely due to l abile carbon in the extracts which stimulated microbial biomass and ac tivity. As a result microbial uptake increased, thereby depleting the plant available pool of N and P, or, for the P-rich Betula extract, de pleting soil in organic N alone, to the extent of reducing plant growt h. This chain of events is supported by the negative correlation betwe en plant growth and sugar content in the three added extracts, and the positive correlation between mi crobial activity, fungal biomass prod uction and sugar content, and are known reactions when labile carbon i s added to nutrient deficient soils.