PLANT-BACTERIAL AND PLANT-FUNGAL COMPETITION FOR NITROGEN AND PHOSPHORUS

Abstract: Competition between and within species has been one of the most studied concepts in ecology. In this thesis the aim was to test the idea of plant-microbial competition for N and, by studying this interaction, to gain a better understanding of different N processes in soil. The research carried out seeks to investigate plant-microbial competition for N based on the definition that competition only can occur when plants and soil microorganisms are mutually limited by N and use the same sources of N. Papers I to V are attempts to test the operational parts of the definition under the influence of different soil variables. It was found that, in an acidified deciduous forest soil with relatively high N mineralization rates, plants and fungi were both P limited, while the bacteria were C limited. The results also suggested that plants competed with fungi for P in this soil. The soil C:N ratio was rejected as an important factor in determining microbial N immobilization or plant-microbial competition for N, but the quality of the soil C and the soil P availability were. Furthermore, plants from deciduous forest soils with relatively high N mineralization rates have the ability to take up amino acids, such as glycine and glutamate, both in hydroponic experiments and in soil. Soil drying and rewetting affected the proportions of the N forms that were taken up by the plants and microorganisms ? plants shifted from an ammonium to an amino acid dominated uptake, while the opposite pattern was found for the microorganisms ? which may reduce the probability of competition.