Carbon and nitrogen balance in beech roots under competitive pressure of soil-borne microorganisms induced by girdling, drought and glucose application

The goal of this work was to increase the understanding of factors regulating nitrogen (N) competition between roots and soil microbes. For this purpose, root assimilate supply was diminished or abolished in beech (Fagus sylvatica L.) seedlings by girdling, drought stress or a combination of both factors. This was revealed by ¹³C tracer abundance in root tips after ¹³CO₂ pulse labelling of the shoots. Analysis of different root tip fractions revealed that only 6% were ectomycorrhizal. Carbon (C) allocation to ectomycorrhizal and vital non-mycorrhizal root tips was ∼26% higher than to distorted root tips. Drought resulted in ∼30% increased ammonium (NH₄⁺) and amino acid concentrations in roots and ∼65% increased soil NH₄⁺ concentrations, probably because of lower consumption of NH₄⁺ by free-living microorganisms. Root uptake of glutamine of 13nmolg^-1 fresh mass h^-1 decreased 2-fold with drought, although the number of vital root tips did not decrease. Carbon content in biomass of free-living microbes increased with glucose application regardless of drought, resulting in significant depletion in soil nitrate (NO₃^-), root NH ₄⁺ and amino acid concentrations. Our results suggest that the rootsoil system of young beech trees was C-limited, and this prevented amino acid metabolism in roots and microbial NO₃^- consumption in the soil, thereby exerting feedback inhibition on uptake of inorganic N by roots. We suggest that rhizodeposition is a key link in regulating the plantmicrobial N balance.