Competition between roots and microorganisms for nitrogen: mechanisms and ecological relevance

Summary Demand of all living organisms on the same nutrients forms the basis for interspecific competition between plants and microorganisms in soils. This competition is especially strong in the rhizosphere. To evaluate competitive and mutualistic interactions between plants and microorganisms and to analyse ecological consequences of these interactions, we analysed 424 data pairs from 41 15 N‐labelling studies that investigated 15 N redistribution between roots and microorganisms. Calculated M ichaelis– M enten kinetics based on K m (Michaelis constant) and V max (maximum uptake capacity) values from 77 studies on the uptake of nitrate, ammonia, and amino acids by roots and microorganisms clearly showed that, shortly after nitrogen ( N ) mobilization from soil organic matter and litter, microorganisms take up most N . Lower K m values of microorganisms suggest that they are especially efficient at low N concentrations, but can also acquire more N at higher N concentrations ( V max ) compared with roots. Because of the unidirectional flow of nutrients from soil to roots, plants are the winners for N acquisition in the long run. Therefore, despite strong competition between roots and microorganisms for N , a temporal niche differentiation reflecting their generation times leads to mutualistic relationships in the rhizosphere. This temporal niche differentiation is highly relevant ecologically because it: protects ecosystems from N losses by leaching during periods of slow or no root uptake; continuously provides roots with available N according to plant demand; and contributes to the evolutionary development of mutualistic interactions between roots and microorganisms. Contents Summary 656 I. Introduction 657 II. General solution: trade of C for nutrients 657 III. Methods 658 IV. Interactions between roots and rhizosphere microorganisms 659 V. Potential of plants and microorganisms for N uptake by their competition 661 VI. Ecological relevance of competition between roots and microorganisms for N 662 VII. Effect of biotic and abiotic factors on N flows between microorganisms and roots 663 VIII. Conclusions and outlook 665 Acknowledgements 666 References 666