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

Abstract

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 ¹⁵N-labelling studies that investigated ¹⁵N redistribution between roots and microorganisms. Calculated Michaelis–Menten 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