Competition for amino acids between wheat roots and rhizosphere microorganisms and the role of amino acids in plant N acquisition

The direct uptake of organic nitrogen compounds from the soil solution by plant roots has been hypothesised to constitute a significant source of N to the plant particularly in N limiting ecosystems. The experiments undertaken here were designed to test whether wheat roots could out-compete the rhizosphere microflora for a pulse addition of organic N in the form of three contrasting amino acids, namely lysine. glycine and glutamate. Amino acids were added at a concentration reflecting reported soil solution concentrations (100 muM) and the uptake into either plant biomass or respiration or microbial biomass and respiration determined over a 24 h chase period. The results showed that the plant roots could only capture on average 6%, of the added amino acid with the remainder captured by the microbial biomass. We therefore present direct in vivo evidence to support earlier work which has hypothesised that organic N may be of only limited consequence in high input agricultural systems. We suggest that this is a result of the higher concentrations of NO3- in agricultural soil solutions, the slow movement of amino acids in soil relative to NO3- the rapid turnover of amino acids by soil microorganisms, and the poor competitive ability of plant roots to capture amino acids from the soil solution. (C) 2001 Elsevier Science Ltd. All rights reserved.