Kinetics of soil microbial uptake of free amino acids |
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Authors: | L C Vinolas J R Healey D L Jones |
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Institution: | (1) School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK e-mail: d.jones@bangor.ac.uk Tel.: +44-1248-382579 Fax: +44-1248-354997, GB |
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Abstract: | Amino acids and proteins typically form the biggest input of organic-N into most soils and provide a readily available source
of C and N for soil microorganisms. Amino acids can also be taken up directly by plant roots, providing an alternative source
of available soil N. However, the degree to which plants can compete against the soil microbial population for amino acids
in soil solution remains poorly understood. The aim of this study was to measure the rate of microbial uptake of three contrastingly
charged 14C-labelled amino acids (glutamate1–, glycine0, lysine0.9+) over a wide concentration range (0.1–5 mM) and in two contrastingly managed soils varying in their degree of erosion, organic-C
content and microbial biomass. Amino acid uptake was concentration dependent and conformed to a single Michaelis-Menten equation.
The mean maximum amino acid uptake rate (V
max) for the non-eroded (control) soil (high organic-C, high biomass) was 0.13±0.02 mmol kg–1 h–1, while half maximal uptake occurred at a concentration (K
m) of 2.63±0.07 mM. Typically, V
max was fourfold lower and K
m twofold lower in the eroded soil (low available organic-C, low biomass) compared to the non-eroded (control) soil. Amino
acid substrate concentration had little effect on the proportion of amino acid utilized in catabolic versus anabolic metabolism
and was similar for both. While the results obtained here represent the summation of kinetics for a mixed soil population,
they indicate that amino acid uptake is saturated at concentrations within the millimolar range. Because the affinity constants
also were similar to those described for plant roots, we hypothesized that competition for amino acids between plants and
microbes will be strong in soil but highly dependent upon the spatial distribution of roots and microbes in soil.
Received: 2 March 2000 |
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Keywords: | Amino acids Uptake kinetics Microorganisms Root-microbe competition Dissolved organic nitrogen |
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