Amino acid biodegradation and its potential effects on organic nitrogen capture by plants

It has been reported that plant roots can directly utilise soil organic-N in the form of amino acids without prior mineralisation by the soil's microbial biomass. To critically assess this, however, requires a knowledge of microbial amino acid-N turnover times in soil. The effects of soil type, depth and temperature on the uptake and partitioning of a mixture of 15 14C-labelled amino acids by the soil's microbial biomass was therefore studied in 10 contrasting soil types. The results indicated that the degradation of amino acids was soil dependent but that the mean half-life in topsoils at 18°C was 1.7±0.6 h, whilst in subsoils the mean half-life was 12.2±3.3 h. On average 34% of the amino acid-C was respired as CO2 whilst 66% was utilised for new cell biomass. Amino acid decomposition increased with soil temperature, however, rapid rates of amino acid uptake and assimilation were also observed at 5°C (mean half-life in topsoil=2.9±1.5 h). Little correlation was observed between amino acid half-life and either microbial yield, soil arginase activity or organic matter content (r 2<0.40), however, decomposition did appear to be weakly related with soil respiration. The high concentration of amino acids used here (5 mM) was intended to simulate amino acid release after root cell lysis. For previously reported lower concentrations in the bulk soil solution, half lives can be predicted to be even less based on microbial amino acid transport kinetics. The significance of this previously overlooked microbial decomposition of amino acids in the utilisation of organic N by plants is discussed.