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 amine acids without prior mineralisation by the soil's microbial biomass. To critically assess this, however, requires a knowledge of microbial amine 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 amine acids by the soil's microbial biomass was therefore studied in 10 contrasting soil types. The results indicated that the degradation of amine acids was soil dependent but that the mean half-life in topsoils at 18°C was 1.72±0.6 h, whilst in subsoils the mean half-life was 12.2 ± 3:3 h. On average 34% of the amine 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 amine 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 amine acid half-life and either microbial yield, soil arginase activity or organic matter content (r²< 0.40), however, decomposition did appear to be weakly related with soil respiration. The high concentration of amine acids used here (5 mM) was intended to simulate amine 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 amine acid transport kinetics. The significance of this previously overlooked microbial decomposition of amine acids in the utilisation of organic N by plants is discussed.