Control of amino acid mineralization and microbial metabolism by temperature

Amino acids constitute a major reserve of soil organic-N and studies demonstrating direct uptake of amino acids by plants has indicated that understanding their bioavailability and fate in soil is important to understanding terrestrial N cycling. The aim of this study was to determine the effects of temperature and sorption on the mineralisation of three amino acids (glycine, lysine glutamate) in soil. Amino acid sorption followed the series lysine > glycine > glutamate, whereas mineralisation rate followed the series glutamate > glycine > lysine. These observations support the concept that sorption reduces the bioavailability of amino acids to the soil microbial population. Although the amino acids were used preferentially for making new biomass rather than respiration, differences were apparent between the individual amino acids with microbial assimilation efficiency (biomass production) following the series, lysine > glycine > glutamate. Our results suggest divergences in the uptake and metabolism of the individual amino acids with a rapid mineralisation of amino acids which readily enter general metabolic cycles (e.g. glutamate) compared to the amino acids which typically form the terminus of metabolic pathways (e.g. lysine). Temperature significantly affected the rate of amino acid mineralisation which increased up to 30°C (Q₁₀ = 2.0) followed by a decline as the temperature approached 40°C. Rapid mineralisation occurred even at very low temperatures (1°C). Amino acid mineralisation across three experimental soil treatments followed the trend acidified > control > eroded soil. In summary, the results indicate that mineralisation is highly amino acid species dependent, has a mesophilic optimum, is retarded by sorption and is most rapid in soils which are not degraded.