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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Mineralization of Amino Acids Applied to Soils

Impact of Soil Sieving, Storage, and Inorganic Nitrogen Additions

^a School of Agricultural and Forest Sciences, Univ. Wales-Bangor, Gwynedd, LL57 2UW, UK

d.jones{at}bangor.ac.uk

The effect of inorganic N additions on the biodegradation and microbial use of organic N pools in soil is poorly understood. To examine the effects of inorganic N on the mineralization rates of amino acids, four soils under contrasting management regimes were subjected to increasing loadings of NH₄NO₃, ranging from 0 to 120 kg N ha^-1. In addition, the effect of soil sieving and storage temperature and time on amino acid mineralization was also investigated. At times ranging from 1 to 40 d after the addition of the inorganic N, the mineralization kinetics of an equimolar mixture of fifteen ¹⁴C-labeled amino acids was followed for a subsequent 24-h period. The rate of ¹⁴CO₂ evolution was soil dependent, with half-lives ranging from 2 h for topsoils to 25 h for subsoils. For all soils, at all times, and at all inorganic-N loadings, the addition of inorganic N appeared to have little effect on the mineralization kinetics of the amino acids to ¹⁴CO₂. In addition, the presence of inorganic N also had no major effect on the C use efficiency of the microbial biomass. It is speculated that N release from the amino acids into the soil by the microbial biomass may also be little affected by inorganic-N additions. Sieving and storage of soil at either 4 or 18°C for up to 40 d had little impact on amino acid mineralization rate. Experiments with potential microbial disrupting agents (autoclaving, CHCl₃ fumigation, HgCl₂, and freeze–thaw) all indicated that the observed mineralization of amino acid C was due to microbial activity. We conclude therefore that inorganic N and soil storage has little effect on the microbial use of readily assimilatable amino acids.

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