Mineralization of amino acids applied to soils: Impact of soil sieving, storage, and inorganic nitrogen additions

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 C-14-labeled amino acids was followed for a subsequent 24-h period, The rate of (CO2)-C-14 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 al linorganic-N loadings, the addition of inorganic N appeared to have little effect on the mineralization kinetics of the amino acids to (CO2)-C-14, 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 degrees 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.