Amino acids as diagnostics of soil and soil water quality
Information on the contribution of amino acids to dissolved organic nitrogen and carbon exported from grassland soil is scarce. Evidence from the literature for other environments, suggests that determination of amino acid patterns of distribution may be a useful method for improved understanding of the interaction of microbial synthesis and degradation of organic N in conjunction with soil physical states. A sample pre-concentration technique and an HPLC methodology were developed that enabled the determination of dissolved free (DFAA) and combined (DCAA) amino acids in natural waters at picomolar concentration. These methods were used to examine the content of amino acids and their distribution patterns in waters from 3 different settings. Firstly, field-sized lysimeters (1 ha) were used to examine dissolved free and combined amino acids in surface runoff and drainage waters from a grassland soil over 3 winter drainage periods. The waters were collected from soils beneath drained and undrained permanent ryegrass swards, receiving 280 kg N haˉ¹yrˉ¹ , permanent ryegrass receiving no mineral N input, and grass/white clover (no mineral N). Total DFAA concentration ranged between 1.9 nM - 6.1 µM and total DCAA concentration ranged between 1.3 - 87 µM. A large library of amino acid distributions was assembled and multivariate pattern analysis techniques were used to determine whether there were distinctive amino acid signatures that could be used as a diagnostics for soil management and condition. Although addition of mineral N fertilizer increased amino acid concentration in waters, there was no detectable effect of fertilizer addition on DFAA distribution patterns. In contrast, both DFAA and DCAA patterns were strongly influenced by soil hydrology alone. However, in the case of DCAA patterns, there was evidence of an interaction between hydrology and fertilizer addition. Secondly, monolith lysimeters were used to determine the DFAA in drainage waters from 4 different grassland soil types, in order to find whether there was evidence of pattern difference with soil texture. Results showed that distribution patterns vary between soil types, and contrary to what might be expected, that clay soils do not necessarily retain basic amino acids. Thirdly, the concentration and patterns of DFAA were determined hourly over a 24 hour period, for a river that received exported soil waters from the field lysimeters mentioned above. Total DFAA concentration correlated with water temperature and NH4+ Compared with exported soil waters, the concentrations of DFAA in river water were several orders of magnitude smaller, although GLY, SER, LYS and MET were in greater relative proportions. Results of the studies show that amino acids have the potential to be used as diagnostics of source, soil condition and management.