Soil Amino Acid Composition Quantified by Acid Hydrolysis and Anion Chromatography−Pulsed Amperometry

Soil organic N accounts for 95−98% of the total soil N content with amino acids (AAs) and amino
sugars (ASs) identified as the major soil organic N compounds, but traditional 6 M HCl with reflux or
sealed digestions for 24 h and various detection systems have accounted for only 30−40% of soil
total N content as AA-N. This study compared traditional HCl extraction methodology with
methanesulfonic acid (MSA) hydrolysis and nonderivatized AA and AS quantification by ion
chromatography with pulsed amperometric detection for determination of the AA composition of plant
litter and soils. MSA (4 M) gave AA-N recovery comparable to or better than 6 M HCl for plant AA
digestions (16 h, 121 °C, 104 kPa). Use of 4 M MSA (0.5−1.5 h, 136 °C, 112 kPa) increased the
total recovery of organic N as AAs, ASs, and NH4+ by 46% from soils (n = 22) compared with 6 M
HCl (12 h, 110 °C, reflux) with a MSA recovery rate of 85.6% of the total N content (n = 22 soils).
The shorter MSA soil digestions (0.5−1.5 h) suggested that the majority of soil organic N was not
present as protein forms found in plant litter analysis (16 h of digestion). MSA ion chromatographic
analysis for soil AA/AS composition is a robust nonderivatization method requiring little sample
preparation that can distinguish between small changes in soil AA composition during one growing
season due to vegetation and tillage managements.
Keywords: Soil organic nitrogen; amperometric detection; soybeans; determination; analysis; corn;
methanesulfonic acid; ion chromatography