Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2003
Publication Date: 10/29/2003
Citation: Martens, D.A., Loeffelmann, K.L. 2003. Soil amino acid composition quantified by acid hydrolysis and anion chromatography - pulsed amperometry. Journal of Agriculture and Food Chemistry. 51:6521-6529.
Interpretive Summary: Soil nitrogen is composed of an organic fraction and an inorganic or mineral nitrogen fraction. The mineral fraction can account for up to 5% of the total soil nitrogen with the remaining 95%, an organic fraction composed of amino acids, amino sugars and other unidentified nitrogen compounds. The organic nitrogen fraction cycles and provides the mineral nitrogen that is then available for plant use. Understanding the amount of organic nitrogen that cycles to mineral nitrogen in a year would provide scientists with a method to predict the amount of nitrogen that a soil could provide, potentially limiting the over application of nitrogen fertilizers. At present, few options are available to measure these nitrogen changes. In this manuscript, a method is detailed that provides a tool for determining the composition of soil organic nitrogen and by using the detailed method, the importance of nitrogen cycling following soybean growth to the following years corn crop is determined. Application of the method will provide a better method for scientists to determine nitrogen cycling and provide better recommendation rates for nitrogen fertilizer application. The benefits may include reducing nitrogen costs to producers while not impacting economic yields and reducing nitrogen nutrient pollution in the environment.
Technical Abstract: Soil organic N accounts for 90 to 98% of the total soil N content with amino acids (AAs) and amino sugars (ASs) identified as the major soil organic N compounds. Previous hydrolysis methods using 6 M HCl with reflux or sealed digestions for 24 h and various detection systems have accounted for only 30 to 40% of soil total N content as AA-N. This study compared the traditional HCl extraction methodology with methanesulfonic acid (MSA) hydrolysis and nonderivatized AA and ASs quantification by ion chromatography with pulsed amperometric detection for determination of the AA composition of plant litter and soils. Methanesulfonic acid (4 M) gave comparable quantitative or better AA-N recovery to 6 M HCl for plant AA digestions (16 h, 121 oC, 104 kPa). Use of 4 M MSA (0.5 to 1.5 h, 136 oC, 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 (16 h, 121 oC, 104 kPa) with a MSA recovery rate of 85.6% of the total N content (n = 22 soils). The 1 h MSA soil digestions suggested that the majority of soil organic N was not present as large proteins found in plant analysis or that the presence of clay minerals catalyzed the hydrolysis process. Methanesulfonic acid-ion chromatographic analysis for soil AA/ASs composition is a robust nonderivatization method requiring little sample preparation that can distinguish between small changes in soil AA composition due to vegetation and tillage managements during a single growing season.