Amino Acid N in Holocene Paleosols As Identified By IR Spectroscopy and Amperometric Techniques.

Organic matter in paleosols is utilized to infer the mechanisms by which carbon can preserved over millennial time scales.  However, there is limited information about the nitrogen forms associated with stabilized organic matter in paleosols.  There have been studies that dealt with the quantity of amino acids (AA) in paleosols, but few have reported on the qualitative nature of these AA.  We investigated paleosols at two locations, the Farwell Site (along the South Fork of the Big Nemah River in southeast Nebraska) and the Claussen site (along Mill Creek, near Lawrence, Kansas). At each location, paleosols and overlying modern soils were sampled and analyzed for the amino acid contents and their distribution.  We examined both the unfractionated soil samples as well as the clay fractions where most of the organic matter occurs.  The clays were extracted from whole soils by repeated suspension and exhaustive density separations, freeze dried, and then analyzed for total organic C and N, amino acid N via amperometric technique after digestion with 4 M methanesulfonic acid, and DRIFT infrared spectroscopy.  Amino acid N as a percentage of total N generally decreased with depth in both the modern and buried unfractionated samples.  Considerable amounts of the total N in the unfractionated Claussen paleosols, particularly in the early Holocene paleosol (P3), were amino sugars - glucosamine and galactosamine.  Infrared spectroscopic scans of HF-treated, freeze-dried clays yielded peaks at 3400 and 1630 cm^-1, indicating the presence of N-H and/or O-H groups, and carboxyl C, respectively.  Additional peaks at 1106 and 1037 cm^-1 are associated with carbohydrates and polysaccharides.