This thesis includes two studies. The first study compared the net N mineralization rate in soils of five cropping systems (corn-soybean rotation, continuous corn, continuous corn with a winter rye cover crop, perennial prairie, and N-fertilized perennial prairie) in Iowa. To consider the effect of plant residues and freezing-thawing on net N mineralization rate, treatments with and without plant residues and with or without freezing were established and followed through a 30-day aerobic incubation. The effect of chemical characteristics of soil samples and the plant residue samples of each cropping system on N mineralization were also investigated. Overall, the cropping systems had significant effect on net N mineralization rate: N-fertilized perennial prairie (a) ≈ continuous corn with winter rye (a) ≥ corn-soybean rotation (ab) ≥ continuous corn (bc) ≥ perennial prairie (c) (lowercase letters indicate least significant differences (p < 0.05). Freezing and thawing treatment increased the net N mineralization rate about twofold in the 30-day period incubation. The simple presence of plant residues did not affect net N mineralization, but the plant residue N per weight of soil was significantly correlated with N mineralization.

The second study measured the concentration of amino acids in soil and plant residues which were extracted from the same samples as the first study. Amino acids were analyzed by using two approaches: high performance liquid chromatography (HPLC) which couples cation exchange chromatography with a ninhydrin colorimetric method, and high performance anion exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD). One objective of this study was to compare the precision and effectiveness of the two approaches. Another objective of this study was to document the concentration of amino acids in partially decomposed plant residues. The concentrations of amino acids in all the soil samples were similar, as measured by the amperometric method and the ninhydrin method. On the other hand, in plant samples, the ninhydrin method could be a better choice than the amperometric method at present, because of the peaks of some amino acids were overlapped by co-eluting carbohydrate peaks. About forty percent of total N in the partially decomposed plant residues was amino acid-N. Compared to the freshly dried plant samples, the total amino acid concentration of the dried and partially decomposed plant residues (mainly roots) was lower.