Studies of amino acid composition of soil organic matter extracted from 10 Iowa surface soils and from surface soils of two long-term rotation experiments showed that, with the exception of asparagine plus aspartic acid and glutamine plus glutamic acid, the other 13 amino acids, expressed as percentages of total amino acid extracted, were very uniform among the soils. The total amino acid extracted was significantly correlated with organic C (r = 0.79[superscript]\*\*) and clay content (r = 0.76[superscript]\*\*), but not with total N. Expressed as percentages of organic C and N in soils, the amounts extracted ranged from 10.9 to 32.4% and from 12.0 to 27.4%, respectively. Crop rotations affected the total amino acid content of soils;A simple and precise method for assay of aspartase activity in soils was developed. It involves determination, by steam distillation, of the NH[subscript]4[superscript]+-N released when soil is incubated with K-aspartate in 0.1 M tris buffer (pH 8.5) and toluene at 37°C for 24 h. Results showed that the optimal pH and temperature for the activity of this enzyme were 8.5 and 40°C, respectively. The activation energy values of this enzyme in three soils ranged from 40.1 to 50.7 kJ mol[superscript]-1, and the average Q[subscript]10 was 1.9. The K [subscript]m values ranged from 0.173 to 0.208 M. Tests with 27 Iowa surface soils showed that aspartase activity was significantly correlated with clay content (r = 0.44[superscript]\*), organic C (r = 0.85[superscript]\*\*\*), total N (r = 0.73[superscript]\*\*\*), amidase activity (r = 0.44[superscript]\*), urease activity (r = 0.88[superscript]\*\*\*), L-asparaginase activity (r = 0.94[superscript]\*\*\*), and L-glutaminase activity (r = 0.88[superscript]\*\*\*). Aspartase activity was inhibited by 24 trace elements added to soils; at 5 mmol kg[superscript]-1 soil, the inhibition ranged from 13% with Fe(III) to 98% with Ag(I). Aspartase activity of soils under three long-term rotations showed that the order was: COMM > CSCS > CCCC. Tillage and residue management practices significantly affected the level of aspartase activity in soils in the following order: no-till/2x mulch > chisel plow/mulch > moldboard plow/mulch > no-till normal > chisel plow normal > no-till bare > moldboard plow normal;Analysis of 171 Iowa surface soils showed that the amounts of fixed NH[subscript]4[superscript]+-N ranged from 40 to 268 mg kg[superscript]-1 soil. The values were not significantly affected by crop rotations and lime application, but were significantly correlated with organic C (r = 0.37[superscript]\*\*\*) and total N (r = 0.39[superscript]\*\*\*). Crop rotation and lime application significantly affected N mineralization at 30°C. Lime application significantly increased the nitrification rate.