With renewed interest in maintaining our soil resources, it is important to establish criteria that can describe and quantify the effect of different crop management practices on soil organic matter (SOM). We conducted this study to assess changes in SOM and other soil properties after long-term (>10 yr) continuous corn (Zea mays L.; CC) and corn-soybean rotation [Glycine max (L.) Merr,; C/SB] with and without fertilizer, Soil samples were collected from two furrow-irrigated CC and C/SB rotations on a Crete silt loam (ene, montmorillonitic, mesic Pachic Argiustoll) and a Eudora loam (coarse-silty, mixed, mesic Fluventic Hapludoll). Long term (350-d) laboratory incubation at optimum moisture and temperature conditions measured potentially mineralizable C (PMC) and N (PMN) as a measure of the active fraction of soil organic C and N, Microbial biomass C (MBC) and N (MBN), organic C and N, pH, and texture also were determined, Crop rotations that included high-residue-producing crops such as corn and addition of fertilizer increased soil organic C and N, Crop rotation did not affect PMC in the Crete soil, but addition of fertilizer significantly increased PMC by 32%. The PMN in both soils was not affected by crop rotation or fertilizer addition, Inclusion of soybean in the rotation decreased the stable and active fractions of organic C and N, Changes in soil organic C and N in response to crop rotation and fertilizer addition were related to the estimated amount of crop residues returned to the soil and to soil texture.
