RESIDUE DECOMPOSITION EFFECTS ON NITROGEN AVAILABILITY TO CORN FOLLOWING CORN OR SOYBEAN

Rates of N fertilization required to attain maximum yields of corn (Zea mays L,) usually are less for corn grown after soybean [Glycine max (L.) Merr.] than for corn grown after corn, but the reason for the difference in N fertilizer requirement has not been clearly established, We studied immobilization and mineralization induced by various management histories and residue treatments to learn about the causes of differences in N fertilizer requirements. Soil samples were collected at five sites in the fall of 1990 from plots that had been planted to soybean and plots that had been planted to corn receiving various N rates, The soil samples were incubated for 42 wk after treatment with N-15-labeled NO3- and with crop residues, All treatments showed a period of net immobilization followed by a period of net mineralization. Net amounts of N immobilization induced by soybean residue were approximately equal to those induced by corn residue from plots with the higher rates of fertilization, However, the soybean-induced immobilization was much more rapid, Rates of mineralization of nonlabeled N were not greater from soils having soybean residue than from soils having corn residue, Because soybean produces less residue under field conditions, the results suggest that differences in N fertilizer requirement are better explained by differences in amounts of N immobilized during residue decomposition than by mineralization of biologically fixed N associated with the soybean.