FIELD CROP RECOVERY AND MODELING OF NITROGEN MINERALIZED FROM LABELED SORGHUM RESIDUES

Efficient use of fertilizer N requires an assessment of the N contribution from decomposing crop residues. The objectives of this study were to quantify and model the amount of mineralized N recovered by a growing crop from N-15-labeled sorghum (Sorghum bicolor [L.] Moench) residues of variable N concentration and composition. The residues were incorporated into the surface soil of field microplots. The microplots were double-cropped with sorghum and wheat (Triticum aestivum L.) for a 3-yr period. The N-15 tag in the crop residue was used to distinguish between residue-derived N and that from soil organic matter. Between 4.5 and 25% of the residue N applied (36-83 kg N ha-1 was applied as crop residue N) was recovered by a sorghum crop 110 d after incorporation. This constituted 56 to 77% of the total N recovered during a 3-yr period. Accumulated N recovered over time was described by modified first-order models. Regression analysis indicated that 93% of the variability in N recovered by 110 d could be explained using the C/N ratio and acid-detergent fiber contents of the residues. Measurements of N mineralized, using the method described above, were compared with predicted N mineralized, using MINIMO (a subroutine of the CERES-Maize model). After adjusting MINIMO parameters using data collected 110 d after residue incorporation, the slope and intercept of a linear fit between measured and MINIMO-predicted N mineralized 1097 d after residue incorporation were not different from one and zero, respectively.