EQUATIONS FOR ESTIMATING THE AMOUNT OF NITROGEN MINERALIZED FROM CROP RESIDUES

The amount of N mineralized or immobilized during the decomposition of a crop residue will influence the amount of N available for crop uptake and will ultimately impact N-management practices and groundwater quality. The objective of this work was to determine quantitative relationship(s) between a crop residue's chemical properties and the potential net amount of N that would mineralize in a season. Eight experiments (six from the literature and two conducted by the authors) were combined to determine general relationships between net N mineralized and residue chemical characteristics. Regression analysis indicated that 75 and 72% of the variability in the measured amounts of N mineralized in the eight experiments could be explained using either the C/N ratio or the square-root transformation of N concentration of the residue, respectively. The break point between net N mineralization and net immobilization was calculated to be at a C/N ratio of 40, which corresponds to a N concentration of about 10 g N/kg (assuming residue C is 400 g/kg). Eighty percent of the variability in the amount of N mineralized could be explained by a regression equation that included N and the lignin-to-N ratio as independent variables. The fitted equations provide estimates of the maximum amount of N that potentially will mineralize in a season from incorporated crop residues of different N contents.