NITROGEN DYNAMICS OF DECOMPOSING CORN RESIDUE COMPONENTS UNDER THREE TILLAGE SYSTEMS

Corn (Zea mays L.) residues returned after grain-corn harvest are a heterogeneous mix of leaves, stems, husks, and cobs with a rather high overall CIN ratio. Considerable N immobilization has been reported from laboratory studies of decomposing corn residues, with less and variable N immobilization reported from field studies. The primary objectives of our study were to determine overall N dynamics for crop residues applied to land under corn grain production under three tillage systems in eastern Canadian conditions, and to see how constituent plant parts contribute to overall patterns of net N immobilization and release. Mesh litterbags (fiberglass screen) containing corn leaves, stems, cobs, or husks were buried or left on the soil surface in plots under no-till, reduced tillage, or conventional tillage, and retrieved over a 2-yr period. Residue N dynamics, including depth effects on residue N, differed greatly by residue type. Cobs, husks, and stems all immobilized N at some point. However, N immobilization was counterbalanced or exceeded by simultaneous N release from other residues, and no net N immobilization was observed for the residues overall (all types combined) for the sampling intervals included in our 2-yr study. Nitrogen dynamics were related to ease of residue decomposition, in turn influenced by residue physical and chemical characteristics as well as by placement depth. Thus cobs immobilized little or no N at any given time because of their slow decomposition, despite very low initial N content. Estimated differences in residue N content between overall tillage systems were relatively small.