Crop residue decomposition and nitrogen dynamics in corn under three tillage systems

Decomposition and N dynamics of grain-corn residues were investigated in a field study in southwestern Quebec, with particular reference to the roles of different plant parts (stems, leaves etc.) in determining overall residue mass loss and N content. A litterbag study was conducted, with surface and buried placements in plots under three tillage systems (no-till, reduced tillage, and conventional tillage, established five years before litterbag placement). Residue mass loss and N content were monitored over a two-year period. Separate data were obtained for leaves, stems, husks, and cobs. Net values for all residues combined were calculated taking into account initial proportions of each plant part at harvest. Overall estimates were made based on residue depth-distribution typical of each tillage system. A spreadsheet-based model of surface residue mass loss was developed, incorporating litterbag mass and other surface-residue data, in order to determine how well litterbag results predicted surface residue mass loss in the field, and to test alternative assumptions regarding residue decomposition and/or burial. Buried residues lost mass more quickly than surface residues, as expected. Thus residue breakdown would be fastest in a conventional system, slowest under no-till, and intermediate with reduced tillage. Substantial decreases in mass and residue N content occurred between fall placement and first sampling in spring, despite low temperatures for much of this period. Mass loss in the first period was substantial for stems as we as husks and leaves. Cobs decomposed most slowly throughout. Nitrogen dynamics, including effects of depth on residue N content, differed greatly by residue type. All the lower-N residues (cobs, husks, stems) immobilized N at some point. However, during the two-year study, N immobilization by one or more residue types was always counterbalanced or exceeded by N release by other residue, at least for the sampling intervals included. P