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. Pa

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A two year study was initiated in 1999 at the Macdonald Campus Farm, on a 2.4 ha site consisting mainly of St-Amable sand to shallow loamy sand (Typic Endoaquent; Humic Gleysol). The site was planted to alfalfa ( Medicago sativa) prior to the establishment of the experimental design and has been planted to corn (Zea mays L.) since 1991. The experiment was set up as a randomized complete block design and consisted of three tillage levels (NT, no-till; RT, reduced tillage; and CT, conventional tillage) and two residue levels (-R, no residues; +R, with residues). This study was performed in order to ascertain the effect of these management systems on soil organic carbon (SOC), particulate organic matter carbon (POM-C), microbial biomass carbon (MBC) levels and soil physical properties. Soil physical properties measured included bulk density, saturated hydraulic conductivity (Ksat), dry and wet aggregate stability, total porosity, macroporosity at -1 kPa and -6 kPa of matric potential, and moisture content. Crop yield was also measured. Results indicated that lower rates of mineralization of POM-C under NT+R in the surface 0--10 cm led to significantly higher SOC at the same depth. There were no significant differences between treatments at the 10--20 cm depth. The MBC was not a good indicator of the differences in soil organic matter levels between treatments. No-till treatments had higher bulk densities, reduced total porosity and macroporosity, increased moisture content, and increased constant-head measured Ksat. Differences in Ksat as measured with the Guelph permeameter, were not significant between treatments, most likely due to increased earthworm activity in NT. There were no significant differences in crop yield in 1999, but a significantly wetter year in 2000 caused lower grain yields in NT+R compared to NT-R and CT+R.

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