RESIDUE QUALITY, LOADING RATE AND SOIL-TEMPERATURE RELATIONS WITH HAIRY VETCH (VICIA-VILLOSA ROTH) RESIDUE CARBON, NITROGEN AND PHOSPHORUS MINERALIZATION

Studies were conducted 1) to determine the effects of crop residue quality, loading rate and their interaction on residue decomposition; and 2) to assess these same effects on predicting residue N and P turnover with thermal units. Two residue qualities were obtained by harvesting field grown hairy vetch (Vicia villosa Roth) in the fall and spring. Loading rates equivalent to 0, 2150, 4300, and 6450 kg ha-1 (dry weight basis) of each of the two residue qualities were added to a soil. Residue loading rate did not affect residue C or N mineralization when expressed as a percentage of added C or N mineralized, respectively. Therefore predicting residue N mineralization with thermal units does not appear limited by differential residue loading rates. Residue quality exhibited a dramatic effect on vetch C mineralization, with approximately 35% of the added C mineralized 30 days after application of the fall vetch and about 17% of added C mineralized 30 days after application of spring vetch. Consideration of residue quality must also be given when determining N mineralization x thermal unit relationships. The effect of residue quality on C and N mineralization did not appear related to residue N content or C/N ratio but rather to lignin and/or hemicellulose contents. Either vetch P mineralization did not occur to a measurable extent in this study or the H2O soluble fraction was not a useful indicator of this process.