A COMPARISON OF DIFFERENT INDEXES FOR NITROGEN MINERALIZATION

Indices of N mineralization in soils of contrasting texture, pH, and o rganic matter contents were compared at different dates during the gro wing season. The indices were derived from a 12-week aerobic incubatio n, determination of the amount of microbial biomass at the start of th e incubation, determination of the increase in NH4+ after boiling with 2 M KCl for 2 h, and extraction of total soluble N with 0.01 M CaCl2. Cumulative mineral N increased linearly with time in the course of th e incubations. Rates of mineralization in soil samples taken in March 1989 and 1990 were significantly correlated with soluble organic N, wh ile correlations between the mineralization rate and the increase in N H4+ after boiling with 2 M KCl for 2 h were poor for sandy soils and a bsent for loamy soils. Correlations between NH4+ after boiling with 2 M KCl for 2h and the soil N concentration were highly significant, but no general relationship was found between the mineralization rate and the soil N concentration. Neither biomass N nor biomass C was signifi cantly correlated with the mineralization rate or with one of the chem ical indices. Among the methods tested, soluble organic N extracted wi th 0.01 M CaCl2 was the only method with any promise for routine measu rement of the mineralization capacity of the individual sites.