NITROGEN ACCUMULATIONS AND RELATIVE RATES OF MINERALIZATION IN 2 SOILS FOLLOWING LEGUME GREEN MANURING

The distribution and dynamics of N following green manuring of full bloom field pea (Pisum sativum 'Sirius') were investigated in the soil mineral, microbial and non-microbial organic (NMO N) fractions at two contrasting field sites in central Alberta; one on a Chemozemic (Dark Brown) soil near Provost and the other on a Luvisolic (Gray Luvisol) soil near Rimbey. Soils were sampled four times during a 1-yr period. The accumulations of N in the whole soil and in the soil mineral and microbial fractions were similar between sites. Net mineralization rates under controlled environments were strongly influenced by pre-incubation soil conditions. The short-term dynamics of N were distinguished best between the two soils by mineralization rates normalized to selected soil fractions rather than on the basis of N accumulation in these fractions. Net N mineralization rates expressed on the basis of soil N, microbial N or NMO N were greater in the Luvisolic soil indicating a more rapid internal N cycling system and greater activity of microbial biomass. These observations were consistent with the hypothesis that higher rates are associated with soils of lower clay content.