DYNAMICS OF N-15 IN 2 SOIL-PLANT SYSTEMS FOLLOWING INCORPORATION OF 10-PERCENT-BLOOM AND FULL BLOOM FIELD PEA

The distribution and dynamics of N-15 following green manuring of N-15-labelled 10% bloom and full bloom field pea (Pisum sativum 'Sirius') were investigated in the soil mineral N, microbial N and non-microbial organic N (NMO-N) fractions and in a subsequent barley crop at two contrasting field sites in central Alberta: one on a Chernozemic (Dark Brown) soil near Provost and the other on a Luvisolic (Gray Luvisol) soil near Rimbey. Soils and plants were sampled four times during a 1-yr period. The 10% bloom and full bloom pea shoots were similar in dry matter production and N and C content. More N was, however, released from the younger pea residues directly following soil incorporation, which we attributed to a larger proportion of labile components. Barley yield, N content and N-15 recovery in the grain were not influenced by legume bloom stage at incorporation, although significantly more N-15 was recovered in the barley straw and roots of the full bloom treatment. Incorporation of full bloom legumes resulted in closer synchrony between the appearance of legume-derived mineral N-15 and early N demand by the barley crop. The decay rate constants for the recalcitrant fraction of the legume residues were not significantly influenced by bloom stage or site over the time intervals of our observations and are, thus, consistent with the theory that decomposition of the recalcitrant fraction of plant residues can be described by a single exponential equation.