SEASONAL MICROBIAL BIOMASS DYNAMICS AFTER ADDITION OF LENTIL AND WHEAT RESIDUES

The dynamics of soil microbial biomass after the addition of plant residues have a considerable influence on nutrient availability for plants, and can be quantified using the chloroform-fumigation-extraction method. The dynamics of microbial C and N following addition of C-14- and N-15-labeled lentil (Lens culinaris Medik.) green manure, lentil straw, and wheat (Triticum aestivum L.) straw were investigated under field conditions at a site located at Outlook, Saskatchewan, on a Bradwell sandy loam (Typic Boroll). Plant residues were incorporated into microplots on 5 Oct. 1988, and the fraction of added C-14 and N-15 in microbial biomass was determined on four dates during the 1989 growing season. Maximum levels of labeled and unlabeled microbial biomass were observed at the time of planting (8 May) in 1989. Of added C-14, 26 and 15% was in the microbial mass in the green manure and straw treatments, respectively, on 8 May; greater microbial accumulation of green-manure C-14 was due to a higher proportion of C-14 being available rather than to a higher efficiency of C-14 assimilation. Microbial N-15 accounted for 65 to 81% of added residue N-15 on 8 May. Plant-residue N-15 was readily available to decomposer microorganisms from all residue types, whereas C-14 was more available from green manure than straw. During the 1989 growing season, microbial C-14 declined by 51 and 40% in the green manure and straw treatments, respectively, while microbial N-15 declined by 54% in all treatments. The decline in microbial N-15 during the 1989 growing season was approximately five times greater than the amount of N-15 mineralized in all sampling periods except the first for the green-manure treatment. The highest levels of labeled and unlabeled microbial biomass observed at the time of planting indicates that microbial biomass may reduce losses of N and other nutrients during periods of low crop demand, and may act as a source of nutrients during active crop growth.