Seasonal Microbial Biomass Dynamics after Addition of Lentil and Wheat Residues

Abstract

AbstractThe 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 14C‐ and 15N‐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 14C and 15N 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 14C, 26 and 15% was in the microbial biomass in the green manure and straw treatments, respectively, on 8 May; greater microbial accumulation of green‐manure 14C was due to a higher proportion of 14C being available rather than to a higher efficiency of 14C assimilation. Microbial 15N accounted for 65 to 81% of added residue 15N on 8 May. Plant‐residue 15N was readily available to decomposer microorganisms from all residue types, whereas 14C was more available from green manure than straw. During the 1989 growing season, microbial 14C declined by 51 and 40% in the green manure and straw treatments, respectively, while microbial 15N declined by 54% in all treatments. The decline in microbial 15N during the 1989 growing season was approximately five times greater than the amount of 15N 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.