NITROGEN IMMOBILIZATION AND MINERALIZATION DURING INITIAL DECOMPOSITION OF N-15-LABELED PEA AND BARLEY RESIDUES

The immobilization and mineralization of N following plant residue inc orporation were studied in a sandy loam soil using N-15-labelled field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) straw. Both crop residues caused a net immobilization of soil-derived inorgan ic N during the complete incubation period of 84 days. The maximum rat e of N immobilization was found to 12 and 18 mg soil-derived N g(-1) a dded C after incorporation of pea and barley residues, respectively. A fter 7 days of incubation, 21% of the pea and 17% of the barley residu e N were assimilated by the soil microbial biomass. A comparison of th e N-15 enrichments of the soil organic N and the newly formed biomass N pools indicated that either residue N may have been assimilated dire ctly by the microbial biomass without entering the soil inorganic N po ol or the biomass had a higher preference for mineralized ammonium tha n for soil-derived nitrate already present in the soil. In the barley residue treatment, the microbial biomass N was apparently stabilized t o a higher degree than the biomass N in the pea residue treatment, whi ch declined during the incubation period. This was probably due to N-d eficiency delaying the decomposition of the barley residue. The net mi neralization of residue-derived N was 2% in the barley and 22% in the pea residue treatment after 84 days of incubation. The results demonst rated that even if crop residues have a relative low C/N ratio (15), t ransient immobilization of soil N in the microbial biomass may contrib ute to improved conservation of soil N sources.