Survival and mineralization and immobilization of ¹⁵N-labelled Serratia cells in a boreal forest raw humus

Abstract

15Nitrogen-enriched Serratia cells and (NH4)2SO4 were added separately to non-sterile, to gamma-irradiated, and to autoclaved black spruce raw humus F horizon (pH 3.4). Survival of the Serratia and the distribution of added- and of soil-N between several inorganic- and organic-N fractions were studied from 0 to 100 days.The zero time distribution of cell-N was similar to that of the soil-N, indicating that soil microorganisms probably contribute N to most of the N fractions considered. Irradiation at 1.5 Mr induced no important change in soil N fractionation, but autoclave sterilization released a small amount of exchangeable NH4+-N. In non-sterile soil, added NH4+-N was immobilized in and added cell-N was mineralized from the organic N fractions (hydrolyzable forms of N, acid-insoluble humin-N, etc.). Addition of either NH4+- or cell-N caused "N interchange" and the mineralization of soil N. In irradiated soil, transformations were much decreased and were not directly related to population size. In autoclaved soil at pH 3.4 change in cell-N and fixation of NH4+-N were negligible. There was no relation between the multiplication or disappearance of an added Serratia population (under different conditions of partial sterility and pH) and the rate of mineralization of the added cell-N.

Soils as Components of Ecosystems

“…Added ammonia N becomes rapidly immobilized and in turn microbial N is remineralized. Both ammonia N and cellular N may inter change with the native soil N and cause-part of it to become remineralized (56). Differences in nitrogen regimes of different soils appear to be, to a large extent, the result of their different immobilization-remineralization properties (81).…”