MINERALIZABLE ORGANIC NITROGEN IN SOIL FRACTIONATED ACCORDING TO PARTICLE-SIZE

A sandy‐loam under wheat‐pasture‐pasture rotation was sampled both after legumeenriched and legumedepleted pasture leys and then fractionated according to particlesize using mainly sedimentation in water. The C/N ratios decreased from 18 to 7 with decreasing particle‐size and were similar for both pasture types, despite ratios of 13 and 31 respectively for organic material collected at the surface of the soil before sampling. Organic matter was most concentrated in fractions having mineral particle‐sizes between 0.3 and 4.0 μm e.s.d. Mineralisation of the N of high C/N fractions was much greater in incubations under waterlogged anaerobic conditions than under aerobic conditions, yet for low C/N fractions, there was little difference. Most of the readily mineralisable soil N resided in fractions of mineral e.s.d. less than 4 μm and much of this was probably recently formed microbial tissue or metabolites. An increasingly greater proportion of the N within size‐fractions was mineralised as the particle size decreased, the increase being greater for aerobic incubations. The proportion of N mineralised in the surface organic material was much greater for legume‐rich than for legume‐poor pasture, but in the soil fractions, a corresponding difference was established only for sand‐sized organic matter. Copyright © 1979, Wiley Blackwell. All rights reserved