Organic matter extracted with 0.01 M CaCl₂ or with 0.01 M NaHCO₃ as indices of N mineralisation and microbial biomass

Abstract.

A pot experiment was carried out with three soils at ambient temperature in which temporal changes in fractions of soil organic matter that were extractable with either 0.01 M CaCl₂ or 0.01 M NaHCO₃ were compared with changes in N mineralisation and microbial biomass C. UV spectral analysis of soil extracts was also carried out on sub-samples taken at the beginning of the experiment. The objective was to quantify the fractions of extractable soil organic matter and determine whether these could be used to estimate the mineralisable organic N content of the soils. The results suggested that part of the NaHCO₃-extractable organic matter originated in the microbial biomass but that non-biomass material was also present. The non-biomass material was not identified directly, but was composed of compounds with high UV absorbance. In the case of CaCl₂, the results suggested that extracellular proteins were contained in the extract and that some material released from the actively growing microbial biomass may also have been present. A supplementary study with 16 soils was carried out to determine the ability of the organic matter solubilised by either extractant to predict soil N uptake by barley seedlings. A significant relationship (P<0.01) was found between N uptake and CaCl₂-extractable material only.