The Use of Naturally Occurring C14 to Measure the Persistence of Organic Components in Soil

Abstract

Applicability of the carbon-dating method of analysis to studies in Soil Science was investigated. Factors affecting the accuracy of the method, e.g. precision, isotopic fractionation, bomb produced C14, 'Suess effect', and 'de Vries effect', were found to either cause insignificant errors, or errors for which corrections could easily be made. Mean residence times (m.r.t's) of humus from the Ap horizon of five Saskatchewan soils ranged fram 250 years for a Gray-Wooded Podzolic soil to 1000 years for the Black Chernozemic soils. The humus of the Dark Brown Chernozemic soils from Lethbridge, Alberta, was older. Humic fractions from a Chernozemic and a Podzolic soil were dated and chemically characterized. The fractions differed significantly in their m.r.t's and optical properties. The 'calcium' humates and non-hydrolysable humic acids of the Chernozem dated 1400 years while the humic acids hydrolysate was, in comparison, extremely young (25 years). There were indications that a close relationship between optical properties and m.r.t. exists. Except for a small fraction (the humic acids hydrolysate) the major portion of soil humus is inert to decomposition. Calculations employing the m.r.t. of the fractions of a Chernozem showed that the humic acids hydrolysate accounts for 80 per cent of the nitrogen released per annum. Data for the soils and fractions showed the humus of Podzolic soils to be much more labile than that of Chernozems, and also served to emphasize the importance of different types of organo-mineral bonds in the stabilization of humus. It is concluded that the carbon-dating technique, supported by other chemical methods of analysis, is a useful research procedure for resolving problems in soil organic matter investigations.