Mineralisation and immobilisation of nitrogen in soil

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Abstract
The processes of mineralisation and immobilisation of nitrogen are defined, and the contribution of mass isotope (15N) studies to our knowledge of these simultaneous transformations in soil is briefly reviewed.Studies of immobilisation of nitrogen in soils treated with organic materials are reviewed with particular reference to the nitrogen content and carbon/nitrogen ratio of the added substances. Laboratory studies on carbon/nitrogen relationships in glasshouse soils incubated with mixtures of sucrose and inorganic nitrogen are summarised, together with comparative data for a wide range of other soils. The amounts of nitrogen immobilised in acid ‘maiden’ soils on incubation with sucrose were increased by the addition of lime and phosphate, but did not reach the levels of activity of old glasshouse soils; these findings are discussed in relation to the carbon/nitrogen ratios of the untreated soils and to their rates of evolution of carbon dioxide.Recent studies on the simultaneous mineralisation of carbon, nitrogen and phosphorus are briefly described. Data are presented for 32 soils showing relationships between initial soil pH and the percentages of the total soil nitrogen mineralised after liming or hydrolysed by autoclaving with acid. The observation that acid soils appear to contain a reserve of nitrogenous constituents which is readily mineralised on liming is discussed with reference to recent views on the chemistry of the nitrogenous components of soil organic matter.Reference is made to some recent work on laboratory incubation tests as a guide to the availability of soil nitrogen and the prediction of crop response to nitrogenous fertilisers.