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Title: Nitrogen supply for organic crops
Author: Stockdale, Elizabeth Anne
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1993
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An integrated series of field, laboratory and pot experiments was carried out between 1990 and 1993 to study the release of nitrogen from organic manures and its subsequent recovery by crops. The aim was to increase understanding of the soil processes controlling N release from manures and therefore enable N supply to be more closely matched to crop demand in organic cropping systems. The study of N release from manures is handicapped by the lack of appropriate methods to measure rates of mineralisation (both net and gross) in the field. The use of isotope dilution techniques under field conditions was found to be difficult due to the slow diffusion of ammonium ions in soils. The release of N from manures was therefore studied indirectly by monitoring plant uptake and changes in the soil mineral N pool. Indices, used to predict N release, were not found to be applicable where additions of manure had been made. Various management strategies aimed at maximising N supply for organic crops were studied. The N released from manures in the first year was shown to be derived mainly from the pool of mineral N added in the manure. The availability of this pool was controlled by the supply of soluble carbon also added in manures, which stimulates the growth of the microbial biomass and therefore leads to immobilisation of the mineral N. The availability of any immobilised N for crop growth is not clear, though some evidence suggested that it was completely recovered by a spring barley crop. The organic N pool of the manure did not seem to be important in supplying N for crop growth in the first year. The use of 15-N-labelled manures enabled the separation of the N taken up by plants into that derived from the soil and that derived from the manure. Manures were labelled non-uniformly by incubation with 15N salts for a short period before application. Where the assumption could not be made that the manure was uniformly labelled, a simple model was developed based on isotope dilution theory, to calculate the percentage of plant N uptake from the manure. 15N was also used to determine the source of the N extracted by a number of methods, used to assess potential N availability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available