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, w ere 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
The use of 15N-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 15 N salts for a short
period before application. W here 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.
The structure of a simple model was described, which could be used to select a
manurial strategy, which maximised crop yields and estimated the potential N
losses for an organic farm. It was suggested that this could be constructed using
an expert system approach, which was able to refer to databases and simple
models to provide the final output. The full development of such a model is not
yet possible, as the availability of the N applied in organic manures is only partly
understood. However, our understanding of the complex processes controlling
the release of N from manures has been increased as a result of the work