Three animal manures cross-labelled with 15N in either the urine, faeces or straw fractions were
prepared. After a storage period of 86 days when the manures were exposed to either composting or
to anaerobic storage, portions of the manures were incubated in six differently textured soils with clay
contents ranging from 11 to 45%. Evolved CO2-C was determined during a 266 day incubation and
inorganic N and 15N in soil were measured at the termination of the incubation. The mineralization
of C was analysed using first-order kinetics, and two C pools with fast (P1) and slow (P2) turnover
rates were estimated. The total conversion of added C (Ps) was estimated as Ps=P1+P2.
The cumulated CO2 production was considerably higher from soils incubated with anaerobically
stored manure compared with soils amended with composted manure. CO2 production levelled off
after c. 60 days in the three sandier soils whereas CO2 continued to be produced throughout the
incubation from the three soils with the highest clay content. More C was assigned to the easily
decomposable P1 pool in the sandiest soils whereas the more recalcitrant P2 pool was larger in the soils
with higher clay content. Because of the different relationships between soil texture and C pools, Ps
ended up being similar for five of the six soils. When taking C losses during the preceding storage into
account, the accumulated C losses during storage and after incubation in soil accounted for 60 and
54% of C initially present in the composted and anaerobically stored manure, respectively.
Net N mineralization which averaged 16% of applied organic N took place in all soils amended
with composted manure. Soils with anaerobically stored manure showed net immobilization after the
266 days of incubation. The amount of N immobilized accounted for up to 30% of the inorganic N
applied with the manure. As anaerobically stored manure generally loses less inorganic N during
storage, it may contain more inorganic N than composted manure at the time of field application.
Because of the immobilization that takes place after application of anaerobically stored manure to
soil, the immediate levels of plant available N in soil may not be as different from soil supplied with
composted manure as could be expected from the inorganic N content in the two types of manure.
However, when considering the manure as a N resource, anaerobic storage is superior to composting.