The demand of nutrients for plant growing largely depends on soil condition, climate and plant species (Finck 1991). While the required amount of the macro nutrient nitrogen can easily be added to the soil by means of mineral fertilisers, there is a problem with regard to composts as the compost N is hardly calculable. The total N-content in composts analysed by Körner et al. (1997) widely ranged from 5.5 to 38.0 kg N / Mg dry compost. Only minor parts of the total compost nitrogen - the water soluble inorganic components (ammonia, nitrate) - are directly available as plant nutrients (LAGA 1995). Analyses of composts composed of household and green waste showed contents of inorganic nitrogen between 0 and 14 % of the total compost N. Composts made of substrates especially rich in N reached contents up to 19 %. In all cases the amounts of nitrite were not relevant (Körner et al. 1997, 1999). The major part of N is bound in organic forms which will slowly be available to the plant and only after years (Döhler, 1996).
The total N contents showed a dependency on the substrate. Thus N contents of household waste composts were in the range of 1.2 - 2.7 % of the compost dry substance compared to green waste composts with values between 0.5 and 2.1 %. Especially high variations were measured in composts produced of special substrates (up to 3.8 % dry matter). Furthermore, the total nitrogen contents showed a slight dependency on compost stability (Körner et al. 1997, 1999).
The different starting conditions and the different composting parameters affect the course of composting and environmental factors such as temperature, water content and pH value. These factors do not only have an influence on the biological degradation processes but also on N turnovers and releases.
The N turnovers and releases showed some common patterns during composting. Thus the ammonification of waste proteins into ammonia proceeded parallel to the substrate degradation. The most significant releases were detected during the thermophilic phase of composting in the form of ammonia carried by the exhaust air. Compared to this, the leachate releases were low (Körner et al. 1998). The transformation of ammonia via nitrite into nitrate (nitrification) could not start until the thermophilic phase was over. The formation of N2 via denitrification processes and its release is possible if nitrate is contained in substrate. The milieu conditions suitable for nitrification and denitrification processes are largely the same during composting processes. One exception is that a nitrification needs O2, the denitrification requires an O2 deficiency (Körner et. al 1999).
Besides common properties, many differences were noticed. Ammonification increased with increasing pH value and when the substrate was efficiently aerated. Both factors were also decisively important for the amount of ammonia present in the exhaust air. The transformations of N were influenced by the type of substrates. Mainly kitchen waste proteins were mineralised in an early stage of composting. Green waste proteins seemed to be less bioavailable. While the degradation of the kitchen waste proteins was terminated to a large extent during the thermophilic phase, the green waste proteins were still degraded after this phase (Körner et al. 1998, 1999).
The knowledge of N turnovers and releases is essential if N contents of composts shall be regulated during composting. The production of composts, i.e., tailor-made regarding their N composition, may be of interest to a lot of applications. To increase attractiveness of composts in plant production the fertilising effect should be improved. It would be helpful to increase the quantity of nitrogen and to shift the ratio of inorganic to organic N in favour of the inorganic N. There are also situations where composts with a low total N content are needed (e.g. for recultivation or forestry).
The following processing scheme shall be an example for the production of N-rich composts which have a high inorganic content. The principle feasibility of the regulation of the compost N during composting shall be shown.