APPARENT AVAILABILITY OF NITROGEN IN COMPOSTED MUNICIPAL REFUSE

The use of composted municipal refuse on agricultural land requires pr ior knowledge of the interactions among compost, soil, and plants. Res earch into the availability of N in highly matured municipal refuse co mpost is particularly important considering the current concern about groundwater contamination by NO3--N. A greenhouse pot bioassay was con ducted to determine the percentage of short-term apparent bioavailable N of a highly matured refuse compost and its relative efficiency in s upplying inorganic N to the soil-plant system in comparison with NH4NO 3. Municipal refuse (after 165 days of composting) was applied at rate s equivalent to 10, 20, 30, 40, and 50 t ha-1 to a ferrallitic soil fr om Tenerife Island (Andeptic Paludult). NH4NO3 was applied at rates eq uivalent to the total N content of the compost treatments. Perennial r yegrass (Lolium perenne L.) was grown in 3-kg pots and the tops were h arvested at regular intervals after seedling emergence. The compost in creased dry matter yield, soil mineral N, and plant N uptake proportio nal to the applied rate. These increases were significantly higher tha n the control at an application rate of 20 t ha-1. After 6 months the apparent bioavailable N ranged from 16 to 21%. The relative efficiency was 43% after 30 days. This suggests that large inputs of inorganic N into soil can be obtained with high rates of this kind of compost, wi th a potential for NO3--N contamination. However, applied at moderate rates in our bioassay (<50 t ha-1), compost showed a low N-supplying c apacity to ryegrass, i.e. a small fraction of the mineralized compost N was used by plants in the course of time. This was ascribed to a par tial biological immobilization. This pattern of N availability in high ly matured municipal refuse compost, positive net mineralization but p artial immobilization, is similar to the pattern of N availability in biologically active soils and is therefore extremely interesting for t he conservation of N in agro-ecosystems.