Mineralization/immobilization of nitrogen and phosphorous in composted growing media

The effects of different temperature and moisture regimes of a composted growing medium on N and P plant-availability and main differences with a peat-based growing medium have been evaluated during laboratory incubation experiments. The influence of root growth has also been tested by plant growth experiments. Microbial biomass and extractable N and P have been determined during a time-course incubation up to 56 days. The microbial biomass carbon (Bc) was relatively high in the composted growing media (2900–3600 μg Bc g-1) but less influenced by treatments. In contrast, Bc was much lower in peat-based growing media (350–600 μg Bc g-1) but more modified by the incubation conditions, especially by drying/rewetting cycles and by root growth. On the other hand, mineral N and extractable P were much less affected in the peat-based growing media than in the composted growing media. The cyclic changes of moisture regimes in particular, produced a much higher mineralization rate. In the composted growing media, small changes in soil microbial biomass were associated with larger changes in microbial activity.