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.
