plant-soil interaction experiments can have large consequences on
the size of the effect that the soil community has on plant performance.
Below we discuss a number of possible mechanisms that
may explain how the inoculation method can influence plantesoil
effects.
The higher biomass production in pots inoculated only with the
microbial community indicates that the main growth reducing
agents were partly sieved out and thus that these agents did not
pass the 20 mmsieve. The most obvious explanation is that the main
growth reducing agents are larger than 20 mm. This corresponds
with the observation that plants were smallest in the treatment
that contained most plant-feeding nematodes. This observation
points at plant-feeding nematodes as a cause of reduced J. vulgaris
growth in pots inoculated with sieved field soil. However, there are
also many pathogenic soil bacteria and fungi present in soils and it
could be that some of them were filtered out by the 20 mm sieve as
well. For example, because soil bacteria and fungi can be associated
to larger soil aggregates (Six et al., 2004; Briar et al., 2011), which
are filtered out by the 20 mm sieve. It is also possible that the
differences are due to differences between soil organisms in their