The ability of four species of vesicular–arbuscular
mycorrhizal (VAM) fungi to increase phosphorus uptake and
growth of clover plants (Trifolium subterraneum L.) at different
levels of soil compaction and P application was
studied in a pot experiment. Dry matter in the shoots and roots
of clover plants decreased with increasing soil
compaction. Colonization by Glomus intraradices Schenck &
Smith
and Glomus sp. City Beach WUM16 increased
plant growth and P uptake up to a bulk density of 1·60 Mg m−3,
although the response was smaller as soil
compaction was increased. Glomus etunicatum Becker & Gerdeman
and
Glomus mosseae (Nicol. & Gerd.)
Gerdemann & Trappe had no effect on the shoot d. wt and P
uptake when the bulk density of the soil was [ges ]1·40
and [ges ]1·60 Mg m−3, respectively. Soil
compaction to a bulk density of 1·60 Mg m−3
had no effect on the percentage
of root length colonized by G. intraradices and Glomus
sp.
City Beach, but total root length colonized decreased
as soil compaction was increased. Decreased P uptake and
growth of clover plants colonized by G. intraradices and
Glomus sp. City Beach, with increasing soil compaction up
to a bulk density of 1·60 Mg m−3, was mainly attributed
to a significant reduction in total root length colonized and
in the hyphal biomass. Soil compaction, which
increased bulk density from 1·20 to 1·75 Mg m−3,
reduced the O2 content of the soil atmosphere from 0·16
to
0·05 m3 m−3. The
absence of any observable mycorrhizal growth response to
any of the four species of VAM fungi in highly compacted soil (bulk
density = 1·75 Mg m−3) was attributed to the
significant decrease in the O2 content
of the soil atmosphere, change in soil pore size distribution
and, presumably, to ethylene production.