THE EFFECT OF SOIL COMPACTION ON GROWTH AND P UPTAKE BY TRIFOLIUM-SUBTERRANEUM - INTERACTIONS WITH MYCORRHIZAL COLONIZATION

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of th e soil increased from 1.0 to 1.6 Mg m(-3). The strongest effects of so il compaction on P uptake and plant growth were observed at the highes t P application (60 mg kg(-1) soil). The main observation of this stud y was that at low P application (15 mg kg(-1) soil), P uptake and shoo t dry weight of the plants colonised by Glomus intraradices were great er than those of non-mycorrhizal plants at similar levels of compactio n of the soil. However, the mycorrhizal growth response decreased prop ortionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were att ributed to the reduction in the root length. Soil compaction had no si gnificant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot(-1)) in highly compac ted soil than in slightly compacted soil (27.8 m pot(-1)). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m(3)m(-3) in slightly compacted soil (1.0 Mg m(-3)) to 0.10 m(3)m(-3) in highly compacted soil (1.6 Mg m(-3)).