EFFECTS OF SOIL COMPACTION ON PHOSPHORUS UPTAKE AND GROWTH OF TRIFOLIUM-SUBTERRANEUM COLONIZED BY 4 SPECIES OF VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI

The ability of four species of vesicular-arbuscular mycorrhizal (VAM) fungi to increase phosphorus uptake and growth of clover plants (Trifo lium subterraneum L.) at different levels of soil compaction and P app lication was studied in a pot experiment. Dry matter in the shoots and roots of clover plants decreased with increasing soil compaction. Col onization by Glomus intraradices Schenck & Smith and Glomus sp. City B each WUM 16 increased plant growth and P uptake up to a bulk density o f 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 a nd P uptake when the bulk density of the soil was greater than or equa l to 1.40 and greater than or equal to 1.60 Mg m(-3), respectively. So il 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 compacti on was increased. Decreased P uptake and growth of clover plants colon ized by G. intraradices and Glomus sp. City Beach, with increasing soi l compaction up to a bulk density of 1.60 Mg m(-3), was mainly attribu ted to a significant reduction in total root length colonized and in t he hyphal biomass. Soil compaction, which increased bulk density from 1.20 to 1.75 Mg m(-3), reduced the O-2 content of the soil atmosphere from 0.16 to 0.05 m(3) m(-3). The absence of any observable mycorrhiza l growth response to any of the four species of VAM fungi in highly co mpacted soil (bulk density = 1.75 Mg m(-3)) was attributed to the sign ificant decrease in the O-2 content of the soil atmosphere, change in soil pore size distribution and, presumably, to ethylene production.