EFFECTS OF SOIL COMPACTION ON PLANT-GROWTH, PHOSPHORUS UPTAKE AND MORPHOLOGICAL-CHARACTERISTICS OF VESICULAR-ARBUSCULAR MYCORRHIZAL COLONIZATION OF TRIFOLIUM-SUBTERRANEUM

We investigated the effect of soil compaction and phosphorus (P) appli cation on morphological characteristics of mycorrhizal colonization an d growth responses, to determine the reasons for reduced responses obs erved in our previous work with compacted soil. Growth, phosphorus (P) uptake and intensity of vesicular-arbuscular (VA) mycorrhizal coloniz ation were studied in clover plants (Trifolium subterraneum L.) with a nd without VA mycorrhizal colonization at two P applications and three levels of soil compaction. Phosphorus was supplied either at constant mass concentration (mg P kg(-1) soil) or at constant volume concentra tion (mg P dm(-3) soil). Increasing bulk density of the soil from 1.1 to 1.6 Mg m(-3) significantly decreased root length and shoot d. wt, b ut increased the diameter of both main axes and first order lateral ro ots regardless of P application. Total P uptake and shoot d. wt of clo ver plants colonized by Glomus intraradices (Schenck & Smith) were sig nificantly greater than those of non-mycorrhizal plants at all levels of soil compaction and both P applications. However, soil compaction t o a bulk density of 1.6 Mg m(-3) (penetrometer resistance = 3.5 MPa at a matric potential of -33 kPa) significantly decreased mycorrhizal gr owth response. There was no evidence that the increased volume concent ration of P at high bulk densities was responsible for the reduced res ponses. Soil compaction had no significant effect on the fraction of r oot length containing arbuscules and vesicles, but total root length c olonized by arbuscules, vesicles or by any combination of arbuscules, vesicles and intra-radical hyphae significantly decreased as soil comp action was increased. The air-filled porosity of highly compacted soil , which varied from 0.07 to 0.11 over the range of matric potentials e ncountered (-33 and -100 kPa), had no significant effect on the intens ity of internal colonization.