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 the soil increased from 1.0 to 1.6 Mg m -3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg-1 soil). The main observation of this study was that at low P application (15 mg kg -l soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot-l) in highly compacted soil than in slightly compacted soil (27.8 m pot-z). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m3m -3 in slightly compacted soil (1.0 Mg m -3) to 0.10 m3m -3 in highly compacted soil (1.6 Mg m-3).
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 WUM 16 increased plant growth and P uptake up to a bulk density of 1n60 Mg m −$ , 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 1n40 and 1n60 Mg m −$ , respectively. Soil compaction to a bulk density of 1n60 Mg m −$ 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 1n60 Mg m −$ , was mainly attributed to a significant reduction in total root length colonized and in the hyphal biomass. Soil compaction, which increased bulk density from 1n20 to 1n75 Mg m −$ , reduced the O # content of the soil atmosphere from 0n16 to 0n05 m$ m −$ . The absence of any observable mycorrhizal growth response to any of the four species of VAM fungi in highly compacted soil (bulk density l 1n75 Mg m −$ ) was attributed to the significant decrease in the O # content of the soil atmosphere, change in soil pore size distribution and, presumably, to ethylene production.
SUMMARYWe investigated the effect of soil compaction and phosphorus (P) application on morphological characteristics of mycorrhizal colonization and growth responses, to determine the reasons for reduced responses observed in our previous work with compacted soil. Growth, phosphorus (P) uptake and intensity of vesicular-arbuscular (VA) mycorrhizal colonization were studied in clover plants {Trifolium subterraneuvi L.) witli and 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"' soil) or at constant volume concentration (mg P dm"'' soil). Increasing bulk density of the soil from l-l to 1' 6 Mg m~^ significantly decreased root length and shoot d. wt, but increased the diameter of both main axes and first order lateral roots regardless of P application. Total P uptake and shoot d. wt of clover plants colonized by Glomus intraradices (Schenck & Smith) were significantly greater than those of non-mycorrhizal plants at all levels of soil compaction and both P applications. However, soil compaction to a bulk density of 1-6 Mgm"^ (penetrometer resistance = 3-5 MPa at a matric potential of -33kPa) significantly decreased mycorrhizal growth response. There was no evidence that the increased volume concentration of P at high bulk densities was responsible for the reduced responses. Soil compaction had no significant effect on the fraction of root length containing arbuscules and vesicles, but total root length colonized by arbuscules, vesicles or by any combination of arbuscules, vesicles and intra-radical hyphae significantly decreased as soil compaction was increased. The air-filled porosity of highly compacted soil, which varied from 0-07 to 0-11 over the range of matric potentials encountered ( -33 and -lOOkPa), had no significant eflect on the intensity of internal colonization.Key words: Arbuscule, compaction, mycorrhizai growth response, P uptake, vesicle.
INTRODUCTIONcompaction, thereby reducing both the volume of soil explored by the roots and the uptake of less Agricultural machinery often causes soil compaction, mobile nutrients like P. Shierlaw & Alston (1984) which increases bulk density (and penetrometer found a significant negative correlation between resistance) and alters soil pore size distribution, shoot P concentration and difi'erent levels of soil movement of air, water and nutrients (Grable & compaction. Siemer, 1968;Hoffmann &Jungk, 1995). These soilThe growth and development of ve.sicularproperties influence plant growth either via mech-arbuscular (VA) mycorrhizal fungi are afiected by anical resistance (Bengough & MuUins, 1991) or by many environmental factors. The best documented poor aeration (Agnew & Carrow, 1985). A root of these is soil P supply, which is negatively cannot penetrate a soil pore smaller than the diameter correlated with mycorrhizal growth response and of the root cap if the soil is strong (Wiersum, 1957). with the extent of fungal colonization (Abbott, Root gro...
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