The inconsistent effect of soil disturbance on colonization of roots by arbuscular mycorrhizal fungi: a test of the inoculum density hypothesis

Reducing the tillage of agricultural soils can increase early-season crop-P uptake. Consistent increases in plant-P have been found in both field- and laboratory-systems with undisturbed (U) compared to disturbed (D) soil. A concomitant stimulatory effect on colonization of roots in U soil by arbuscular mycorrhizal (AM) fungi has been found in some cases, but in others the colonization has been similar in U and D treatments. Disruption of the extraradical mycelium that remains from the previous crop is the mechanism by which soil disturbance restricts mycorrhizally mediated P uptake for the subsequent crop, with a tandem change in colonization not necessary, but sometimes seen. Nonetheless, a complete account of these processes will need an understanding of the conditions under which the extent of colonization is affected. Soil-P does not explain when a difference in colonization will appear. Among ecosystems in Western Australia, high inoculum density in a pasture was reported previously to preclude the appearance of a difference in colonization in response to soil disturbance, whereas for other ecosystems with lower inoculum densities a difference in colonization was seen. Here, we determined if a similar mechanism operates for an agricultural soil collected mid-season during the growth of a maize (Zea mays L.) crop in Ontario, Canada. Blending various proportions of pasteurized and non-pasteurized soil gave a range of inoculum densities. Maize was taken through two 3-week growth cycles in pots, and for the D treatment the soil was passed through a 5 mm sieve between cycles. All plants became colonized with AM fungi. Reducing the inoculum density served to limit colonization to similar low levels in both U and D soils. Stimulation of colonization and of shoot-P uptake in the U-compared with the D-treatment was greater for plants under the higher inoculum conditions tested. We conclude that the inoculum density during crop growth of the soil studied here is moderate, and that this density makes it possible, if other conditions are met, for a reduction of colonization of roots in response to soil disturbance. Whether or not a difference in colonization will appear following disturbance of a soil such as the one studied here probably depends on the interaction between the environment and the plant. Possible interactions are discussed. The high inoculum density of ecosystems such as the pasture studied in Australia likely overrides any effect of soil disturbance and ensures roots of all plants become well-colonized by AM fungi. (C) 2000 Elsevier Science B.V. All rights reserved.