Combined effects of soil disturbance and fallowing on plant and fungal components of mycorrhizal corn (Zea mays L.)

Soil disturbance may reduce the effect of mycorrhizae on plant growth and n utrient uptake through its effects on the integrity of the extraradical hyp hal network. A growth-chamber experiment was conducted to evaluate the surv ival of extraradical arbuscular mycorrhizal (AM) fungal hyphae when detache d from the host root system, and to understand the effects of soil disturba nce on the ability of these hyphae to colonize plant roots and to reestabli sh mycorrhizal associations in previously disturbed soils. The experiment c onsisted of establishing AM fungi in pots divided into two compartments by a nylon mesh (37 mu m), by growing corn (Zea mays L.) in one of the compart ments for 6 weeks in an unsterilized agricultural field soil. The mesh prev ented the growth of corn roots from one side of the pot to the other, while allowing the passage of the AM hyphae. After establishment of AM fungi the following treatments were performed: soil in the two compartments was eith er disturbed by sieving through 2 mm mesh (D) or undisturbed (U) leading to four combined disturbance treatments: (1) both compartments undisturbed (U U); (2) root compartment disturbed and root-free compartment undisturbed (D U); (3) root compartment undisturbed and root-free compartment disturbed (U D); and (4) both compartments disturbed (DD). The effects of fallows of fou r different durations; 0, 30, 60 and 90 d were also measured in the same ex periment giving a total of 16 treatments. Soils were disturbed at the begin ning of the experiment in the root compartment, and after each fallow perio d in the root-free compartment. Immediately after disturbance of the soil i n the root-free compartment, corn was planted and grown for 30 d to test th e combined effects of fallow and soil disturbance on AM formation and nutri ent content. Soil disturbance had no adverse effect on AM efficiency if tes t plants were planted immediately after disturbing the root-free compartmen t. However, AM efficiency decreased with increasing length of fallow. Lengt hs of total and metabolically active extraradical hyphae in the root-free c ompartments were measured before each fallow. Significantly less hyphal len gths were observed in pots where the soil of the root compartment had been disturbed. Test plant shoot weight was highest in UU and lowest in DD treat ed pots. Phosphorus content by the test plants was twice as high in UU as i n DD. Test plants in undisturbed (UU) pots had greater Zn and Cu contents t han in DU, UD or DD pots. Content of P, Zn and Cu in test plants was reduce d by about 40%, 63% and 70%, respectively, by 90 d of fallow, compared to 0 d fallow. (C) 1998 Elsevier Science Ltd. All rights reserved.