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 nutrient uptake through its effects on the integrity of the extraradical hyphal network. A growth-chamber experiment was conducted to evaluate the survival of extraradical arbuscular mycorrhizal (AM) fungal hyphae when detached from the host root system, and to understand the effects of soil disturbance on the ability of these hyphae to colonize plant roots and to reestablish mycorrhizal associations in previously disturbed soils. The experiment consisted of establishing AM fungi in pots divided into two compartments by a nylon mesh (37 micrometers), by growing corn (Zea mays L.) in one of the compartments for 6 weeks in an unsterilized agricultural field soil. The mesh prevented 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 either disturbed by sieving through 2 mm mesh (D) or undisturbed (U) leading to four combined disturbance treatments: (1) both compartments undisturbed (UU); (2) root compartment disturbed and root-free compartment undisturbed (DU): (3) root compartment undisturbed and root-free compartment disturbed (UD); and (4) both compartments disturbed (DD). The effects of fallows of four different durations: 0, 30, 60 and 90 d were also measured in the same experiment giving a total of 16 treatments. Soils were disturbed at the beginning of the experiment in the root compartment, and after each fallow period in the root-free compartment. Immediately after disturbance of the soil in the root-free compartment, corn was planted and grown for 30 d to test the combined effects of fallow and soil disturbance on AM formation and nutrient content. Soil disturbance had no adverse effect on AM efficiency if test plants were planted immediately after disturbing the root-free compartment. However, AM efficiency decreased with increasing length of fallow. Lengths of total and metabolically active extraradical hyphae in the root-free compartments were measured before each fallow. Significantly less hyphal lengths 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 treated pots. Phosphorus content by the test plants was twice as high in UU as in DD. Test plants in undisturbed (UU) pots had greater Zn and Cu contents than in DU, UD or DD pots. Content of P, Zn and Cu in test plants was reduced by about 40%, 63% and 70%, respectively by 90 d of fallow, compared to 0 d fallow.