ELEMENT UPTAKE BY MYCORRHIZAL SOYBEAN FROM SEWAGE-SLUDGE-TREATED SOIL

Components of sewage sludge that reduce the availability of P or are toxic to mycorrhizal fungi will alter the uptake of P, Cu, and Zn by plants. A greenhouse study was conducted to (i) determine effects of sewage sludge on P availability, (ii) determine the effects of sludge on mycorrhizal (MR) uptake of P, Cu, and Zn, and (iii) confirm MR supression of Cu and Zn uptake by P. 'Beeson' soybean (Glycine max [L.] Merr.) was grown for 7 wk in soil (Ravenna, a fine-loamy, mixed, mesic Aeric Fragiudalf) treated factorially with (i) 0, 60, 150, or 270 mg/kg P as monocalcium phosphate; (ii) four sources of sewage sludge providing 100 mg/kg P or a nontreated control; and (iii) Glomus fasciculatum (Thaxter) Gerdemann and Trappe emend. Walker and Koske as MR inoculum or no inoculum. Sludge reduced P uptake at 150 mg/kg P or higher in nonmycorrhizal (NMR) plants with little difference in plant growth among sludges. In MR treatments, growth and P-uptake responses to sludge ranged from very beneficial with two sludges to a complete inhibition of the MR response with another sludge. This inhibition was persistant and apparently due to suppression of MR fungi by toxic levels of NH4+. Mycorrhizae substantially increased shoot Cu and Zn uptake only at low soil-P levels. Foliar Mn concentrations increased with P in NMR plants, but not in MR treatments. Our conclusions are that (i) sludge may reduce the availability of fertilizer P, (ii) sludge may exert differential effects on MR uptake of minerals, and (iii) inhibition of MR activity by P is the major reason for P-induced decreases in Cu and Zn for soybean.