Metal response of Douglas-fir: a comparison of foliar metals and phytochelatin production in trees planted in soils amended with biosolids or metal salts

This study was designed to explore the relationship between metals in soil, foliar metal concentrations and phytochelatin (PC) production in Douglas-fir trees planted in soils amended with biosolids or metal salts. Both a greenhouse and field study were conducted to determine whether PC production could be correlated with increased foliar metal or soil metal concentrations. Two seedlings per pot were planted in one of seven greenhouse treatments. The control soil, a sandy loam forest soil, was amended with modern biosolids at rates of 20 Mg ha-1 or 40 Mg ha-1 or metal salts added to similar metal concentrations as biosolids treatments to represent low to moderate metal application rates. Two additional treatments using 100% historic biosolids and the control soil amended with metal salts represented a high metal application rate. The field study included trees amended with modern biosolids at a rate of 21 Mg ha-1 or 16 Mg ha -1 and corresponding trees of the same age that had not received biosolids applications. Soils in both studies were tested for total and NH4NO 3- extractable Cd, Cu, Pb and Zn, as well as pH, C, N, C:N and electrical conductivity. Tree foliage was analyzed for total foliar Cd, Cu, Pb and Zn, as well as phytochelatin, glutathione and cysteine content.;Metal concentrations in greenhouse soils ranged from 2.2-38.0 mg kg -1 Cd, 11-833 mg kg-1 Cu, 49-1361 mg kg-1 Pb and 33-726 mg kg-1 Zn depending on metal application rate. Total metal concentrations in field soils ranged from 3-4.8 mg kg-1 Cd, 15.0-41.0 mg kg-1 Cu, 65.0-79.5 mg kg-1 Pb and 40.0-99.0 mg kg-1 Zn. Greenhouse foliar metal concentrations were not significantly different among low and moderate metal application rates, however, foliar concentrations in high metal treatments were significantly higher than control with the exception of Pb. This followed a similar pattern as total soil metals, which varied only slightly among low and medium metal treatments, however, were significantly higher at high metal rates. No differences in foliar metal concentrations were observed in field samples. Foliar metal concentrations in either study were below toxic levels suggested by existing literature. PC production was not significant by any of the factors tested in the greenhouse study. In the field study, PC production was significantly higher in treatments without biosolids applications. In addition, no correlation was found between PC production and total soil metals or foliar metals in either the greenhouse of field study. These results indicate that foliar PC production in Douglas-fir should not be used as an indicator of metal-stress in trees exposed to similar concentrations of metals as those used in this study.