Copper activity in soil solution. II. Relation to copper accumulation in young snapbeans. [Phaseolus vulgaris]

Soil solution Cu activity measured in soil saturation extracts was compared with Cu accumulation in young snapbeans (Phaseolus vulgaris L.). Copper was supplied through sewage sludge mixtures of varied Cu concentrations and through Cu-salt additions to the soil. Treatments were a factorial combination of four levels of Cu additions and four levels of total sludge additions. Soil and sludge pH values were limited to pH 5.0 to 5.5. Plant yield differences were minor. Data are reported showing the relationships of root, shoot, and total plant Cu contents with Cu activity, with DTPA-extractable Cu, with soluble Cu, and with sludge Cu concentration. The Cu source (sludge vs. salt) produced distinctly different curves in most every case. Sludge treatments demonstrated nonlinear relationships of Cu activity vs. Cu accumulation; these were fit with exponential functions. Higher shoot Cu concentrations occurred with the Cu salt treatments. Higher root Cu levels occurred at lower Cu activity values with sludge as the Cu source. This probably reflects the superior ability of the sludge to replenish or maintain the Cu supply in soil solution. Total soluble Cu was not related to plant Cu. The proportion of Cu in the sludge was closely linked with shoot Cu (r = 0.94); root Cu levels also reflected the proportion of Cu in the sludge, but higher sludge rates resulted in increased root Cu. Increased labile, soluble, Cu-organic complexes with increased sludge rates would explain both the observed differences between salt and sludge treatments and the positive correlation of root Cu and sludge rate. Physiological control of Cu translocation within the plant is implicit in this reasoning. Continual monitoring of soil solution ion activity or a buffering index is needed to improve predictions of plant ion uptake. Additional Index Words: Donnan dialysis, ion-selective electrode, plant ion uptake, sewage sludge, trace metals. Minnich, M.M. , M.B. McBride, and R.L. Chaney. 1987. Copper activity in soil solution: II. Relation to copper accumulation in young snapbeans. Soil Sci. Soc. Am. J. 51:573-578. R WHICH LINKS ION ACTIVITIES tO plant response has a long history (reviewed by Sparks, 1984). Only recently have studies shown this relationship in soil. Aluminum concentrations in coffee leaves (Coffea arabica L.) have been shown to correlate with soil A1 activity (Pavan et al, 1982). Similarly, Cd activity has been found to correlate with Cd uptake in soil-grown swiss chard (Beta vulgaris Cicla) (Bingham et al., 1983, 1984). Dragun et al. (1976) attempted to demonstrate that Cu uptake in corn (Zea mays L.) hybrids was related to Cu activity in a solution in equilibrium with soil solids; this study suffered from too much variability in the data to draw any conclusions. Of course, ion activity is only one of many factors that influence plant ion uptake. Khasawneh (1971) suggested the following factors: (i) intensity, expressed 1 Contribution from Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. Received 13 Dec. 1985. 2 Former Graduate Research Assistant, Associate Professor, and Research Agronomist, respectively. Address of senior author: Asst. Professor, Dep. of Soil and Water Science, Univ. of Arizona, Tucson, AZ 85721. Address of third author: USDA-Agricultural Research Service, Soil-Microbial System Laboratory, Beltsville, MD 20705. by ion concentration or ion activity; (ii) replenishment, the capacity of the soil to supply an ion; and (iii) relative intensity, best explained as the physiological consequences of variations in ionic composition. Although scientists often try to devise ways to study each of these factors separately, plants grown in soil necessarily reflect the combination of these factors. In solution culture one can isolate each factor with reasonable certainty (e.g., Khasawneh, 1971; Hendrickson and Corey, 1983). In soils, however, one can at best minimize the effects of two factors while studying the third. Khasawneh's list is not the kind of list that is applied directly to soil test procedures. Soil organic matter, clay content, pH, and quantity of extractable metals are readily measurable parameters that affect the basic factors and are often found to correlate with microelement uptake by plants. For macronutrients, nutrient uptake models (e.g., Claassen and Barber, 1976) have provided a fairly comprehensive and certainly constructive approach to the study of plant nutrient uptake. These models input solution concentration rather than ion activity, an approximation that works fairly well for macronutrients. The objective of this study was not to provide a soil test correlate or a comprehensive approach to plant Cu uptake, but to study the intensity factor in soil with minimal variation in other key parameters. Barber (1984) has suggested that ion activity is an important factor if uptake is active (energy-dependent), but may not be important if uptake is passive. Plant Cu uptake is believed to be an active process (Bowen, 1969). In sugarcane (Saccharum offidnarurri), Zn and Cu were reported to compete for the same absorption sites (Bowen, 1969). Also, Schmid et al. (1965) noted that Cu inhibited Zn uptake by barley roots (Hordeum vulgare). Copper interactions with Fe and P have been reported which indicate that high Cu levels reduce uptake of these elements (Reuther and Smith, 1952; Spencer, 1966; Adriano et al., 1971). In this study Cu was supplied by a high-Cu sludge and a Cu salt. The sludge would be expected to have good replenishment capabilities and the Cu salt less so. The pH of the sludges was adjusted and mixed with a soil of similar pH to maintain a fairly constant pH across all treatments. The possibility of a Cu/Zn interaction in the sludge treatments was minimized by adding small portions of a high-Zn sludge to maintain a constant level of total Zn in the soil. MATERIALS AND METHODS Mardin silt loam Ap (coarse-loamy, mixed, mesic Typic Fragiochrept) was amended with CuSO4 or mixtures of three sludges as described in Minnich and McBride (1987). A factorial treatment was used with Cu added at 75, 150, 225, and 300 mg Cu/kg, and sludge added at 0, 40, 80, and 120 g/kg. The sludges had been titrated to pH 5.5. The soil had an initial pH of 5.7 in a saturated paste and a pH of 5.0 in a saturation extract. Air-dried soil and sludges were mixed, then placed in 0.11m . diam plastic pots, four replicates per treatment. Subsam-