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Journal of Environmental Quality 30:78-84 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
HEAVY METALS IN THE ENVIRONMENT

Cupric Ion Activity in Peat Soil as a Toxicity Indicator for Maize

M.B. McBride

Department of Crop and Soil Sciences, Cornell Univ., Ithaca NY 14853

Corresponding author (mbm7{at}cornell.edu)

Received for publication April 17, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Copper phytotoxicity in soils is difficult to assess because Cu accumulates at and damages roots, and is not readily transferred to shoots. Soil chemical properties strongly influence Cu speciation, so that total soil Cu alone is not a broadly useful indicator of potential toxicity to plants. The present study measured free Cu2+ activity in Cu-enriched peat soils using the ion selective electrode. The soil Cu2+ activity was related to the severity of phytotoxicity as measured by several indicators in a maize (Zea mays L.) bioassay, including leaf chlorosis, root stunting, and reduced shoot growth and Fe concentration. A soil Cu2+ activity of 10-7.0 to 10-7.5, corresponding to total Cu of about 275 mg/kg in the peat soil, caused phytotoxicity in maize seedlings. It is proposed that Cu2+ activity is more directly related to phytotoxic effects than other soil tests, such as extractions with strong acids or chelating agents, because it is the free Cu2+ in soil solution that has the most direct toxic effects on roots. There was very limited uptake of Cu into maize shoots, and even when Cu2+ activity and total soil Cu were raised into the extreme toxicity range of 10-5 and 4000 mg/kg, respectively, shoot Cu remained less than 35 mg/kg. These results indicate the inadequacy of the USEPA risk assessment of potential for Cu toxicity to crops amended with sewage sludge, which assumed a no-effect level of maize shoot Cu of 40 mg/kg.

Abbreviations: NOAEL, no observable adverse effect level • ISE, ion-selective electrode • Kd, partition or distribution coefficient, the ratio of total to dissolved metal • pCu, negative logarithm of the free Cu2+ ion molar concentration in soil solution


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Copper application to agricultural land in quantities greatly in excess of that required by crops occurs in the case of sewage sludge application (McBride, 1995), some animal manure (particularly swine) application (Coppenet et al., 1993), and in certain field, orchard, and vine crops where Cu salts are used as fungicides (Merry et al., 1986; Magalhaes et al., 1985). Copper is also applied prophylactically on peat soils for vegetable crop production because virgin peats are commonly Cu deficient (Murphy and Walsh, 1972). Long-term Cu accumulation in soils has resulted in specific instances of phytotoxicity in vineyards (Magalhaes et al., 1985; Drouineau and Mazoyer, 1962; Delas, 1981), temperate and tropical orchards and plantations (Alva, 1993; Dickinson et al., 1988), pig farms (Coppenet, 1981; Unwin, 1981), and sewage sludge farms (Unwin, 1981). McBride (1995) has cited a number of studies of Cu phytotoxicity in soils amended with pig manure or sewage sludge, with general experience indicating that 300 to 500 kg/ha total Cu loading, Mehlich 3-extractable Cu of 180 to 200 mg/kg, or an EDTA-extractable soil Cu level of 120 mg/kg is sufficient to produce phytotoxicity in near-neutral soils for several crops (Alva, 1993; Unwin, 1981; Coppenet, 1981). Differences in crop sensitivity, soil texture, and pH may alter these toxicity thresholds considerably, so that it is inadvisable to apply such soil threshold values indiscriminately.

One problem in recognizing incipient Cu toxicity in soils is the generally unreliable nature of plant tissue tests and soil tests intended to estimate soil Cu bioavailability. Copper concentrations in leaves of many crops are not very sensitive to extractable or total soil concentrations of Cu (Jarvis and Whitehead, 1981; Logan et al., 1997; Chang et al., 1992). Copper ions are strongly absorbed by plant roots (Minnich et al., 1987; Lexmond and van der Vorm, 1981), inhibiting fine root development and trace element (particularly Fe) uptake without concomitant uptake into shoots of excessive Cu. In fact, root stunting and damage by Cu can actually reduce Cu concentrations in plant tops (Lexmond and van der Vorm, 1981). Dragun and Baker (1982) concluded that Cu concentration in the top growth of maize is not suitable as an indicator of Cu toxicity. Nevertheless, the risk assessment used to derive the USEPA 503 rule for land application of sewage sludges (USEPA, 1992) calculated an "acceptable" Cu loading for soils on the single observation that 40 mg/kg Cu in maize shoots did not decrease top growth in one cited hydroponic study, and identified this tissue concentration as the no observable adverse effect level (NOAEL) for Cu in maize (Chang et al., 1992; USEPA, 1992). This decision led to a permitted soil loading of 1500 kg/ha total Cu, which was justified by calculating a low probability of maize shoots reaching the 40 mg/kg concentration at the permitted Cu loading (USEPA, 1992).

Inspection of the published studies of Cu toxicity suggests that a more cautious and defensible NOAEL could have been chosen for maize. For example, Cottenie et al. (1976) reported growth inhibition at 20 mg/kg in maize shoots, and MacNicol and Beckett (1985) reported an upper critical level (10% yield reduction) for Cu toxicity of 21 mg/kg in maize shoots. Later studies (Borkert et al., 1998; Mocquot et al., 1996) reconfirmed maize leaf or shoot concentrations of 20 to 21 mg/kg Cu as critical thresholds of toxicity.

Irrespective of the choice of an NOAEL for a single crop, it is apparent that other scientific analyses of phytotoxic potential have used assumptions and interpretations very different from those of the USEPA in arriving at Cu loading limits. Agricultural soil concentration limits (including the background concentration) range from about 50 to 200 mg/kg total in several European countries and Ontario, which in effect restricts Cu loading to no more than about 10% of that allowed in the USA. The recommendations for land application of sewage sludges in the northeastern USA are similarly cautious, advising an application limit of ~150 kg/ha Cu on heavier-textured soils (Pennsylvania State University, 1985). The general approach employed in the USEPA risk assessment for phytotoxicity of Cu, Zn, and Ni, using 50% growth retardation in immature plants as the phytotoxicity threshold, has been challenged as not scientifically sound (Schmidt, 1997; McBride, 1995).

The weak correlation of plant shoot Cu to soil Cu concentration appears to have led some to the conclusion that total soil Cu has little relevance to Cu solubility, bioavailability, or toxicity. In fact, free Cu activity is sensitive to total soil Cu as well as pH and soil organic matter content (McBride et al., 1997), and dissolved Cu in soil solution is a near-linear function of total soil Cu concentration when pH is held constant (Sauvé et al., 1997). A soil test sensitive to dissolved free Cu2+ would arguably be a more direct and reliable indicator of phytotoxic potential than either a plant tissue test or a measure of total or reagent-extractable Cu. Studies linking Cu bioavailability in soils to estimated or measured free Cu2+ activity in soil solution suggest that the Cu2+ activity may predict Cu uptake into indicator crops as well as biological toxicity in the soil (Sauvé et al., 1996, 1998; Dragun and Baker, 1982). The Cu2+ activity test is based on evidence that, while most of the Cu in soil solution is usually complexed with dissolved organic matter, free Cu2+ is more toxic to biota and a more sensitive measure of toxicity than total dissolved Cu (Campbell, 1995). Nevertheless, any specific concentration of free Cu2+ appears to be more toxic to organisms at high pH than at low pH (Meador, 1991; Lexmond and van der Vorm, 1981; Lexmond, 1980). Thus, the toxic effects of free Cu2+ are modulated by other factors.

The purpose of the present study is to create a wide range of free Cu2+ activities in an organic soil with a high adsorption capacity for Cu, and relate the measured Cu2+ activities to biological indicators of phytotoxicity in maize, including tissue Cu, growth, and visual symptoms. This will give a more universally applicable measure of Cu toxicity status, the threshold Cu2+ activity in soil solution that produces toxicity in maize. Depending on soil pH, organic matter content, clay content, form of Cu added, and other variables, different soils are expected to have quite different total Cu concentrations that produce this threshold activity.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Three peat soils that had been under vegetable cultivation for different lengths of time were collected from a muck farming region of Orange Co., New York, and stored field-moist in large plastic buckets. Copper (as CuSO4) is applied routinely to these soils as part of the management for onion (Allium cepa L.) production. Two peats had been in production for more than 60 yr (60Y) and about 20 yr (20Y), whereas one peat (0Y), sampled about 60 m (200 ft) from 20Y, had never been cropped and therefore had received no copper amendment. These peats are referred to hereafter as 0Y, 20Y, and 60Y to identify them by the number of years they have been cropped. The sampled soils were well-mixed prior to taking subsamples for chemical analysis. The trace element content of the three soils are reported in Table 1, based on inductively coupled plasma (ICP)–emission spectrophotometric analysis of duplicate samples (oven-dried at 70°C) digested by a standard nitric–perchloric acid wet ash procedure (Greweling, 1976). This method has been tested for ability to recover trace elements from an NBS soil standard (San Joaquin #2709), giving better than 95% recovery of total Cd, Zn, Cu, and Ni, and about 88 to 90% recovery of total Mn and Fe (data not shown).


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  		Table 1. Characteristics and elemental composition of Orange Co., NY peat soils with varying durations of vegetable cropping. All element concentrations are reported as mg/kg (dry wt.) except where indicated. Numbers in parentheses are standard deviations

 
The pH values of the three peats were measured in water using a glass-reference combination electrode, after weighing 10 g (moist wt.) into 20 mL distilled water. The Cu2+ activity, measured as -log (Cu2+), or pCu, was measured in 0.01 M CaCl2 using an Orion (Beverly, MA) ion selective Cu electrode, as described below. Organic matter content was estimated by weight loss on ignition of the peats (dried at 70°C) when ashed for 2 h at 600°C in a muffle furnace.

A large sample of 20Y peat was further homogenized by repeated hand mixing, and subdivided into 0.5-kg (field-moist wt.) quantities to be used for amendment with Cu and maize seedling bioassays. Subsamples of this peat were dried in the oven at 70°C to obtain the moisture content of the field-moist soils. Individual 0.5-kg quantities of the field-moist peats were wetted with 150 mL of distilled water to bring them to near saturation. Sufficient CuSO4, dissolved in 100 mL of water, was added to separate 0.5-kg quantities of the peats to add 0 (control), 200, 400, 800, 1000, 1500, 2000, 3000, and 4000 mg/kg of Cu on a dry-weight (70°C) basis. After thorough mixing, the Cu-spiked soils were equilibrated in open plastic containers until they reached a nearly dry state (4 d time), and were then further homogenized with a large porcelain mortar and pestle, returned to their containers, and sealed.

The Cu2+ activities and pH of the Cu-amended peats were determined by weighing out 10 g field-moist peat into 20 mL of 0.01 M CaCl2, shaking on a wrist-action shaker for 30 min, and determining pCu by ion-selective electrode (ISE) and pH with a combination glass-Ag/AgCl reference electrode. The ISE was calibrated using a combination of dilute Cu2+ salts and Cu2+ activity buffers prepared using iminodiacetate (IDA) to control Cu2+ activities in the 10-7 to 10-12 range (Sauvé et al., 1997).

The pH and pCu values were measured on subsamples of these Cu-amended peats 9 d later, with no significant changes noted. Approximately 1 wk following these measurements, each Cu-amended peat was split in two, placed in small styrofoam pots, and brought to field-moist condition with distilled water. Three maize seeds were planted in each of the duplicate pots at the 0 to 4000 mg/kg Cu levels, and grown at about 22°C under wide-spectrum fluorescent plant lights (General Electric, Cleveland, OH). The maize seedlings were harvested after 3 wk by cutting the whole plants at 0.5 cm from the soil surface. The combined weight of the three plants from each replicate was measured after drying the tissue at 80°C. The whole plant tops were then analyzed for total Cu and other trace elements by dry-ashing dried plant tissue and bringing the ash into solution with nitric and hydrochloric acid, followed by inductively coupled plasma (ICP)–emission spectrophotometric analysis of the digests (Greweling, 1976). This plant digestion method has been tested for accuracy using the NBS apple leaf standard (#1515), with measured Cu and Zn within 1% of the certified values, and Mn and Fe about 5% lower than the certified values (data not shown).

At harvest, 10 g of field-moist soils were taken from each pot and analyzed for pH and pCu as described above. The day before harvest, each pot had been gradually wetted to saturation with sufficient distilled water to displace about 25 mL of leachate into a collecting beaker. A separate subsample of each leachate was analyzed for total dissolved Cu using flame atomic absorption spectrophotometry. The pH of subsamples of these leachates were measured directly, and, after bringing the CaCl2 concentration up to about 0.01 M using concentrated CaCl2, the pCu of the leachates was measured as described before.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Chemical Characteristics of the Peats
As shown in Table 1, the nutrient and trace metal content was lowest for the virgin peat (0Y), with 20 and more than 60 yr of cropping leading to progressively higher soil concentrations of most elements except carbon. For some elements, notably K and P, this trend can be attributed to the cumulative effect of annual fertilizer applications. However, for Cd, V, Cr, Ni, Pb, Mn, Fe, and Al, concentrations increasing with duration of cropping would seem to be explained by organic matter decomposition and soil subsidence. Clay and other silicate mineral mixing into the surface appears not to be responsible for enrichment in Fe, Al, and several other trace metals in the 20Y peat because the topsoil Si concentration actually decreased after 20 yr of cropping. Organic matter oxidation leads to an increase in the concentration of the most strongly solids-bound metals concomitant with a decrease in soil organic matter (Table 1). Bioaccumulation of some of the more plant-available elements from deeper in the soil profile into the crop biomass (e.g., Cd, Ni, Zn, Mg, Ca) could also be responsible for part of the increased surface soil concentration with prolonged cropping.

The high Cu and Zn concentrations in the cropped peats (20Y, 60Y) compared with the virgin peat (0Y) cannot be accounted for by enrichment arising from organic matter decomposition, as these two metals had a higher enrichment factor than all other elements except K (see Table 1), which tends to have enrichment on the order of three- to fourfold following 20 yr of cultivation (comparing peat 20Y and 0Y, which are in close proximity in the field). Presumably, repeated applications of both Cu and Zn as micronutrient amendments explain this excess in the soil. Nevertheless, 60 yr of cropping did not produce higher soil Zn and Cu concentrations than 20 yr, which may mean that Cu and Zn losses by leaching or other processes could have occurred over this long cropping time. It should be noted, however, that the bulk density of 60Y peat is higher than that of the 20Y. Thus, per hectare, a similar concentration by weight in the more decomposed soil translates into a larger quantity of Zn or Cu in the topsoil. Therefore, although the 60Y peat with the longest cropping history has lower Zn and Cu concentrations in the topsoil than the 20Y peat, it could actually have received a higher metal loading per hectare.

Although both cropped peats contained higher Cu concentrations than the virgin peat, this did not cause an increase in free Cu2+ activity (measured by pCu in Table 1). Presumably, the higher pH of the cultivated soils accounts for this, and particles of marl were evident in both cropped peats, but not in the virgin peat.

Cupric Ion Activities in Copper-Amended Peat
The addition of CuSO4 to the 20Y peat increased free Cu2+ activity, measured by ISE, as a function of total Cu loading. The relationship, shown in Fig. 1 , reveals that the first (200 mg/kg) and second (400 mg/kg) addition levels of Cu had a particularly marked effect in decreasing pCu (increasing Cu2+ activity). The pCu decreased from about 8.6 in the unamended (control) soil (which, however, already contained 76 mg/kg total Cu), to about one and two pCu units lower with 200 and 400 mg/kg Cu addition, respectively. Greater Cu loadings produced progressively smaller decreases in pCu, as is expected for this logarithmic function of free Cu2+ activity. Little change in pCu was observed in the interval of 5 and 9 d after mixing Cu into the peats (Fig. 1). However, after growing and harvesting maize, there was a shift in soil pCu, toward lower Cu2+ activities at the higher Cu addition levels, and higher Cu2+ activities at the 0 or 200 mg/kg Cu addition levels (Fig. 1). The pCu measurement on leachates collected just before maize harvest also had, at all Cu addition levels, lower Cu activities than measured prior to planting, with the pCu values shifted by about 0.5 to 1.0 units, as illustrated in Fig. 2 . Thus it appears that Cu2+ activity was affected by the rhizosphere, possibly crop uptake of free Cu2+ or exudation of Cu-chelating compounds. The soil pH was only slightly higher at harvest compared with preplanting (pH ranged from 5.3–5.4 in the control peat to 5.0 for the peat with highest Cu loading). Total dissolved Cu in the leachate, plotted in Fig. 2 along with pCu, increases nearly linearly with Cu loading.



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  		Fig. 1. The dependence of soil Cu2+ activity, as expressed by the -log (Cu2+) or pCu, on total Cu in 20Y peat soil. The solid line is fitted to pCu measurements at 5 and 9 d after Cu addition (pCu = 12.98 - 2.30 log CuT, r = 0.997) and the broken line is fitted to the pCu measurements at the time of maize harvest (pCu = 11.63 - 1.81 log CuT, r = 0.994). Total soil Cu is symbolized by CuT in these equations. Error bars in this and subsequent figures display ±one standard deviation unit

 


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  		Fig. 2. The dependence of total dissolved Cu (mg/L) and Cu2+ activity (pCu) in leachate on total concentration of Cu in peat soil

 
Based on the ISE-measured free ionic Cu2+ concentrations and total dissolved Cu concentrations in the collected leachates, the percentage of soluble Cu in the free cation form was calculated, with these results shown in Table 2. Although dissolved Cu in the unamended (control) peat was more than 98% in complexed form, presumeably with dissolved organic matter, this fraction decreased systematically with higher Cu loadings, so that more than one-half the soluble Cu in the peat with 4000 mg/kg added was free ionic Cu2+. There was therefore a clear trend for the more toxic (free-ionic) form of Cu to increase more markedly in response to Cu addition to the soil than all of the dissolved forms. This response could indicate a limited capacity of dissolved organic matter to complex Cu added to solution.


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  		Table 2. The percentage of dissolved Cu in soil leachate that was in the free Cu2+ form and the partition coefficients (Kd) for Cu at the time of maize harvest

 
The partition coefficients (Kd) for Cu in the peat varied with Cu salt addition (Table 2), actually increasing with Cu addition up to the 1500 mg/kg level, and decreasing at higher additions. This behavior suggests that recently added Cu salts do not have higher solubility than Cu already present from past amendment, and is probably an indication that there is limited dissolved organic matter to solubilize Cu.

Maize Response to Soil Cupric Ion Activity
Maize seedlings showed a significant reduction of growth at all Cu addition levels, relative to the peat unamended with Cu, as measured by dry weight after 3 wk growth (Fig. 3) . Furthermore, all Cu addition levels, including 200 mg/kg, produced obvious interveinal chlorosis, most marked in the youngest leaves. Chlorosis became increasingly severe at the highest Cu addition levels, with most of the leaf tissue developing a faded yellow appearance; however, yield as measured by plant dry weight was not very sensitive to Cu addition beyond 200 mg/kg (Fig. 3).



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  		Fig. 3. The dependence of maize shoot weight (yield) and soil pCu on the concentration of Cu added to the soil

 
Iron concentrations in the maize shoots were decreased to levels considered to be deficient (50–150 mg/kg) (Marschner, 1995), even by the lowest (200–400 mg/kg) additions of Cu to the peat (Fig. 4) . Shoot Cu increased to the greatest extent with the first 200 mg/kg addition of Cu to the peat, with smaller increases at higher additions. Total shoot Cu remained less than 35 mg/kg at all Cu addition levels, actually showing evidence of decreased tissue Cu at the highest soil Cu loadings. Copper addition to the soil had little effect on shoot Mn concentrations, with tissue Mn greater than 35 mg/kg at all soil Cu levels. The critical Mn tissue concentration for deficiency is about 10 to 20 mg/kg (Marschner, 1995).



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  		Fig. 4. The concentration of Cu and Fe in maize shoots as a function of Cu added to peat soil

 
There was an inverse relationship between shoot Cu and Fe, as shown in Fig. 4. This pattern is consistent with the known ability of Cu to strongly inhibit Fe uptake (Wallace et al., 1992; Marschner, 1995), either by competing with Fe at transport sites of the root, competing with phytosiderophores released by the root (Marschner, 1995), or by inhibiting growth of fine roots (Lexmond and van der Vorm, 1981). Inspection of the maize roots washed free of soil after the tops were harvested showed a root system that became progressively smaller and thicker with fewer fine roots as soil Cu increased. Stunted roots, with low surface area, could account for the observation in Fig. 4 that the two highest soil Cu loadings actually caused a decrease in tissue Cu. Lexmond and van der Vorm (1981) showed that root stunting by Cu toxicity can retard Cu uptake to shoots. These results are an indication that plant shoot or leaf analyses for Cu are a potentially misleading indicator of Cu phytotoxicity. Figure 3 indicates the extent of top growth inhibition attributable to Cu toxicity, revealing that seedling weight is not much affected by the severity of the toxic stress once the initial effect of the pCu decrease from 8 to 7 occurs. However, progressively more severe damage was evident as chlorosis and root stunting as pCu decreased further below 7 at higher soil Cu loadings. It appears that shoot weight in a short-duration bioassay with maize does not reliably gauge the level of toxicity.

Phytotoxicity was evident in maize seedlings at lower soil solution Cu concentrations (<0.20 mg/L) than has been found with near-neutral mineral soils (unpublished data). This can probably be attributed to the low pH of the peat (about 5), which causes a greater fraction of the soluble Cu to be in the more phytotoxic cationic Cu2+ form (see Table 2). Typically, more than 95% of dissolved Cu in nonacid mineral soils, even when moderately contaminated with Cu, is complexed with organic ligands (Sauvé et al., 1997).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Indicators of Phytotoxicity in Maize
When the soil chemical data and the maize bioassay results are considered together, it is apparent that in this peat soil, a pCu in the 7 to 8 range is phytotoxic to maize, as evidenced by reduced growth in tops and roots, obvious chlorosis, decrease in tissue Fe below the deficiency threshold, and increase in shoot Cu. This pCu is in general agreement with independent estimates of the threshold soil Cu2+ activity that induces symptoms of toxicity in several crops (Sauvé et al., 1998). Shoot Cu may be the least reliable indicator of Cu phytotoxicity in maize because of the strong barrier to Cu transfer from roots to shoots. Chang et al. (1992) and Logan et al. (1997) presented data for maize and other crops, grown on Cu-contaminated sewage sludge–amended soils, which revealed little change in crop Cu concentration in response to substantial increases in total soil Cu. The present study further indicates that short-term "yields" measured with seedlings fail to reflect the progressively more toxic concentrations of free Cu2+ in soils that stunt roots and exacerbate chlorosis in the shoots. That these severe effects on the young plants did not reduce shoot weight to a greater extent relative to controls can be attributed to the artificial limitations on growth by the use of small pots and lower than natural light intensity. Also, with short-term bioassays, maize seedlings use seed reserves of energy and micronutrients to achieve initial growth. In the field, a similar degree of chlorosis and stunted roots produced by high Cu2+ activity could be expected to have a more severe effect on economic yield by greatly limiting photosynthesis and water and nutrient uptake. Small yield reductions in young plants, because of the exponential nature of growth, could magnify to large reductions in yield at maturity. For these reasons, the criterion of 50% yield reduction used by USEPA as the lowest observable adverse effect level (LOAEL) for phytotoxicity, based on short-term greenhouse pot or hydroponic experiments that measure vegetative growth, may grossly underestimate practical yield reductions at a particular plant tissue metal concentration.

Interestingly, none of the maize plants in the present study reached the USEPA NOAEL of maize plants (40 mg/kg Cu in shoot), despite severely damaged plants and soil Cu as high as 4000 mg/kg. Numerous other studies have indicated that a more reasonable threshold concentration for maize phytotoxicity is 20 mg/kg in shoots (e.g., Borkert et al., 1998; Mocquot et al., 1996). The barrier to root–shoot transfer of Cu means that large increases in total soil and Cu solubility must occur before maize shoot Cu can approach or exceed 20 mg/kg.

Because of the expected higher solubility and potential toxicity of Cu salts recently added to soils compared with Cu added in sewage sludges or manures (McBride, 1995; Minnich et al., 1987), the total soil Cu concentrations that produced phytotoxicity in this study cannot be taken to derive soil Cu loading limits. Nevertheless, the measured pCu value reflects the level of dissolved Cu2+ ion that causes growth reduction and is therefore a superior indicator of Cu toxicity because it already accounts for differences in the form of added Cu and aging effects. Furthermore, the Kd values for added Cu were actually higher than for Cu added in the past (Table 2), suggesting that aging effects do not significantly diminish Cu solubility in organic-rich soils after the first few weeks of reaction.

The Long-Term Fate of Soil Copper
In three peat soils with variable lengths of cropping history, the uncropped (virgin) soil was lower in total Cu (10 mg/kg) than the peats cropped for 20 or 60 yr (60–80 mg/kg), attributable to repeated CuSO4 applications in vegetable production. There was, however, no increase in free Cu2+ activity resulting from the greater Cu concentration, due in part to a pH increase and perhaps other soil chemical changes associated with long-term cultivation. Continuous cropping, by promoting organic matter decomposition and soil subsidence, led to increases in a number of trace elements in the surface soil. Although 60 yr of cropping would be expected to lead to a greater soil Cu concentration than 20 yr, this was not what was found, raising the possibility of long-term leaching losses of Cu. However, it is possible that the 60-yr cropped soil had received less frequent or less heavy applications of Cu. Also, the higher bulk density of the more decomposed (60-yr) peat could account for a greater Cu content per hectare despite a similar soil Cu concentration, and therefore leaching loss may not have to be invoked to explain the similar Cu concentrations after 20 and 60 yr of cropping.

The partition coefficient, Kd (units of L/kg), measured for Cu in the 20-yr cropped soil, was on the order of 2000 to 5000, indicating a low potential for leaching where Cu is well-mixed with the soil. In practice, limited mixing and preferential flow of water could allow greater loss of Cu than suggested by this high Kd. The phenomenon of Cu loss from mineral soils in coffee (Coffea arabica L.) orchards where Cu is applied as a fungicide has been described by Lepp and Dickinson (1994), with mass balance calculations indicating leaching losses of 41 to 55%. However, it seems unlikely that relative losses in peat soils, with their intrinsically high affinity for Cu, could be as high as this.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The free Cu2+ activity in Cu-enriched peat soils is correlated to the severity of phytotoxicity as measured by several indicators in maize, including leaf chlorosis, root stunting, and reduced shoot growth and Fe concentration. A soil Cu2+ activity of 10-7 to 10-7.5 caused phytotoxicity in maize seedlings, corresponding to total Cu of about 275 mg/kg in the peat soil. This Cu2+ activity measurement is more directly related to phytotoxic effects than other soil tests, such as extractions with strong acids or chelating agents, because it is the free Cu2+ in soil solution that has the most direct toxic effects on roots. Nevertheless, a given Cu2+ activity, if strongly buffered by soluble complexes of Cu, which serve as Cu carriers, is likely to be more toxic than the same Cu2+ activity that is weakly buffered. Thus, a pCu of 7.5 may be phytotoxic at a soil pH of 7, but not at pH 5, attributable to the greater amounts of soluble organically bound Cu generally found at higher pH.


   ACKNOWLEDGMENTS
 
The author thanks Dr. L.A. Ellerbrock for collecting the peat samples, Dr. L. Tyler for ICP analyses, and Christine Besemer for laboratory assistance. The financial support of USDA Agricultural Ecosystems Program, Award no. 91-34244-5917, is acknowledged.


   REFERENCES
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 ABSTRACT
 INTRODUCTION
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 RESULTS
 DISCUSSION
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