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Management effects on cadmium accumulation by peanut and soybean

M.J. Bell1, M.J. McLaughlin2, G.A. Barry3, N.V. Halpin4 and G. Cozens2

1QDPI, Kingaroy Qld.
2
CSIRO L&W, Adelaide SA.
3
QDNR, Indooroopilly Qld.
4
QDPI, Bundaberg Qld.

ABSTRACT

Agronomic studies to minimise Cd accumulation by peanut and soybean were undertaken on acidic, sandy soils in the Bundaberg region (Qld). Individual amendments (lime, zinc, and organic matter additions) had no impact on phyto-available Cd in the soil, but a combination of these amendments reduced phyto-available Cd and Cd uptake (P<0.05) by peanut plants. Soil Zn application actually increased plant Cd uptake but reduced Cd concentration in peanut kernel, while liming increased kernel Cd concentration in the year of application. The impact of Zn fertiliser on kernel Cd suggests an important Cd-Zn interaction during translocation to developing pods. Peanuts and soybeans accumulated similar amounts of plant Cd, but peanut kernels contained higher Cd concentrations than soybean seeds. More frequent irrigation reduced Cd concentration in peanut kernel but not soybean seed, presumably via an effect on root activity in deeper soil layers where there was a greater proportion of phyto-available Cd.

KEY WORDS

Peanut, soybean, soil amendments, cadmium, irrigation frequency.

Introduction

The heavy metal cadmium (Cd) poses a significant threat to food safety due to its mobility in the soil - plant system. Historically, most of the Cd in Australian agricultural soils has been added as an impurity in phosphatic fertilisers and continued additions, albeit at a reduced rate as strict quality standards are enforced for fertiliser products, represent a serious threat to sustainable production systems. Similarly, the increasing use of organic waste materials (eg. biosolids, mill mud) and animal manures (eg. piggery sludge and poultry manure) as fertilisers in broadacre cropping and in horticulture poses some risks from heavy metal contaminants like Cd (1, 2).

Summer grain legumes like peanut and soybean are particularly at risk, due to their ability to accumulate high concentrations of Cd in the harvested product (3, 5, 7). This risk is being exacerbated by recent industry expansion into irrigated, high rainfall environments that offer more reliable production. These regions (at least in Queensland) are often characterised by sandy, acidic soils with a history of phosphatic fertiliser application – conditions that represent significant risks of high phyto-available Cd.

On high-risk soils, agronomic packages that minimise phyto-available Cd in the soil and/or restrict the concentration of Cd in the harvested product are required to enable grain legumes to form part of the crop rotation. Bell et al. (3) have reported significant variation in Cd concentration between peanut cultivars, so choice of variety will be an important part of that package. Research has also shown that soil properties (pH, clay mineralogy, organic matter content and salinity) and availability of other nutrients (eg. Zn) will influence Cd phyto-availability and plant uptake (6, 9). Preliminary findings (4) showed that raising soil Zn status was important in peanuts grown on a sandy soil, but liming to raise soil pH had no significant impact. Part of the reason for the lack of a lime response may have been the contribution to plant Cd uptake from deeper soil layers (below the 15-20cm depth of lime incorporation).

Bell et al. (4) also proposed that irrigation scheduling might have an impact on Cd uptake by plants, via effects on root distribution in soil layers with varying concentrations of phyto-available Cd. Initial studies with peanut in 1997/98 were inconclusive after being affected by unseasonal rain, and there are no reports of this phenomenon in soybean. This paper investigates the impact of soil amendments, organic matter and irrigation scheduling on Cd accumulation in peanut and soybean.

Materials and Methods

Experiments were undertaken in the field at Bundaberg during the 1998/99 and 1999/2000 summers. The first experiment (the soil amendment study) was established on a red podsolic soil at Kolan. Peanuts and sugarcane have been grown in rotation over a number of years at this site and peanuts typically return kernel Cd levels of 0.2 mg Cd/kg (fresh weight basis). There were eight treatments in a randomised complete block design, with four replicates. Treatments included –

Control (farmer practice); (ii) incorporated cane trash (10 t DW/ha); (iii) mill mud (150 t/ha wet weight); (iv) lime to pH 6.5 in the top 30cm (3 t/ha); (v) substitution of K2SO4 for normal KCl in basal fertiliser (50 kg K/ha); (vi) 100 kg zinc/ha as zinc sulphate heptahydrate; (vii) feedlot manure at 10 t DW/ha; and (viii) a treatment combining Zn, lime, K2SO4 and cane trash.

All treatments were broadcast on the soil surface one month before planting in October 1998. The materials were incorporated into the surface soil using a rotary hoe and then the profile was inverted to a depth of 30cm using a reversible plough, before normal seedbed preparation with tined implements. Soil samples to 60cm depth were collected at flowering (30 days after planting), at the same time as a destructive plant sample. Total aboveground biomass was determined at maturity, along with yield and kernel quality. A similar sampling protocol was followed in the 1999/2000 season, with the residual effect of the 1998 treatments being determined.

The second experiment (the irrigation scheduling study) was established on an acidic yellow earth at Calavos. Treatments were a factorial combination of frequent and infrequent watering using either above-ground sprinklers or subsurface trickle (20cm depth), with the frequency (30% or 60% deficit of plant available water in the top 60cm) determined by monitoring soil water status using an Enviroscan capacitance probe. Subplots consisted of either peanuts (cv. Conder) or soybeans (cv. Melrose), planted in October of 1998 and 1999. Destructive plant samples were taken at flowering in each species and again at maturity. Leaf loss in soybean meant only seeds were analysed from the maturity sample, while peanut kernels and tops were analysed separately.

In both experiments, plant material (shoot) was oven-dried (60C) and both shoots and seeds (in the case of peanuts, whole kernels with testa intact) were ground using a stainless steel grinder. Samples were totally digested in a concentrated solution of HNO3 / H2O2, after which the solution Cd concentration was determined by graphite furnace atomic absorption spectrophotometry (GF-AAS). All plant Cd concentrations are expressed on a dry weight basis, while those of seeds are on a fresh weight basis, with moisture contents ranging from 2 to 5%.

CaCl2-extractable Cd (weakly-bound surface Cd) was determined by extracting soils for 4 h in 0.01 M CaCl2 solution using a soil:solution ratio of 1:5. Concentrations of Cd in extracts were determined by GF-AAS using Zeeman background correction. Other more routine soil analyses, including DTPA-extractable Zn, were undertaken using standard procedures described in (10).

RESULTS AND DISCUSSION

All treatments in the soil amendment study produced significant effects on soil chemical properties (eg. the effects of Zn fertilisers on DTPA Zn in the top 20 cm). However, only one (that combining Zn, lime, K2SO4 and cane trash) significantly reduced phyto-available Cd (extracted using 0.01M CaCl2) in the top 20cm of the soil profile in either the year of application or 12 months later (Table 1). This was despite significant changes in soil pH in the lime treatment – an effect that has been shown to lower phyto-available Cd in other crops in the year following application (8). It was interesting to note the apparent synergistic effect of the combined amendments, as none had produced any changes in phyto-available Cd when applied in isolation. This phenomenon is being investigated in more detail in current studies.

None of the treatments in this trial produced any significant effects on either biomass production or pod yields. Analyses of plant Cd data in the year of application (Fig. 1) show that the only treatment to significantly reduce the Cd concentration in vegetative biomass at flowering was the ‘combined’ treatment – consistent with the effect on phyto-available Cd shown in Table 1. Interestingly, the ‘zinc’ treatment raised Zn concentration in vegetative biomass from 19.5 mg/kg to 80.7 mg/kg, but also significantly raised tissue Cd concentration (Fig. 1).

In contrast to plant uptake, a number of treatments caused significant effects on kernel Cd concentration (Fig. 1). The ‘combined’ treatment reduced kernel Cd, as expected from Cd concentration in the vegetative biomass. However, the ‘zinc’ treatment also significantly reduced Cd concentration in kernels, despite raising plant Cd concentration, and suggested significant interactions between Zn and Cd during phloem loading and/or remobilisation of nutrient reserves during pod filling. Conversely, the ‘lime’ treatment actually raised kernel Cd concentration despite not having any significant effects on Cd concentration in vegetative biomass.

Results from the ‘irrigation scheduling’ study highlighted species differences between peanut and soybean in terms of the relative risk of high Cd concentrations in harvested product (Fig. 2). Both species did not contain significantly different Cd concentrations in vegetative biomass, and both showed Cd concentrations in harvested product as high as (soybean) or much higher (peanut) than in vegetative plant parts – a very different situation to other grain crops.

Peanut did show a significant reduction in kernel Cd concentration with more frequent irrigation (either overhead sprinkler or subsurface trickle), while soybean was unresponsive. The lack of response in vegetative material at flowering may simply reflect the relatively low root activity in deeper soil layers early in the growing season.

Table 1. Effect of soil amendments on pHw, DTPA-extractable Zn (mg/kg) and 0.01M CaCl2-extractable Cd (g/kg) at flowering in the 1998 (year of treatment application) and 1999 peanut crops.

 

1998/99

1999/00

pHw

DTPA Zn

CaCl2 Cd

pHw

DTPA Zn

CaCl2 Cd

10
cm

20
cm

40
cm

10
cm

20
cm

40
cm

10
cm

20
cm

40
cm

10cm

20cm

40cm

10cm

20
cm

40
cm

10
cm

20
cm

40
cm

Control

6.4

6.4

5.5

0.4

0.4

0.1

3.0

3.0

4.4

6.2

6.3

6.0

0.3

0.2

0.1

4.2

3.7

2.1

Cane trash

6.5

6.5

5.4

0.3

0.4

0.1

2.3

2.3

3.4

5.9

6.1

5.8

0.3

0.3

0.1

3.9

3.5

2.3

Mill mud

6.6

6.4

5.5

0.8

0.6

0.1

2.9

2.4

4.0

5.9

6.0

5.9

0.5

0.5

0.1

4.2

3.9

2.1

Lime

7.2

7.2

5.8

0.4

0.2

0.1

3.2

2.1

3.2

6.3

6.8

6.0

0.3

0.2

0.1

3.7

3.5

1.6

K2SO4

6.6

6.6

5.8

0.3

0.2

0.1

2.3

2.5

2.5

6.0

6.4

6.0

0.3

0.2

0.1

3.8

3.4

1.6

Manure

6.5

6.8

5.6

1.4

1.0

0.1

2.2

1.5

4.0

6.0

6.2

5.9

0.9

0.6

0.2

3.4

3.3

1.8

ZnSO4

6.4

6.2

5.5

13.3

10.7

0.2

3.3

4.4

3.9

5.8

6.0

5.7

9.5

7.7

1.9

4.1

3.5

3.3

Trash + Lime + Zn + K2SO4

7.2

7.2

5.6

18.0

21.9

0.4

0.6

0.4

4.0

6.5

6.6

6.3

9.9

11.6

2.8

1.6

1.4

1.2

lsd (0.05)

0.4

0.5

ns

5.3

9.8

0.2

1.0

1.2

ns

0.2

0.3

ns

1.1

1.7

0.5

0.7

0.6

1.2

Fig. 1 Effects of soil amendments on Cd concentration (mg/kg) in vegetative biomass at flowering and kernels at maturity for peanut cv. NC7. Data are from the year of treatment application. Vertical bars indicate lsd values (P< 0.05).

The reasons for the irrigation response are unclear, but there are two possible explanations. Firstly, frequent irrigation (especially with overhead sprinklers) may have improved access to higher soil Zn reserves in surface soil layers (0-20 cm DTPA Zn = 0.465 mg/kg) relative to those in the subsoil (20-60 cm DTPA Zn = 0.144 mg/kg). Whilst Zn concentrations in either seed/kernel or vegetative material were not significantly lower (p< 0.05) with infrequent irrigation, there was a consistent trend for lower Zn concentrations with infrequent irrigation. Alternately, while total Cd concentrations were lower in the deeper soil layers, there was a higher proportion of phyto-available Cd (eg. the proportion of total Cd that was phyto-available in the 0-10cm, 10-20cm, 20-40cm and 40-60cm layers was 10.5%, 6.6%, 17.4% and 26.4%, respectively). This increased phyto-availability may have resulted in greater plant Cd uptake when roots were in these deeper soil layers. These data suggest that management factors such as irrigation scheduling, that affect root distribution and activity in different parts of the profile, may be an important part of minimising Cd accumulation in peanut. Further work on this aspect of plant Cd uptake is continuing.

Figure 2. Effect of irrigation frequency on Cd concentration (mg/kg) in vegetative biomass (tops) and seeds of peanut (cv. Conder) and soybean (cv. Melrose). Vertical bars indicate lsd (P<0.05).

CONCLUSIONS

Accumulation of the heavy metal Cd poses a serious risk to cultivation of summer grain legumes (especially peanuts) on acidic, sandy soils. There are a number of soil and crop management strategies that show promise for minimising Cd uptake. A combination of lime and organic matter will reduce phyto-available Cd, but further work is needed to understand the mechanism for this response. Maintaining adequate plant Zn status will be a key strategy in peanut, with further work needed to determine whether application method (to soil or foliage) affects this result. Infrequent irrigation and a resulting increase in root activity in more acidic subsoil layers also increased kernel Cd in peanut, although the mechanism of this response is also unclear.

Acknowledgments

Funding support for this work from GRDC and SRDC (Project DAQ437) and the Peanut Company of Australia is gratefully acknowledged, as is the field assistance of Mr Luca Pippia and access to the properties of Mr Don Halpin Mr Pat Hawe to conduct these studies.

REFERENCES

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6. McLaughlin, M.J., Tiller, K.G., Naidu, R. and Stevens, D.P. 1996. Aust. J. Soil Res. 34, 1-54.

7. McLaughlin, M.J., Hamon, R.E., McLaren, R.G., Speir, T.W. and Rogers, S. L. 2000. Aust J. Soil Res. (In press).

8. Mordvedt, J. J., Mays, D. A. and Osborn, G. (1981). J. Environ. Qual. 10, 193-197.

9. Oliver, D.P., Hannam, R.J., Tiller, K.G., Wilhelm, N.S., Merry, R.H. and Cozens, G.D. 1994. J. Env. Qual. 23, 705-11.

10. Rayment, G. E., and Higginson, F. R. 1992. Australian Laboratory Handbook of Soil and Water Chemical Analyses. (Inkata Press: Sydney).

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