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VII. Soil As and Vegetation
E. SMITH ETAL.
Steevens et al., 1972; Anastasia and Kender, 1973; MacLean and Leangille, 1981;
Jiang and Singh, 1994).
A. SOILAs AND PLANTUPTAKE
The accumulation of As in the edible parts of most plants is generally low
(Vaughan, 1993; O'Neill, 1995). Plants seldom accumulate As at concentrations
hazardous to human and animal health because phytotoxicity usually occurs before such concentrations are reached (Walsh and Keeney, 1975). Thus, the major
hazard for animal and human systems is ingesting As-contaminated soils or consuming contaminated water.
Uptake of As by plants occurs primarily through the root system, and the highest As concentrations are reported in plant roots and tubers (Anastasia and Kender,
1973; Marin et al., 1993). Therefore, tuber crops (e.g., potatoes) could be expected to have higher As concentrations than other crop types when grown in polluted soils. This appears not to be the case, since potatoes grown in a sandy soil that
received As additions ranging from 45 to 720 kg As ha-' accumulated only 0.5
mg As kg-' in the potato tuber (Jacobs et al., 1970). In contrast, the external potato peels had As concentrations of up to 84 mg As kg- I . This was attributed to contamination from soil adhering to the surface peels.
1. Crop Tolerance to As
A considerable variation in plant sensitivity to As exists among plant species
(Jacobs et al., 1970; Jiang and Singh, 1994). Vegetable crops grown in three soils
(Lakeland loamy sand, Hagerstown clay loam, and Christiana clay loam) in a
greenhouse trial exhibited a range of sensitivities to sodium arsenate (0-500 mg
As kg- I ) . Plant sensitivity followed the order green beans > lima beans = spinach
> radish > tomato > cabbage (Woolson, 1973).
The symptoms of phytotoxicity may vary between species. Tomato plants
grown in soils with high As background concentrations (100-130 mg As kg- I )
showed leaf dieback from the tip and poor-quality fruit set (Fergus, 1955). Fruit
trees grown on replanted orchard sites commonly exhibit retarded early growth, to
which As toxicity may contribute (Davenport and Peryea, 1991). Similarly, rice
grown on former cotton-producing soils that had a history of repeated MSMA applications showed indications of susceptibility to straighthead disease (abnormally developed or sterile flowers resulting in low grain yields) under flooded soils
conditions (Wells and Gilmor, 1977). The range of soil-As concentrations that may
be phytotoxic is summarized in Fig. 6. Although the data are not extensive, they
highlight both the broad range of concentrations of soil As over which toxicity
symptoms may occur and the narrow margin that exists between background con-
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
Figure 6 Range of As concentrations in soils at which crops may exhibit phytotoxic symptoms.
(A) Woolson, 1973; ( B ) Jacobs ef a/., 1970; (C) Woolson ef a/., 1973; (D) Woolson et al., 1971b; (E)
Steevens ef al., 1972; (F)Wells and Gilmor, 1977.
centrations of As (<40 mg As kg- in most soils) and phytotoxic concentrations.
As discussed earlier, the total concentration of As in soil is a poor indicator of
its bioavailability. The concept of bioavailability generally refers to some fraction
of the total amount of As present in the soil that best correlates with plant availability (Pierynski et al., 1994). Woolson et al. (1971a) related plant-available As
to the total soil As and found that the amount of water-soluble As was better correlated with plant growth than with total soil As. Plants absorb nutrients and metals from the soil solution, and the “bioavailable” As soil pool may be a better indicator than total As concentration of plant phytotoxicity in the soil (Woolson et
al., 1971; O’Neill, 1995). Sadiq (1986) reported that As concentration in maize
was correlated with the water-extractable As but not with the total As concentrations in calcareous soils. Jiang and Singh (1994) found a similar relationship between the As concentrations of barley and ryegrass plants grown in greenhouse experiments and soils (typic udipsamment and humaquept) fortified with As.
Reviewing the literature on As, Sheppard (1992) concluded that soil type is the
only significant variable when considering plant phytotoxicity for inorganic As. It
was reported that inorganic As was five times more toxic to plants in sands (mean
= 40 mg As k g ’ ) than in clay (mean = 200 mg As kg-’) soils. Arsenic phytotoxicity is expected to be greater in sandy soils than in other soil types, since sandy
soils generally contain low amounts of Fe and A1 oxides and clays. These soil constituents have been implicated in the adsorption of As from solution in soils.
E. SMITH ETAL.
The nature of As species in the soil solution may also determine the phytotoxicity. Although As is primarily present as AsV or As"' in the soil-water environment (Bohn, 1976), MMAA and DMAA compounds may also be present. Marin
et af. (1992) reported that both As']' and MMAA were found to be phytotoxic to
rice plants grown in nutrient solution, whereas AsV did not affect plant growth at
the same concentration (0.8 mg As liter-l). The degree of As uptake by rice followed the trend As"' > MMAA > AsV > DMAA. However, these observations
were made after nutrient solutions were amended with different As compounds to
produce concentrations (0.05,0.20, and 0.80 mg As literp1)that were much higher than those encountered under normal field conditions.
Furthermore, the effect of soil As on plant growth may be complicated by the
presence of competing anions, notably P. Phosphate and As exhibit similar physiochemical behavior in the soil, and both synergistic and antagonistic effects of P
on uptake of As by plants have been reported. Davenport and Peryea (1991) have
reported a reduction of As uptake by plants after the application of P. In other studies, P has been found to increase As availability and therefore to increase the As
concentration in plants (Woolson, 1973). The increased availability of As is particularly noticeable in sandy soils, where fewer adsorption sites are present and
added Pmay displace some of the bound As ions into soil solution (O'Neill, 1995).
Apart from P, N fertilizers, lime, and SO:- have also been observed to alleviate
or depress the availability of As to plants (Thomas, 1977; Merry et al., 1986).
VIII. SOIL As AND MICROORGANISMS
The effects of metal pollution on biotic communities have been extensively studied, and many of these studies have focused on the adaptation of bacterial communities through the development of resistance or tolerance mechanisms.
Biological transformations are important in redistributing As in soils (Fig. 7).
Arsenic may have a direct influence on the microbial populations present in the
soil. Decreases have been reported in microbial populations in soils that have been
polluted with As compounds (Malone, 1971; Bisessar, 1982; Maliszewska et al.,
1985). Bisessar (1982) reported that the decrease in population counts of bacteria,
actinomycetes, fungi, and nematodes was significantly correlated (p 5 0.05) with
concentrations of As in soil collected from several sites near a secondary Pb
smelter (Table VIII). Although As may have influenced a decline in the soil microflora population, the decline of the soil biological population was probably due
to the combined effects of all metals present. Speir et af. (1992), in contrast, reported that very few negative effects were attributable to Cu, Cr, and As in a pot
trial conducted to assess the feasibility of using CCA and boric-treated sawdust as
a soil amendment. The total concentrations of elements in the pot trial (45, 136,
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
I I Consumptton
Figure 7 Soil-air cycle (WHO, 198 1; reprinted by permission of the publisher).
63, and 32 mg kg- for Cu, Cr, As, and B, respectively) were considerably lower
than those present in Bisessar's (1982) work.
Considerable variation in tolerance to As compounds applied to the soil has been
shown by the soil microflora. Maliszewska et al. (1985) reported that As"' compounds were more toxic than As" compounds to microorganisms important to
maintaining soil fertility.Arsenite compounds were applied at 500, 1500,and 3000
mg As kg- I and As" compounds at 1000,5000, and 10,000 mg As kg- to sandy
and alluvial soils. The influence of As"' and As" compounds on the soil microflora differed, depending on the microflora and the soil type. In sandy soil, As"' depressed the growth of bacteria, whereas As" stimulated their proliferation. Overall, both As compounds had little effect on the development of actinomycetes and
fungi flora and suppressed the growth of Azotobacter sp.
Maliszewska et al. (1985) also measured a decrease of approximately 30% in
dehydrogenase activity in the soils. The decrease in activity was greatest in the
sandy soil and with the application of As"'. Dehydrogenase is an unspecific enzyme for assessing the effect of As compounds on the soil microflora; other authors have found that enzymes involved in more specific biochemical reactions are
inhibited by the addition of As to the soil environment. Tabatabai (1977) reported
that As"' greatly inhibited the urease activity of some soils at a concentration of
375 mg As"' kg- I , but the addition of As" at a similar concentration to the soil
had no effect on activity. Bardgett er al. (1 994) investigated microbial properties