Báo cáo khoa học: "general overview with a seasonal assessment silver fir forest in the Vosges mountains (France)"

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Review article Aluminium toxicity in declining forests: general overview with a seasonal assessment a in silver fir forest in the Vosges mountains (France) a JP Boudot, T Becquer, D Merlet, J Rouiller CNRS, Centre de Pédologie Biologique, UPR 6831 associated with the University of Nancy I, 17, rue Notre-Dame-des-Pauvres, BP 5, 54501 Vand&oelig;uvre-lès-Nancy, France 22 June 1993; 4 (Received September 1993) accepted A general overview on Al toxicity to plants is given, including the following aspects: Summary — symptoms; mechanisms; mitigating environmental factors; and diagnostic possibilities. An Al toxicity index is proposed to replace the classical but poorly performant Ca/Al ratio and is used in a declining fir stand in the Vosges mountains (eastern France). A potential Al toxicity phase was observed in winter only, namely during the vegetation rest phase. As nutrient uptake is expected to be potentially low during this season, this finding suggests that Al toxicity is probably not strongly involved in the local forest decline. However, a low influence may occur with respect to the winter growth of mycor- rhized fine roots. aluminium toxicity I aluminium speciation I acid rain I forest decline Résumé — La toxicité de l’aluminium dans les forêts dépérissantes. Connaissances géné- rales et application au cas d’une sapinière vosgienne. Les auteurs passent en revue les princi- pales connaissances portant sur la toxicité de l’aluminium vis-à-vis des végétaux. Un index de toxici- té aluminique est proposé en remplacement de l’habituel rapport Ca/Al, très insuffisant. Son application au cas d’une sapinière dépérissante des Vosges permet de mettre en évidence l’exis- tence d’une phase de toxicité aluminique potentielle en hiver. Les besoins en nutriments étant très faibles durant cette saison, cette phase n’a probablement qu’une influence mineure sur l’état dépé- rissant du peuplement. Un faible impact pourrait néanmoins être envisagé dans l’éventualité où les essences locales présenteraient normalement une croissance optimale de leurs racines fines myco- rhizées en hiver, comme cela a été établi pour une espèce américaine de sapin. toxicité aluminique / spéciation de l’aluminium / pluies acides / dépérissement forestier
INTRODUCTION This paper provides a general overview of Al toxicity to plants. An Al toxicity index will be then proposed and the occurrence Two of the most striking features of acid of Al toxicity investigated in a declining sil- soils are their high exchangeable Al con- ver fir (abies alba Mill) forest in the Vosges tent and their low base cation status. Al- highlands. though acid soils have proved to be unsuit- able for a number of agricultural species, most of them have till now allowed the de- GENERAL OUTLINE velopment of forest ecosystems. The natu- OF ALUMINIUM TOXICITY TO PLANTS ral occurrence of soluble, organically com- plexed Al has been recognized for many Although some plants can accommodate years in podzolic soils, where the translo- high amounts of Al in their foliage without cation of organic forms of Al was repeated- serious injury (as high as 1 350 mg Al·kg -1 ly suggested and demonstrated (Konono- of dry needles in the case of Picea abies va, 1961; Duchaufour, 1970; Bartoli et al, (L) Karst (Ogner and Teigen, 1980)), many 1981; Nilsson and Bergkvist, 1983; David species are sensitive to soluble Al in soil and Driscoll, 1984; Dahlgren and Ugolini, solutions, which can be highly toxic under 1989; Baur and Feger, 1992; Berggren, certain conditions. 1992). The existence of soluble inorganic Al in acid brown soils, mainly arising from acidification due to biological processes Symptoms of Al toxicity (nitrification, mineralization of organic sul- phur), is a more recent observation (Ulrich et al, 1980; Van Breemen et al, 1987; Nys, In a number of crop species, Al toxicity is 1987; Becquer, 1991; Baur and Feger, indicated by a coralloid morphology of the 1992; Becquer et al, 1992). A number of root system, which exhibits scarce root forest trees have adapted to such chemi- hairs, scarce, short and thick secondary cal environments. Due to atmospheric pol- roots and short, swollen, stubby and lution and related acid deposition, how- gnarled primary roots. Root tips may addi- tionally turn brown in the most severe cir- ever, Al content in soil solution is now as- cumstances and, as for tree species, the sumed to increase. Moreover, important above-ground organs may wilt and die due changes in Al speciation are expected to to inhibition of water uptake (Foy, 1984; occur in many acid ecosystems, with pos- Arp and Strucel, 1989; Grimme and Lind- sible partial decomplexation of soluble hauer, 1989). In contrast, Ca deficiency organic Al due to pH decrease. High in- leads to short, slender and straight primary puts in nitric, sulphuric and chlorhydric ac- roots with brown tips (not swollen). ids are nowadays quantified in a number of ecosystems throughout the world. Coralloid roots due to Al injury are not Whether the vegetation will adapt to such for tree species and specific reported environmental alterations is uncertain. The symptoms cannot be found. Roots are toxicity of soluble Al was clearly demon- shortened, exhibit a necrotic morphology strated for many agricultural species. More and turn dark brown. Secondary root for- recently, it was hypothesized that Al toxici- mation is restricted and the branching pat- ty was also involved in forest decline (Ul- tern of all the underground system is re- rich et al, 1980; Hüttermann and Ulrich, duced. Leaves may exhibit a chlorotic appearance and it was demonstrated that 1984).
the yellowing of some European conifers uptake, in connection to low root elonga- due to magnesium deficiency (Zöttl tion and efficiency (Arp and Strucel, 1989; was and Hüttl, 1986; Landmann et al, 1987), Grimme and Lindhauer, 1989), applies to the latter being related either to base cat- both Ca and Mg but also to iron and 2+ 2+ ion depletion or to Al or Mn toxicity (Hecht- important anions such as SO PO ,, 2- 3- 44 Buchholz et al, 1987; Godbold et al, 1988; - Cl and NO (Foy, 1984; Cambraia et al, - 3 Godbold, 1991; Göransson and Eldhuset, 1989); vi) inhibition of important enzymatic 1991; Schlegel et al, 1992). Calcium defi- systems such as acid phosphatases (Pet- ciency has sometimes been reported to terson et al, 1988), ATPases, calmodulin also occur in declining stands growing on (Haug, 1984), and nitrate reductase (Cam- acid soils, and this may also be related to braia et al, 1989); vii) shift from an aerobic Al toxicity (Joslin et al, 1988; Shortle and metabolism to anaerobic one, with in- Smith, 1988). creased activity of the corresponding enzy- matic system (Copeland and De Lima, 1992); viii) phosphate precipitation in roots Mechanisms involved in Al toxicity by accumulated Al, with concomitant P de- ficiency in the above-ground organs (Schaedle et al, 1989; Asp et al, 1991). The main mechanisms that were recog- nized to operate in the detrimental action of monomeric forms of Al against plants Mitigating factors for Al toxicity are as follows: i) competition between Al species, Ca and Mg for the meriste- 2+ 2+ matic root absorbing sites acts toward a The detrimental action of soluble Al to lowering of Ca and Mg uptake (Asp et al, plants can be ameliorated both by biologi- 1988; Bengtsson et al, 1988; Schröder et cal factors and soil chemical conditions, al, 1988; Lindberg, 1990; Rengel, 1990; such as total base cation concentration Tan and Keltjens, 1990; Godbold, 1991; and the identity of the particular Al species Göransson and Eldhuset, 1991; Schiman- in soil solutions. sky, 1991),but not, despite a number of conflicting reports, towards a direct inhibi- Biological factors tion of potassium uptake (Petterson and Strid, 1989; Rengel and Robinson, 1990; It was hypothesized that, in the field, my- Horst et al, 1992); ii) inhibition of the meris- corrhizae will protect trees against Al toxic- tematic cell division originates mainly from ity. A number of conflicting reports, how- the inhibition of DNA replication and relat- ever, have shown that this assertion ed mitotic activity as a consequence of Al- should not be generalized. Although the DNA linkages, and leads to low root mycorrhizal fungi Pisolithus tinctorius growth and efficiency (Matsumoto et al, Coker and Couch and Paxillus involutus Fr 1979; Tepper et al, 1989); iii) strong inhibi- have been shown to protect at least partly tion of cytokinines synthesis and transloca- pitch pine (Pinus rigida Mill) and Norway tion also reinforces the inhibition of the root spruce, respectively, from Al toxicity (Wil- system development (Pan et al, 1989); iv) kins and Hodson, 1989; Cumming and alteration of the root membrane structure Weinstein, 1990; Kasuya et al, 1990; and functioning (Hecht-Buchholz and Foy, Hentschel et al, 1993), mycorrhizal infec- 1981; Foy, 1984), including the blockage tion by Lactarius rufus (Scop) Fr does not of Ca channels (Huang et al, 1992; Rengel and Elliot, 1992); v) low nutrient and water protect Norway spruce (Jentschke et al,
Al was found to be tox- 1991).Additionally, Among the monomeric inorganic spe- ic for cies of aluminium, Al AlOH Al(OH) ,,+ 3+ 2+ 2 number of mycorrhizal fungi a (Browning and Hutchinson, 1991; Jong- and Al(OH) (due to its polymerisation to - 4 bloed and Borst-Pauwels, 1992; Zel et al, Al once absorbed by roots) are currently 13 1992), so that mycorrhizal infection per se regarded as toxic. A great controversy ex- may be reduced by Al (Boxman et al, ists with respect to their relative toxicity, 1991).As a consequence, a low density of however, and recent data suggest that mycorrhizae was reported in the field in AlSO must be included here, despite re- + 4 Al-exposed stands (Schlegel et al, 1992). peated reports about its so-called non- toxicity. This will be discussed below. No generalisation can be drawn and the hypothesis of the alleviation of Al toxicity toxicity of Al was clearly demon- 3+ The by mycorrhizal fungus is far from being strated Parker et al (1988a) for wheat. by verified. According to Noble et al (1988a) and No- Other biological factors include the so- ble and Summer (1988), the toxicity of mon- called strain effect. Both various cultivars onuclear Al species for soybean decreased in the order Al > AlOH > Al(OH) This 3+ 2+ 2 of cereals and provenances of Norway . + spruce were proved to exhibit contrasting view is not so far away from the conclusion of Bruce et al (1988), for which Al and 3+ Al resistance capacities to Al toxicity, ow- 2+ AlOH are the only toxic inorganic mono- ing to important differences in metabolism and root membranes properties (Geburek meric Al species. Other data suggest con- versely that AlOH and Al(OH) are 2+ et al, 1986; Wilkins and Hodson, 1989; + 2 much more toxic for soybean than Al 3+ Blamey et al, 1992). (Alva et al, 1986a). Polymeric forms of Al occurred in this case and the presence of Chemical factors the very toxic Al could not be ruled out, 13 invalidating this conclusion as a conse- Some inorganic and organic anions allevi- quence. According to Kinraide and Parker ate the toxicity of Al by forming soluble (1989, 1990) and Kinraide (1991), wheat complexes (species) of low toxicity or de- and possibly a number of monocotyledons void of toxicity. Additionally, some cations would be sensitive to Al but not to the Al- 3+ act by competing with Al at the root ab- OH mononuclear species. Dicotyledons sorbing sites. would be sensitive to Al-OH monomers at least and perhaps also to Al This is not . 3+ Alleviation of Al toxicity by inorganic totally convincing, however, since: i) their and organic anionic ligands. Toxic assumption that H+ is less toxic than Al3+ and non-toxic species of aluminium is clearly an accommodation in contradic- Hydroxyls, fluoride, sulphate, phosphate, tion with literature data (Shuman et al, silica and organic matter are the most im- 1991),particularly with the repeated obser- portant relevant ligands for Al with respect vation that a low concentration of Al exerts to terrestrial and aquatic ecosystems. a beneficial effect on root elongation as a Some of the resulting Al species (see ta- consequence of the replacement of a ble I for a complete list) either are not toxic strong H toxicity by a lower Al one (Viets, + or have a lower toxicity, and the latter may 1944; Fawzy et al, 1954; Thornton et al, be partly related to their cationic charge, to 1986a, 1986b; Keltjens, 1990; Huang and their stability in the root environment and Bachelard, 1993); and ii) the variations of to the target organism. Others remain the respective proportions of Ca Mg , 2+ 2+ toxic. and Al were not taken into account. What-
the reality and according to Rost- species and that the toxic Al-sulphate ever new Siebert (1983), Hüttermann and Ulrich ion pair must be identified as AlSO . + 4 (1984), Hutchinson et al (1986), Thornton The fluoride complexes of Al prevailing et al (1987), Asp et al (1988) and Nosko in the acid range, namely AlF AlF ,+ 2+ 2 , and Kershaw (1992), the European spruce AlF and, more rarely, AlF have been * 3 , - 4 (Picea abies) and several American proved to be non-toxic (AIOHF AlOHF ,* +2 spruce appear to be sensitive at least to and Al(OH) being neglected due to their - F 3 3+ Al with a good certitude; data are, how- short half-life (Nordström and May, 1989)) ever, lacking with regards to their sensitivi- (Cameron et al, 1986; Tanaka et al, 1987). ty to AlOH and Al(OH) 2+ . + 2 Such species do not prevent root growth Although it does not constitute a toxic and do not inhibit Ca or Mg uptake (Mac- species per se, the aluminate ion Al(OH) - 4 Lean et al, 1992). should be included in the harmful forms of The toxicity of the monomeric Al-PO 4 Al, as it is expected to transform easily into and Al-Si complexes remains mostly un- the very toxic Al polymer within the 13 known but White et al (1976) and Alva et al roots, from which the free space remains (1986a) demonstrated that adding PO 3- 4 in the acid range as long as the external ions will induce a dramatic formation of pH was < 8.9 (Kinraide, 1990). Converse- non-toxic Al-PO polymers, to such a large 4 ly, Al(OH) does not constitute a toxic * 3 extent that the residual concentration of Al- species (Alva et al, 1986a; Tanaka et al, 4 PO monomers can probably be neglect- 1987). ed. Additionally, the non-toxicity of Al-PO 4 Strong controversies exist about the and Al-Si species have been proved with toxicity of AlSO The existence of a toxic . + 4 respect to Chlorella pyrenoidosa (Helliwell species of Al-sulphate was demonstrated et al, 1983), so that the same situation can by Van Praag et al (1985), Alva et al be eventually expected with regards to ter- (1986b), Joslin and Wolfe (1988) and restrial plant species. Tang et al (1989), with respect to the Euro- Polymeric forms of Al occur in acid solu- pean beech (Fagus silvatica L), the Ameri- tions above pH 3.5-5.5, depending on the can red spruce (Picea rubens Sarg), soy- concentration and the ionic strength. The bean (Glycine max (L)) and rice (Oriza existence of both toxic (Bartlett and Riego, sativa L). Conversely Pavan and Bingham 1972a; Wagatsuma and Ezoe, 1985; Wa- (1982), Cameron et al (1986), Kinraide gatsuma and Kaneko, 1987; Parker et al, and Parker (1987a), Tanaka et al (1987), 1988a) and non-toxic (Blamey et al, 1983) Noble et al (1988a, 1988b) and Wright et al (1989) claimed the non-toxicity of Al polymers is now well documented. The former was recently identified as the "Al AlSO regarded as the prevailing Al- , + 4 " 13 sulphate ion pair in their experimental con- polymer AlO Its tox- . 7+ O) 2 (H 24 (OH) 12 Al 4 ditions. In most of these experiments, how- icity was often considerably higher than ever, the SO ratio was high to very that of Al (Parker et al, 1989; Shann and 3+ /Al 4 high, ranged from 0.1 to 2 700 and was al- Bertsch, 1993). About 1 to 11.5 times less 1. As a consequence, the most always > Al as Al than as Al (ie about 13 to 150 3+ 13 prevailing sulphate ion was not AlSO but + 4 times on a molar basis) was needed to ob- a more recently discovered one (approxi- tain an inhibition of 20% of either soybean mately Al(SO (Alva et al, 1991). 2.4- 2.7 ) 4 or wheat root elongation. Moreover, plant As the latter was found to be non-toxic, species tolerant to monomeric Al remain there is a great probability that the so- highly sensitive to Al suggesting the oc- , 13 called "non-toxic AlSO was in fact this " + 4 currence of mechanisms other than those
listed above. The and Jardine, 1989) and would not be al- has been 13 Alpolymer to occur in soils lowed to maintain in the aqueous phase (Hunter recently reported other than in minor proportions, if any and Ross, 1991),under an adsorbed state in a podzol humus. As chelating organic (Brown and Newman, 1973; Bache and matter is regarded as an inhibitor of Al13 Sharp, 1976). Additionally, sulphate ions formation, this presence is very surprising. are known to restrict Al formation and 13 Additionally, the occurrence of Al(OH) is - 4 phosphate to precipitate Al polymers (Bart- believed to be a prerequisite to the forma- lett and Riego, 1972a; Blamey et al, 1983; tion of Al (Bertsch, 1987) and it is not 13 Alva et al, 1986b; Parker et al, 1989). easily conceivable that podzol humus of- Organic complexes of Al are wide- fers favourable conditions to its formation. spread in acid soils. According to Arp and Clay surfaces, however, would be highly Ouimet (1986) and Asp and Berggren favourable to Al hydrolysis and polymerisa- (1990), plant roots do not absorb Al com- tion, even in unsaturated solutions (Tenna- plexed with colloidal organic acids (at least koon et al, 1986), so that some generalisa- the largest fulvic and humic acids). Al com- tion of this finding cannot be ruled out. The plexed with non-colloidal organic acids toxicity of adsorbed Al if any, remains so , 13 (simple carboxylic acids and perhaps small far unknown but in the event that it equili- fulvic acids) are easily absorbed by roots, brates with soil solution, it would constitute however, to such an extent that complexa- a source of a high toxicity, especially dur- tion has been reported to enhance Al ab- ing soil acidification phases (Bertsch, sorption (Van Praag and Weissen, 1985; 1989). Arp and Ouimet, 1986; Arp and Strucel, The occurrence of Al in soil solutions 13 1989). Both absorbed and non-absorbed remains undocumented, due in part to the organic complexes of Al are currently re- lack of diagnostic tools compatible with ferred to as non-toxic (Brogan, 1964; Bar- natural soil water composition (Al concen- lett and Riego, 1972b; Rost-Siebert, 1983; tration being mostly too low for 27 NMR Al Van Praag and Weissen, 1985; Van Praag studies). That the ferron kinetic analysis et al, 1985; Hue et al, 1986; Suhayda and procedure can be used successfully in nat- Haug, 1986; Tan and Binger, 1986; Arp ural soil solutions must be verified in true and Strucel, 1989; Asp and Berggren, samples, as many interfering substances 1990; Suthipradit et al, 1990), so that the are able to darken the expected clarity of ability to synthesize and to exude chelating kinetic curves (Parker and Bertsch, 1992). organics can be regarded as a mechanism The presence of Al in natural soil water 13 of Al resistance (Horst et al, 1982; Miyasa- should be regarded as uncertain for sever- ka et al, 1991). al reasons. Al polymers are allowed to ap- pear only in supersaturated solutions with Alleviation of Al toxicity respect to gibbsite (when the saturation in- by competing elements dex is calculated without taking the possi- Some cations have been proved to miti- bility of Al formation into account) 13 gate Al toxicity by competing with mono- (Stumm and Morgan, 1981; Bloom and meric Al species and by lowering Al activi- Erich, 1989; Kinraide and Parker, 1989), ty. Non-toxic divalent cations are more and this is known to occur in natural soil efficient than monovalent ones so that the solutions. Once formed, however, Al poly- following general classification can be put mers are readily adsorbed onto anionic soil forward: Ca≈ Mg ≈ Sr >>> 2+2+ 2+ K+ Na+ organic and inorganic surfaces (Brown and = (Vidal and Broyer, 1962; Rhue and Gro- Hem, 1975; Parker et al, 1988a; Zelazny
phate and organic anions and which also gan, 1977; Alva et al, 1986c; Hecht- Buchholz and Schuster, 1987; Hecht- forms several pH-dependent hydroxy spe- as a substitute of Al in Buchholz et al, 1987; Kinraide and Parker, cies, has been used studies to assess transloca- 1987b; Tanaka et al, 1987; Rengel and toxicological tion pathways and mechanisms (Clarkson Robinson, 1990; Edmeades et al, 1991; and Sanderson, 1969). As scandium is 10 Tan et al, 1991; Blamey et al, 1992). De- pending on the studies, Mg was reported 2+ to 30 times more toxic than Al, this can be to be either more efficient, less efficient or validated only in the case of short-term la- as efficient as Ca however, and a sur- , 2+ boratory experiments (Yang et al, 1989). prising lack of amelioration of Al toxicity by the latter was even, but rarely, observed. Al content of plant organs The competing effect of K and Na+ is + about 200 times weaker than that of cal- Neither leaf nor root Al content can be cium (Kinraide and Parker, 1987b). Stron- used as a realistic tool for the assessment tium is only a minor element in natural soil of Al toxicity. Al content in needles of vari- solutions and can be neglected in field ous Picea species from north America and conditions. Europe is not related to Al concentration in soil solutions (Joslin and Wolfe, 1988). Moreover, as mentioned above, Picea Diagnostic tools for the assessment abies can accommodate up to 1 350 mg of Altoxicity in soils -1 Al·kg needles without damage. Addition- ally and according to McCormick and Bor- Root den (1972), Huett and Menary (1980), Wa- elongation gatsuma (1983) and Schaedle et al (1986), high proportion of root Al originates from Root elongation measurement is generally a non-metabolic processes, accumulates in regarded as a better indicator of Al toxicity than either roots or leaf dry weight. These cortical cells without further significant pen- values are currently used to calibrate the etration inside roots (Godbold et al, 1988; detrimental effect of various species of Al Schlegel et al, 1992) and is not toxic. Only in toxicological studies. The use of this cri- the Al which is related to the meristem terion in natural forest ecosystems is, how- area is regarded as directly toxic. ever, time consuming and poorly suitable. Exchangeable soil Al concentrations Tracer studies Saigusa et al (1980) stated that Al toxicity 45 48 and 86 (the latter re- + Rb , 2+ 2+ Ca Mg appeared when 1 N KCI exchangeable soil garded as a substitute of K have been Al was in excess of 2 meq·100 g A . -1 ) + used to assess Al influence on nutrient up- weaker ionic strength of the extractant was take with a good accuracy (Asp et al, recommended by other authors. Both 0.01 1988; Bengtsson et al, 1988; Godbold et M SrCl and CaCl Al were -extractable 2 - 2 al, 1988; Petterson and Strid, 1989; Asp found to be well correlated with total Al, and Berggren, 1990; Lindberg, 1990; Schi- monomeric Al, fine root Al content (in inor- mansky, 1991; Horst et al, 1992; Rengel ganic soil layers only) (Joslin and Wolfe, and Elliot, 1992). Due to some common 1988; Joslin et al, 1988; Conyers et al, properties with Al, 46 a trivalent cation Sc, 1991 a, 1991 b), response to Al toxicity (Kel- which may be complexed by both phos- ly et al, 1990) and root growth (Baligar et
al, 1992). Although a toxicity threshold was decreased. Picea rubens, P mariana (Mill) found to be reached for 10 mg extractable Britt, Fagus silvatica L and Acer sacchar- -1 Al·kg soil, it is clear that such procedures um Marsch belong to this group. A consid- address the source of toxic Al more than erably higher Al threshold has been report- genuine toxic Al per se, the latter being ed for Picea rubens (3 700 μM·l which ), -1 only a part of soluble Al in soil solutions. would pertain in this case to the following Therefore, exchangeable Al can constitute group (Schier, 1985). at best an indicative value only. Tolerant species are those which are sensitive to Al concentrations ≥ 800 μM·l -1 only. Species such as Pinus strobus L, P Aluminium concentrations sylvestris L, Picea sitchensis (Borg) Car, in soil solutions Car, Pseudotsuga douglasii (Lindley) Abies balsamea Mill, Fagus grandifolia Schaedle et al (1989) proposed the classi- Ehrh, Betula pendula Roth and Quercus fication of some important forest trees into rubra L pertain to this group (Schaedle et 3 groups, according to their sensitivity to al, 1989; Göransson and Eldhuset, 1991). soluble Al. A considerably lower Al threshold has also Sensitive species are those which ex- been reported for Quercus rubra (120-280 hibit sensitivity for Al concentrations ≤ 150 ) -1 μM·l (Kelly et al, 1990), which would . -1 μM·l Root tips turn brown and swollen pertain in this case to one of the previous 2 and elongation is inhibited. Foliar organs groups. are depleted in calcium and magnesium It can be noted that important discrep- and strong necrosis may occur. As roots ancies occur for several species, due ei- and sometimes shoots growing area are ther to uncontrolled strain effects or to ig- destroyed, natural maxima in Al concentra- nored nutrient factors. Indeed, a number of tions (Al pulses) affect durably plant devel- species are more tolerant to Al in Ca- and opment in the field and these species re- Mg-rich solutions. Roots of Picea abies do cover only slowly once Al stress has not show any injury as a consequence of ceased. Picea abies, P glauca (Moench) 1 700 μM·l Al in nutrient solutions when -1 Voss and Gleditsia triacanthos L belong 2+ Ca 1 300 μM·l and Mg 300 μM·l 2+ -1 - here, the latter being sensitive to Al con- = = 1 but are strongly damaged when these 2 centrations as low as 12 μM·l (Schaedle -1 elements reach only 130 and 30 μM·l re- , -1 et al, 1989; Sucoff et al, 1990). Higher tox- spectively (Hecht-Buchholz et al, 1987). icity thresholds (ranging from 200 to 700 Thus, the concept of a given Al concentra- ) -1 μM·l have been reported for Picea tion threshold for a given species is prob- abies, which could belong to the following ably not appropriate for the majority of group as well (Göransson and Eldhuset, plant species. 1991; Van Praag et al, 1985). Intermediate species are those which exhibit sensitivity for Al concentrations Aluminium index toxicity ranging from 150 to 800 μM·l Roots are . -1 apparently not damaged and only root and/ To of the previous dis- overcome some or shoot growth was affected. As growing the calculation of a toxicity in- crepancies, points are not destroyed but only inhibited dex that takes into account all the factors in their functioning, Al pulses affect root controlling Al toxicity is a useful and prom- development only temporarily and such ising approach to assess Al toxicity in a species recover rapidly once Al stress has given ecosystem.
According to Lund (1970), Rost-Siebert Al to wheat, the latter being regard- icity of ed as insensitive to Al-OH monomers: % (1983, 1984), Hüttermann and Ulrich }} 3+ 3+ 100{Al / [{Al + (1984), Wolfe and Joslin (1989) and Kelly root growth inhibition = 1.2 + 2.4{Ca + 1.6{Mg + }1.5 2+ 1.5 } 2+ et al (1990), the Ca/Al ratio in soil solu- 0.011{Na+}+0.011{K .8 Blamey et al 1.8 + ]. tions would be one of the best expressions 1 } for assessing Al toxicity, mainly with re- (1992) put forward an even more sophisti- cated index for dry weight productivity of spect to root development. It would reflect the competing conditions which occur be- wheat. These kinds of index are not stan- tween Ca regarded as the most impor- , 2+ dardised for other plant species, denied the probable toxicity of Al-OH and AlSO tant base cation, and soluble Al at the + 4 root monomers and do not involve the well- absorbing sites. Al toxicity would be a real- ity for all values of this ratio < 1 or 2 in the demonstrated toxicity of Al The same re- . 13 mark applies to the Al activity ratio (AER) case of Picea abies and Fagus silvatica. A of Bessho and Bell (1992): AER strong root mortality would occur for val- 1000 = [3{Al / (3{Al + 2 {Ca + 2{Mg + }) 2+ 2+ }} 3+ 3+ ues of this ratio around 0.2. The observa- tion of Bennet et al (1987) that Zea mays L } })]. ++ {K + {Na root cell division was inhibited when 1/2 Given these imperfections, the previous log Ca ≤ 1/3 log Al (on a molar basis) 2+ 3+ considerations make it tempting to modify reflects a closely allied concept. Obvious- the initial Ca/Al ratio and to propose the fol- ly, these expressions are imperfect, as lowing formulation as a general expression they do not include the beneficial effect of intended to assess any risk of Al toxicity: important elements such as Mg and do not ATI (aluminium toxicity index)= [4{Ca } 2+ take into account the non-toxicity of some } } }) +} 2+ + + 3 4{Mg + 0.02{K + 0.02{Na / [9{Al + Al species. Even the data of Rost-Siebert + 4{AlOH + {Al(OH) + {AlSO + }++ 2+ 2 4 }} (1983, 1984) and Hüttermann and Ulrich 117-1345{Al + 9-103{Al(OH) In this }- 13 4 }]. (1984) stretched the very limits of the Ca/ expression, brackets denote molar activi- Al ratio, which was validated only at pH < ties and each element is weighted by a co- 4 in absence of organic matter. Additional- efficient intended to reflect its relative ben- ly, ionic activities instead of concentrations eficial or detrimental effect. This coefficient should always be used in such studies is based on values produced by Grauer (Adams and Lund, 1966; Pavan and Bing- and Horst (1991) and on the relative effect, ham, 1982; Pavan et al, 1982; Tanaka et detailed above, exerted by each of these al, 1987; Thornton et al, 1987). elements or species. The toxicity threshold The calcium-aluminium balance (CAB = may be derived from literature data by re- }] - } } 2+ 3+ 2+ [2log{Ca [3log{Al + 2log{AlOH + calculating speciation whenever possible, log {Al(OH) of Noble et al (1988a, }]) + 2 and falls in the range 0.9 to 2 for Picea 1988b) and Noble and Sumner (1988) abies and Fagus silvatica (from Rost- of these imperfections. overcomes some Siebert, 1984; Hüttermann and Ulrich, it must be completed by taking Obviously, 1984; Neitzke, 1990). It can be either con- into account both the beneficial effect of siderably lower for some Pinus (0.1-0.2) 2+ Mg and the toxicity of AlSO Al(OH) , +- 44 and Betula (0.006) species (from Truman and Al at least. Other imperfections of 13 et al, 1983; Göransson and Eldhuset, the CAB were discussed by Grauer and 1987; Raynal et al, 1990), considerably or Horst (1991). for Gleditsia triacanthos (> 4.3) and higher Various other approaches have been some cereal species (4.8 to > 10) (from tried. Kinraide and Parker (1987b) pro- Hecht-Buchholz and Schuster, 1987; Suc- posed the following expression for the tox- off et al, 1990).
Before assessing Al toxicity, it must be Al complexes has been repeatedly report- ensured that a minimum amount of Ca is ed, however, ranging from about 0 to 34%, depending mainly on the Al/C ratio of the present so as to prevent absolute Ca defi- sample (Backes and Tipping, 1987; ciencies. The minimum Ca requirement is water Berggren, 1989; Dahlgren and Ugolini, known to be pH-dependent, and Ulrich et 1989; Kerven et al, 1989a; Van Benscho- al (1984) claimed that Ca deficiency oc- ten and curs for values of the Ca/H molar ratio in Edzwald, 1990). Decomplexation was negligible at low Al/C ratios, as is the soil solutions < 0.1 for conifers such as Pi- case in podzol solutions, increased pro- cea abies, and < 1 for more demanding once the Al/C ratio exceeded gressively species such as Fagus silvatica. 300-500 μM Al·g organic matter and -1 It goes without saying that it would be could reach 25% of initial organic Al for highly desirable to take into account any values of this ratio around 1 000. In Al-rich strain effect, as this has proved to be im- acid brown soils, this ratio ranges from portant at least in cereals and Picea abies 3 000 to 12 000 and the resin method obvi- (Geburek et al, 1986; Wilkins and Hodson, ously cannot be used. Moreover, un- 1989; Blamey et al, 1992). charged or negatively charged monomeric and polymeric colloidal inorganic species cannot be fixed by the resin and were re- OPERATIONAL PROCEDURES covered as organic Al (Lydersen et al, FOR ALUMINIUM SPECIATION 1990; Alvarez et al, 1992). The same im- perfections were observed with chelating The calculation of any valid toxicity index resins (Campbell et al, 1983; Hodges, requires the determination of Al speciation. 1987; Kerven et al, 1989a). Additionally, it Many procedures have been attempted but was shown that the oxine extraction failed operational artifacts have often been re- to recover quantitatively organic Al (La- ported. The most advanced techniques will lande and Hendershot, 1986; Royset and be listed below. Sullivan, 1986) and the cumulative effect of all these imperfections will result in strong uncertainties with respect to the reli- The Driscoll procedure ability of the results. The most widespread procedure of Al spe- Colorimetric procedures ciation is that of Driscoll (1984), which may be accompanied by some minor operation- al changes (Berggren, 1989; McAvoy et al, The oxine rapid extraction procedure 1992). This method is founded: i) on the (15 s) can be performed both at pH 8.3 use of a strong cationic exchange resin, (Lazerte, 1984) and pH 5 (James et al, set to sample pH and ionic strength, to 1983; Clarke et al, 1992). The pH 8.3 ex- separate organically complexed Al from in- traction was at the time assumed to pro- organic Al; and ii) on a rapid extraction (15 vide a good estimation of both inorganic s) of both inorganic and organic monomer- and organic monomeric Al species. Organ- ic Al by 8-hydroxyquinoline (= oxine) at pH ic Al, however, is not quantitatively recov- 8.3. With respect to the resin step, inorgan- ered and strong interferences (eg, Cu, Mn, ic Al is assumed to be fixed by the resin, Fe, Zn) occur. The pH 5 extraction does while organic Al passes through quantita- not significantly extract the Al-F complexes but organic Al is partly extracted in variable tively. Variable decomplexation of organic
the latter from 26 to ble Al may allow, under certain conditions, proportions, ranging the quantitative determination of several 55%, depending on the C/Al ratio (Lalande categories of Al, including monomeric Al, and Hendershot, 1986; Kerven et al, polymeric Al and colloidal, non-reactive 1989a; Whitten et al, 1992). Mn does not 13 Al (Jardine and Zelazny, 1986, 1987a, interference by Fe significantly interfere, 1987b; Parker et al, 1988b; Parker and be corrected, but that of Cu cannot be can Bertsch, 1992). Strong interferences with eliminated. A recent improvement of the Mn strongly limit, however, the application procedure by Clarke et al (1992) seems to of the method to natural solutions. Addi- suppress the partial extraction of organic tionally, organic, phosphate and fluoride Al and limits strongly the main interfer- anions tend to make the kinetics obscure ences. This improvement will deserve and poorly interpretable at anions/Al ratio great attention in the future. fairly relevant to surface and soil water Eriochrome cyanine reagent (McLean, composition. 1965) allows the measurement of inorgan- ic monomeric Al and unfortunately of vari- able proportions (75 to 95%) of organic Al Fluoride-selective electrode procedures (Adams and Moore, 1983; Kerven et al, 1989a). The use of the aluminon reagent The measurement of both free F and total - has been attempted as an alternative but it F by fluoride-selective electrode would al- was not very reliable (Wright et al, 1987; low theoretically the calculation of Al specia- Alva et al, 1989; Kerven et al, 1989b). tion. The reliability of the method depends None of these reagents are a good substi- on the F/Al ratio, the pH and the organic tute to the oxine reagent. carbon content of the solutions (Driscoll, The use of pyrocatechol violet (PCV) 1984; LaZerte, 1984; Hodges, 1987; Munns leads to similar results to the oxine rapid et al, 1992). Small F determination errors extraction at pH 8.3 and suffers compara- lead to small errors in Al speciation at pH 4 ble imperfections (Whitten et al, 1992). but to very high errors at pH 5.5. A low sen- Nevertheless, Achilli et al (1991) used this sitivity was observed for high values of the reagent with success to perform an organ- Al/F ratio. As a consequence, poor reliability ic complexation of all the monomeric Al would be expected in many natural waters. species and subsequently to measure pol- Alternatively, Ares (1986a, 1986b) de- ymeric Al after separation by a cationic veloped a procedure based on the inter- resin procedure. On the other hand, Men- pretation of the reaction kinetics of added zies et al (1992) proposed a modified PCV F with soluble Al species. The limits of this method in order to distinguish between method have been poorly investigated. soluble and suspended Al. The latter was flocculated with La and organic Al was 3+ decomplexed by the addition of Fe be- 3+ Procedures using fluorescence fore colorimetry. Although the behaviour of polymeric Al was not investigated, this Browne et al (1990) developed a proce- method seems to be very useful, in addi- dure using 2,3,4,5,7-pentahydroxy-flavone tion to those addressing inorganic Al mon- (morin) fluorescing chelating reagent as a omers. for Al; the fluorescence measurement al- The analysis of the colorimetric reaction lowed the calculation of initial Al specia- kinetic of the ferron reagent (8-hydroxy-7- tion. Interferences due to naturally fluo- iodo-5-quinoline-sulphonic acid) with solu- rescing organic matter cannot always be
both initial Al and all species that under- 3+ corrected and this method cannot apply to go dissociation during the chromatographic carbon-rich soil solution. Additionally, a pathways, namely the Al-OH and Al-SO 4 number of cations tend to lower the yield of monomers. Uncharged and probably nega- the reaction so that each sample should tively charged species such as AlF and * 3 have its own blank. This is poorly compati- AlF are eluted in the dead volume or in - 4 ble with series analysis. the eluent front and cannot be recovered in Shotyk and Sposito (1990) emphasized a defined chromatographic peak. The be- that the fluorescence quenching of organi- haviour of outersphere organic complexes cally complexed Al may be used for a of Al and the Al-PO and Al-Si monomers 4 quantitative determination of organic Al in along the chromatographic pathways re- simple aqueous solutions. Interfering ele- mains unknown, but data reported by Whit- ments remain unstudied and this method is ten et al (1992) suggest strongly that natu- in need of further developments before it is Al little ral organic undergoes suitable for natural water, if ever. decomplexation, if any. Whether or not Al, Al-PO and Al-Si polymers are eluted or 4 fixed by the resin is unknown. The main in- Alspeciationusing ion terest of this method resides in the fact chromatography that the calculation of every monomeric species involved in the third peak by equi- librium calculation provides a fairly good Al speciation was recently attempted by basis for the evaluation of Al toxicity. The ionchromatography coupled with post- use of the separation columns appears to column reaction with either tiron, pyrocate- be compatible with natural water samples chol violet or 8-hydroxiquinoline-5-sulpho- only at medium to low F/Al ratios. For the nate, and either UV or fluorescence detec- values of this ratio higher than 1.5 there is tion (Anderson and Bertsch, 1988; Bertsch a strong redistribution of AlF towards + 2 and Anderson, 1989; Willett, 1989; Gibson (AlF + AlF In addition, there is an im- + *). 23 and Willett, 1991; Jones, 1991; Whitten et portant but variable decomplexation of or- al, 1992). Fluorescence detection is suita- ganic Al, even with respect to some inner- ble for very low Al concentrations (as low sphere complexes. Uncharged species as 35 nM), UV detection to higher ones. be recovered in such * 3 AlF can one as Tiron would be preferred to pyrocatechol peak, however, and polymeric Al is fixed violet due to the inability of the latter to re- on the column, perhaps providing a useful veal some organically complexed Al. Either tool for the quantitative determination of guard columns or separation columns polymeric Al. The latter would equal unre- have been used. When the guard columns covered Al if all organic Al can react with are coupled with a 0.08-0.1 M K elu- 4 SO 2 Tiron. tion, 3 peaks are separated. The first in- volves the monovalent species that did not undergo dissociation during the chromato- Al speciation using electric methods graphic pathways, which were identified as AlF and at least the inner sphere organic + 2 complexes of Al. The second involves the Schmid et al (1989) obtained a reliable of Al in synthetic solutions, 3+ divalent species that were not dissociated quantification during the chromatographic pathways, ie natural soil leachates and aqueous soil ex- AlF and Al-humic acids complexes. tracts, by isotachophoresis, without any in- + 2 + 2 The third involves as a single Al species 3+ terferences of Al-sulphate ion-pairs. The
interferences of Al-F, Al-PO and Al-Si and reached about 1.4 kg NH, ·yr -1 -N·ha 4 4 15 kg NO 22 kg SO -1 -S·ha 4 , ·yr -1 -N·ha 3 which are widespread in acid complexes, ·yr 31 kg Cl·ha and 0.4 kg free ·yr -1 , -1 soils, were not investigated. As a conse- ·yr -1 ·ha + H (Becquer, 1991). Fluoride in- quence, this method requires additional puts occurred very rarely. studies before it can be applied to natural soil and surface waters. Leaching soil waters were continuously collected during 3 yr at depths of 15, 30 and 60 cm with zero tension Polyethylene- Alspeciation using plate-lysimeters, with the sampling fre- chemical equilibrium programs quency being determined by precipitations events. The determination of their chemi- cal composition was performed by colorim- The distribution of the various species of etry (NH ion chromatography (inorganic ), + 4 Al can be predicted by chemical equilibri- anions), flame-emission spectrophotome- um models, providing their equilibrium try (total K and Na) and inductively coupled constants are known. Sophisticated pro- plasma emission spectrometry (total Al, grams such as MINEQL (Schecher and + Ca, Fe, Mg, Mn and Si). Organic carbon McAvoy, 1992) and GEOCHEM-PC (Par- content was determined with a Carlo Erba ker et al, 1992) are now available for per- analyser, and the pH was measured with a sonal microcomputers under the DOS op- pH meter connected to a combined glass erating system. The former is character- electrode. All these data allowed the calcu- ised as being user-friendly and the lation of Al speciation with the MINEQL + database can be easily updated. Tempera- program, of which the database has been ture values can be specified and a number previously updated both by introducing of species and minerals can be added. new Al species and mineral and using re- Both organic and polymeric Al can be con- vised equilibrium constants (table I). The sequently computed with reasonable as- organic matter ("fulvate") molar concentra- sumptions. tion was derived from the carbon concen- tration by assuming that only 13% of the total C pertained to complexing functional APPLICATION: A CASE STUDY groups (mean of potentiometric titration IN A DECLINING SILVER FIR FOREST data) and that each organic molecule held IN THE VOSGES MOUNTAINS 2 of these functional groups (diprotic mod- (NORTH-EASTERN FRANCE) el). Ionic activities were calculated using the extended Debye-Hückel equation, the a values (ion-size parameter) being mostly To illustrate the previous considerations, those listed by Truesdell and Jones (1974) will briefly report hereafter some data we and Ritsema (1993), to which we added from a study performed in a declining sil- that of 12.6 for the Al polymer (Bottero et 13 ver fir (Abies alba) forest, located in the al, 1982b). According to the previous con- Mortagne watershed between St-Dié and siderations, the aluminium toxicity index Rambervillers (Haut-Jacques pass, E 6° (ATI) listed above and both the Ca/Al and 51’ - N 48° 17’). The stand has developed the Ca/H ratios were calculated on a sea- on an acid brown soil derived from the sonal basis. weathering of a triasic silty sandstone. The main analytical features of the soil are giv- Figure 1 shows the seasonal variations en elsewhere (Becquer, 1991). Atmos- of the ATI and those of both the Ca/Al and pheric inputs were found to be moderate the Ca/H ratios. With respect to the cal-
cium status, it must be emphasized that to this seasonal pattern origi- exceptions the Ca/H ratio was always > 0.1 and nated from occasional fluoride inputs de- ranged from 0.3 to 142. Values in the rived from atmospheric pollution and con- range 0.3-1 occurred mainly during au- comitant formation of non-toxic Al-F tumn and winter. Minimal calcium require- complexes. The values of the Ca/Al ratio ment can be satisfied throughout the year, followed the same seasonal pattern as thus, for coniferous trees at least. As a those of the ATI, but were often considera- consequence, the aluminium toxicity hy- bly lower, particularly in the upper hori- pothesis warrants consideration. With re- zons, where values close to the Al toxicity spect to the ATI, we must emphasize the threshold or below also occurred during absence of both Al and Al(OH) in the 13 - 4 spring and summer. As the ATI excludes soil solutions studied. Therefore, the toxici- the non-toxic species of Al and takes into ty index was not influenced by species with account the beneficial effect of Mg, it is be- very strong weighting coefficients. It can yond any doubt that it is more reliable than be observed that the ATI was affected by the Ca/Al ratio and that the latter overesti- strong seasonal variations, with values < mates Al toxicity in waters rich in fluoride 1.5 during autumn and winter only, empha- and organic matter. sizing the occurrence of an Al toxicity con- Whether the seasonal phases of Al tox- text during the rest phase of the vegetation icity indicated by the ATI reflected natural and also its spring disappearance. Some processes or anthropic pollution was inves- tigated in a companion study (Becquer et al, 1990, 1992). Seasonal protons budgets were found to be mainly under the depen- dence of nitrate flux. Despite the poor con- dition of the stand studied, a total uptake of nitrate was observed along the whole growth period of the vegetation, whether these ions originated from natural nitrifica- tion or from atmospheric inputs, resulting in a net alkalinisation phase. During au- tumn and winter, conversely, atmospheric inputs increased and nitrification continued at a rather high rate. Nitrate uptake was very low during this period, however, pro- ducing a net acidification phase. We ob- serve here that at the acidification period corresponds both a potential Al toxicity phase and a context of low Ca availability. These poor conditions disappeared during the alkalinisation phase. Although atmos- pheric inputs were higher in winter than during summer, the acidification/potential Al toxicity phases were obviously more re- lated to the seasonal vegetation rest phase than to the atmospheric inputs per se. The latter must be regarded as a minor compo- nent of Al toxicity, the main component
resting in the intrinsic ecosystem proper- and all the beneficial cations is a prerequi- ties and functioning. This point of view is site to any assessment of Al toxicity. The not so far from that of Baur and Feger application of these considerations allowed (1992), for whom natural soil processes us to observe a winter seasonal occur- have a greater influence than acid deposi- rence of a potential Al toxicity phase in a tion upon Al mobilisation in forested declining silver fir forest from the Vosges ecosystems with low to moderate acid highlands receiving moderate acid loads loads. The fact that even low winter acid (3.78 keq·ha By contrast, the use ). ·yr -1 loads may have cumulative effects on mo- of the Ca/Al ratio would erroneously sug- bilisation of base cations from the soil gest the occurrence of Al toxicity during a should not be overlooked, however, as this large part of the year, particularly in the could be regarded as responsible for the surface soil horizon. Nutrient uptake during poor base status of many acid soils being potentially low, it seems winter (Falkengren-Grerup and Eriksson, 1990; doubtful that the observed winter Al toxicity Hallbäcken, 1992; Joslin et al, 1992). context indicated by the ATI may constitute a decisive factor for the declining condition As it occurred during the vegetation rest of the stand studied. No generalisation can phase, the influence of such a winter Al be drawn with respect to other forests. toxicity context on forest decline can be questioned. Nutrient uptake during winter may be regarded as potentially low, and REFERENCES the impact of the toxic Al species on tree nutrition would be negligible during this season. Vogt et al (1980), however, have Achilli M, Ciceri G, Ferraroli R, Culivicchi G, shown that the development of the mycor- Pierri S (1991) Aluminium speciation in aque- ous solutions. Water Air Soil Pollut 57-58, rhized fine roots of Abies amabilis Dougl 139-148 occurred mainly during winter. In the Adams F, Lund ZF (1966) Effect of chemical ac- present study it can be hypothesized, tivity of soil solution aluminium on cotton root therefore, that such a process may be re- penetration of acid subsoils. Soil Sci 101 (3), stricted during winter. A better understand- 193-198 ing of tree root dynamics is needed to an- F, Moore BL (1983) Chemical factors af- Adams swer this question. root growth in subsoil horizons of fecting coastal plain soils. Soil Sci Soc Am J 47, 99- 102 CONCLUSION Alva AK, Edwards DG, Asher CJ, Blamer FPC (1986a) Effects of phosphorus/aluminium molar ratio and calcium concentration on By inhibiting Ca and Mg uptake, the toxic plant response to Al toxicity. Soil Sci Soc Am species of Al can be theoretically regarded J 50, 133-137 as constituting a factor contributing to for- Alva AK, Edwards DG, Asher CJ, Blamey FPC est decline. Alone or in conjunction with (1986b) Relationships between root length of other environmental factors, they could be soybean and calculated activities of alumin- involved in the discoloration of conifers on ium monomers in nutrient solution. Soil Sci acid soils. Neither total Al concentrations Soc Am J 50, 959-962 (or activities) nor too simple indexes, such Alva AK, Asher CJ, Edwards DG (1986c) The as the widely used Ca/Al ratio, can ac- role of calcium in alleviating aluminium toxici- count satisfactorily for the influence of sol- ty. Aust J Agric Res 37, 375-382 uble Al. The calculation of a suitable toxici- Alva AK, Sumner ME, Li YC, Miller WP (1989) ty index involving only the toxic Al species Evaluation of three aluminium assay tech-
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